An application of sequence stratigraphy in modelling oil yield distribution the stuart oil shale deposit, queensland, australia

Keywords: sequence stratigraphy, oil shale, oil yield, kerogen, Fischer Assay, computer modelling, lacustrine, lacustrine parasequence, Stuart Oil Shale Deposit, The Narrows Graben, Rund

AN APPLICATION OF SEQUENCE STRATIGRAPHY

IN MODELLING OIL YIELD DISTRIBUTION THE STUART OIL SHALE DEPOSIT, QUEENSLAND, AUSTRALIA

Graham John Pope

B.Sc (Applied Geology) UNSW

A thesis submitted for the degree of Master of Science from the

Queensland University of Technology

School of Natural

Resource Sciences

August 2000

Supervisors: Dr S Lang, National Centre for Geology & Geophysics, University of

Adelaide (formerly Senior Lecturer, Queensland University of Technology)

Dr A White, Andrew White & Associates

Keywords: sequence stratigraphy, oil shale, oil yield, kerogen, Fischer Assay,

computer modelling, lacustrine, lacustrine parasequence, Stuart Oil Shale Deposit, The Narrows Graben, Rundle Formation, Tertiary, Australia

ABSTRACT

The Stuart Oil Shale Deposit is a major oil shale resource located near Gladstone on the central Queensland coast It contains an estimated 3.0 billion barrels of oil in place in 5.6 billion tonnes of shale Commissioning of a plant capable of producing 4,500 barrels per day has recently commenced The shale is preserved in Tertiary age sediments of The Narrows Beds in the southern part of The Narrows Graben The oil shale sequence consists of repetitive cycles composed of oil shale, claystone and lesser carbonaceous oil shale in the 400 metre thick Rundle Formation The formation is the main oil-shale bearing unit in the preserved half-graben sequence up

to 1,000 metres thick

Previous studies on the lacustrine sedimentology of the Rundle Oil Shale Deposit in the northern part of The Narrows Graben have recognised eight facies that exhibit unique and recognisable cycles The cycles and sequence for the Kerosene Creek Member of the Rundle Formation is correlatable between the Rundle and Stuart deposits The nature of these facies and the cycles is reviewed in some detail In conjunction with the principles of sequence stratigraphy, the ideal oil shale cycle is described as the equivalent of a parasequence within a lacustrine system The

lacustrine parasequence is bounded by lacustrine flooding surfaces The organic material in the oil shale consists of both Type I (algal dominated) and Type III (higher plant matter dominated) kerogen Where Type I kerogen dominate, oil yields greater than about 100 litres per tonne are common In contrast where Type III kerogens are dominant, yields above 100 litres per tonne are rare The variation in oil yield is described for the Stuart lacustrine system The variation is consequent on the balance between production, preservation and degradation of the kerogen in the parasequences within systems tracts A system for the recognition of oil shale

deposition in terms of lacustrine systems tracts is established based on oil yield assay parameters and the assay oil specific gravity

The oil yield and oil specific gravity variation within the Rundle Formation is

modelled by member and the nature and distribution of oil yield quality parameters

in terms of the contribution of organic and inorganic source material are described

The presence of significant oil yield (greater than 50 litres per tonne) is dependent on the dominance of lacustrine transitional systems tracts and to a lesser extent,

lacustrine highstand systems tracts within the parasequence sets deposited in a balanced lake system in a generally warm wet climate during the middle to late Tertiary

CONTENTS ABSTRACT I ABBREVIATIONS IV ACKNOWLEDGEMENTS VI INTRODUCTION 1

Tertiary Oil Shale Deposits of Queensland 16

Sedimentation – Mineral and Organic Matter 31 Degradation and Preservation 31 Sedimentary Facies and Cycles in the Narrows Graben 32 Sequence Stratigraphy in The Narrows Graben 40 Sequence Stratigraphic Patterns in lacustrine Systems 45 Cyclicity in the Rundle Formation 49 Sedimentological Factors Contributing to Oil Shale Character 50

Background to the Datasets 53

Dataset Compilation And Verification 53 Datasets and Files Generated 55

RESULTS 56

Model and Data Visualisation 96

DISCUSSION 109 CONCLUSIONS 113 REFERENCES 114

LIST OF FIGURES:

Figure 2: Oil Shale classification based on maceral composition and environment of deposition (after

Hutton et al 1980 & Hutton, 1982) 10

The Stuart Oil Shale Deposit is classified as a lamalginite dominated lamosite with kerogen precursors related to the present-day blue-green algae Pediastrum 10

Figure 3: Principal types an devolution paths of kerogen types I, II and III 12

Figure 4: General scheme of hydrocarbon formation and kerogen evolution as a function of burial 14 Figure 5: Location of the major Tertiary oil shale basins and oil shale deposits of Queensland 17

Figure 6: Regional geology and structural setting of The Narrows Graben 22

Figure 7: Interpretive subcrop geology map of the Stuart and Rundle Oil shale Deposits, The Narrows Graben 24

Figure 8: Schematic cross-section showing the relationship of the Curlew and Worthington Formations and Rundle Formation members of The Narrows Graben sequence 26

Figure 9: Palaeoreconstruction of seafloor spreading around Australia at 45Ma 28

Figure 10: Kerosene Creek Member correlation diagram for the Rundle and Stuart Oil Shale Deposits 33

Figure 11: Schematic block diagrams illustrating the geological evolution of The Narrows Graben 35 Figure 12: Ideal composite parasequence for oil shale in The Narrows Graben (parasequence lithological cycle based on composite cycle of Coshell, 1986) 42

Figure 13: Sequence and parasequence composition for the Kerosene Creek Member of the Rundle Formation, The Narrows Graben 44

Figure 14: Cumulative thickness plot for parasequences of the Kerosene Creek Formation 45

Figure 15: Schematic diagrams showing cross-section models and the relationship of sequences in the Rubielos de Mora Basin (from Anadon et al., 1991) 47

Figure 16: Illustration of interaction of eustacy and subsidence to produce parasequences and sequences in marine settings (from Van Wagoner, et al., 1990) 48

Figure 17: Drillhole locations for the studied dataset, Stuart Oil Shale Deposit 54

Figure 18: Drillhole locations for the boxed area shown on Figure 17 55

Figure 19: Histogram and cumulative frequency plots-sample thickness 58

Figure 20: Histogram & cumulative frequency plots - sample bulk density 64

Figure 22: Proportional effect by drillhole location - sample bulk density 66

Figure 23: Histogram & cumulative frequency plots -total moisture 68

Figure 24: Scatter plots of total moisture by formation and member 69

Figure 25: Proportional effect by drillhole location - total moisture 70

Figure 26: Histogram & cumulative frequency plots - oil relative density 72

Figure 27: Scatter plots of oil relative density by formation and member 73

Figure 28: Proportional effect by drillhole location – oil relative density 74

Figure 29: Histogram & cumulative frequency plots - oil yield mass percent 76

Figure 30: Scatter plots of oil yield mass percent by formation and member 77

Figure 31: Proportional effect by drillhole location - oil yield mass percent 78

Figure 32: Histogram & cumulative frequency plots - oil yield mass percent dry 81

Figure 33: Scatter plots of oil yield mass percent dry by formation and member 82

Figure 34: Proportional effect by drillholes location - oil yield mass percent dry 83

Figure 35: Histogram & cumulative frequency plots –oil yield MFALT0M 85

Figure 36: Histogram & cumulative frequency plots -MFA gass+loss% 87

Figure 37: Scatter-plot - %OIL_DRY v BD (2m samples) and best-fit curve 89

Figure 38: Scatter-plot %OIL_DRY v BD (2m samples) Curlew Formation and Humpy Creek Member sub-groups 89

Figure 39: Scatter-plot %OIL_DRY v BD (2m samples) oil specific gravity populations 90

Figure 40: Scatter-plot %OIL_DRY v BD (facies samples) distinguished by Formation and Member 90

Figure 41: Downhole variogram – 2m sample data 92

Figure 42: Omni-directional variogram for Rundle Formation Members - 2m sample data 92

Figure 43: Minemap model cross-sections for the Stuart Oil Shale Deposit 94

Figure 44: Minemap long-sections for the Stuart Oil Shale Deposit Vertical extent of Cells plotted represent Members of the Rundle Formation and Curlew Formation 95

Figures 45: Oil yield and Oil SG contour plots for the Teningie, Ramsay Crossing, Brick Kiln, Humpy Creek and Munduran Creek Members of the Rundle Formation 100

Figures 46: Oil yield and Oil SG contour plots for the Telegraph Creek Member MAT and Kerosene Creek Member sub-units D2 to B1 of the Rundle Formation 104

Figures 47: Oil yield and Oil SG contour plots for the Kerosene Creek Member sub-units B2 & A of the Rundle Formation and the Curlew Formation 107

LIST OF TABLES

Table 1: Stuart Oil Shale Deposit Resource Estimate 6 Table 2: Summary of Primary Characteristics of some Queensland Tertiary Oil Shales 19 Table 3: Stratigraphic Table – Stuart Oil Shale Deposit 25 Table 4: Summary Criteria for Facies Recognition 36 Table 5: Environments of Deposition for Facies and Sub-facies 37 Table 6: Summary of Narrows Graben Lacustrine Systems Tract Facies Parameters 43 Table 7: List of Intersections used in compiling data analysis 56 Table 8: Summary statistical data for MFA qualities for oil shale units in the Stuart Oil Shale Deposit, two-metre composite assay data 59

LIST OF APPENDICES

Appendix 1: Modified Fischer assay Procedure

Appendix 2: Tables of General Statistical Data by formation and member for modified Fischer Assay

results

ABBREVIATIONS

AR_moist Total Moisture (includes air dried, oven dried & and analysed

percent moisture) basis MASS%OIL Oil Yield weight percent (oven dry basis)

percent moisture) basis measured by modified Fischer Assay

The work contained in this thesis has not been previously submitted for a degree or diploma at any other tertiary institution To the best of my knowledge and belief, the thesis contains no material previously published or written by another person except where due reference is made

Signed:

ACKNOWLEDGEMENTS

I would like to thank Southern Pacific Petroleum NL and Central Pacific Minerals

NL for both the use of and access to the geological and assay data on the Stuart Oil Shale Deposit, software modelling programs and computing resources In addition, discussions with other company geologists and consultants involved in oil shale exploration not only at Stuart but also on other oil shale deposits have contributed to

my interest in understanding of oil shale processes over many years Special thanks

go to Dr L Coshell, Mr D Dixon, Mr J Ivanac, Mr A Lindner and Mr R McIver

My supervisor Dr S Lang is acknowledged for his continued enthusiasm and

assistance during the various changes of emphasis in the work over the years The support of Dr A White of Andrew White and Associates and the assistance of the staff of the School of Natural Resource Sciences at Queensland University of

Technology are also acknowledged

In particular, thanks must also go to my family; Sue, Alexander and Katrina for their continued patience and support particularly over final stages of preparation

Rationale

Oil shale is found in a diverse range of sedimentary environments – marine, fluvial and lacustrine are the principal settings (Demaison & Moore, 1980, Katz, 1990) Lacustrine oil shale deposits represent a significant proportion of hydrocarbon

resources worldwide Not only are they in many instances the prime source rocks for economic oil fields in many petroleum basins but also in the right setting make attractive targets for exploitation in their own right Composed almost entirely of algal remains, there is a direct relationship between oil shale pyrolysis yield and the abundance and preservation of organic matter In a lacustrine setting, high frequency changes in lake level and sediment influx impart a characteristic cyclical sequence of newly deposited organic matter often overprinted by degradation The principles of sequence stratigraphy can be used to quantitatively map the cyclical arrangement and oil yield parameters in a lacustrine oil shale sequence and add to the understanding and modelling of oil yield within oil shale deposits

The relationship between the pyrolysis yield of organic-rich shale and the content and type of kerogen has long been the focus of study of hydrocarbon source rocks (Demaison & Moore, 1980, Fleet et al., 1988, Smith, 1990) In times of extreme fuel shortages and before the discovery of conventional oils, oil shale played an important role in providing liquid hydrocarbons With a rising demand for transportation fuels, the oil price shocks of the early 1970’s and uncertainties with the traditional source

of supply from the Middle East, the focus on alternative commercial sources for oil often shifted to investigation of oil shale

This study examines the relationships between the organic type and oil content in one

of the largest Tertiary lacustrine oil shale basins in Australia, The Narrows Graben

on the central Queensland Coast The graben contains up to 1000 metres of sediment dominated by oil shale Since the recognition of the large size of the oil shale

resource in the Rundle Deposit, the main oil shale unit, the Rundle Formation, has been investigated for the potential to recover the oil in the Rundle and Stuart Oil Shale Deposits (Lindner & Dixon, 1976) In this work the sedimentological and

stratigraphic relationships of the oil shale members of the Rundle Formation in the Stuart Oil Shale Deposit are reviewed in the context of the oil yield of the sediments

Traditionally, drilling and assaying has been the primary evaluation tool in the

exploration phase of the examination of deposits The fine-grained nature of oil shale deposits in thick sequences has long been a problem in establishing a

correlation within an oil shale basin Detailed work in The Narrows Graben

established a stratigraphy based on cycles within the sequence (Coshell, 1986) The sedimentary cycles exhibit an oil yield pattern related to the nature and content of kerogen preserved in each cycle The grade or oil yield is related to these cycles.More recently, the principles of sequence stratigraphy have been applied to the interpretation of lacustrine sequences (Liro, 1993)

Aim of the Thesis

The principal aim of this work is to review the sedimentological, structural and depositional character of oil shale and to integrate these aspects with the recent advances in the sequence stratigraphy of lacustrine systems using the Stuart Oil Shale Deposit as an example Definition of sequences with defined organic source properties can then be used during the process of modelling the grade distribution and variability for the deposit Application of the relationships between sedimentary features and the grade of shale oil will result in a more robust resource model on which to base the economic exploitation of the shale oil in the deposit

In addition, comment on the application of the study findings to the development of a method of estimation of probable oil yield based on facies and cycles within the framework of sequence stratigraphy can be made

Scope and Objectives

The shale oil is contained in five oil shale members of the Rundle Formation The resource is based on the subsurface results of drilling with assay control based on the modified Fischer Assay (MFA) Detailed facies relationships and cyclic sedimentary sequences that show distinctive qualitative oil yield characteristics are well defined for the northern section of the Narrows Graben (Coshell, 1986) These relationships have been applied to the Kerosene Creek Member, the uppermost member of the Rundle Formation, and correlation throughout The Narrows Graben has been

demonstrated (Coshell, 1986, Coshell and McIver, 1989) Correlation based on facies relationships has been described for the complete Rundle Formation in the northern section of The Narrows Graben The facies description and logging for the majority of the Rundle Formation at Stuart has not been completed A similar facies and cycle relationship is expected for the balance of the Rundle Formation at the Stuart based on the two drillholes that have been logged by facies The recognition

of the same facies and cycles in these holes suggests the remainder of the Stuart sequence elsewhere will correlate with the Rundle sequence to the north

This thesis will:

1 Examine the cyclic nature of the oil shale stratigraphy at Stuart and discuss the various aspects of oil yield in relation to oil shale facies in a sequence

stratigraphic framework, and

2 Use the tools of computer modelling software to examine the broad environment

of deposition, and distribution of oil yield within the deposit

Given the general geological relationships outlined above, it is proposed to examine:

1 The population statistics of the main Modified Fischer Assay components

including total moisture content, shale oil content, oil specific gravity, together with rock bulk density for both the total deposit (Rundle Formation) and the individual stratigraphic members as appropriate,

2 Any lithological (facies) controls/associations of the various populations found in the above exercise, and

3 The basis for the development of a model for the spatial distribution of shale oil and, where supported by appropriate data, oil shale facies and member variation (thickness)

It is also proposed to explore the more recent aspects of the application of sequence stratigraphy to lacustrine basin settings, architecture and structure of the sedimentary packages observed within these systems The deeper basinal facies are of particular interest since these sequences are the most likely host to thick sequences of oil shale

Of particular interest will be:

1 The application of sequence stratigraphic principles to assist in the

understanding of the facies relationships and basin structure/tectonics of

lacustrine systems and, links (if any) to eustatic sea level changes in the marine environment and the influence of climate changes on lake level,

2 The determination of key surfaces - transgressive surfaces and sequence

boundaries - using facies and cycles and the grade distribution,

3 To determine the lacustrine system tract equivalents of highstand (HST),

transitional (TST) and lowstand (LST), and associated flooding surfaces (FS) in

an oil shale system,

4 To determine the relationships between system tracts and the grade distribution within the deposit, and

2 Enable a more precise estimation of the spatial distribution (and geostatistical continuity) of shale oil yield within the deposit,

3 Determine of the level of investigation required to adequately define the shale oil resource within oil shale deposits, for audit and commercial viability studies in particular (drill-hole spacing, sampling intervals etc), and

4 Add to the understanding of thick sequences of lacustrine shale in a sequence stratigraphic framework that may lead to the prediction of oil plays in lacustrine settings

The parameters mentioned in points 2 and 3 above are commonly used in resource investigation of coal and mineral deposits (and their limitations well known) This is not the case for oil shale deposits due to the paucity of recent commercial

exploitation of these deposits Estimates of the continuity and variability of

important oil shale qualities (such as oil yield and moisture content) can be made geostatistically, however the value of such estimates must clearly be understood in the light of the sedimentological and stratigraphic framework of the deposit under examination The Stuart Oil Shale Deposit represents an excellent opportunity to establish these parameters for future application during the exploitation of the

deposit

Economic Background

In April 1999, Southern Pacific Petroleum NL, Central Pacific Minerals NL

(SPP/CPM) and Suncor Energy Australia (Suncor) commenced commissioning a

A$250 million R&D demonstration plant to test a new technology for the production

of shale oil from the Stuart Stage 1 Project The Stuart Stage 1 development is based

on the Kerosene Creek Member of the Rundle Formation Initially, the plant was

scheduled to complete commissioning by late 1999, producing oil at a design rate of

4,500 barrels per day (bpd) The first production of raw shale oil was in August 1999

with two hot test runs using oil shale completed at rates of up to 50% of design

capacity Runs in November although mechanically successful, were halted after

reports of odour in a neighbouring community The need to resolve the air emissions

issues to meet regulatory requirements and address concerns of the companies and

those of the neighbouring rural community have delayed commissioning activities

Depending on the success of the demonstration plant (Stage 1) and the scale-up to a

commercial-sized module (Stage 2), the project could be expanded to a fully

commercial scale by 2007 (Stage 3), producing approximately 85,000 barrels per

day

The Stuart Oil Shale Deposit has an estimated total resource of 3.0 billion barrels

(Bbls)1 contained in 6.5 billion tonnes of shale at an average grade of 91 litres per

tonne at zero percent moisture (LT0M) The distribution of the categories of the

resource is given in Table 1

Table 1: Stuart Oil Shale Deposit Resource Estimate

Resource at cut-off grade of 50 Litres per tonne at zero moisture (LT0M)

Resource Category Tonnes

x 10 9

Moisture (wt %)

Grade (LT0M)

In-situ Oil (Bbbls)

To efficiently extract any resource, a thorough understanding of the distribution and variation of grade (oil yield) within the resource is highly desirable Understanding the quality parameters that contribute to the resource grade and variability greatly enhance the efficient extraction and development of the resource

Oil shale has been exploited in one form or another for centuries from

countries throughout the world The organic content allows its direct use as a heat source in light, heat and in power generation or, use of the derived oil products as transportation fuels and power generation It is used as a decorative stone, was and is used for jewellery and ornaments in cottage industries, provides the basis for oil production and electricity generation in China and Estonia and most importantly is

an important source rock for the generation of oil

With the onset of the modern industrial revolution, kerosene, oil and wax products from oil shale provided the bulk of the fuel requirements for machinery and the blossoming internal combustion engine in the mid to late 19th century Commercial exploitation of oil shale began to suffer following the discovery of “conventional” crude petroleum in Pennsylvania, USA in 1859 In Australia, production of oil from oil shale for indigenous consumption began in 1865 Earlier, exports of N.S.W torbanite to England and the U.S.A for gas production to enhance the lighting power

of the local reticulated gas used in street lighting proved profitable (Cane, 1979) Australian shale oil production could not sustain competition with the imported crude oil produced by the emerging American oil companies and indigenous production waned Strategic shortages that arose during the major global wars and serious recessions of the late 19th and early 20th centuries saw periodic revival of the

domestic industry Australian production declined rapidly following the removal of the duty on imported kerosene by the Federal Government in 1904 although the last production did not come until 1952 when the mines and shale works at Glen Davis in NSW finally closed

The onset of increased consumption coupled with the “oil shock” in 1973 (when the spot price of oil rose rapidly to US$25 from a base of US$10) together with the subsequent price rises to a high of US$45/barrel during 1981, served to revive the interest in oil shale as an alternative, competitive source for basic transportation and heating fuel (Figure 1) Small developmental mines were established and pilot plants built in the U.S.A, utilising oil shales of the Green River Formation in Colorado, and

on the Irati Formation in Brazil Although the price of oil has remained subdued since the oil price collapse in the mid 1980’s, recent tightening of oil production by OPEC has seen a steady increase in price to levels reminiscent of the early 1980’s Despite the large degree of Australian self-sufficiency in oil products, the rising oil price during the 1970’s provided justification for a renewed consideration of oil shale

as an alternative indigenous source of liquid hydrocarbons and as a strategic

resource

YEAR 0

1900 Texas-Oaklahoma Rush 1938 Kuwait -Saudia

Arabia Discoveries

1967 6-Day War

1973 Arab Oil Embargo

1986 Oil Price Collapse

1990 Iran-Iraq War

2000 Price Surge

Figure 1: Effect of oil discoveries and oil industry shocks on the price of oil

(Modified from Yergin, 1991)

Currently, Australia’s demonstrated and inferred shale oil resources amount to

approximately 283 billion barrels of shale oil, 90% of which is an inferred resource contained in the Cretaceous Toolebuc Formation of the Eromanga and Carpentaria Basins of Central Queensland (Gibson, 1980, Gibson and Rutland, 1981, Bureau of Resource Sciences, 1998) The demonstrated resources regarded as sub-economic amount to about 22 billion barrels of shale oil and are accounted for by the oil shales

in organic-rich lacustrine sediments of early Tertiary age preserved in number of elongate basins over a distance of 500km along the central coastal region of

Queensland (Bureau of Resource Sciences, 1998) The Stuart Oil Shale Deposit is one of these occurrences

Definition and Classification of Oil Shale

Although there is no standard definition, the term oil shale is generally applied to a finely laminated rock containing sufficient kerogenous material to yield

hydrocarbons when heated (hence their importance as a source rock for generation of hydrocarbons for petroleum reservoirs) Kerogen is essentially that part of an

organic rock that is neither soluble in aqueous alkaline solvents nor in common organic solvents but is generally recoverable by destructive distillation (Tissot & Welte, 1978) Although coals will also yield oils (or bitumen) on pyrolysis, unlike oil shales they also yield appreciable oil to solvent extraction Oil shales that are too thermally immature and have generated little or no hydrocarbons on diagenesis and burial can be exploited as a source for commercial extraction of oil

There have been a number of definitions applied to oil shale using energy balance but in general an oil shale can be defined as a kerogen bearing rock which yields at least as much energy as that used in the extraction process Tissot and Welte (1978) defined an economic oil shale as one that had an organic content of at least 5% by weight (a yield of about 25 litres/tonne) Taylor (1987) considered 15 USgal/ton (63 litres/tonne) a rich oil shale Synonymous terms used to describe deposits of organic matter (apart from coals) that have been variously applied to oil shales include black shale, cannel coal, carbonaceous shale, kerosene shale and kerogenous shale

Hutton et al (1980) proposed a framework for the classification of oil shales based

on algal types following petrographic examination of the organic matter in a number

of occurrences in the Tertiary and Cretaceous deposits of Queensland and the Green River Formation in the USA (Figure 2) The Stuart deposit is classified as a

lamosite, dominated by lamalginite with lesser telaginite, sporinite, resinite and vitrinite

`

vitrinite inertinite sporinite

vitrinite lamalginite

liptodetrinite ltelalginite inertinite sporinite

corpohuminite sporinite vitrinite bitumen

vitrinite telalginite sporinite resinite bitumen

Westfield Torbane Hill, N.S.W.

Mersey River, Tas Toolebuc Formation,

Qld.

Posidonia Shale, Toarcian,Paris Basin.

Green River Formation, U.S.A

Stuart Rundle Condor

freshwater lake

in a lake peat mire (swamp)

lacustrine Marine (shallow

sea)

shallow marine Stratified (saline)

lacustrine

shallow freshwater lacustrine

Figure 2: Oil Shale classification based on maceral composition and environment of deposition (after Hutton et al 1980 & Hutton, 1982)

The Stuart Oil Shale Deposit is classified as a lamalginite dominated lamosite with kerogen precursors related to the present-day blue-green algae Pediastrum.

Tissot & Welte (1978) proposed the use of a diagram using H/C and O/C atomic ratios, pioneered by Van Krevelen to characterise coals and their evolution paths on progressive burial, as a useful indicator of kerogen type and their maturation path The three basic types of kerogen are distinguished by their differing atomic ratios (Tissot et al, 1974), and to some extent these can be used to indicate depositional environment and algal precursors (Figure 3) In this instance whole rock samples from Stuart fall into the Type I and Type II evolution paths The more vitrinite dominated and mixed oil shales from the uppermost portion of The Narrows Graben (the Curlew Formation) tend to plot in the Type III or Type II evolution paths

respectively (Generally the H/C ratios of Type II are indicative of a marine origin but mixed lacustrine do exhibit similar ratios)

Detailed work on the role of cyanobacterial mats as contributors to the organic

inventory of sedimentary environments and studies on the organic matter and its relationship to diagenesis highlights the importance of algal precursors and the nature and degree of depositional environment and diagenetic change in the preservation and degradation of organic matter in sediments (Bauld, 1981, Philip, 1981)

Although pervasive anoxic conditions such as those found in anoxic lakes can

provide a favourable environment for organic matter deposition (Demaison &

Moore, 1980), conditions such as sedimentation rate, preservation, water chemistry, climate and time windows are equally important contributors (Kelts, 1988)

The earliest oil shales of the geologic record are distinguished by the dominance of Type I kerogens, or those rich in hydrogen primarily due to their algal origin and where the bulk of the volatile components are released at temperatures above 500oC.Rocks rich in algal organic matter constitute prime source material for oil generation Higher plant material, the Type III kerogens with atomic hydrogen to carbon ratios less than about 1.35, lack the significant volatile components generated above 500oC.Input of Type III kerogens became more significant from the Silurian Although they are a source for some oils, rocks bearing this type of organic matter are

generally considered as source material for gas generation on burial

KEROGEN TYPES AND EVOLUTION PATHS

Green River Formation (Paleocene-Eocene) Uinta Basin USA Algal kerogens (Botryococcus etc) Various oil shales Narrows Graben Kerogens (Stuart filled, Rundle unfilled symbol) Lower Toarcian shales, Paris Basin

Silurian shales, Sahara, Algeria & Libya Various oil shales

Late Cretaceous, Douala, Cameroon Lower Manville shales, Alberta, Canada

Stuart Type I Kerogen

Stuart Type III Kerogen

Incr eas ing bur ial

Figure 3: Principal types an devolution paths of kerogen types I, II and III

Changes in the atomic ratios H/C and O/C show the change in kerogen composition during increasing burial (modified after Tissot and Welte, 1978, Stuart and Rundle data from Crisp, et

Oil shales are found in both marine and fresh water settings, as both are capable of providing an environment appropriate to the preservation of algal matter The

marine setting is generally host to algal remains, whereas some of the lacustrine settings provide an environment where algal, higher plant and spore/pollen material can be preserved in the one system Organic matter can accumulate in both

carbonate and silicic dominated sequences Despite the depositional setting having

an initial influence on the preservation of organic matter and its thermal evolution, the composition of the host mineral matter plays a major role during the migration and entrapment of any hydrocarbon generated on burial There is adsorption of hydrocarbons dependent on the surface area of the argillaceous minerals in the matrix, and on clay minerals in particular (Hartman-Stroup, 1987) The nature of the host sediment is also critical from mining, processing and materials handling point of view should the oil shale be used directly to produce oil by pyrolysis

It has long been appreciated that oil shales containing mixed kerogen types preserved

in lacustrine environments are capable of generating both oil and gas on increasing depth of burial With increasing depth of burial, kerogens initially loose oxygen with the breakdown of heteroatomic bonds followed by the progressive breakdown of carbon chains and thermal cracking with increasing temperature and pressure (Figure 4a) The changes in the H/C and O/C ratios can be mapped and the zones of oil and gas generation determined (Figure 4b)

As a consequence, there has been a renewed appreciation of the environmental, structural and climatological settings of these rocks in lacustrine settings following the association with an increasing number of recently discovered commercial oil and gas fields (Katz, 1990, Smith, 1990) Where the degree of burial and maturation has been insufficient to generate oil and gas these sediments host some of the thickest and moderately rich oil shales known in the world

Figure 4: General scheme of hydrocarbon formation and kerogen evolution as a function of burial

4a (top) -The evolution of hydrocarbon formation is shown in insets for three structural types Depths are only indicative and correspond to an average on Mesozoic and Palaeozoic source rocks Actual depths vary according to varying geological conditions

4b (bottom) - The successive evolution stages and principal products are indicated on a van Krevelen diagram (from Tissot and Welte,1978)

OILSHALES IN AUSTRALIA

Earliest reports of oil from Precambrian rocks have included the Middle Proterozoic black shales of the Battern Subgroup and Umbolooga Subgroup from the McArthur River Basin, in the Northern Territory These sequences developed partly in

lacustrine environments developed in half-rift lake complexes and are more widely known as host to the McArthur Pb-Zn deposits (Womer, 1986, Plumb et al 1990, Swarbrick, 1974)

The oil shale (torbanite) deposits of the Sydney Basin are found associated with the Permian Illawarra Coal Measures These deposits developed in short-lived fresh to brackish, lakes where algal growth flourished The kerogens are dominated by

Botryococcus related algal matter These torbanite layers are thin (1.5 metres

maximum) but due to the high lipid content of Botryococcus can be exceptionally

rich which makes up for their limited lateral extent Similar deposits of torbanites are found at the Alpha deposit in the Permian Colinlea Formation of the Galilee Basin in Queensland (Madre, 1986)

The accumulation of sediments in lacustrine systems continued into the Mesozoic in Australia particularly in the intracratonic basins on the east of the continent Perhaps the most extensive of the Australian oil shales are those of the Cretaceous Toolebuc Formation in the northern Eromanga Basin and the southern Carpentaria Basin more commonly referred to as the Julia Creek oil shale However, these shales are marine rather than lacustrine and were deposited in the more protected areas of an

epicontinental sea during an Early Cretaceous (Albian) marine incursion from the north (McMinn & Burger, 1986) The organic matter of the Toolebuc shales consists mainly of bituminite and micrinite with lesser liptodetrinite, lamalginite, telalginite and inertodetrinite (Saxby, 1986)

The Tertiary saw the accumulation of a number of oil shale deposits in half-grabens developed in eastern Queensland as a response to the tensional regime initiated during the opening of the Tasman Sea in the Late Cretaceous (Grimes, 1980) This regime continued into the early Tertiary with the opening of the Coral Sea to the northeast The later phases of the movement associated with the formation of the Tasman Sea introduced a preference for a strike-slip movement that in some

instances reactivated older tensional faults This contributed to initial Tertiary basin formation and the syn-sedimentary structure common in the Tertiary oil shale basins.

Deposits of organic-rich lacustrine sediments of early Tertiary age have been preserved in elongate basins over a distance of 500km along the central coastal region of Queensland The Hillsborough (Condor), Yaamba and Duaringa Basins and The Narrows (Rundle & Stuart), Nagoorin and Lowmead Grabens are generally structurally asymmetric, with faulted, generally syn-depositional margins and contain substantial thickness of oil shale (Figure 5) Although these are the more well-

known and documented basins because of their significant shale oil resources, other Tertiary oil shales are known from the Strathpine, Redbank, Waterpark Creek,

Biloela, Casuarina and Plevna basins (Swarbrick, 1974; Day, 1981; Gibson, 1980.)

Typically fine-grained siliciclastic sediments dominate all the basin sequences The sediments generally have a moderate to high moisture content The rocks are soft, weather readily and outcrop is poor to non-existent, thus the dimensions, structure and stratigraphy of the deposits have been largely determined by drilling and

geophysical investigations Generally the type and richness of organic material persist laterally for many kilometres throughout the length of individual basins and where well stratified have provided correlatable attributes for the definition and mapping of potential resource units

The Tertiary age basins are located in the northern part of the Palaeozoic New

England Fold Belt During the Middle to Late Palaeozoic, this area formed part of a back-arc accretionary province in which deep to shallow marine and some terrestrial sediment accumulated (Murray et al., 1987, Murray & Cranfield, 1989) Acidic intrusives and extusives were emplaced both syn-depositionally and post-

depositionally during the Late Palaeozoic and Mesozoic

HERBERT CREEK YAAMBA

Oakey-Acland Withcott Strathpine

Carnarvon Creek Orallo

Figure 5: Location of the major Tertiary oil shale basins and oil shale deposits of Queensland

These sedimentary and igneous suites formed the basement to the basins that

developed in the early Tertiary in response to an extensional tectonic regime initiated

in conjunction with the opening of the Coral Sea (Grimes, 1980) The provenance of the sediment fill in the basins was dominantly siliceous volcanic, sedimentary and metamorphic rocks with high clay content Sandstones and conglomerates occur in most basins largely as the earliest deposits at the base of the sequence, as marginal facies, on intra-basin highs or as sedimentary wedges associated with the syn-

depositional movement along marginal faults and basement structures

A number of basins have been explored extensively in recent times to assess the potential of the oil shale units for commercial oil extraction As a consequence, a large volume of published and unpublished data exists (see below) The primary characteristics of a number of the better documented basins have been summarised

(Table 2) The characteristics focus on the age, tectonic setting, sedimentary

sequence and depositional history, nature of the preserved organic matter and the depositional environment for the oil shale units (and to some extent the non-oil shale intervals) of the sedimentary package The oil shale deposit at Stuart, in The

Narrows Graben, is the subject and focus of this thesis

GEOLOGY AND SEDIMENTOLOGY

History and Previous Work

Oil shale in The Narrows area was first reported by Ball (1914) Following

inspection of two petroleum prospecting areas, Ball (1921) later described the

ignition of shale by an open flame The shale was extracted from a shallow (13m deep) shaft near high watermark At the time, shale had also been exposed by

cyclonic storms at the edges of the deep-water channel of the Narrows In 1941 the Queensland State Government undertook reconnaissance drilling in the area (Ball, 1946) Associated Australian Resources conducted limited work and Carpentaria Exploration Company completed a small diamond drilling programme in the late 1960s before Southern Pacific Petroleum N.L and Central Pacific Minerals N.L commenced exploration at Rundle in the northern portion of the graben in 1973 (Lindner & Dixon, 1976) The first borehole at Stuart in 1977 proved in excess of 380m of Tertiary sediments existed in the southern portion of the graben To the end

of 1996, 97 drillholes, with an aggregate 15,893m, have been completed (Henstridge

& Missen, 1982; Coshell & McIver, 1989)

With the commitment to the construction of Stage 1 and subsequent EIS process for Stage 2, an additional 6,020 metres of drilling have been completed to the end of 1998

Table 2: Summary of Primary Characteristics of some Queensland Tertiary Oil Shales.

Middle to Late Eocene Mid to Late Eocene Mid to Late Eocene

half-graben, initial tensional opening with partial syn- depositional faulted margins (?growth faulting)

Hillsborough Basin:

asymmetric graben tensional Post- depositional transverse faulting.

-Duaringa Basin:

asymmetric graben - tensional Post- depositional transverse faulting.

Lowmead Graben:

asymmetric tensional, ?syn- & post- depositional strike-slip faulting.

graben-Nagoorin Graben:

asymmetric graben- tensional Syn & post- depositional strike slip faulting.

lignite; minor siltstone, dolomite, sandstone;

clayey sandstone;

conglomerate dominated basal section

oil shale, carbonaceous oil shale, lignite carbonaceous, shale, claystone, sandstone siltstone

sandy to silty claystone, oil shale, sandstone, siltstone, carbonaceous shale, limestone, dolomite

oil shale, claystone, carbonaceous oil shale, carbonaceous shale, sandstone siltstone

oil shale, carbonaceous oil shale, carbonaceous shale/cannel coal, claystone, sandstone conglomerate.

Depositional

Sedimentary Features:

recognisable cyclic facies (? climate-related)

in oil shale sequences

cyclic sandstone facies upward-fining to oil shale; massive, thick, laminated oil shale

cycles grading upwards oil shale to claystone evident in upper resource seam; upward- fining sandy facies

oil shale laminated basal sands cyclic graded bedding

montmorillonite & quartz;

minor illite, kaolinite, felspar, carbonates; rare siderite, gypsum, analcime, apatite, pyrite,

Dominant quartz, kaolinite, montmorillonite; minor siderite,buddingtonite, apatite; trace pyrite, calcite.

montmorillonite, quartz, amorphous silica; lesser kaolinite,illite,siderite;

trace felspar, pyrite, gypsum & anatase

kaolinite, quartz, opaline silica, illite, minor montmorillonite, felspar siderite, pyrite, alunite, anatase

quartz, kaolinite, opaline silica, montmorillonite; lesser siderite ; trace pyrite, felspar, gypsum,calcite: rare trace analcime

Organic Matter in Oil

Shales:

lamalginite dominant;

minor telalginite, vitrodetrinite & liptinite

Type I assemblage

Lamalginite, vitrinite (dominant in carbonaceous oil shale), telalginite, sporinite, rare bitumen and inertinite

Type I & mixed Type I &

III

Upper oil shale:

lamalginate, telalginite dominant, rare vitrinite, sporinite Type I

Lower oil shale:

lamalginite dominant, rare telalginite, vitrinite, inertinite, sporinite

Type I-II, immature

lamalginite, leaf resinite vitrinite, and sclerotinite rare telalginite

Type I

Oil shale/lamosite:

lamalginite, sparce telalginite, trace vitrinite, inertinite, sporinite

Carbonaceous shale:

inter-laminated lamosite

& coaly laminae

Cannel coal: clarinite

and vitrinite,

Table 2: Summary of Primary Characteristics of some Queensland Tertiary Oil Shales (continued)

fish & reptile fragments, microflora.

spores & pollen, acritarchs;

poor macro-fossils in oil shale units

Upper Oil Shale: spores

& pollens, fish bones,

diatomite-Melosira.

Lower Oil Shale:

ostracods, gastropods, microflora.

bivalves,gastropods, ostracodes,

dicotyledons, palm fronds

ostracodes, gastropods, tortoise shell fragments, reptile teeth, coprolites, microflora.

Depositional History:

(base to top)

1.contracting alluvial fan 2.open lacustrine to limnic mudflat 3.limnic mash (swamp) 4.fluvial

1.fluvial,continental

2.fluvial, deltaic.

3.marginal swamp/fluvial

4.stratified saline lake

5.fluvio-deltaic.

1 fluvio-lacustrine

2 lacustrine, marginal swamps

3 fluvial, aerobic

4 shallow fresh-water lake

5 fluvial

1.alluvial 2.deep stratified lake

3.limnic swamp 4.deltaic

1.fluvial to ?deltaic

2 fresh water lake with periodic development of limnic swamps and/or fluvial systems

3.fluvial

Oil Shale Depositional

Environment:

dominantly fresh-water lacustrine - open water, swamp, anoxic mudflat &

pedogenic subenvironments

Stratified lake, possibly increasing salinity during deposition

predominantly lacustrine, marginal swamp

influences in the lower unit subject to climate variations

shallow restricted saline lake, evolving to deep open stratified with reduced salinity & anoxic sediment-water

interface.

shallow freshwater lake with periodic incursion of limnic swamps and anoxic fluvial systems Organic matter is largely allochthonous

Volcanic

Associations:

oil shale intruded by Late Oligocene alkaline dolerite (26.8Mya)

early tuffaceous facies in basal sequence

?associated with ?E

Cretaceous Whitsunday Volcanics.

alkaline basalt flows at base of the sequence

acidic ash -fall tuff marker at the base of upper oil shales

clay mineralogy reports minerals with tuffaceous affinities - source unknown

oil shale sequence intruded by multiple- phase alkaline Late Oligocene dolerite/

basalt (28Mya)

Total Stratigraphic

Thickness:

at least 1000m up to 3,000m greater than 1300m at least 700m at least 1200m

Lower: 200m low grade

200m 600m

Shale Oil Resource:

(in-situ)

Rundle: 2.6 Bbbls,105 LT0M

Stuart: 3.0 Bbbls, 91 LT0M

9.65 Bbbls, 67 LT0M 4.1 Bbbls, 79 LT0M 740Mbbls, 82 LT0M Nagoorin: 2.65 Bbbls

Nagoorin Sth: 467 Mbbls

Data compiled from Coshell(1983,1986), Coshell & Loughlan(1986), Dixon & Pope(1987), Doyle(1992), Finegan(1991), Green & Bateman(1981), Green

et.al.(1983), Henstridge & Missen(1982), Henstridge & Coshell(1984), Henstridge & Hutton(1984), Hutton(1985), Lindner(1983), Lindner & Dixon(1976),

McConnachie & Henstridge(1985), McIver et.al.(1991) and Spiro et.al.(1993) and unpublished data from Southern Pacific Petroleum N.L and Central Pacific

Geological Setting of The Narrows Graben

The Stuart Oil Shale Deposit which, together with its sister deposit Rundle to the north, forms part of the preserved Tertiary sediment fill of The Narrows Graben which overlies Palaeozoic sediments of the Coastal Block (Figure 6) In the

Gladstone region, the Coastal Block consists primarily of the Curtis Island Group (Kirkegaard et al, 1970), which was subdivided into the Shoalwater and Wandilla tectonostratigraphic terranes to the east and west respectively by Fergusson et.al (1988) They interpreted these terranes as part of a subduction complex present during the Devonian to Carboniferous in this part of the New England Orogen

(Murray et al., 1987)

The structure forming the basin was initially interpreted as a graben (Kirkegaard et

al 1970; Lindner & Dixon, 1976; Henstridge & Coshell, 1984) but in more recent interpretations a half-graben structure is considered to fit the cross-sectional

geometry of the basin more closely (Finegan, 1991; McIver et al 1991) Lindner & Dixon (1976) did however note the asymmetric nature of the internal basin sequence

In the Stuart-Rundle region, the bounding faults of The Narrows Basin parallel the Tungamull-Boyne River-Yarrol Fault system which marks the boundary between the forearc sediments of the Yarrol Basin (Yarrol terrane of Fergusson et.al., 1988) and the subduction complex, pelagic and turbidite deposits of the Doonside and Wandilla Formations (Wandilla terrane of Fergusson et al., 1988) (Murray, 1974; Murray and Cranfield, 1989) A line of discontinuous serpentine bodies marks the fault in the Gladstone Rockhampton region North of Rockhampton, extensive ultramafics associated with undated metamorphics (Kirkegaard et.al., 1970) grouped as the Marlborough terrane, separate the Wandilla and the Yarrol terranes

The graben formed in the Devonian to Carboniferous sediments of the Wandilla terrane during an extensional tectonic regime activated in response to the opening of the Coral Sea Basin during the early Tertiary (Grimes, 1980) This period of

instability was heralded by the emplacement of rhyolite and trachyte plugs (White Rock and Mount Larcom) close to the western fault margin of the graben in the Late Cretaceous (Kirkegaard, et.al., 1970; Donchak and Holmes, 1991)

Sedimentation in the graben commenced prior to the Middle Eocene with deposition

of clastic material primarily as outwash fans (Worthington Formation) derived from the surrounding Palaeozoic rocks (Coshell, 1986) This was followed in the Middle

Figure 6: Regional geology and structural setting of The Narrows Graben

Eocene by quieter, lacustrine conditions with deposition of both kerogen-bearing and non-kerogen-bearing clay of the Rundle Formation as the basin matured An

increasingly more fluvial regime resulted in the deposition of carbonaceous material, clay and kerogen-bearing clays of the Curlew Formation At least 1,000 metres of

LEGEND

Wandilla Terrane

Stuart Oil Shale Deposit

2 4 6 8

kilometres

sediment accumulated by the Late Eocene (Lindner and Dixon, 1976; Henstridge and Missen, 1982; Henstridge and Coshell, 1984; Coshell and McIver, 1989; McIver

et al., 1991)

Subsequent to the filling of The Narrows Graben, the Wandilla terrane experienced deep weathering, peneplanation and gravel deposition during a period of relative tectonic stability common to much of southeast Queensland during the Middle Eocene to Early Pliocene (Day et al., 1983) Lateritised gravels and sands have been recognised at Rundle (Coshell, 1986) and Stuart (Finegan, 1991) and are probably part of a more widespread, Late Miocene lateritic surface recognised in the Fitzroy Region (Grimes, 1980) The bounding faults of the basin have generally remained inactive since this time, although Finegan (1991) observed off-set bedding in the lateritised Tertiary gravels and suggested some oblique-slip in addition to strike-slip movement along the Western basement fault (WBF) at Stuart Coshell (1986) also suggested some strike-slip movement along the WBF at the Rundle deposit in the northern part of The Narrows Graben Quaternary gravel and sand were deposited as slopewash and alluvial fans close to the emerging Mount Larcom Range whilst saline muds and local beach sands were deposited in more recent times and unconformably overlie much of the younger strata of the basin Present day streams deposit and redistribute gravel and sand as streambed loads Exploration of the oil shale deposits

in the graben has established the subcrop pattern and cross-sectional asymmetric nature for the preserved Tertiary sequence (Figure 7)

Stratigraphy of The Narrows Graben

The stratigraphy of The Narrows Graben was first described by Lindner and Dixon (1976) and later by Henstridge and Missen (1982) Coshell and Henstridge (1984) formally defined The Narrows Graben sequence and the stratigraphy of the Stuart Oil Shale Deposit was detailed by Coshell and McIver (1989) The stratigraphic

relationships are summarised in Table 3 and Figure 8

Figure 7: Interpretive subcrop geology map of the Stuart and Rundle Oil shale Deposits, The Narrows Graben

The cross-section asymmetry is shown in the Cross-section (A-B) The interpreted limits of the Curlew Formation and members of the Rundle Formation within The Narrows Graben are shown

Table 3: Stratigraphic Table – Stuart Oil Shale Deposit

Colluvial gravel sand & soil, minor conglomerate &

sandstone

Maximum possibly 15m

Curlew Formation

Claystone interbeds of lignite oil shale sanstone

At least 160m

Kerosene Creek Oil shale, lesser interbeds ofclaystone & lignite 41-61m

Telegraph Creek Claystone, lesser interbeds of oil shale minor dolomite. 55-95m

Munduran Creek Oil shale, lesser interbeds ofclaystone rare lignite 24-55m

Humpy Creek Oil shale, lignite and lesser claystone interbeds 9-31m

Brick Kiln Member Oil shale, minor interbeds of claystone rare lignite 52-151m

Ramsay Crossing

Oil shale, claystone dominant in upper section rare lignite

35-70m

Teningie Creek

Oil shale, claystone dominant in upper section, rare lignite

Fining upward sequences of conglomerates, sandstone and claystone

At least

289 m

Shoalwater Formation

Arenite, mudstone & minor chert

Chert, mudstone, minor tuff

& tuffaceous arenite

At least

3 000m

Stratigraphic table from Pope (1999)

Figure 8: Schematic cross-section showing the relationship of the Curlew and Worthington Formations and Rundle Formation members of The Narrows Graben sequence

Key to geological units (see Figure 7) Vertical exaggeration approximately 5:1 Modified from Sinclair Knight Mertz (1999)

Coshell and Henstridge (1984) described the stratigraphy of The Narrows Graben sediments and defined the eight members of the Rundle Formation The Narrows Graben sediments, The Narrows Beds, are divided into three formations (oldest to youngest):

sequences of coarse conglomerates, sandstone and claystone, derived from the

surrounding Paleozoic rocks The Worthington Formation is at least 289 m thick

this formation is subdivided into eight members, six containing predominantly oil

shale and two containing predominantly claystone It was formed in a hydrologically closed freshwater lake, with algae deposited in the fine mud providing the source of kerogen, the name given to the organic content of oil shale, which releases

hydrocarbons on pyrolysis The uppermost kerogen bearing unit, the Kerosene Creek Member, is the oil shale to be mined in the Stuart Project The Rundle

Formation is up to 600m thick; the upper 400m contain the bulk of the oil shale

Curlew Formation The base of the Curlew Formation comprises a lignite rich carbonaceous oil shale, indicating swamp conditions with increased higher plant growth The sequence then grades upward into claystone and minor interbedded carbonaceous oil shale The Curlew Formation is up to 100 m thick in the graben

Sedimentation in Tertiary Oil Shale Sequences

The number of Tertiary basins which developed along the Queensland coast together with the significant amount of oil shale and carbonaceous shale that accumulated in the preserved sections during the same period in geologic time has led a number of authors to speculate on a set of unique conditions favouring oil shale formation (Lindner, 1983; Green & Bateman, 1981; Henstridge and Missen, 1982; Grimes, 1980; McIver et.al 1991; McConnachie & Henstridge 1985) Most if not all of the basins in Table 2 developed as a result of the tensional regime following the opening

of the Coral Sea in the early Tertiary (63.5Ma) Spreading in the Coral Sea and northern Tasman Sea had ceased by 55Ma and continental sedimentation was taking place by the middle Eocene on the eastern Queensland coast (Figure 9) All of the basins experienced similar depositional histories and sediment types and all have, to some extent undergone significant syn-depositional and post-depositional faulting, folding and erosion resulting in their present form Lindner (1983) referred to the possible influence of plate movement on climate during the first cycle of deposition and erosion proposed by Grimes (1980) for the early Tertiary The early Tertiary was a period of gradual lowering of sea level from very high levels during the late Cretaceous This phase was completed toward the end of the lower Oligocene with a pronounced fall in sea level and a sustained period of low sea-levels in a cooler dryer climate (Haq et al 1987, Emery & Myers, 1995) A period of subsequent stable

conditions during the Oligocene was followed by a major period of lateritisation in the early Miocene

The sedimentation in the Stuart deposit in The Narrows Graben and other deposits had largely ceased by the late Oligocene although sedimentation in the Hillsborough Basin may well have been continuing but waning The cross-sectional asymmetry in most of the examples described above is not uncommon in rifted systems, as

described from other world examples earlier, and is frequently associated with

tensional regimes resulting from continental relaxation The timing of these events is important, as McConnachie & Henstridge (1985) pointed out when discussing the

Figure 9: Palaeoreconstruction of seafloor spreading around Australia at 45Ma

Base figure from Veevers et al (1991) Oil shale basin locations added Continental

sedimentation (yellow) developed inland from the shoreline (blue dashed) over the previous 4Ma Spreading in the Coral Sea (CS) and northern Tasman Sea (NTS) has ceased about 10Ma earlier

early development of the Lowmead Graben, since significant sediment accumulation was probably taking place at Lowmead during a period of apparent stability in the Maryborough region Both the Lowmead and Nagoorin Grabens show relocation of the centre of the deposition for the later lacustrine sedimentary facies relative to the early fluvial, high-energy phases The relocation may be related to early strike-slip movements during the initial sag phase of the basin or simple syn-depositional faulting as observed in The Narrows and Hillsborough Basins

The facies assemblage that accumulated is remarkably similar in each deposit, not only in the dominance of very fine silicic material over carbonates, but also for the overall depositional cycle (although this is largely a function of source material as carbonate facies represent only a small amount of basement rocks in most of these areas) Accumulation in each basin commenced with fluvial deposition, evolved through a prolonged lacustrine or fluvio-lacustrine period where significant oil shale deposits accumulated and culminated in a shallowing of the lake and return to a final fluvial or fluvio-deltaic phase after a transitional period where limnic marsh or swamps were common particularly at basin margins At Condor, oil shale developed

in a deep possibly stratified lake similar to that proposed for the Green River

Formation (Green & Bateman, 1981, Doyle, 1992, Little, 1992) Lacustrine

conditions also persisted in the Duaringa Basin for a long period although the lake depth remained shallow enough to permit significant aeration and bioturbation destroying any significant accumulation of organic matter during a large part of the lacustrine sedimentation history (Dixon & Pope, 1987)

Oil Shale Facies - Degradation and Preservation

Oil shale deposits have been recorded from Precambrian to Recent times Earliest life-forms are now generally considered to have been present during the Proterozoic and cyanobacteria-like organisms were diverse by at least 3400 Ma By far the majority of these fossils occur in marine strata, although older lacustrine sequences can be difficult to recognise and may well have long since given up any organic matter to intense metamorphism and erosion There are instances where organic

matter can be preserved for up to 2Ga albeit as almost pure carbon (shungite for example, Melezhik et al., 1999) In some instances, the presence of organic material has been instrumental in the localisation of metalliferous deposits of significance (e.g Century, North Queensland; McArthur River, North Queensland, the Basin and Range gold deposits of Nevada, USA) More ancient settings also have similar structural and depositional features such as basin asymmetry and growth faults present in the more modern Tertiary equivalents

Both the geographical and geologic age spread of preserved oil shale occurrences reflect not only the multiplicity of accumulation and preservation controlled by palaeoclimate and tectonic setting, but also the diversity of source for organic

precursors to the kerogenous material of oil shales Kerogenous material can be preserved in clastic sediments in both marine and lacustrine settings, and oil shales can often be found in lake sediments preserved in intracratonic or intermontane basins and marginal tectonic settings often associated with rifting and active margin settling (Smith, 1990) Palaeolatitude and palaeoclimate also play a significant role

in the accumulation of oil shale in major lake systems Permanent stratification of lake waters combined with a continuous history of a permanent anoxic layer at the base of the hypolimnion and an active accumulation of generally fine-grained clastic sediment favours preservation of organic material on the lake floor (Demaison and Moore, 1980)

Sediments can accumulate in lake basins that may be hydrologically open or closed depending on the input/output vs evaporation rates within the system Open lakes tend to be more stable, and shallower with dilute to mesosaline conditions in contrast

to the deep (pelagial) saline to hypersaline water conditions more typical of the closed lake system (Demaison & Moore, 1980, Kelts, 1988)

Kelts (1988) suggested that optimal conditions for accumulation of organic matter are provided in a large, relatively deep, mesosaline, alkaline, closed lake basin in a sub-tropical setting Adequate nutrients, a stressed environment with low predator numbers, seasonal stratification, low sulphate concentration and concentration of organic matter by dissolution of silica and minimum clastic input are considered as