A large amount of electromagnetic soundings in the Far East of Russia, in the transition zone from continent to marginal seas where several large industrial oil-and-gas bearing basins ar
Trang 1ABOUT INVESTIGATION OF THE TEKTONOSPHERE DEEP STRUCTURE AND THE PREDICTION OF OIL-AND-GAS PRESENCE IN
THE DELTAS OF THE RED AND MEKONG RIVERS
V.M Nikiforov R.G Kulinich, N.G Shkabarnya, I.V Dmitriev
V.lirichev Pacific Oceanological Institute, Vladivostok, Russia,
nikiforovv@mail.ni
1 Introduction
The importance of the deep factors in the formation of oil-and-gas deposits is recognized by most researchers, regardless of their views on the origin of hydrocarbons: Organic or inorganic The spatial relationship between the oil-and-gas areas and deep faults, weakness zones in the Earth's crust, heat flow anomalies can be considered as established fact [1] A large amount of electromagnetic soundings in the Far East of Russia, in the transition zone from continent to marginal seas where several large industrial oil-and-gas bearing basins are located, has revealed the geoelectric criteria prediction of oil-and-gas presence, taking into account the fluid regime of deep horizons of the crust and lilhosphere In addition,
as revealed in recent years, the concentration of oil-and-gas deposits is controlled
by zones of anomalous conductivity in the crust and subcrustal lilhosphere [2] Detection of such zones using electromagnetic soundings, tracking them over large areas, the study of relations with the structural plan, tectonics, material constitution contributes to a more efficient prediction and conducting oil and gas exploration Such an approach may be useful in studying the resources of the East-Vietnam Sea, characterized by the fact that the patterns of distribution of hydrocarbon deposits have specifics, significantly different from the classical ones
2 Geoelectric section of the tectonosphere in the transition zone from continent lo the marginal sea (for the example of the Russian Far East)
The concepts of normal and abnormal differentiation of the electrical resistivity are important for understanding the overall tectonosphere structure, its petrology, fluid dynamics and direction of the underlying processes, ways of the deep substance transportation There are different approaches to constructing models of the electrical resistivity distribution in the depth [3] Important role in this series is the standard gradient-section model the assumption L.L Vanyan [4] The standard (normal, planetary) model was obtained as a result of the interpretation of magnetic-variation curve constructed from the data analysis worldwide network of magnetic observatories and magnetotelluric sounding for all the earth shields According to this model, the electrical resistivity of crustal rocks and upper mantle decreases steadily from 200,000 Ohmm at a depth of about 20 km to 20 Ohmmal
a depth of 300 km
V.M Nikiforov
Trang 2The standard model is deep sequence of electrical resistivity values of the dry rocks under conditions of temperatures and typical pressiu-es for areas with normal heat flow equal to 45 mW/m^ The increase in heat flow or fluid saturation of deep rocks lowers electrical resistivity relative to a standard incision The increase in heat flow or fluid saturation of deep rocks reduces the electrical resistivity level relative to the standard modeL
Experimental data analysis on the results of deep electromagnetic soundings in the transition zone fi'om continent to marginal seas of the Russian Far East, showed
a significant difference between the regional deep geoelectric model and the standard model Instead of a monotonic decrease with depth, the electrical resistance changes abruptly, forming three main layers The parameters of these layers beneath the continent and the marginal sea are different (Fig 1)
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300
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A 20-50 Ohmm ^
A A A ^ ^
norniiit pUncl Lfcoclccirk
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/ A 5-20 Ohmm
A y^
section {by I.J Vany.tn)
+-, v
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48,7 65.0 SI.2 07,5
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Fig 1 Geoelectric section of transmit zone from continent to marginal sea
1 - Earth's crust; 2 - subcrust lilhosphere; 3 - asthenosphere, 4 - upper mantle; 5
- borders of tectonosphere layers; 6 - anisotropic electric conductivity zones in subcrust lilhosphere identifiable with the largest shearing systems of region; 7 -anisotropic electric conductivity zones in lower part of Earth's crust identifiable with hydration of crust basic composition rocks
Thickness of the upper layer, which is relatively high-resistivity (1000 ohm-m), beneath the continent is 35-40 kin, and under the marginal seas bottom is reduced
to 20-35 km Beneath it lies the subcrustal lilhosphere formation that differs from the standard model in electrical resistivity significant reduction up to 80-150 ohmm The bottom of this layer is located beneath the continent at a depth of 150
km and beneath a marginal sea at a depth of 80-100 km Below it the layer of low-resistivity (5-20 ohmm), usually identified with the asthenospheric layer, is bedded Below the asthenospheric layer the section closes to the standard model Low levels of electrical resistivity of the asthenospheric layer beneath the continent where the heat flow does not exceed 45 mW/m^ can not be explained by
V.M Nikiforov
Trang 3the phenomenon of the dry rocks melting [4]
This circumstance forces suggest that the upper mantle rocks melting occurs ig the presence of fluid, which lowers the melting point of the ultrabasic rocks to
1200 C, achievable at a depth of 150 km, even in conditions of normal heat flow Relatively low electrical resistivity values in the subcrustal lithosphere can't be associated with the melting of rocks
A possible reason for the decrease of the resistivity level is the saturation of lithospheric rocks with graphite, could be explained by following: in the vertical migrations of deep gases, including H2, H2O, CO, CO2 at a temperature below 700
°C, the Boudoir reaction proceeds:
2CO = C (graphite) -1- COj
Decreasing temperature [5] should cause the start of the process nf hydrocarbons formation under the scheme:
CO -I- H2 = CP -I- HC -I- CO2 + H2O, where CP - compression products, HC - hydrocarbons
Water which is formed by chemical reactions at temperatures below 500 °C interacts with the host rocks:
9 Enstatite-I- Water = Antigorite -i- Forsterite;
Antigorite -I- Water = 9 Talc + Forsterite;
2 Forsterite + 3 Water = Serpentinite -i- Brucite
Hydrous minerals: talc, antigorite, serpentinite - occupy a greater volume than the original anhydrous minerals Because of this the healing of deep gas migration channels occurs A regional cover, which prevents the promotion of fluids in the crust, IS established This provides a high level of electrical resistance
mJ^T.\ !*" " T " " 8f==1^^ !<= ^^«ion in the transition zone from continent to
margina seas is formed, probably, under the influence of the rising fluid, with C,
L Tn 't'™'".'=°'"P°"™'^- R«g"'^f changes in temperature, depths and pressure
i fftrlnliatinrnf f ? ° '^'^^'•'"' "^'''''"'' ^^"'^ products provide layered
a tia n r ^ u ' e ' h ' ? '''"'""y-.'^' =°"^ce of the rising gas flow is probably
margina?sea '^"""8^"^°"^ '" composition under the continent and the
3 Features of the electroconductive structure of the subcrustal lithosphere
^ I S r t T i U l T l * ' ' ' ' ^ * ' ° ' ^"-"Plication of the deep geoelectric
o n d u l ; X h i 7 r ? ^'""'""^ '^°''- F ' g " « 2 shows the electrical :
As is s en the en f r u'^°'^^"' '" ^^'^"™ ' ° ™^J" f^""^ ' " * e region,
tnospnere of the marginal sea (80-100 ohm-m) The boundary between
208
Trang 4these geoelectric areas within the investigated area is near the coastline of the Asian continent However, the transition between them takes place gradually through the intermediate Sikhote-Alin anisotropic conductive belt The boundaries
of this belt correspond to the Cenfral Sikhote-Alm fault in the west and Coastal fault in the east It is significant that the direction of maximum conductivity of the belt at an angle of about 60 degrees to the general direction of the belt This fact allows us to consider the belt as one of the biggest strike-slip faults in the region A similar belt extends from the Liaodong Bay to Shantarskye islands It coincides with the well-known trans-regional fault - Tan-Lu,
Fig 2 Relation between abnormal electroconductive zones in subcrust lithosphere and Far Eastem major faults diagram
Deep foults: ILIT llanItunsky, MFA MishanFushunAlchansky; US -Ussuriysky; P R - Furmanovsky; CSACentralSikhoteAIinsky; KHIN -Khingansky; KUR- Kursky; LIM-Limucharsky; WSAK- West-Sakhalinsky; CSAK - Central-Sakhalinsky; ESAK- East-Saldialinsky; POOR Pogranichniy; PBRV Pervomaysky; TUG Tugorsky; PAUK -Paukansky; NAO- Naolihe; WAN-Verhne-Anuysky; P R - Pribrezhny; MED- Meridian tectonic line; ESA-East-Sikhote-Alinsky
extends in a northeasterly direction In addition to these fault zones in the lithosphere blocking are involved the faults of east-north-easterly direction along which is traced Mishan anisotropic belt and a series of linear low-resistivity (50-80 ohm) zones in the lithosphere
The concentration of mineral deposits near the above-stated continental lithosphere heterogeneities indicates that they are zones of high permeability, and removal of deep substance occurs through tiiem In the marginal-marine lithosphere are developed high-resistivity heterogeneity, which are also in agreement with the well-knovm major faults High resistivity of anomalous zones possibly indicates the thrusting nature of the faults Within the investigated area these anomalous zones conti-ol the location of oil and gas deposits
4 Conductive structure of the consolidated crust
In the upper, relatively high-resistivity layer (more than 1000 ohm-m),
corresponding to the Earth's crust, there are two types of the section homogeneity breaking: linear low-resistivity (15-25 ohm-m) zone and anisotiropic-conductive
Trang 5beh (Fig 3) The first is developed mamly in tire lower crust at deptiis of 15-25 h^
Zd constitate a body of hmidreds kilometers m length and width of 10-15 km Ue
maximum conductivity of bodies observed in the direction of the stiike This aUows us to interpret fliese anomalies as stretching faults developed m tiie lowet
crust Aiusotropic belts characterized by the fact that the direction of maximum conductivity makes the angle about 60 degrees to the directioa
of the belt strike They are developed at depths of 8-15 km, crossing the different strata On the investigated area identified the following belts: Tan-Lu, West-Primorsky, East Sikhote-Ahnsky, Tatarsky, West-Okhotomorslq', These belts are consistent with the general arrangement of the heterogeneities in the underlymg lithosphere considered above, although they are not i direct continuation of heterogeneities in the Earth's crust The configuration
,.„„j,*:i,™-' ^.^•,.„„j,*:i,™-'^jrti"^^^^,.„„j,*:i,™-'^^^
' ! ^ » £ ^ " "
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• • : J ^ i
&^,
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r z n i
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^ ^
"yjif^m
^HI
*?•
• i
*
s^'^
"1 i*
Ijffife
ly'Wfoh
1 ^
•
r
Fig 3 Relation between abnormal
electroconductive zones and Far Eastem major
faults diagram
of belts boundaries in the crust is determined by the superposition of the faults influence in different directions (in conteast to the subcrustal lithosphere, where the heterogeneities the faults are subject to predominantly a single direction)
An important feature of the anisoteopically-conductive belts' structure is the presence of galvanic contact with the near-surface (sedimentary) layer through the fi'actures system, developed along the border belts This fact is the main condition
to detect anomaUes, in the absence of contact along the botmdary of anisotropic belt the above-mentioned formation are not visible to magnetotelluric sounding Concerning the nature of conductivity, we note the following At the bottom of the Eartii's cmst, water can't be in the free state for a long time At temperatures of this range of depths is consumed in the hydration of olivine and pyroxene -minerals that make up the basic rocks But in terms of the pressiu'e of the lower crust at temperahires below 500°C tiie following reaction is possible:
CO2+ 2H2 = C -1- 2H2O After the water consumption in the anomalous zone remains the graphite, which provides lowering tiie electiical resistivity of rocks Thus, the linear conductive zones are likely to tiace faults that healed after the next activation
In tiie middle of tiie Earth's crust at temperahires below 250 ° C conditions are
Trang 6favorable for tiie reaction:
CH4-l-C02 = 2C + 2H20 The products of this reaction in the form of water-graphite mixture can be filled fractured space arising from the interplay of shifts, stretching for hundreds and thousands kilometers and thus provide a high anisotropic-conductivity belts, as shown in Figure 2 It is important to note that the low-resistivity anisoteopic-conductivity zone detected by the graphite-water mixture may also contain the original gas components, mcludmg methane This mixture can interact with organic matter, sedimentary rocks (if the anisotropic conductive complex captures the sedimentary strata, as is the case in northern Sakhalin)
5 The relationship between oil-and-gas deposits and anomalous conductivity formations of the Earth's crust
Oil-and-gas deposits known in the Russian Far East approach to the boundaries
of anisotropic- conductivity belts exactly to the places along which the anisotropic conductive rocks are in contact with the rocks of the sedimentary layer [6, 7] This regularity is clearly seen along the westem boundary of the anisotropic belt of
Tan-Lu (Fig 4), where Bohayvan depression's deposits are concenttated At the same even a single gas deposit "Urgal" in north-west region is also confined to the
westem boundary of the Tan-Lu anisotropic belt
Near the westem border of West-Primorsky belt oil deposit is discovered in the Yanji area by Chinese researchers All hydrocarbon deposits on Sakhalin Island approach to the eastem border
of the Tatarsky and the westem border of the West-Okhotomorsky aiusotropic conductive belts Two fields: Central Tamlevskoe (located
in the north of Sakhalin Island) and Izylmetevskoe (in the middle part) located inside the Tatarsky belt, are associated with the contact of aiusotropic rocks with sedimentary strata Hydrocarbon deposits chain, which takes place along the west coast of Hokkaido Island (Japan) is probably the continuation of tiie soutii eastem boundary of the Tatarsky belt
In the East-Sikhote-Alin belt the hydrocarbon deposits are not found A feature of this belt is anisotropic conductive formation rises directly to the siuface m its southem part The geological stmcture of this region contains granites and rhyolites
Fig 4 Disposition of oil and gas deposits
relative to abnormal electroconductive zones
in earth crust diagram
Trang 7Late Cretaceous-Paleogene age
Probably due to lack of tight covers, water-gas components of the fluid depth dissipated into the atmosphere, and graphite particles that remain in the rocks skeleton (possibly in conjunction with ore minerals) gave the rocks of the considered belt the anisotropic electrical conductivity
In connection with the above stated it would be interesting to compare the electrically conductive structure of the crust of the "White Tiger" area (Cuu Long Basin, Viet Nam), where the basement is covered with thick layer of sediments and Sikhote-Alin (Far East Russia) and, for the reason of this comparison, make an assessment of mineral deposits specific
From analysis of the deep electromagnetic soundings we can make some conclusions as following:
1 Geoelectric section of the tectonosphere, in the transition zone from continent
to the marginal sea, is very different from the standard (planetary) model
2 Vertical stratification of the tectonosphere section is closely related to the gas-geochemical processes that are characteristic for different temperature and pressure intervals
3 Breakings of the horizontal geoelectric layering structure caused by the superposition of tectonic factors on "normal" process of the region degassing
4 Thank to the peculiarities of the geodynamic regime of permeable zones (electrically conductive traces), where transport of deep fluid into the crust can (or could) be happened, they can be detected by electromagnetic sounding
5 Studies of the electric structure of anomalous deep zones allow specifying the nature of oil and gas deposits' formation, and to prediction of new oil-gas areas on this basis
6 Proposal for the deep electromagnetic studies of the East Vietnam Sea-bottom
On the assumption of similar struchiral and tectonic position of the marginal seas m the transition zone from the Asian continent to the Pacific Ocean and
nf ,°h7 h^,? f T ^^^"larities, we proposed to start the electromagnetic study
1 ,„ ,h M f^^l^^'^ ^ i ^ ' " ^ " ! Sea from a soundings along two profiles BTsin in h 1 ° " ^ "'" °" "'" ' " " " ' " ' ^ continental shelf of Vietnam and Hanoi
Basin in the north respectively (Fig 5)
Derh,her^l'nl°"f°!i.*'5',"•""' ^ P"°"''^= '^ based on the following In the
se imen trv'form" , ' ^ ^ ^ " " 8 ^asin, composed of terrigenous and carbonate
ccumuTatl of ni ,K " " ""T' " ^ P " ^ ' ' " "White Tiger" The worid's largest ssocTat d with?o -l " ^ ' ' ' " ' ™ ^''''"""' ™^ks was detected there Deposk is
associated with a granite protrusion with size 22 km at 6 km, broken by fauhs into
Trang 8several blocks The surface of the central, most elevated blocks, can be traced to a depth of 3,000 m Established rBservou fliickness in the granite is more than 1000
m The thickness of the weathering cmst does not exceed 20 m and oil-saturated reservoir is mainly connected with the interior of a crystalline massif OUgocene clay formation, overlying granite massif, is the caprock of the reservoir In the
Cenozoic sediments near the
"White Tiger" small oil fields are also found
Capacitive space of granites formed a huge number of micro-cracks, cavities and pores Rocks are obvious signs
of secondary changes, particularly of the formation of zeolite [8] Zeolites replace denser feldspar and clay rocks, leading to a granites decompaction [9]
Exclusive feature of the bearing rocks of the "White Tiger" is they contain direct evidence of deep fluids implantation The direct products of the implantation of this fluid for a variety of fractures are the included mafic polymineral material [10] Predominance in its composition of solid carbon (hydrocarbon) phases, the presence of native metals and carbides indicates that the basis
of fluid was super-deep nature multicomponent gas [11,12] Considerable amount of various clay minerals indicates the assimilation of water from the mantle during the passage through the channels of its low viscosity The presence of metallic particles dispersed native not only in the granite massif, but also in the terrigenous reservoirs zeolite shows a leading role of the plume-tectonic factor in the formation of oil fields in the "White Tiger"
Thus the available data indicates that deep fluids plays veiy important role in the formation of the "White Tiger" deposit This closely echoes the conclusions that we independently obtained from tiie analysis of electromagnetic soundings in
Fig 5 Planned electromagnetic research of
Vietnam Sea bottom plotted on tectonic struchiral
map fragment (by Phung Van Phach, et al., 2010)
Trang 9the Far East of Russia In this regard, it is expected tiiat as a result of electromagnetic soundings in the Mekong Basin, will be obtained a simila, structure of anomalous conductivity at different deptiis and revealed relationship of oil deposits' location with the elements of the anomalous conductivity Electromagnetic sounding within the Hanoi Basin is also filled with Cenozoic sediments up to 15 kilometers, will establish an analogy or contrast the deep conditions from conditions under the Mekong Basin
The proposed set of electromagnetic studies will provide an opportunity to specify the criteria for geoelectiic oil-and-gas content discovered in the Far East of Russia, an adjustment to the conditions of the East Vietnam Sea and use them to search for oil and gas stmctures in the new, as yet poorly studied areas
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V.M Nikiforov