The lecture notes in earth sciences

Evaluation of Geothermal Conditions in the Geological Past Edited by GL~nter Buntebarth and Lajos Stegena Springer-Verlag Berlin Heidelberg NewYork London Paris Tokyo... La]os Stegena

Evaluation of Geothermal Conditions in the Geological Past

Edited by GL~nter Buntebarth and Lajos Stegena

Springer-Verlag

Berlin Heidelberg NewYork London Paris Tokyo

Dr GLinter Buntebarth

Technische Universlt~.t Clausthal, Institut fur Geophysik

Arnold-Sommerfeld-Str 1, D-3392 ClausthaI-Zellerfeld, FRG

Prof Dr La]os Stegena

Institute of Environmental Physics, EStvSs-Unwersity

Kun B61a T~r 2, H-1083 Budapest, Hungary

ISBN 3-540-16645-9 Springer-Verlag Berlin Heidelberg N e w York

ISBN 0-38?-16645-9 Springer-Verlag N e w York Heidelberg Berlin

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During the l a s t decades, remarkable progress in heat flow studies has been made and

a rough p i c t u r e of the global surface heat flow density d i s t r i b u t i o n can now be drawn Simultaneously, the question of over which time period the surface heat flow

is constant arose

There is a big f i e l d of model c a l c u l a t i o n s , based on the changes i n r a d i o a c t i v e heat generation of the Earth, on plate motions, on s t r e t c h i n g hypotheses or on other ideas, which r e s u l t in geotherms in the geological past Although these speculative paleogeotherms seem to be r e a l i s t i c e s p e c i a l l y in oceanic areas they

do not belong to the scope of t h i s book In continental areas however, i t is not possible to f i n d a simple time dependence of the surface heat f l o w density How- ever, petroleum research and tectogenetic studies are very interested in the geo- thermal h i s t o r y of sedimentary basins and other continental areas To obtain

s a t i s f a c t o r y r e s u l t s , a more or less d i r e c t determination of paleo heat flow den-

s i t y or geothermal gradient would be necessary to give more c e r t a i n boundary con-

d i t i o n s f o r c a l c u l a t i n g o i l generation, and f o r c o n t r o l l i n g tectogenetic hypo- theses

There are many methods a v a i l a b l e in the geosciences to determine temperatures in the geological past Most of these models are able to estimate temperatures at which a mineral or a mineral assemblage was formed These methods, however, are mostly unsuitable to reach the main goal of paleogeothermics in general, which i s

to determine the (regional) heat flow density v a r i a t i o n s during the geological past f o r bigger geological u n i t s , such as sedimentary basins

The methods applied most in sedimentary basins have been deduced from the degree of

c o a l i f i c a t i o n of organic matter Although much e f f o r t has been made to explain

a n a l y t i c a l l y the organic metamorphism, the r e s u l t s found up to now have been

i n s u f f i c i e n t However, the widespread a p p l i c a t i o n of t h i s thermometer to estimate ancient thermal conditions is also r e f l e c t e d in the contents of t h i s very volume where the i n t e r p r e t a t i o n of the degree of c o a l i f i c a t i o n of organic matter plays

an important r o l e

viewpoint which favours methods s u i t a b l e to determine a paleothermal state of the upper c r u s t

Further c o n t r i b u t i o n s of t h i s book deal with

essential c o r r e c t i o n f a c t o r in heat flow density determinations,

evaluate thermal conditions in the past geological h i s t o r y ,

c o n d i t i o n of the Central Alps

Most of the c o n t r i b u t i o n s were presented at the symposium "Paleogeothermics" which was held at the 18 General Assembly of the I n t e r n a t i o n a l Union of Geodesy and Geophysics, August 15-27, 1983 in Hamburg/FRG

I t has been the f i r s t time t h a t such a symposium has been organized by the I n t e r - national Heat Flow Commission, and t h i s book presents an attempt to define paleo- geothermics under the auspices of the I n t e r n a t i o n a l Heat Flow Commission

Relations between c o a l i f i c a t i o n and paleogeothermics in Variscan

and A l p i d i c foredeeps of western Europe

TEICHMOLLER/TEICHMOLLER

The c o r r e l a t i o n of v i t r i n i t e reflectance with maximum temperature

in humic organic matter

Geothermal e f f e c t of magmatism and i t s c o n t r i b u t i o n to the maturation

of organic matter in sedimentary basins

The a p p l i c a t i o n of the degree of c o a l i f i c a t i o n of organic matter has received p a r t i c -

u l a r a t t e n t i o n as a means of estimating the geothermal h i s t o r y of sedimentary basins because the degree of c o a l i f i c a t i o n is mainly influenced by the temperature of the environment and the time of exposure at t h i s temperature Several empirical i n t e r -

p r e t a t i o n methods are reported which have been developed f o r s p e c i f i c basins and which are e s p e c i a l l y v a l i d f o r these areas

During c r y s t a l growth, l i q u i d s and other phases can be entrapped in the host c r y s t a l These entrapped phases preserve the temperature and the pressure conditions which were present at the time of c r y s t a l growth

Chemical reactions are temperature s e n s i t i v e Therefore, s o l u t i o n e q u i l i b r i a and isotope exchange reactions are applied to estimate paleothermal c o n d i t i o n s , or to compare the calculated reaction temperature with the present thermal state in p a r t i c -

u l a r areas

A recent s u c c e s s f u l l y tested method which deals with the transformation of minerals during diagenesis is reported Clay minerals, z e o l i t e s and quartz polymorphs are transformed in sedimentary rocks of s i m i l a r composition at d i s t i n c t temperatures Another method is reported which analyses the color a l t e r a t i o n of conodonts This method is applicable f o r sedimentary rocks from the Late Cambrian to the T r i a s s i c period when the conodonts l i v e d

Lecture Notes in Earth Sciences, Vol 5 Paleogeothermics Edited by G Buntebarth and L Stegena

rocks Because each r a d i o a c t i v e system has i t s own closure temperature, radiometric age determinations give the ages at which a rock cooled down to the respective c l o - sure temperature

Since organic l i f e grows on the earth, i t is included in the geological cycle The remains of the organic matter are covered by sediments or deposited together with

c l a s t i c d e t r i t u s I f the circumstances are favourable, the organic matter is pre- served and subsides w i t h i n a sedimentary basin During subsidence i t undergoes i n - creasing pressure as well as temperature, and both gradually a l t e r the o r i g i n a l

m a t e r i a l The a l t e r a t i o n of organic matter is known as diagenesis and process of

c o a l i f i c a t i o n There are two factors which govern predominantly the rank o f c o a l i -

f i c a t i o n , which are the temperature in the depth where the organic matter existed during i t s h i s t o r y , a n d the time o f i t s exposure An i n t e r p r e t a t i o n of the degree of

c o a l i f i c a t i o n based on the e f f e c t of temperature and time of exposure to t h a t tempe-

r a t u r e , can be of l i m i t e d value only More care must be taken on the o r i g i n o f organic material and the f i r s t steps in i t s s t r u c t u r a l and chemical decomposition

in d i f f e r e n t environments

The oldest coals which seem to be o f plant o r i g i n are preserved in rocks of Algonkium age in North America Several l o c a l i t i e s with coal embedded in a sedimentary sequence are known in the Lower Devonian Since Middle and Upper Devonian, when plenty of plants grew on the continent and on the submerged shore, coal seams are more common The most prominent bituminous coal deposits are of Carboniferous age in Europe and North America, and o f Permian, T r i a s s i c and Jurassic age in South A f r i c a , Eastern

A u s t r a l i a and India

Since Cretaceous, much more v a r i e t y in the f l o r a has been created which implies more heterogeneity in the plant remains from which the coaly matter o r i g i n a t e s The coals are formed not only from d i f f e r e n t plant communities but also at d i f f e r e n t environ- mental conditions which are summarized by M TEICHMOLLER & R TEICHMDLLER (1981)

I t is important, t h a t the plants or t h e i r remains have to be deposited under condi- tions with r e s t r i c t e d oxygen supply Usually, t h i s condition is present in swampy areas I f a sedimentary basin with swampy areas subsides g r a d u a l l y , the organic mat-

t e r can be deposited in layers o f some thickness A warm or temperate to cool climate with high humidity throughout the year is necessary to r e t a i n the condition favoured

f o r organic deposition

There are a few peat-forming plant communities which grow in d i f f e r e n t swamp types,

i e moss swamp, f o r e s t swamp, open reed swamps and p a r t l y submerged areas with water

nomic~l coal seams y i e l d from deposition in swamps, in general As well as in coal seams, organic matter is also present in a dispersed form in many minerogene sedi- mentary rocks Plant remains in r i v e r deltas and on the shores of lakes and oceans, barks, other p l a n t d e t r i t u s , and also coal which can be redeposited, can be covered

by c l a s t i c sediments and buried I f the environmental conditions are favourable f o r preservation, the organic substances undergo the diagenesis during the subsidence, and w i l l become coaly p a r t i c l e s l i k e the plant remains in swamps However, there is

a d i f f e r e n c e The plant remains are exposed not only to the mechanical treatment during the t r a n s p o r t by water, but also to the o x i d i z i n g atmosphere and to the bacte-

r i a l a c t i v i t y at the surface which favours the preservation of e s p e c i a l l y r e s i s t e n t

p a r t i c l e s This means t h a t the o r i g i n a l organic substance is not e x a c t l y the same as

in seams The composition of the organic matter bearing rocks is of some importance too The organic matter is often oxidized in sandstones, e s p e c i a l l y in red-coloured ones, but i s r a t h e r seldom in limestone Usually c l a y and s i l t s t o n e s are the rock types from which the organic p a r t i c l e s can be observed and i n t e r p r e t e d f o r paleogeo- thermal i n v e s t i g a t i o n s

There are a d d i t i o n a l factors i n f l u e n c i n g the composition of the organic substance which y i e l d s the coaly matter Whereas organic deposits under t e r r e s t r i a l and sub- aquatic conditions are comparable, marine-influenced and c a l c i u m - r i c h swamps produce coals r i c h e r in ash, sulphur and nitrogen These conditions imply t h a t a d i f f e r e n t

a c i d i t y of water may produce coals of same d i s t i n g u i s h a b l e p r o p e r t i e s , even with the same o r i g i n a l m a t e r i a l I t seems t h a t the b a c t e r i a l a c t i v i t y is a most important

f a c t o r c o n t r o l l i n g the decomposition of plants and thereby at l e a s t the o r i g i n a l materials f o r the coals Therefore, a l l environmental properties which favour or prevent b a c t e r i a l l i f e also define the properties of the coal V i t r i n i t e is a most common c o a l i f i c a t i o n product which is formed from organic deposits under some acid

c o n d i t i o n I f the environment is neutral to weakly a l k a l i n e , the b a c t e r i a l a c t i v i t y

is very high Since the protein of the bacteria is also accumulated, the organic substances y i e l d hydrogen-rich bituminous products which form b i t u m i n i t e and weakly

r e f l e c t i n g v i t r i n i t e s during subsidence (M TEICHMOLLER & R TEICHMOLLER, 1981)

Peat is the f i r s t stage in the diagenetic process of the organic matter P e a t i f i c a -

t i o n can s t a r t a f t e r the b u r i a l of plant remains with the help of the b a c t e r i a , which are a c t i v e down to some meters of depth With continuing subsidence, the i n - creasing overburden pressure causes the water to be squeezed out of the organic

20 to 50 ° C At the upper l i m i t of the temperature range, l i t t l e methane i s s p l i t o f f (van HEEK et a l , 1971), and the transformation from peat to brown coal is u s u a l l y reached in a depth range between 200 m and 400 m At temperatures of about 70 to

v o l a t i l e bituminous coal gradually changes to semi-anthracite, large q u a n t i t i e s of methane develop

The rank of coal is determined in a general way by appearances and/or by i t s proper-

t i e s , e.g b r i g h t brown coal and gas coal This q u a l i t a t i v e scale is not s u f f i c i e n t

f o r a n a l y t i c a l i n v e s t i g a t i o n s The composition of organic matter in sediments is

90 % kerogen and 10 % bitumen (hydrocarbon, r e s i n , asphaltene) The f r a c t i o n soluble

in organic solvents, is c a l l e d bitumen, whereas the other f r a c t i o n , i n s o l u b l e in organic matter, is termed kerogen There are methods to estimate the m a t u r i t y by examining the soluble organic matter: percentage carbon in bitumen, carbon preference index (odd carbon number compounds to even carbon number), p a r a f f i n p r o f i l e , percen- tage wet gas Other, more important methods, examine the kerogen as a maturation index These methods are the kerogen a l t e r a t i o n index KAI, thermal a l t e r a t i o n index TAI, p y r o l y s i s , elementary CHO a n a l y s i s , and atomic H/C r a t i o

A l l these chemical rank parameters are not applicable in general f o r rocks with f i n e -

l y dispersed organic matter, because the chemical methods need some amount of organic

p a r t i c l e s The rank determination with microscope is successful The method is not

d e s t r u c t i v e f o r the sample, and is easy to apply V i t r i n i t e i s the most common coal maceral, and is the one taken in order to measure i t s o p t i c a l r e f l e c t i v i t y at the polished sample under o i l , applying monochromatic l i g h t This method is applicable to both the coal from seams and the coaly p a r t i c l e s dispersed in sedimentary rocks

V i t r i n i t e r e f l e c t i v i t y is a r a t i o of the i n t e n s i t y of the r e f l e c t e d l i g h t and the source l i g h t , expressed in percent, using v i t r i n i t e (= woody kerogen) as the r e f l e c -

t o r The value is often simply c a l l e d R o, % R o, or % Rm the subscript "o" designates

t h a t the measurement was made in o i l , and "m" means the mean r e f l e c t i v i t y , instead

of Rmax, the maximum r e f l e c t i v i t y , which should be applied at r e f l e c t i v i t y values above 4 % Rm

The r e f l e c t i v i t y c o e f f i c i e n t gives a continuous scale f o r the c o a l i f i c a t i o n of humi-

MOLLER, 1970) Huminite and v i t r i n i t e are maceral groups of humous components, where huminite is the precursor of v i t r i n i t e in peat and brown coal During the progress in c o a l i f i c a t i o n huminite i s converted i n t o v i t r i n i t e between the c o a l i f i - cation stages of d u l l and b r i g h t brown coal

I f some rocks are so poor in organic matter t h a t concentrates must be prepared by chemical or physical methods, i t is much more d i f f i c u l t to determine the correct degree of c o a l i f i c a t i o n The surroundings of the p a r t i c l e s are often h e l p f u l

coal macerals, i e v i t r i n i t e , poses the greatest d i f f i c u l t y in the determination of the degree of c o a l i f i c a t i o n in rocks For t h i s determination the so-called "kero- bitumen" which can be found in bituminous shales is of some importance The b i t u - minous matter r e f l e c t s in the lower rank of c o a l i f i c a t i o n less than v i t r i n i t e , but more in the rank of a n t h r a c i t e The d i s t i n c t i o n between recycled and authochtonous organic matter is often d i f f i c u l t in rocks, but nearly impossible in concentrates There are a l o t of problems a r i s i n g from the selection of macerals f o r measurements, which are described more d e t a i l e d e.g in STACH et a l (1982), ROBERT (1985), TISSOT

& WELTE (1978)

Besides the r e f l e c t i v i t y of v i t r i n i t e in shales, sandstones and limestones with d i s - persed coaly p a r t i c l e s , the spectral fluorescence measurements on s p o r i n i t e has been introduced as an i n d i c a t o r of the degree of diagenesis I f s p o r i n i t e is i r r a d i a t e d with u l t r a v i o l e t l i g h t (A=365 + 30 nm), a v i s i b l e fluorescence can be observed from

y e l l o w to dark red colour However, the s p o r i n i t e fluorescence spectra are observed

at low grades o f diagenesis only, i e from the stage of peat to t h a t of high v o l a -

t i l e bituminous coal (OTTENJANN et a l , 1974)

Both parameters, the r e f l e c t i v i t y of v i t r i n i t e and the s p o r i n i t e fluorescence, are used together to f i n d a more correct degree of diagenesis The i n t e r p r e t a t i o n of the rank of c o a l i f i c a t i o n f o r paleogeothermics is based on the f a c t that the temperature

is the most important f a c t o r t h a t increases the degree of c o a l i f i c a t i o n , but the duration of heating must also be taken i n t o consideration The influence of pres- sure, however, seems to be n e g l i g i b l e Based on HUCK & KARWEIL (1955), LOPATIN (1971) gave a simple scheme f o r describing the degree of c o a l i f i c a t i o n Supposing that the

c o a l i f i c a t i o n process is to be treated as a f i r s t order chemical r e a c t i o n , the Arrhenius' equation i s v a l i d and the v e l o c i t y of the reaction (k) depends exponen-

f a r from room temperatures, because t h e i r a c t i v a t i o n energy

and the parameter which describes the rank o f c o a l i f i c a t i o n

t *

where T ( t ) is the temperature of the layer during the time i n t e r v a l dt, and t * is

For p r a c t i c a l reasons, LOPATIN introduced the sum instead of the i n t e g r a l , d i v i d i n g the whole temperature h i s t o r y of the layer i n t o 10 ° C temperature i n t e r v a l s He then

a r b i t r a r i l y chose the 100 to 110 ° C temperature i n t e r v a l (which is the mean domain

of o i l generation) as the base i n t e r v a l and assigned to i t an index value of n=O,

to the 120- 130 ° C i n t e r v a l n = 2 , to the 9 0 - 1 0 0 ° C i n t e r v a l n = - 1 , and so on The maturity parameter calculated in t h i s manner was c a l l e d the Time Temperature Index ( T T I ) ,

nmin

nmax where ~ t n is the time i n t e r v a l ( i n Ma) the l a y e r spent in the n-th 10 ° C temperature

i n t e r v a l , and nmax and nmin are the n-values of the highest and lowest temperature

i n t e r v a l s occurring in the thermal h i s t o r y of the l a y e r

l a t i o n of the Time Temperature Index f o r a layer

l y i n g at a depth of 2300 m, aged 20 Ma TTI is

c h a r a c t e r i s t i c f o r the maturity of organic matter

Fig I demonstrates the method of c a l c u l a t i o n of TTI, f o r a hypothetical layer 20 Ma old and l y i n g at present at a depth of 2300 m Let us suppose t h a t the subsidence and

b u r i a l h i s t o r y of the layer during geologic time was determined as shown by the curve

of Fig I Let us then suppose t h a t the present geothermal gradient is 50 mK/m, and the gradient was constant during the whole sedimentary h i s t o r y , as shown in Fig I ,

by the horizontal s t r a i g h t geotherms In t h i s case f o r the layer of Fig I ,

TTI= ;5.2

Based on 402 thermal maturity (R o) data from 31 worldwide wells, WAPLES (1980) de- termined a correlation between TTI values calculated for each borehole from burial

h i s t o r i e s , supposing the v a l i d i t y of present geothermal conditions during the geolo- gical past, and R o values measured (Fig 2)

l e f t hand column, with and without correc- tion of compaction (STEGENA et a i , 1 9 8 1 )

Using porosity-depth functions and/or other considerations, the sedimentary

h i s t o r i e s are corrected f o r the e f f e c t of compaction during the geological past (Fig 3, s o l i d l i n e s ) (DU ROUCHET, 1980; STEGENA et a l , 1981; FALVEY & DEIGHTON, 1982)

Based on present borehole temperatures, the geotherms f o r each 10 ° C round i n t e r - val are constructed in the time-depth section (Fig 4, l e f t ) with the present heat flow during the geological past The constancy of heat flow during the past does not r e s u l t in p a r a l l e l and e q u i d i s t a n t s t r a i g h t l i n e s ; i t is possible to take i n t o consideration the probable changes with time and depth of thermal con-

d u c t i v i t y of the l a y e r s , with the aid of the l i t h o l o g y and b u r i a l h i s t o r y of the borehole

density was constant through the sedimentary h i s t o r y ( l e f t ) , and t h a t the bore- hole was heated up during the l a s t 5 Ma f o r the present heat flow value (STEGENA

et a l , 1981)

A f t e r c a l c u l a t i n g the TTI values f o r each layer of the borehole the TTI-s are

(Fig 2)

hole The discrepancy between calculated and measured values is a t t r i b u t e d to the

v a r i a t i o n s of heat flow during the geological past Using p l a u s i b l e hypotheses, one makes a change in the past heat flow (Fig 4, r i g h t ) and repeats the compa-

between calculated and measured v i t r i n i t e reflectances is achieved (Fig 5)

crease ( l i n e a r l y ? ) to i t s present value ( - 1 0 0 mW/m2)

in the Pannonian basin) and the v i t r i n i t e reflectances calculated from the f o l l o w - ing heat flow s t o r i e s : the heating-up o f the boreholes began at co, 5, 2, I Ma ago ( a f t e r STEGENA et a l , 1981)

The above scheme serves b e t t e r to understand the p r i n c i p l e s of the paleogeothermal

c a l c u l a t i o n s , but does not present a f i n a l s o l u t i o n of the question There are some fundamental problems in the o i l geochemistry which are not solved s a t i s f a c -

t o r i l y and which can influence the above sketched model

I t became usual to assume that increases in v i t r i n i t e reflectance values were

v a l i d i n d i c a t o r s of the extent to which organic matter maturated and o i l genera-

t i o n had occurred (WAPLES, 1983) However, there is an uncertainty in some R

o measurements, because the values have a wide spread, and sometimes i t is hard to

d i s t i n g u i s h low r e f l e c t i n g r e s i n i t e and high r e f l e c t i n g fusunite from v i t r i n i t e s (HO, 1978) During the beginning o f o i l generation, bitumen impregnations lower the v i t r i n i t e r e f l e c t a n c e In a l l red-coloured rocks organic matter is o x i d i z e d ;

in limestones v i t r i n i t e is very r a r e l y preserved and i f i t occurs, the reflectance value d i f f e r s from the value o f v i t r i n i t e in the same rank RONSARD & OBERLIN (1984) suggest t h a t , as with any other e l e c t r o n i c property of any s o l i d , r e f l e c - tance depends on three parameters: chemical composition, atomic structure and microstructure The same value f o r reflectance can thus be measured f o r materials

d i f f e r e n t in t h e i r microstructure and chemical composition, which can be of d i f f e -

rent ranks or not They suggest the use of transmission electron microscopy (TEM)

by using successive heat treatment in an i n e r t atmosphere to 1000 ° C, which bet-

t e r characterizes the maturation of organic m a t e r i a l s

I t is g e n e r a l l y supposed t h a t pressure does not have a s i g n i f i c a n t e f f e c t on the maturation o f organic matter and on the amount o f hydrocarbon generated I t is to

be noted however, that the r o l e o f pressure in o i l generation has never been exa- mined properly (WAPLES, 1983)

The maturation of organic matter e x h i b i t s a very complex process, i n v o l v i n g a l o t

of p a r a l l e l chemical reactions with various a c t i v a t i o n energies, and the whole process can hardly be described by a f i r s t - o r d e r k i n e t i c expression (SIEVER, 1983) This was also shown by p y r o l y s i s experiments (CUMMINGS & ROBINSON, 1972) LASAGA (1981) has compiled a t a b l e of a c t i v a t i o n energies f o r geochemical reactions that

TISSOT & ESPITALIE (1975), TISSOT et a l (1975), and JONTGEN & KLEIN (1975) have modelled the thermal a l t e r a t i o n of kerogen with a set of f i r s t - o r d e r rate equa-

l a t e d This process although g i v i n g a b e t t e r t h e o r e t i c a l approximation, is hardly

a p p l i c a b l e f o r paleogeothermal a p p l i c a t i o n s

LOPATIN (1971) tested his model on a very d i f f i c u l t w e l l , MUnsterland I/FRG Recali- bration of Lopatin's method with l a r g e r and more r e l i a b l e data sets (WAPLES, 1980; KETTEL, 1981) has v e r i f i e d the general v a l i d i t y of the model i t s e l f , but has modi-

f i e d Lopatin's o r i g i n a l T T l - v i t r i n i t e r e f l e c t i v i t y c o r r e l a t i o n LOPATIN & BOSTICK (1973) and LOPATIN (1976) l a t e r suggested some improvements to the o r i g i n a l scheme LOPATIN (1976) used fewer and l a r g e r temperature i n t e r v a l s ; instead o f ~T= 10 ° C,

1.37T 2

~T = ~ 3 7 T (T in Kelvin, E a c t i v a t i o n energy=42 kJ/mole)

120 ° C<T<170 ° C, and 30 ° C f o r AT above 170 ° C

The diagenesis of organic matter accelerates e x p o n e n t i a l l y with temperature In the whole process, the time which the layer under consideration passed away at maximum temperatures, plays a decisive r o l e HOOD et a l (1975) worked out a model, in which the period spent w i t h i n 15 ° C of the rock's maximum paleotemperature was taken i n t o consideration For the maturation of organic matter, and i n d i r e c t l y , f o r the v i t r i -

n i t e r e f l e c t i v i t y , they created a scale of thermal maturity c a l l e d the "level of organic metamorphism" (LOM), which is c o n t r o l l e d only by the maximum temperature sur- vived by the l a y e r , and by the " e f f e c t i v e heating time" spent by the layer w i t h i n

15 ° C of the rock's maximum temperature (Fig 6) S t a p l i n ' s s i m i l a r scale (TAI, thermal a l t e r a t i o n index) is based on microscopic s t r u c t u r e v a r i a t i o n and the colour- ing of organic debris

l i e v e d to be a c t i v e l y subsiding and so s a t i s f y i n g the h i g h l y probable assumption

t h a t samples from these basins are now at maximum temperature since b u r i a l But ESR signals are not only dependent on temperature but are also subject to v a r i a t i o n s in kerogen type, diagenetic changes in kerogen, weathering and geologic time

PRICE (1982) improves the idea t h a t v i t r i n i t e r e f l e c t i v i t y depends f i r s t of a l l on maximum temperature A p l o t of R o versus present temperature from a number of areas

t h a t have not undergone major geologic m u t i l a t i o n , increases in a s t r i c t l y l i n e a r

f a s h i o n ( r = 0 9 7 ) yet b u r i a l times f o r these d i f f e r e n t areas range from 0.3 to

240 Mao He suggests t h a t some geochemical postulates are in e r r o r and t h a t time has

l i t t l e e f f e c t on organic maturation I t appears t h a t v i t r i n i t e r e f l e c t i v i t y can be used as an absolute paleogeothermometer from 20 ° C to at least 400 ° C

MIDDLETON & FALVEY (1983) propose, f o r s i m p l i c i t y , t h a t maturation (C) and R o are

r e l a t e d by the equation

In R o = A + BC

Empirical studies give A=-2.275 and B=0.177

For maturation C, they accept Lopatin's o r i g i n a l idea with i n s i g n i f i c a n t m o d i f i c a t i o n (AT= 10.2 instead of 10 ° C) and f o r s i m p l i c i t y use the logarithm of the previously given i n t e g r a l

t

C = I n ~ 2 T ( t ) / 1 0 " 2 d t

(as used by ROYDEN et a l , 1980 and DE BRAEMAEKER, 1983)

time:

t

(Ro)a = b~o exp[c T(t)] dt

where a=5.635, b = 2 7 1 0 -6 Ma - I and c =0.068 ° C -I

Given the thermal h i s t o r y of an organic sediment T ( t ) , t h i s equation can be used to

p r e d i c t the v i t r i n i t e r e f l e c t i v i t y of the sediment a f t e r a time t Nor does WELTE& YOKLER's (1981) equation add more to t h a t formulated by LOPATIN (1971) and WAPLES (1980):

R o [%] = 1.301 l g ( T T I ) - 0 5 2 8 2 BUNTEBARTH (1978) t r i e d to c a l c u l a t e paleogeothermal gradients, as f a r as possible

w i t h o u t theory I t is c l e a r t h a t a r e l a t i o n s h i p e x i s t s between the coal rank,

measured by the mean o p t i c a l r e f l e c t i v i t y of v i t r i n i t e (Rm), and the i n t e g r a l of depth and duration of b u r i a l A c o r r e l a t i o n has been evaluated between the square of

v i t r i n i t e r e f l e c t i v i t y and the b u r i a l h i s t o r y :

Furthermore, i t is clear t h a t , in t h i s r e l a t i o n s h i p , the coal rank is proportional to

a function of the geothermal gradient,

0 Geothermal gradients measured at present in the four boreholes in Fig 7 allowed the c a l i b r a t i o n of the empirical equation The a p p l i c a b i l i t y of t h i s equation f o r other areas is investigated in BUNTEBARTH & MIDDLETON ( t h i s volume)

Fig 7 Relation between the mean v i t -

r i n i t e reflectance (Rm) and the i n t e - gral of depth and time, in four bore- holes of the Upper Rhinegraben (BUNTEBARTH, 1979)

Based on maximum measured v i t r i n i t e r e f l e c t i v i t y data (HACQUEBARD, 1977) and burial

h i s t o r y of 28 wells l y i n g in the Central Prairies Basin, Canada, MAJOROWICZ & JESSOP (1981) estimated a lower average paleogeothermal gradient (27 mKm -I ) f o r the early Oligocene time than the present day one (30.6 mKm -I in average) (Fig 8) For the

c a l c u l a t i o n , they used the method of KARWEIL (1956) with BOSTICK's (1973) modifica- tions and the method proposed by HOOD et a l (1975)

w i t h the histogram of present geothermal gradients,

in the Central P r a i r i e s basin, Canada (MAJOROWICZ

& JESSOP, 1981)

EGGEN (1984) worked with a l o t of v i t r i n i t e r e f l e c t i v i t y data but present heat f l o w estimations only He stated t h a t in the Viking Graben (Norwegian North Sea) the c a l - culated paleo heat flow density (approx 55 mWm -2) f i t s well with the present heat flow estimation ( 5 0 - 6 0 mWm-2); on the f l a n k of the Viking Graben, however, an

= 2

- 2 observed m a t u r i t y , while the present day estimation l i e s at 70 mWm

WANG JI-AN et a l ( t h i s volume) found t h a t the c o a l i f i c a t i o n gradient increases from 0.25 to 0.65 % Ro/km from middle to e a r l y Eocene, in the western part of Liaohe o i l

f i e l d , North China, and a c o a l i f i c a t i o n gradient of about 0.4 was determined in the

e a r l y T e r t i a r y sediments of the Central Hebei o i l f i e l d KARWEIL's (1956) and

LOPATIN's (1971) methods were used f o r paleotemperature reconstructions In c o n t r a s t , RYBACH (1984) gives 0 0 9 - 0 0 5 % Ro/km c o a l i f i c a t i o n gradients f o r the Northern Alpine Foreland (Molasse basin)

Labrador shelves, based on t h e o r e t i c a l l y derived thermal e v o l u t i o n , and on LOPATIN's theory A s i m i l a r work was carried out by ROYDEN et a l (1980), f o r the Falkland Plateau and f o r three places of the North A t l a n t i c

BUNTEBARTH (1983,1985) estimated the paleotemperature gradient as well as the heat

f l o w density in a few sedimentary basins in the F.R.G

In the Ruhr Basin, the foredeep of the Rhenish Variscan mountains, f o r which many data are a v a i l a b l e (BUNTEBARTH et a l , 1982), the heat f l o w decreased during West- phalian C from about 125 mW/m 2 to about 105 mW/m 2 Because of the low thermal con-

d u c t i v i t y of the coal, the temperature gradients reached mean values of 79 ° C/km before, and 65 ° C/km a f t e r t h i s decrease in heat f l o w data obtained

The thermal regime of the back-deep of the Rhenish Variscan mountains, t h a t of the Saar Basin, is nearly the same as t h a t of the Ruhr Basin during the Westphalian (BUNTEBARTH, 1983) A s i m i l a r high heat flow is indicated in other European Carboni- ferous basins, e.g ROBERT (1985)

Within the Lower Saxony Basin, the Upper Carboniferous coal beds were heated by

i n t r u s i v e bodies during the Upper Cretaceous - about 200 Ma a f t e r sedimentation Model c a l c u l a t i o n s t h a t take the cooling of the Massif of Bramsche i n t o account,

i n d i c a t e t h a t temperature gradients between 60 and 80 ° C/km existed w i t h i n the coal bearing s t r a t a From model c a l c u l a t i o n s , p r i o r to the magmatic heating, the tempera- ture gradient did not exceed 30 to 40 ° C/km during maximum b u r i a l (BUNTEBARTH, 1985)

The paleogradient derived f o r the borehole Urach 3 (Swabian Alb) f o r t u i t o u s l y agrees with the measured present day gradient The paleogradient of 43 ° C/km corresponds to Cretaceous to Lower T e r t i a r y times, because c o a l i f i c a t i o n ended p r i o r to the Upper

T e r t i a r y u p l i f t (BUNTEBARTH & TEICHMOLLER, 1982)

The thermal regime of the middle Upper Rhine Graben changed during the T e r t i a r y Temperature gradients during the Lower T e r t i a r y were higher than those during the Upper T e r t i a r y The values ranged, r e s p e c t i v e l y , from 48 to 78 ° C/km, and from 34 to

50 ° C/km (BUNTEBARTH, 1978) A d i r e c t r e l a t i o n s h i p may e x i s t between volcanic a c t i v -

i t y in the Graben, and the high thermal gradients, both documented f o r the period immediately a f t e r the opening of the Graben

From these few data, i t can be t e n t a t i v e l y concluded t h a t the Upper Cretaceous was

a time of widespread high thermal gradients Furthermore, high gradients also ex- isted during the Upper Carboniferous in northern Germany, and during the Lower Ter-

i d e n t i f i e d MACKENZIE & McKENZIE (1983) have investigated the rates of three reac-

t i o n s which occur before and during the e a r l y stages of o i l formation Two of the reactions are isomerization reactions, at C-20 in a sterane and at C-22 in a hopane hydrocarbon; the t h i r d reaction converts C-ring monoaromatic to t r i a r o m a t i c s t e r o i d

hydrocarbons A l l three reactions were assumed to be f i r s t order and monomolecular; the isomerization reactions are r e v e r s i b l e , with a rate of conversion of the R to the S form of 1.174 and 1.564 resp., while the aromatization reaction was assumed to

be i r r e v e r s i b l e

This method excels by i t s c l e a r t h e o r e t i c a l (thermodynamic) p r i n c i p l e s , the Arrhenius equation is c e r t a i n l y v a l i d f o r these reactions The problem, however, is t h a t f r e - quency f a c t o r and a c t i v a t i o n energy cannot be determined in the l a b o r a t o r y or only very i n a c c u r a t e l y , because of the slowness o f the reactions Because of t h i s ,

McKENZIE's (1978) stretching theory f o r the e v o l u t i o n of sedimentary basins was used

to c a l i b r a t e the reactions This theory involves a thermal h i s t o r y , which can be derived s u f f i c i e n t l y accurately from the b u r i a l h i s t o r y Based on chemical analyses

of North Sea and Pannonian Basin cores, and using more or less determined or hypo-

t h e t i c a l stretching models, the kinematics of the three reactions were determined

in Table I

Frequency f a c t o r (s-l)

A c t i v a t i o n energy (kJ mol -I )

91

91

200 Fig 9 shows the r e s u l t s f o r the Pannonian basin, which are in a c e r t a i n agreement with the t h e o r e t i c a l curves derived with the assumption t h a t the stretching r a t e , B ,

is 2 (SCLATER et a l , 1980) A s i m i l a r study was c a r r i e d out by HOFFMANN et a l (1984) f o r the Malakam Delta, Kalimantan, Indonesia, and SAJGO et a l (1983) f o r the Pannonian basin A r t i c l e s of SAJGO & LEFLER ( t h i s volume) give d e t a i l e d information about the a p p l i c a b i l i t y of some marker reactions to paleogeothermal determinations

hydrocarbon aromatization f o r samples of the borehole HOD in the Pannonian basin The curves are calculated t h e o r e t i c a l l y , based on the thermal h i s t o r y of the basin from McKENZIE's (1978) stretching theory The basin is assumed to have been formed

by sudden extension ( B = 2 ) , 15 Ma ago The marks on the curves are present tempera- tures at 5 ° C i n t e r v a l s (above) and present depths at 200 m i n t e r v a l s (below) (MACKENZIE & McKENZIE, 1983) The l e f t lowest diagram shows the approximate thermal

h i s t o r y which belongs to various ~ values

2 F l u i d i n c l u s i o n thermometry

In nearly a l l minerals, whether ores, rock forming minerals or others, small amounts

of f l u i d s are entrapped in the host c r y s t a l which preserve the physical and chemical conditions of the surrounding medium during the time of the c r y s t a l growth I t is generally assumed t h a t no subsequent change in the entrapped material takes place (LEMMLEIN, 1956; ROEDDER, 1967)

At the time of entrappment, the f l u i d i n c l u s i o n is a homogeneous phase c o n s t i t u t i n g mainly of water, s a l t ( i n general sodium c h l o r i d e ) and some amount of carbon d i o x i d e , and also s i l i c a t e melt Since the thermal expansion of the f l u i d / m e l t i s greater than t h a t of the mineral, a vapour bubble is formed w i t h i n the c a v i t y when the tem- perature decreases

The f l u i d i n c l u s i o n s are formed by c r y s t a l growth when the advancing faces, edges and corners of the growing c r y s t a l are disturbed (primary i n c l u s i o n s ) , by f r a c t u r i n g and healing of c r y s t a l s during mechanical disturbances or by overgrowth of a crystal (secondary i n c l u s i o n s ) (PAGEL & POTY, 1983)

From rock or mineral samples, t h i n sections (-80pm) are prepared which are mounted

on a glass plate and polished on both sides The t h i n section is heated up under a microscope by using a heating stage (e.g OHMOTO & RYE, 1970) At a c e r t a i n temper-

a t u r e , the bubble disappears in the i n c l u s i o n The heating is then reversed to cooling u n t i l the entrapped f l u i d / m e l t becomes an inhomogeneous phase again The temperature at t h i s condition is measured and c a l l e d the homogenization temperature This p a r t i c u l a r temperature is related to the temperature of formation However, the pressure of formation has to be involved Generally, an increase in pressure re- quires a greater temperature to complete homogenization (ROEDDER, 1967; SIGURDSON, 1974; POTTER, 1977; BOWERS & HELGESON, 1983a,b) The pressure c o r r e c t i o n is d i f f e - rent f o r s i l i c a t e melt i n c l u s i o n s with shrinkage vapour bubbles ( o n l y - 2 0 ° C/kb) and f o r more compressible f l u i d s with water and carbon dioxide (ROEDDER, 1982) Since the f l u i d i n c l u s i o n s contain a substantial concentration of sodium c h l o r i d e , the pressure correction can be applied, i f t h i s concentration is known The thermo- dynamic properties of aqueous s o l u t i o n s are affected remarkably by the concentra-

t i o n of sodium c h l o r i d e The pressure c o r r e c t i o n reported by POTTER (1977) is based

on : o l u m e t r i c properties of the NaCI-H20 system The c o r r e c t i o n by BOWERS & HELGESON (1983a,b) works at pressures above 50 MPa and high temperatures from 350 to 600 ° C;

a d d i t i o n a l to the graphs of the ternary system, FORTRAN programs are given to gene- rate the pressure correction (BOWERS & HELGESON, 1985) which is based on a modified REDLICH & KWONG (1949) equation of state matching the pressure-volume-temperature data reported by GEHRIG (1980)

P r i o r to the pressure c o r r e c t i o n , the sodium c h l o r i d e content must be analyzed The

s a l i n i t y of the f l u i d in i n c l u s i o n s can be estimated by the depression of the f r e e z - ing point during cooling The higher the s a l i n i t y , the higher is the depression of the freezing p o i n t ROEDDER (1962) reports the freezing point data f o r pure s o l u t i o n s

of sodium c h l o r i d e Applying t h i s data to f l u i d i n c l u s i o n s , the s a l i n i t y o f the brine can be given in sodium c h l o r i d e equivalent only, since the f l u i d consists of other

c o n s t i t u e n t s , and t h i s mixed s a l t s o l u t i o n causes some u n c e r t a i n t y in the determina-

t i o n of the sodium c h l o r i d e content (ROEDDER, 1976) The freezing point of the i n c l u - sion is determined during cooling of the sample using cooled nitrogen gas When the

i n c l u s i o n is frozen, the gas flow is reduced so t h a t the c r y s t a l s begin to melt The melting temperature of the l a s t ice c r y s t a l determines the freezing point of the i n -

c l u s i o n The pressure is taken e i t h e r as the actual pressure corresponding to the

b u r i a l depth of the sample, or as the paleo-pressure which is estimated from the sedimentary h i s t o r y

CURRIE & NWACHUKWU (1974) and MAGARA (1978) used t h i s p r i n c i p l e f o r determination of paleo-geothermal gradients in Canadian Cardium sandstone as f o l l o w s : Thin sections were made from f r a c t u r e - f i l l i n g materials (mainly quartz) of sandstone cores from 5 boreholes of a s i n g l e r e s e r v o i r Those quartz f i l l i n g s t h a t contained f l u i d i n c l u - sions were heated and m i c r o s c o p i c a l l y observed The ranges of homogenization temper- atures and calculated paleogeothermal and measured present geothermal gradients are shown in Table 2

of Cardium sandstones (by CURRIE & NWACHUKWU, 1974 and MAGARA, 1978)

The paleo-pressures were determined by sedimentary history, taking the rock compac- tion (using sonic log) into consideration Fig 10 shows the diagram of interpreta- tion: the measured maximum homogenization temperatures (A-F) are followed along the

l i n e of specific volume of water t i l l the calculated paleo-pressures are reached; this gives the paleotemperatures A ' - F' For the lowest homogenization temperatures ( a - f ) , the present pressure of cores (calculated by depth and density) were applied

Fig 10 Graphs showing ranges of homogenization temperature of Cardium sandstone

in f i v e (A- F) boreholes in western Canada and the interpreted paleotemperatures ( a ' - f ' , A ' - F') (from MAGARA, 1978, modified)

The homogenization temperature method excels in i t s s i m p l i c i t y , and has been applied since the l a s t century However, there were some uncertainties which biased the re- sults, and which may have been the reason why the method was seldom applied t i l l the

f i f t i e s when the technique and the foundations were improved

Inclusions can contain not only water but many other materials: mixed s a l t , o i l , hydrocarbons, carbon dioxide, etc (FREY et a l , 1980; POTY & PAGEL, 1983) FREY

et a l (1980) demonstrate that the f l u i d composition changes continuously in fissure quartz of the external parts of the central Alps, from higher hydrocarbon, through methane to water-bearing f l u i d inclusions The change of the f l u i d composition is dependent mainly on the temperature increase during the progressive Alpine meta- morphism The continuous change of composition may cause some systematic uncertain- ties in both the pressure correction (ZAGORUCHENKO & ZHURAVLEV, 1970) and the e s t i - mation of the formation temperature ROEDDER (1962,1963) proposed to begin the heat-

ing experiment at low (-35 ° C) or even very low (-180 ° C) temperatures (POTY & PAGEL, 1983), because the freezing temperatures are useful f o r d i s c r i m i n a t i n g among gas,

l i q u i d and s u p e r c r i t i c a l f l u i d , and among l i q u i d water, l i q u i d o i l and l i q u i d carbon dioxide

F l u i d i n c l u s i o n s may also r e - e q u i l i b r a t e during b u r i a l , or could possibly leak, but experimental studies of quartz show t h a t most f l u i d i n c l u s i o n s w i l l not decrepitate

at an i n t e r n a l overpressure lower than 800 bars and t h a t the smallest can withstand overpressures as high as 4 kilobars (TUGARINOV & NAUMOV, 1970; LEROY, 1979)

In many cases, f l u i d i n c l u s i o n studies give minimal temperatures and only occasio-

a l l y actual temperatures of i n c l u s i o n formation I f the f l u i d was homogeneous at the time of t r a p p i n g , the homogenization temperature w i l l be a minimum temperature The trapping temperature of the f l u i d in basins which are 5 - 7 km deep may be higher

by a value up to 80 ° C, in comparison to the f l u i d i n c l u s i o n temperature

POTY & PAGEL (1983) suggest t h a t f l u i d i n c l u s i o n techniques seem to give more

d e t a i l e d data than organic matter and i l l i t e c r y s t a l l i n i t y techniques

Case h i s t o r i e s

PAGEL (1975), using the f l u i d - i n c l u s i o n microthermometry of d e t r i t a l quartz grains

of sandstones, received a 35 ° C/km geothermal gradient f o r sedimentary basin in Athabasca (Canada) "which is c h a r a c t e r i s t i c f o r a c t i v e sedimentary basins" (Fig 11) The f l u i d s of i n c l u s i o n e x h i b i t e d a NaCI concentration of 30 %

atures by microthermometry of d e t r i t o quartz grains from Athabasca sandstones of

f i v e (RL3 e t c ) Canadian boreholes ( a f t e r PAGEL, 1975)

VISSER (1982) studied petroleum source rock from Venezuela by f l u i d i n c l u s i o n thermo- metry Thin ( < 8 0 pm) sections were made, polished on both sides and mounted on cover-

s l i p s with epoxy resin to give the necessary support The h e a t i n g - f r e e z i n g experiments were performed on a stage as described by POTY et a l (1976) The f l u i d i n c l u s i o n data

of secondary i n c l u s i o n s in quartz- and c a l c i t e - f i l l e d veins showed t h a t the maximum diagenetic temperature is in good agreement w i t h the actual measured formation tem- perature of 157 ° C (Fig 12)

temperatures in c a l c i t e (black) and quartz ( w h i t e ) , in a petroleum source rock from Vene- zuela (VlSSER, 1982)

4 0 8 0 8 0 1 D O 1 2 0 1 4 0

T h i n ° C

Based on the primary i n c l u s i o n s - w h i c h were entrapped during the i n i t i a l c r y s t a l growth - in d i a g e n e t i c q u a r t z o v e r g r o w t h s , TILLMAN & BARNES (1983) stated t h a t the host rock (sandstone) temperatures in the Northern Appalachian Basin vary from 176

to 147 ° C, with an average of 155 ° C; the b u r i a l depth, at the end of the Paleozoic

or e a r l y in the Mesozoic, were at depths of 3.5 km (= 40 ° C/km) The average geo- thermal gradient measured today in central and western New York is about 25 ° C/km The milky and clear c a l c i t e samples belong to the Oswego f a u l t system and m i r r o r a wide range of temperature of l a t e r hydrothermal events (Fig 13)

F l u i d i n c l u s i o n studies in a c t i v e geothermal f i e l d s are reported, e g , by BROWNE (1973) and BROWNE et a l (1974) from the Broadlands/New Zealand; by FRECKMAN (1978) from the Salton Sea geothermal f i e l d , Imperial Valley/USA; and by TAGUCHI et a l (1979) from Hatchobaru/Japan

In the Broadlands geothermal area i t was found that the borehole temperatures coin- cide with the homogenization temperature in quartz which indicates a constant ther- mal regime The homogenization temperatures in quartz, anhydrite, and c a l c i t e of the Salton Sea area are closely associated with the borehole temperatures, but vein cal-

c i t e e x h i b i t s some f l u c t u a t i o n s which may be due to episodic changes in f a u l t i n g ,

f r a c t u r i n g and subsequent f l u i d flow at d i f f e r e n t temperatures The homogenization temperatures in quartz and anhydrite in the Hatchobaru geothermal f i e l d indicate a higher thermal regime than was measured, which indicates that the climax of heat flow existed in the past, and the geothermal r e s e r v o i r is already cooling down

3 Geochemical thermometry

In a saturated porous rock a solution equilibrium is established a f t e r a certain time and at a given temperature An equilibrium can e x i s t e i t h e r within the pore f l u i d among d i f f e r e n t isotopes, or between the rock matrix and the pore f i l l i n g The tem- perature dependence of the reactions considered here is used to estimate the temper- ature of the environment where the equilibrium is established

The geochemical thermometry is mainly applied to f l u i d and vapour dominated geother- mal systems and is based on the following requirements:

(a) f r e e l y a v a i l a b l e elements (species) in the (rock-water) system

(b) e q u i l i b r a t e d reactions in the deep reservoirs

(c) slow r e - e q u i l i b r a t i o n during upward migration

The f r a c t i o n a t i o n of stable isotopes of various elements, e.g in solution, has been recognized as a temperature indicator because of the temperature dependence of the

f r a c t i o n a t i o n f a c t o r (e.g HOEFS, 1973; O'NEIL, 1979) The isotope r a t i o s of ele- ments hydrogen (D/H) and oxygen (180/160) are used in d i f f e r e n t reactions f o r tem- perature estimation

The analyzed isotope r a t i o r e f l e c t s the equilibrium condition of an isotope exchange reaction, as f o r example

HD+H20 ~ Hz+HDO (HULSTON, 1976; ARNASON, 1976; GIGGENBACH & LYON, 1977)

CH3D+H2 ~- CH 4+HD (HULSTON, 1976; ELLIS & MAHON, 1977)

H~60+(HS1801603)- ~- H~Bo+(HS1604)- (HULSTON, 1976; ELLIS & MAHON, 1977)

The time needed f o r r e - e q u i l i b r a t i o n is only a matter of days or weeks, respective-

l y , or months f o r the l a t t e r reaction Because of the shortness of t h i s time, these isotope exchange reactions are unsuitable f o r paleothermometry

As well as isotope exchange, the s o l u t i o n e q u i l i b r i u m between the rock matrix and the pore f l u i d i s used f o r temperature determination Only a very small part of the rock component enters i n t o s o l u t i o n I n d i v i d u a l minerals and amorphous components such as glass phases in eruptive rocks or opal have very d i f f e r e n t s o l u b i l i t y Rock- forming minerals such as quartz and feldspar are much less soluble than s a l t s The Si02 content of thermal water is f r e q u e n t l y used to determine the temperature

p r e v a i l i n g in subsurface water r e s e r v o i r s Even though there are many l i m i t i n g fac- tors f o r the v a l i d i t y of e q u i l i b r i u m c o n d i t i o n s , the Si02-thermometer y i e l d s a use-

f u l temperature estimation However, temperatures calculated from t h i s thermometer are generally too low (KOLESAR & DEGRAFF, 1978) Inaccuracies a r i s e , e g , because

of varying s o l u b i l i t i e s of d i f f e r e n t Si02 polymorphs such as quartz, chalcedony,

c r i s t o b a l i t e and amorphous Si02 (FOURNIER & ROWE, 1966; FOURNIER, 1981) Experience shows t h a t low temperatures (to T~100 ° C) should be calculated with the chalcedony

s o l u b i l i t y and higher temperatures with t h a t of quartz

Even though the SiO 2 thermometer has been used f o r some time with success to e s t i - mate an actual r e s e r v o i r temperature (FOURNIER & ROWE, 1966; MAHON, 1966; ARNORSSON, 1975; ELLIS & MAHON, 1977), i t has considerable disadvantages which are mostly r e l a - ted to the absolute content of dissolved Si02 These disadvantages do not favor

t h i s method f o r paleotemperature estimations, although the r e - e q u i l i b r i u m of an

e x i s t i n g s i l i c a - w a t e r e q u i l i b r i u m is a slow process

The advantage of the Na-K and the Na-K-Ca thermometer is t h a t instead of the absolute content, the r a t i o s of t h e i r concentrations are used to estimate the e q u i l i b r i u m temperature (WHITE, 1965; FOURNIER & TRUESDELL, 1978; PACES, 1975; ELLIS & MAHON,

1977, FOURNIER, 1981) The temperature obtained from the Na, K and Ca content of groundwater is often higher than t h a t which is determined by the Si02 content Si02 can p r e c i p i t a t e out during the mixing of cold water with thermal water I f the t h e r - mal source is w i t h i n a sedimentary basin, the s a l i n i t y of the p r e f i l l i n g f l u i d is important Also considerable perturbation of the thermometer can be caused by mono-

Although the r e - e q u i l i b r a t i o n is very slow, slower than f o r s i l i c a , the Si02, Na-K and Na-K-Ca thermometer are of l i t t l e use f o r paleothermometry.The problem arises because water, being mobile, migrates up- and downward through l a y e r s , and the d i s - solved s i l i c a or Na/K r e f l e c t s f i r s t the temperature-differences caused by the mi- gration process; these geothermometers are therefore used f o r the determination of the "basic-temperature" In very special cases only, i f water migration is excluded

or e x a c t l y known, s i l i c a and K/Na geothermometers can be used f o r paleogeothermo- metry

KHARAKA et a l (1980) have reported 180 i s o t o p i c analyses from subsurface waters from a well of Brazoria County, Texas The s t r a t a under consideration were deposited

in marine or near-marine s e t t i n g s During b u r i a l , formation water t y p i c a l l y becomes enriched in 180 as a r e s u l t of reactions with the surrounding sediment A p l o t of oxygen isotope values f o r calcium carbonate with depth shows a trend toward

l i g h t e r values at greater depth (MILLIKEN et a l , 1981), supporting the water i s o - tope data, but with a big s c a t t e r ELDERS et a l (1984)'s r e s u l t s from the 180 con-

t e n t of c a l c i t e s from sandstones in Cerro Prieto geothermal f i e l d boreholes show

s i m i l a r s c a t t e r but are useful (Fig 14) The f o u r t y p i c a l curves of Fig 14 r e f l e c t

at f i r s t the cold and hot water migration processes and give an important c o n t r i b u -

t i o n to deciphering the past f l u i d migration processes and hence, to estimating the paleogeotemperatures

-~i 180 IN CALCITES FROM SANDSTONES

samples recovered from four d i f f e r e n t wells charac-

t e r i s t i c of the Cerro Prieto geothermal f i e l d The shaded areas show the range of values measured The " b o i l i n g " curve shows t h e ~ 8 0 of c a l c i t e in

e q u i l i b r i u m with b o i l i n g water with&t80 = -8.33 %o (ELDERS et a l , 1984)

F r a c t i o n a t i o n of stable isotopes of the elements hydrogen, carbon, oxygen, and s u l f u r

in two minerals are also used f o r temperature determination (HOEFS, 1985) This i s o - tope thermometry has become well established since the c l a s s i c paper of UREY (1947) The p r i n c i p l e of t h i s thermometry is t h a t the p a r t i t i o n i n g of two stable isotopes of

an element between two minerals depends on the temperature The isotope r a t i o of compounds ( I ) and (2) is compared with t h a t of a standardized sample and the r e l a t i v e

d i f f e r e n c e is known as " d e l t a " ~-value The f r a c t i o n a t i o n f a c t o r ~ is the r a t i o of the i s o t o p i c composition of two compounds ( I ) and (2) and is approximately the d i f f e - rence of t h e i r ~-values:

1031n o~(1,2 ) ~ 8(1 ) - ~s(2 )

Since the f r a c t i o n a t i o n f a c t o r ~ is temperature dependent (BOTTINGA & JAVOY, 1973):

- exp(xl/T x2)

where X l , 2 are constants and T is the absolute temperature, the difference of the

~-values is a f u n c t i o n of T -x2 Experimental r e s u l t s are consistent with x 2 = 2 f o r high temperature (O'NEIL, 1979), so t h a t

100 l n ~ = A/T 2 The constant A must be known in order to determine the formation temperature of two

c o e x i s t i n g minerals

The temperature determined is taken as the l a s t e q u i l i b r i u m temperature However, an

e q u i l i b r i u m temperature cannot be assumed from a l l samples at a l l t e r r a i n e s The

i s o t o p i c f r a c t i o n a t i o n can change due to chemical a l t e r a t i o n or r e c r y s t a l l i s a t i o n of minerals and other k i n e t i c e f f e c t s which i n h i b i t a complete r e - e q u i l i b r a t i o n (HOERNES

& HOFFER, 1985) Isotopic temperatures may sometimes i n d i c a t e a thermal condition or

an event which is d i f f i c u l t to i n t e r p r e t , e.g when isotope exchange reactions took place during retrograde metamorphism (HOEFS, 1985) The a p p l i c a t i o n of a d d i t i o n a l paleothermal methods may confirm or a s s i s t in understanding uncertain r e s u l t s in t h i s case Generally, data from the Salton Sea geothermal f i e l d / C a l i f o r n i a (FRECKMAN, 1978) and from mines (HOEFS, 1985) demonstrate t h a t f l u i d i n c l u s i o n analysis and

i s o t o p i c geothermometry are in good agreement

Left: l i n e s i n d i c a t i n g range of possible conditions f o r formation of i l l i t e s with

&180 values of +15, +17, +19 and +21 per m i l l e , at various temperatures, andS80 values of the ambient water Right:&180 values of two cogenetic minerals uniquely define the temperature of formation and the i s o t o p i c composition of ambient water The cogenetic mineral p a i r have been formed at about 106 ° C in the presence of am- bient water w i t h ~ 8 0 of about 3 per m i l l e

SAVIN & LEE (1984) argue that most minerals once formed in oxygen isotopic e q u i l i - brium with the ambient water, are extremely resistant to subsequent isotopic exchange with environmental waters at sedimentary and most diagenetic temperatures, except when they undergo chemical or mineralogical alteration As a r e s u l t , the 180/160 ratios of minerals can provide information about t h e i r conditions of formation I f the 180/160 ratios of two cogenetic mineral phases (e.g i l l i t e and quartz, Fig 15) can be measured, then both the temperature of formation and the 180/160 ratio of the ambient water can be calculated

In sedimentary rocks, some authigenic minerals undergo a diagenesis during b u r i a l

w i t h i n the uppermost few kilometers of depth The low temperature o f up to 200 ° C causes an a l t e r a t i o n of clay minerals, s i l i c a polymorphs, and z e o l i t e s The trans- formation temperature of each series of authigenic minerals in d i a g e n e t i c a l l y a l t e r e d

a r g i l l a c e o u s sediments can be used f o r evaluating the geothermal h i s t o r y (AOYAGI, 1979)

Most of the investigations are based on the transformation of clay minerals (BURST, 1969; PERRY & HOWER, 1970,1972; AOYAGI et a l , 1975; HOWER et a l , 1976) Montmoril-

l o n i t e as an expanding clay mineral transforms at increasing temperature and pres- sure due to water and C02 loss, to potassium-poor s m e c t i t e / i l l i t e which are not expandable The interlayer water of montmorillonite which is released during the transformation to i l l i t e is considered to play an important role in petroleum migra- tion (POWERS, 1967; BURST, 1969; PERRY & HOWER, 1972) AOYAGI & ASAKAWA (1977) argued that both interlayer and i n t e r s t i t i a l water expelled during the diagenesis were responsible for o i l migration

The processes resulting in the diagenesis of montmorillonite begin at the depth corresponding to 80 ° C geotemperature and generally end at 120 ° C (BURST, 1969; JONES, 1970) A part of montmorillonite remains below this depth because the absorb- able potassium available is not enough for the m o n t m o r i l l o n i t e - i l l i t e transformation (JONES, 1970), or transforms in a phase which is to be grouped with the pyrophyllites (WEISS & ROLOFF, 1965)

AOYAGI & ASAKAWA (1984) report a temperature of 104 ° C f o r the transformation o f

m o n t m o r i l l o n i t e to mixed-layer m o n t m o r i l l o n i t e ( s m e c t i t e ) / i l l i t e , and one of 137 ° C

f o r mixed l a y e r minerals to i l l i t e which can be assumed f o r the Neogene a r g i l l a c e o u s sediments o f Japan

In the middle Upper Rhine Graben/FRG, the t r a n s i t i o n zone between Montmorillonite and mixed l a y e r minerals is generally found in the Graue Schichtenfolge Formation (Oligocene) in a depth corresponding w i t h a temperature of about 70 ° C The depth ranges between 700 m and 1300 m HELING & TEICHMOLLER (1974) excluded from t h a t a

s i g n i f i c a n t influence of overburden pressure

Another measure of the diagenesis of clay minerals is the " i l l i t e c r y s t a l l i n i t y " (KOBLER, 1967) which gives a continuous scale f o r the degree of diagenesis I t s

sured w i t h an X-ray d i f f r a c t o m e t e r The a p p l i c a t i o n to the very lowgrade metamor- phism in external parts of the Central Alps show an obvious r e l a t i o n , however be- ing divergent at d i f f e r e n t sites between i l l i t e c r y s t a l l i n i t y , coal rank and f l u i d

i n c l u s i o n data (FREY et a l , 1980) The i n v e s t i g a t i o n s seem to e x h i b i t a substantial influence of local conditions in each area I t has also been thought t h a t the mont-

(WAPLES, 1980), and the progress in transformation r e f l e c t s the thermal h i s t o r y of the sediment WAPLES (1980) stated a c e r t a i n c o r r e l a t i o n between the temperature- time-index (TTI) as calculated f o r diagenesis of organic matter and the proportion

of expandable clay layers (Fig 16) Samples l y i n g s i g n i f i c a n t l y l e f t of the l i n e ( i n a thermally immature r e g i o n ) , probably represent material which contained less than 100 % expandable layers when i t was o r i g i n a l l y deposited This is in agreement with the generally accepted view, t h a t the m o n t m o r i l l o n i t e / i l l i t e r a t i o also depends

with regressive, and montmorillonite with transgressive phases (CHAPMAN, 1973)

ELDERS et a l (1984) determined the progressive zones of hydrothermal a l t e r a t i o n minerals in sandstones at the Cerro Prieto geothermal system, Baja C a l i f o r n i a , Mexico, based on a l o t of deep boreholes The wide temperature-bands and t h e i r overlap give some idea of the s u i t a b i l i t y of t h i s method f o r paleothermal a p p l i c a t i o n s (Fig 17);

f o r the Cerro Prieto f i e l d however, where very high temperature changes occurred as

a consequence of a young (~50000 y) hot ( ~ I 0 0 0 ° C) shallow ( ~ 6 km) thermal plume, ELDERS et a l (1984) constructed r e a l i s t i c thermal h i s t o r i e s , f o r the l a s t 50000 years

A transformational sequence of z e o l i t e s is reported by AOYAGI & KAZAMA (1977) f o r paleotemperature determinations I t has been recognized in Neogene argillaceous rocks from deep boreholes in Japan This a l k a l i z e o l i t e reaction series comprises

4 zones:

Fig 16 Time Temperature Index of maturity versus % expandable layers in mixed-layer clays (WAPLES, 1980)

PYRITE AND/OR PYRRHOTITE QUARTZ

IIJIMA et a l (1984) report a transformation temperature to the next zone of 53 ° C,

85 ° C, and 122 ° C, r e s p e c t i v e l y , which has been estimated from the bottom hole tem- perature of the Miti-Kuromatsunai borehole in Hokkaido, Japan A paleothermal gra- dient of 31 ° C/km has been evaluated, whereas the present gradient has a value of

52 ° C/km The area was u p l i f t e d and a thickness of 600 m was eroded since the l a s t 0.5 Ma

The transformation is e s s e n t i a l l y time-dependent which has been deduced not only from

f i e l d observations but also from experimental and t h e o r e t i c a l studies (AOYAGI & ASAKAWA, 1984)

For evaluating paleotemperatures, the s i l i c a minerals give a transformational se- quence during diagenesis of sedimentary rocks At lower temperatures amorphous s i l i c a

is stable which is transformed to low-temperature c r i s t o b a l i t e and f i n a l l y to low- temperature quartz with increasing temperature (MITSUI & TAGUCHI, 1977) The trans- formation temperatures are 45 ° C and 56 ° C, r e s p e c t i v e l y

A l l three transformation series of minerals (Fig 18) are used to evaluate the paleo- temperature gradient of the M i t i Hamayuchi borehole in northern Hokkaido, Japan (AOYAGI & ASAKAWA, 1984) The depth of the f i r s t appearance of authigenic minerals

quartz; 2800 m mixed l a y e r minerals The transformation temperature of each boundary

is 45 ° C, 56 ° C, 69 ° C, and 104 ° C, r e s p e c t i v e l y From these data, a paleotempera- ture gradient of 27 ° C/km is calculated which was v a l i d during the Neogene

Another 16 deep boreholes in the Niigata basin, Honshu, Japan were analyzed by AOYAGI & ASAKAWA (1984) The paleogeothermal gradient during the Neogene ranged from

19 to 36 ° C/km The paleogeothermal gradient in o i l and gas f i e l d s of the area gene-

r a l l y cover the upper range between 30 and 40 ° C/km

During the Paleozoic era a group of animals l i v e d in the sea, whose remains are to

be found in great numbers in many sedimentary rocks of that era Because these hard, mineral remains resemble teeth, they are c a l l e d conodonts The nature of the animal group i s , however, unknown They have no descendant in t h e i r e v o l u t i o n , since they became e x t i n c t during the Upper Cretaceous, and any essential parts o f the animal which could have found i t s place in the c l a s s i f i c a t i o n of the animal kingdom such as the s o f t , perhaps t i s s u e l i k e parts cannot be reconstructed Although t h i s group of animals, o f which several hundred forms are d i s t i n g u i s h a b l e , is rather enigmatic,

t h e i r remains, i e the conodonts,are of great importance in geology f o r dating and mapping the sedimentary layers in which they occur (LINDSTRUM, 1964) The size of the conodonts r a r e l y exceeds I mm, l y i n g mainly between 0.1 and I mm

Conodonts were widely spread in the seas in which they occurred since Late Cambrium They passed through an e v o l u t i o n which produced so many c h a r a c t e r i s t i c and wide- spread forms t h a t they are valuable f o r s t r a t i g r a p h i c mapping The most prosperous period in the e v o l u t i o n culminates perhaps in the Late Devonian During Carbonife- rous and Permian, the conodonts occurred less f r e q u e n t l y A r e l a t i v e climax can be seen from Middle Triassic sedimentary rocks In most parts of the world the conodonts became e x t i n c t before the Cretaceous age This group of animals outlasted a period

of more than 300 Ma

The conodont remains of the animals consist of calcium phosphate with some minor amounts of carbonate, f l u o r , and sodium (PIETZNER et a l , 1968) As t h i s carbonate

a p a t i t e is r a t h e r r e s i s t e n t to physical and chemical changes of the environment, the conodonts are well preserved, even i n t o the garnet-grade metamorphic facies where they underwent temperatures as high as 500 ° C (EPSTEIN et a l , 1977)

Conodonts are abundant in black shales as well as in limestones The more f i n e - grained the rocks, the b e t t e r is the chance to f i n d enough m a t e r i a l LINDSTRUM (~959) suggested t h a t the conodont frequency is i n v e r s e l y proportional to the sedimentation

r a t e There might be some environmental influence which favours conditions o f q u i t e ,

perhaps warmer sea water than t h a t o f streaming water in which sand and other coarse- grained material are deposited Under favourable conditions several thousand cono- donts can be found per kilogram rock m a t e r i a l In coarse-grained m a t e r i a l , e.g sandstone, or in limestone deposited at a high r a t e , conodonts are absent or as rare

as a few examples per kilogram

In paleogeothermics, the c o l o r of the conodonts is of special i n t e r e s t I t ranges from pale y e l l o w , through d i f f e r e n t brown tones, to black The c o l o r a t i o n seems to

be due to the carbonization of some trace amounts of organic matter, probably amino acids as reported by PIETZNER et a l (1968) This carbonization is e s s e n t i a l l y de- pendent on the temperature and on the time at which the conodonts were exposed to that e f f e c t i v e temperature

EPSTEIN et a l (1977) reported heating experiments with conodonts, and introduced a

c o l o r a l t e r a t i o n index in which f i v e steps in c o l o r a l t e r a t i o n can be discriminated

by comparing the conodont under the microscope with a c o l o r standard The l a b o r a t o r y heating of pale yellow conodonts of the c o l o r a l t e r a t i o n index " I " , i e the f i r s t step with which the a l t e r a t i o n s t a r t s , comprises a temperature range from 300 ° C to

600 ° C and a duration o f the heating up to 50 days The experiments allowed EPSTEIN

e t a l (1977) to draw an A r r h e n i u s - p l o t f o r each conodont a l t e r a t i o n index (Fig 19), from which the e f f e c t i v e temperature of natural colored conodonts can be read

EPSTEIN et a l (1977) reported t h a t conodont colors correspond with the colors from

f i e l d c o l l e c t i o n s , and t h a t t h e i r a l t e r a t i o n is progressive, cumulative, and i r r e -

v e r s i b l e The temperature and time dependence has been mentioned above In p r i n c i p l e , the Arrhenius equation is supposed to describe the process which causes the c o l o r

a l t e r a t i o n

Since the density of the c o l o r also depends on the thickness of the specimen which

is observed, there is a l i m i t e d accuracy in t h i s method From t h i s , EPSTEIN et a l (1977) concluded that temperature i n t e r v a l s below 50 ° C cannot be discriminated using the q u a l i t a t i v e c o l o r a l t e r a t i o n index

The v a l i d i t y of the Arrhenius equation seems to be much more founded in t h i s case than i t is f o r coaly p a r t i c l e s The organic compounds, which might be amino acids only, are less complex than those derived from plant remains Furthermore, the environment w i t h i n the conodonts, i e w i t h i n the carbonate a p a t i t e , i s rather con-

s t a n t , whereas a great v a r i e t y e x i s t s f o r plant remains in d i f f e r e n t sedimentary rocks Other advantages o f the conodont c o l o r a l t e r a t i o n are that the method can be applied f o r Cambrian rocks, in which conodonts are already abundant, but v i t r i n i t e

is s t i l l rare, and that the temperature range is wider and reaches 600 ° C instead of about 350-400 ° C f o r coaly matter

The conodont c o l o r a l t e r a t i o n can also be used to support v i t r i n i t e reflectance methods From using the f i r s t method, the thermal h i s t o r y of a basin can be estimated using marine carbonate rocks, whereas the second method estimates the thermal h i s t o r y from more c l a s t i c rocks Both methods seem unaffected by t e c t o n i c events, n e i t h e r

f o l d i n g nor f a u l t i n g

Case h i s t o r y

EPSTEIN et a l (1977) applied the conodont c o l o r a l t e r a t i o n to Middle Ordovician rocks from Monterey, V i r g i n i a in the Valley and Ridge province They found a color

a l t e r a t i o n index o f 4 to 4.5 According to geologic observations, the maximum time

f o r heating was 270 Ma which corresponds with a temperature of 185-220 ° C applying Fig 19

The e a r l i e s t possible time f o r u p l i f t in t h i s area could be during Late Pennsylva- nian which r e s u l t s in a heating time of up to 210 Ma The corresponding tempera- ture ranges from 190 to 230 ° C, which is not much d i f f e r e n t from the f i r s t r e s u l t and which demonstrates t h a t the time dependence is less important f o r long heating periods In the v i c i n i t y o f Monterey, i t is supposed that a 4770 m thick sequence

of sedimentary rocks covered the Middle Ordovician I f a surface temperature of

20 ° C is taken into account, a thermal gradient between 43 and 52 ° C/km is estimated

f o r the time of the deepest b u r i a l , i e before Late Pennsylvanian

Radiometric age determinations are based on the assumption t h a t r a d i o a c t i v e systems such as K-Ar, Rb-Sr and U spontaneous f i s s i o n are closed during a time span which is then determined as being the age This means that n e i t h e r the d i f f u s i o n of daughter nuclides nor the population of f i s s i o n tracks changes The age which is determined from each system denotes the age since closure As d i f f e r e n t systems close and open

at d i f f e r e n t temperatures, combined radiometric age determination techniques can be applied to evaluate the thermal h i s t o r y of a rock sample The time f o r age determina-

t i o n begins when the rock passes through a c e r t a i n temperature during c o o l i n g Seve-

r a l temperatures and ages are determined from a rock sample, so that the cooling

With the combined a p p l i c a t i o n of both age determination techniques, the a p a t i t e

f i s s i o n track and the Rb-Sr system in b i o t i t e y i e l d s in the Urach I I I borehole, southern Germany, a paleotemperature gradient of 55 to 60 ° C/km which was a c t i v e during the Cretaceous period (HAMMERSCHMIDT et a l , 1984), whereas the present temperature gradient has a value o f 40 ° C/km (HAENEL & ZOTH, 1982)

ZAUN & WAGNER (1984) determined the annealing temperature o f f i s s i o n tracks in zircon using the paleotemperature data of the Urach I l l borehole I t was found

t h a t the f i s s i o n tracks are stable up to a temperature of 130 ° C The closing

temperature, t h a t is the temperature of which h a l f of the f i s s i o n tracks anneal,