The multi-staged “golden zones” of hydrocarbon exploration in superimposed petroliferous basins of onshore China and its significance

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 42, Issue 1, February 2015 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2015, 42(1): 1–13.

RESEARCH PAPER

The multi-staged “golden zones” of hydrocarbon exploration in superimposed petroliferous basins of onshore China and its significance ZHAO Wenzhi*, HU Suyun, LIU Wei, WANG Tongshan, JIANG Hua PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China

Abstract: The hydrocarbon generation model proposed by Tissot et al. points out the temperature and depth of “liquid HC window”, which has become a “golden zone” for hydrocarbon exploration. It has been proved by exploration that multi-staged “golden zones” for hydrocarbon exploration is commonly developed in the superimposed petroliferous basins of onshore China. There are three factors for the formation of multi-staged “golden zones” of HC exploration in the superimposed basins: (1) source kitchens developed with multi-periods and multi-centers which have verified to lead multi-stages of HC generation with large scale, (2) multi-staged reservoirs develop with large scale, (3) hydrocarbon accumulations occur with the multi-periods and late effectiveness. The conventional source kitchens, dispersed liquid HC-cracking gas kitchens and effective reservoirs with large scale join together to control the distribution of “golden zones” in timing and space. Explorational “golden zones” have the characteristics of inherited stacking and lateral variation. Paleo-highs, paleo-slopes, paleo-platform margins, and multi-period inherited fault zones control the distribution of hydrocarbons in the explorational “golden zones”. The concept of multi explorational “golden zones” helps to deepen the knowledge of new hydrocarbon distributional regularity which revealed recently in China. It shows that there exist economic resources in the deep section of the superimposed basins of onshore China. The hydrocarbon discovery history in the superimposed petroliferous basins has the feature of multi-peaks of proven reserve increase and lasting a quite long, which indicates a huge potential for future exploration. Key words: superimposed basin; multi-staged HC exploration golden zone; conventional source kitchen; dispersed hydrocarboncracking gas kitchen; large scaled reservoir rock; multi-period hydrocarbon accumulation; deep section; China onshore

Introduction Under the guidance of “liquid HC window” theory proposed by Tissot et al., a series of large and medium oil and gas fields in shallow- and middle-depth petroliferous basins have been discovered worldwide in the past few decades. In recent years, some Norwegian researchers presented the concept of “golden zone” for hydrocarbon exploration[1]. Its core connotation is that 90 percent of global oil and gas resources concentrate within the subsurface section with the temperature from 60 °C to 120 °C; beyond this section, especially in those sections with temperature higher than 120 °C, it is unlikely to find oil and gas. It has been demonstrated by petroleum exploration that multi-stage superimposed petroliferous basins in China differ greatly from single-cycle single-stage basins or multi-cycle basins with successively inherited features outside China, specifically, multi-stage superimposed petroliferous basins in China feature multi-periods of tectonic and sedimentary evo-

lution, multiple source rocks of various types, multiple reservoirs, various source-reservoir-seal assemblages and multimeasure hydrocarbon accumulations. As a result, understanding on hydrocarbon distribution and discovery of large oil and gas fields would generally go through a tortuous road and hydrocarbon reserves would also increase in a multi-peak multi-stage pattern in a long period. For example, Sinian Weiyuan gas field, Carboniferous gas fields such as Wubaiti and Dachigan, and Permian-Triassic reef-bank gas fields such as Puguang and Longgang have been discovered since the 1960s in the Sichuan Basin and a large tight gas field was discovered in the Xujiahe Formation due to the exploration aiming at the Triassic in Central Sichuan since 2009. A giant Sinian-Cambrian gas field was found recently in GaoshitiMoxi area, it is the largest discovery of petroleum exploration in the Sichuan Basin for nearly a hundred years. So far, commercial oil and gas flow has been found in more than 10 measures in the Sinian, Paleozoic and Mesozoic in the Sichuan Basin and the exploration depth exceeded 6 000 m.

Received date: 29 Sep. 2014; Revised date: 25 Nov. 2014. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the China National Science and Technology Major Project (2011ZX05004); PetroChina Science and Technology Major Project (2014E-32-01). Copyright © 2015, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

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nation since the discovery of Weiyuan gas field, the drilling of Well Gaoshi1 in 2011 marks another milestone in its exploration, followed by the discovery of a large Cambrian Longwangmiao gas field in Moxi with proved gas reserves of 4404×108 m3. The scale of reserves is anticipated to be over a trillion cubic meters[4]. Besides, Permian Xixia-Maokou Formations and Triassic Jialingjiang and Leikoupo Formations also have the potential of finding petroleum on large scale and may become new golden zones for exploration. Exploration activities have proved that superimposed petroliferous basins such as Tarim and Ordos also have multiple exploration golden zones. It is worthy of noting that the meaning of multiple golden zones in a superimposed basin depends on the intrinsic mechanisms of hydrocarbon accumulation, which is different from the conventional concept of oil-bearing in multi-measures, which will be elaborated in this paper later.

Other basins, e.g. Tarim, Ordos, Junggar and Songliao, have experienced similar process of exploration and discovery. Based on hydrocarbon exploration in the Sichuan, Tarim and Ordos Basins, the authors advanced the new viewpoint that there are “multiple golden zones” in superimposed petroliferous basins in this paper, in the hope to shed light on hydrocarbon potential, the discovery pattern of large oil and gas fields and the deadline of exploration depth in superimposed petroliferous basins, and to push forward the progress of petroleum exploration theories and technologies.

1

The meaning of multiple golden zones

In the 1970s, Tissot et al. proposed a model of kerogen-based hydrocarbon generation and defined some relevant concepts, such as hydrocarbon threshold, liquid HC window and dry gas phase, and the ranges of spatial distribution and temperature for kerogen evolution. A golden zone for exploration, entitled by Norwegian researchers, refers to a depth range of liquid HC window corresponding to the geotemperature from 60 °C to 120 °C and thermal evolution Ro from 0.6% to 1.2%. Marine strata in deep superimposed basins in China, featuring old ages, high-mature source rocks and sufficient evolution of organic matter, generated oil on large scale at the early stage, and liquid hydrocarbons retained in source rocks cracked into natural gas on a large scale at the later high- and post-mature stages, resulting in a double-peak pattern of hydrocarbon generation[2−3]. In addition, there are usually multi-measure source rocks of multi-stages in a superimposed basin, and multiple source kitchens in one source rock series which might experience different oil and gas generation histories due to differential evolution. This, together with multi-stage reservoir rocks formed in the geologic history, results in multi-stage accumulations and thus hydrocarbon occurrence in multiple series of strata vertically and multiple belts and prospects laterally. If a series of strata rich in hydrocarbon is regarded as a golden zone, a superimposed petroliferous basin would have multiple golden zones for exploration, instead of one (Fig. 1). Marine carbonate sedimentary sequences deposited in the Sichuan Basin from the Sinian to the Middle Triassic, after the Late Triassic due to the uplifting of surrounding mountain systems and the basin was enclosed, thus the deposits in the basin turned into continental clastic sedimentary sequences. There are at least five golden zones in the basin, i.e. the Sinian-Cambrian, the Carboniferous, the Permian-Lower Triassic, Upper Triassic Xujiahe Formation, and the Jurassic from bottom up. From exploration history of the basin, the Carboniferous golden zone has been prospected for more than 20 years with proved natural gas reserves of 2412×108 m3. The Permian-Lower Triassic reef-bank golden zone has been explored for 16 years with proved gas reserves of 2922.3×108 m3. The Triassic Xujiahe golden zone has been explored for 9 years with proved gas reserves of 7065.79×108 m3. In the Sinian-Cambrian golden zone, after decades of exploration stag-

2 2.1

Forming conditions of multiple golden zones Multi-stage development of source kitchens

The occurrence of multiple golden zones in a superimposed petroliferous basin is closely related to the multi-stage development of source kitchens, which refers to hydrocarbon generation at multiple stages by various source rocks occurring in different series of strata vertically and in different sags laterally due to differential subsidence and multi-stage evolution. It can be detailed in the following three aspects. (1) The source rock features great diversity and massive distribution. In terms of source rock types, since superimposed basins generally experienced Early Paleozoic marine deposition, Late Paleozoic transitional deposition and Mesozoic-Cenozoic continental deposition stages, marine, transitional and continental source rocks occurred accordingly. Marine source rocks, mainly composed of argillaceous rocks occur usually in depressions caused by differential subsidence inside superimposed craton basins and in slope-shelf of marginal depressions. The Cambrian-Ordovician source rocks in the Manjar Depression, the Tarim Basin are a good example. Transitional source rocks, mainly consisting of coal-measure rocks, are generally formed during the depression development stage of a superimposed basin. The Carboniferous-Permian source rocks in the Ordos Basin and Permian and Upper Triassic source rocks in the Sichuan Basin are some of such source rocks. Continental source rocks, mainly lacustrine argillites, and also some coal-measure source rocks, occur in semi-deep to deep lacustrine environment in a lake basin or swamp environment, the Triassic in the Ordos Basin and the Triassic-Jurassic in the Kuqa Depression, the Tarim Basin are some good representatives. In terms of distribution, various source rocks overlap vertically and join laterally to cover extensive areas. For example, the Tarim Basin has Lower Cambrian, Lower Ordovician and Middle-Upper Ordovician marine argillaceous source rocks, Carboniferous-Permian transitional argillaceous source rocks, and Triassic-Jurassic continental argillaceous and coal-measure source rocks in vertical −2−

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strated by exploratory drilling that source rocks in the Sichuan, Ordos and Junggar Basins also have similar features. (2) Scattered liquid hydrocarbons retained in source rocks

direction (Fig. 2), and several source-rock depocenters such as Manjar and Awat in lateral direction, resulting in a total area of source rocks of about 35×104 km2. It has been demon-

Fig. 1

Schematics of multiple exploration “golden zones” in the Sichuan, Tarim and Ordos Basins

−3−

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Fig. 2

Distribution of Cambrian-Ordovician and Triassic-Jurassic source rocks in the Tarim Basin

would be thermally cracked into large amounts of gas in high-mature to post-mature stage of the source rock, acting as a new kind of gas kitchen which has been proved by the researches in the last decade. Before the year 2000, high-mature to post-mature source rocks were generally treated as exhausted rocks without hydrocarbon-generating potential and commercial value for exploration. But actually, there would be a considerable amount of liquid hydrocarbons trapped in source rocks after hydrocarbon expulsion in the liquid window. Based on the results of hydrocarbon generation and expulsion modeling and kinetic gas-generating experiments for the 973 Program, we reached two conclusions: first, massive liquid hydrocarbon expulsion usually happen at the liquid window with Ro ranging in between 0.6% and 1.2%, source rocks generally have a hydrocarbon expulsion efficiency of 40% to 60%, among all kinds of source rocks, oil shale has the highest hydrocarbon expulsion efficiency of up to 80 percent[5]; second, massive gas generation from kerogen degradation takes place usually in the early mature to high-mature stage of source rock with Ro of 1.2−1.6%, while gas generation by residual liquid hydrocarbon cracking happens in the high-mature to post-mature stage of source rock with Ro of more than 1.6%, later than gas generation by kerogen degradation, but the amount of gas generated could be 2 to 4 times that generated by kerogen[6]; all considered, the main gas generation stage of source rocks has Ro from 1.5% to 3.2%. Therefore, the considerable amount of liquid hydrocarbons left in source rocks can generate much more natural gas than kerogen degradation at high-mature to post-mature stage, making it a kind of gas source kitchen for effective gas accumulations at the later stage. (3) Differential evolution of source kitchens gives rise to multiple peaks of hydrocarbon generation and expulsion and −4−

multi-stage hydrocarbon accumulation. Since there are usually multiple sets of source rocks in a superimposed basin, and the burial history and hydrocarbon generation history of same set of source rock could differ widely in different structural areas due to differential evolution, and every set of source rock enters “oil generation peak” and “gas generation peak” at different time and stays in “oil window” and “gas window” for different durations, multiple hydrocarbon generation and expulsion peaks would occur. For example, Middle and Lower Cambrian source rocks widespread in the Tarim Basin, owing to different thermal evolution degrees caused by tectonic subsidence or uplifting, this source rocks in different areas entered mature stage at different time, namely those in depressions matured first at the early stage of evolution, and then all the source rocks in the basin matured by the end of Permian, and those mature source rocks in depressions became high-mature at the later stage of evolution. As a result, Middle and Lower Cambrian source rocks generated hydrocarbons in different zones and geologic epochs (Fig. 3). The same is true for Middle and Lower Ordovician source rocks. In summary, a superimposed basin with multiple sets of source rocks of different types may have several oil and gas windows in a long period of hydrocarbon generation due to differential evolution together with gas generation from residual liquid hydrocarbon cracking at the later stage of evolution, so multiple source kitchens occur at multiple stages, laying foundation for multi-stage hydrocarbon accumulation in different series of strata and multiple exploration golden zones. 2.2

Multi-stage development of reservoirs

Multi-stage development of reservoirs refers to the occurrence of several sets of effective reservoir beds in a superim-

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Fig. 3

Middle-Lower Cambrian source-rock maturity contour in different geologic times in the Tarim Basin

grain banks controlled by large inherited paleohighs form reservoirs of different scales at the top of the Cambrian, and the tops of Ordovician Penglaiba, Yingshan and Lianglitage Formations (Fig. 4). These reservoirs are the final results of two factors: first, grain-type sedimentary rocks with some reservoir space and permeability, second, dissolution caused by later multi-stage tectonic events improving the reservoir properties. In spite of differences in form and process, the dissolution of meteoric water to carbonate rocks would end up with basically the same result of complicated pore-cavernfracture system in large area. In the Ordos Basin, a series of large-scale superimposed sand groups occurring in Upper Paleozoic clastic sedimentary system evolved into pervasively distributed sandstone reservoirs with strong heterogeneity after diagenetic reconstruction. (2) The multi-stage development and evolution of reservoirs in geologic times is a key condition for multi-stage reservoir occurrence. Deposition, diagenesis and alteration all play important roles in the multi-stage reservoir development in a superimposed basin. This section elaborates the role of multi-stage constructive diagenesis in the development of multi-layer reservoirs. First, multi-cycle tectonic movement caused the formation of multistage large unconformities in the basin, which would facilitate leaching and dissolution. Second, carbonate rocks may be partially altered and dolomitized by diagenetic fluids in subsurface temperature and pressure conditions, with reservoir properties, connectivity and scale improved[7]. Moreover, the faults and unconformities formed during tectonic activities can act as the pathways for deep thermal fluid travelling upwards, the thermal fluid may corrode enclosing rocks, forming dissolved pores and caverns. For example, in the Tarim Basin, six sets of Sinian-Ordovician

Fig. 4 Model of multi-layer carbonate reservoirs in Central Tarim Basin

posed basin as a result of multi-cycle sedimentary and tectonic evolutions and various geological factors. The reservoirs comprise clastic rocks, carbonate rocks, volcanic rocks and metamorphic rocks from shallow and middle zones to middle and deep zones and even to ultra-deep zones. Vertically superimposed and laterally joined into large pieces, these reservoir beds may trap oil and gas on a large scale if there are sufficient oil and gas sources and appropriate source-reservoir assemblages. Multi-stage development of reservoirs in a superimposed basin may be explained by the following factors. (1) Multi-stage sedimentary evolution is the key cause behind multi-stage reservoir development, which includes both the multi-stage development of high-energy sediments with good reservoir properties (e.g. sands, reefs and grain banks) occurring in different sedimentary environments and the multi-stage positive reconstruction of reservoirs in the primary sedimentary environment and facies background by later diagenesis. For example, Lower Paleozoic marine carbonate rocks in Central Tarim, the Tarim Basin, in which high-energy −5−

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Dengying reservoirs and Cambrian Longwangmiao reservoirs reach 3–5% and (1–6)×10–3 m2 respectively in the Sichuan Basin[12]; (2) Reservoirs and source rocks form favorable assemblages for hydrocarbon accumulation. For example, in the Sichuan Basin, Cambrian Longwangmiao reservoirs of grain bank facies and Sinian reservoirs of fracture-vug type are in the immediate vicinity of source rock centers. Good quality reservoirs and sufficient hydrocarbon supply come together and give birth to the giant gas field. Reservoir heterogeneity denotes the inhomogeneous petrophysical properties and inner structures of reservoirs as a result of complicated deposition, diagenesis and tectonic processes. For example, Upper Paleozoic sandstone reservoirs in Sulige gas field, the Ordos Basin, and Xujiahe sandstone reservoirs in Guang’an, the Sichuan Basin all feature large area, low porosity, low permeability and strong heterogeneity. The cavern-fracture system in layered Ordovician karst reservoirs in the Tarim Basin is also characterized by strong heterogeneity. Reservoirs are the basis for hydrocarbon accumulation. Findings from the study on formation conditions and preservation mechanisms of deep high quality reservoirs have changed the traditional understanding that there is little chances that large scale high quality reservoirs occur in deep zones, are becoming important bases guiding the exploration of oil and gas in deep layers, and are providing theoretical guidance for the search of exploration golden zones in superimposed basins.

karst reservoirs occurred during the Caledonian and Hercynian orogenies. Dolomitized reservoirs are developed widely in the Upper Cambrian and Lower Ordovician Penglaiba Formation due to burial diagenesis. Host rocks around fault belts were altered by thermal fluid in Late Hercynian into hydrothermal dolomite reservoirs. (3) Acidic fluid produced by hydrocarbon generation which may enter reservoirs together with oil and gas during hydrocarbon expulsion, is another important factor for the constructive development of deep reservoirs. Generally organic acid released by mature organic matter during hydrocarbon generation can dissolve those soluble constituents in the rock[8], which would improve the reservoir porosity and permeability significantly. In addition, carbonate reservoirs may also be altered by acid solution of acidic gases such as H2S and CO2 from thermochemical sulfate reduction (TSR) in deep high-temperature environment. Zhang Shuichang et al. concluded through study that the porosity and permeability of carbonate reservoirs would be improved by 56.06% and 1012.86% respectively with H2S or CO2 saturated acidic fluids from TSR at the temperature of 90 °C[9]. The combined effects of the above factors lead to the multi-stage development, large-scale distribution, effectiveness and strong heterogeneity of reservoirs. Large-scale distribution refers to that various kinds of reservoirs developing in different periods are all large in scale. For example, in the Tarim Basin, the superimposed area of reef-bank reservoirs in 6 platform marginal zones developing in 4 stages during the Cambrian-Ordovician reaches 2.6×104 km2 (Fig. 5); burial dolomite reservoirs of the Upper Cambrian-Lower Ordovician Penglaiba Formation and hydrothermal dolomite reservoirs controlled by faults reach (3-5)×104 km2 in area, and Lower Paleozoic karst reservoirs reach 5×104 km2 in Central Tarim and Bachu prospect[10−11]. Reservoir effectiveness has two aspects of meaning: (1) Despite old age and large burial depth, some reservoirs in the deep zone are still effective and large in scale due to the combined effect of a variety of geologic factors. For example, the porosity and permeability of Sinian

2.3 Multi-stage accumulation and effectiveness of late accumulation Multi-stage accumulation indicates hydrocarbons accumulate at several stages due to multi-stage hydrocarbon generation of source kitchens and multi-phase tectonic activities during or after massive hydrocarbon migration. The accumulations include primary hydrocarbon accumulations through secondary migration from source kitchens, readjusted accumulations by oil and gas released from existing reservoirs into new series of strata and new traps due to later stage tectonic

Fig. 5 Distribution of Middle-Late Ordovician platform margin in the Tarim Basin (O1—2y indicates the Middle-Lower Ordovician Yingshan Formation and O2y indicates the Middle Ordovician Yijianfang Formation.)

−6−

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movement, multiple accumulations of oil and gas originated from the same source kitchen in different areas due to differential burial, and hydrocarbon accumulations changing from oil to gas as a result of thermal evolution. The effectiveness of late accumulations has two aspects of meaning. (1) Retained hydrocarbon cracking occurs late in a high-mature to post-mature source kitchen, leading to late accumulation. In addition, oil and wet gas generation would be slowed down or postponed in a deep zone with high pressure and medium temperature (due to low geothermal gradient), thus putting off accumulation; (2) The chances of oil and gas preserved in reservoirs formed later are much higher due to short time available for oil and gas dissipation. According to statistics, most gas reservoirs discovered in onshore China were formed relatively late, which confirmed the effectiveness of late accumulation from another side. Multi-stage accumulation and late accumulation are mainly attributed to the following three factors. (1) Multi-phase tectonic activities are the internal cause for multi-stage accumulation. Superimposed basins usually experienced four major tectonic activities in the Caledonian epoch at the end of the Early Paleozoic, the Hercynian epoch in the Late Paleozoic, the Indosinian-Yanshanian epochs in the Mesozoic and the Himalayan epoch in the Late Cenozoic, which have great impacts on hydrocarbon generation, migration and accumulation. First, tectonic movements provide driving force for hydrocarbon migration and accumulation, and tectonic stress impels fluids to move on a large scale by causing changes to rock geometry and pore fluid pressure. Second, multi-phase tectonic activities not only give rise to structural traps but also cause hydrocarbon redistribution. In a superimposed basin, especially the deep zone, reservoirs were formed early and thus would be greatly influenced by later tectonic movements, the early reservoirs would be readjusted or even destroyed, each tectonic movement would cause the spatial redistribution of original oil and gas and new hydrocarbon accumulations would occur in the epoch without large-scale tectonic movements, leading to multi-stage hydrocarbon accumulations in different series of strata. (2) Multi-stage hydrocarbon generation and charging are the basis for multi-stage accumulation. For example, Sinian Dengying gas reservoirs in Central Sichuan had been charged with hydrocarbon in three epochs according to fluid inclusions analysis. Inclusions in the first epoch are liquid state with the homogenization temperature from 80 °C to 110 °C, indicating liquid hydrocarbon charging into ancient oil reservoirs. Inclusions in the second epoch are gaseous-liquid state with the homogenization temperature from 110 °C to 160 °C, indicating gas and liquid hydrocarbons charging into reservoirs during gas generation from kerogen degradation and oil cracking. Inclusions in the third epoch include gaseous hydrocarbons or mixed gas-brine, reflecting gas charging during gas generation from oil cracking on a large scale and mixed fluids charging of gas and brine during tectonic uplifting (Fig. 6).

Fig. 6 Histogram of fluid inclusions homogenization temperature, Dengying Fm., Gaoshiti-Moxi, Sichuan Basin

(3) Late hydrocarbon generation on a large scale and final tectonic setting determine the effectiveness of late accumulation. Statistics on main hydrocarbon accumulation stage of large oil and gas fields discovered in large onshore superimposed basins shows both ancient marine carbonate series and Mesozoic and Cenozoic continental clastic series were mostly fixed after the end of the Cretaceous, and the Paleogene is the main stage when large oil and gas fields formed (Table 1). Late accumulation makes it possible to avoid the destruction to oil and gas fields by multi-phase tectonic activities. This is also why many large oil and gas fields can survive in such complicated geologic setting in China. Table 1 Summary of late accumulations in major onshore oil and gas fields, China (modified from references [7]-[9]) Basin

Tarim

Sichuan

Reservoir bed

Type

Epoch of accumulation

Kela2, Di’na1, Di’na2, Dabei2, Kokyar

K, E

Dry gas

E—N

Tahe1, Tahe2, Lunnan, Sangtamu

O, T, J

Oil

N—Q

Central Tarim, Hotan River

O—C

Oil

K—E

Yingmai7, Yingmai2

K—E, O

Oil

E—N

Luojiazhai, Dukouhe, Puguang, Longgang, Yuanba, etc.

P—T

Dry gas

N—Q

Tiandong, Datianchi, Wolonghe, Fuchengzhai

C

Dry gas

N—Q

Moxi, Gaoshiti, Longnüsi, Hebaochang

Z—∈

Dry gas

N—Q

Weiyuan, Ziyang

Z

Dry gas

N—Q

Guang’an, Moxi, Zhongba, Xinchang, Pingluoba

T3x

Dry gas

N—Q

Sulige

P

Wet gas

K

Jingbian, Yulin, Zizhou, Uxin Banner, Shenmu

C-P

Dry gas

K

Junggar

Kutubi

E

Dry gas

E

Qaidam

Sebei-I, Sebei-II

Q

Biogenic gas

Q

Ordos

−7−

Field

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In summary, multi-stage accumulation and late accumulation give rise to multi-layer hydrocarbon accumulation and preservation, and are crucial to the formation of multiple golden zones.

3 3.1

Oil and gas distribution Source-dominated distribution

Source-dominated distribution refers to that most hydrocarbon reservoirs concentrate in the effective source kitchen regions or in ranges closely connected with effective source kitchens in a superimposed basin; in other words, hydrocarbon accumulation and distribution are closely controlled by the source. It has been found that a superimposed petroliferous basin in China mainly have two types of source kitchens, one is the kerogen-type kitchen generating oil and gas by thermal cracking of organic matter (kerogen) in source rocks; the other is the oil-type kitchen generating gas through the cracking of liquid hydrocarbons left in the ancient oil reservoir or source rocks in high-over mature stage, which is called liquid hydrocarbon-type kitchen. It has been proved by exploration activities that both kitchens are efficient source kitchens and could generate hydrocarbon on a large scale. Oil and gas discovered so far in each series of strata in each superimposed basins are mainly dominated by these two kinds of source kitchens. Cambrian-Ordovician source rocks in the Tarim Basin have three hydrocarbon generation centers on plane. The Manjar Depression is the major center with an area of (10−12)×104 km2 and hydrocarbon-generating intensity of up to 160×108 t/km2, followed by Awat center with an area of (8−10)×104 km2 and hydrocarbon-generating intensity of 40×108 t/km2 at maximum. Lying at the north margin of the Manjar and Awat source centers, the North Tarim Uplift is high in oil and gas concentration, where some large oil and gas fields such as Lunnan-Tahe, Halahatang, Yingmaili, etc. have been discovered, with the reserves of over 20×108 t oil equivalent; while in Central Tarim Uplift held between Manjar and Awat source centers, discovered oil and gas reserves by far are nearly 10×108 t oil equivalent. In the Sichuan Basin, although Sinian-Cambrian strata are very old, hydrocarbon distribution there is still dominated by source kitchens. Recently a large Sinian-Cambrian gas field, the oldest gas field in China with the largest mono-bloc reserves, was discovered close to the source rock center in Gaoshiti-Moxi prospect (Fig. 7). It is needed to be noted that the above two kinds of kitchens are related to each other. The kerogen-type kitchen is the base and the liquid hydrocarbons-type kitchen is the derivative product of the former. The former generates both oil and gas at an early stage, and the latter mainly generates gas at a later stage. Big oil and gas fields, especially carbonate gas fields, are usually the products of hydrocarbon supply by both kinds of kitchens. For example in the Tarim Basin, the Manjar Depression is a source center with these two kinds of kitchens. Recently Well Gucheng6 aiming at the latter gas kitchen

Fig. 7 Gaoshiti-Moxi gas field overlapped with source rock distribution in the Sichuan Basin

tapped commercial gas flow, demonstrating the significance of this kind of source kitchen to natural gas accumulation. 3.2

Reservoir-dominated hydrocarbon enrichment

The existence of effective reservoirs in a certain scale is the prerequisite to massive hydrocarbon accumulation and enrichment. A superimposed basin usually has two types of large scale reservoirs, i.e. (1) clastic reservoirs evolving from sands (sand groups) depositing in an inherited drainage system on a gentle terrain after comprehensive effects of construction and destructive diagenesis and (2) carbonate reservoirs of bank facies or fracture-cavity type formed by corrosion, in which fracture-cavity units usually occur in clusters. In general both clastic reservoirs and carbonate reservoirs of bank facies or fracture-cavity type are large in scale. For example in the Tarim Basin, Ordovician reservoirs of reef and bank facies at platform margins in Central Tarim Fault Zone-I are 1−20 km wide from north to south and 260 km long from east to west. Altogether 5 episodes of organic reefs of progradation type are recognized with a total thickness of 300−500 m, in which reservoirs are 30−50 m thick in total. Proved, probable, and possible reserves of nearly 10×108 t of oil equivalent in total have been discovered in Central Tarim Fault Zone-I, making it a major play for reserves and production increase in the basin. Sinian Deng2 and Deng4 karst reservoirs and Cambrian Longwangmiao reservoirs of bank facies have been found in Gaoshiti-Moxi in the Sichuan Basin with an overlapping area of over 3 000 km2. So far, proved, probable and prognostic reserves booked exceed a trillion cubic meters and proved gas reserves in Moxi Longwangmiao Formation alone reach 4 404×108 m3. Sulige field in the Ordos Basin has two sets of reservoirs, i.e. Upper Paleozoic sandstone reservoirs and Lower Paleozoic carbonate karst reservoirs. The former, with a proved and basically proved gas-bearing area of over 4×104 km2, has become the major gas pay in the basin. The latter has a proved gas-bearing area of over 4 500 km2 and cumulative reserves of 6 500×108 m3[13]. In summary, in superimposed basins in China, dolomite re−8−

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reservoirs of sedimentary reef or bank facies in carbonate strata, epigenetically corroded or leached reservoirs, dolomite reservoirs formed by burial and hydrothermal alteration, large-scale sandstone reservoirs in intra-continental depressions and gentle slopes in foreland basins are all affected by multi-cycle evolution, and large-scale effective reservoirs may occur in each series of strata or in each stratigraphic section. These reservoirs either in close contact with source kitchens extensively or connected with source kitchens through faults or unconformities would be the main intervals enriched by hydrocarbons, and intervals with concentrated economic reserves.

deep zone (Fig. 8). It is demonstrated by the discovery of Gaoshiti-Moxi gas field in Central Sichuan paleo-uplift in the Sichuan Basin recently that natural gas from cracking of semi-gathered semi-scattered liquid hydrocarbons at the low-relief slope contributes a lot to this large gas field. Fig. 9 shows that bitumen content is related to the slope of ancient terrain or landform. Structural high at Central Sichuan paleo-uplift has a large slope gradient, corresponding to a high bitumen content of up to 5%, while at the structural low on the slope the bitumen content decreases gradually to 2−3%. As a residue left by liquid hydrocarbon cracking, the content of bitumen indicates the concentration of liquid hydrocarbons in an ancient oil reservoir. The structural high was relatively rich in liquid hydrocarbons and gas would be generated by cracking in the ancient oil reservoir. Liquid hydrocarbons distributed in semi-gathered semi-scattered pattern with decreased slope gradient and disperse at the structural low or in the depression, and then gas would mainly be generated by cracking of retained hydrocarbons. Gas in the fields at the structural high of the uplift discovered recently mainly came from crude oil cracking, demonstrating the contribution of early oil accumulations and semi-gathered semi-scattered oil to the gas field.

3.3 High efficiency of hydrocarbon accumulation in “semi-gathered, semi-scattered” oil zones Owing to the limitation of current exploration, there are still many issues that need to be answered on hydrocarbon generation in ancient source rocks. The exploration of several Paleozoic basins shows that oil reservoirs are mainly controlled by source kitchens with differential evolution, and occur in paleohighs or slopes in large scale. As for gas reservoirs, ancient source rocks generally experienced oil generation peak and gas generation peak successively after sufficient thermal evolution. On the one hand, these source rocks had high hydrocarbon generating efficiency and generated large amounts of hydrocarbon; on the other hand, these source rocks have unique gas generation and accumulation features, that is gas generation by oil cracking besides thermal degradation of kerogen, the oil cracked gas, generated later with little dissipation, is high in accumulation efficiency and large in accumulation scale. Oil cracked into gas in three models: (1) oil cracking inside an ancient reservoir with the rise of temperature, (2) liquid organic matter cracking inside a source rock in high-mature to over-mature stage, and (3) semi-gathered semi-scattered oil cracking on the migration path from source rocks to reservoirs with the increase of temperature. The potential and significance of oil cracked gas have been discussed[14–17] before and here we mainly discuss the potential and significance of semi-gathered semi-scattered liquid hydrocarbons. Generally speaking, the tectonic framework of a superimposed basin usually features large uplift and large depression. In these basins, the distribution of hydrocarbon is mainly dominated by large paleohighs and relatively stable large paleoslopes, and the enrichment is related to paleotopographic slope during accumulation. Liquid hydrocarbons usually accumulate into ancient oil reservoirs at the structural high in slopes with high gradient. In a slope area with small gradient, liquid hydrocarbons are basically in scattered state but gather locally due to poor differentiation and concentration of liquid hydrocarbons as well as flat landform on the whole, which is named “semi-gathered semi-scattered” distribution by the authors. This part of oil can be preserved underground for a long period and cracked later to form large gas fields in the

3.4 Inherited superimposition and lateral migration of golden zones Inherited superimposition and lateral migration refer to oil and gas assemblages of different series of strata and different

Fig. 8 Accumulation model of gas generated by semi-gathered semi-scattered liquid hydrocarbons

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Fig. 9

Overlapping map of paleostructure of top Sinian and bitumen content before Permian deposition in the Sichuan Basin

types superimposed vertically due to multi-cycle sedimentary evolution, and inheritance and migration laterally due to the migration of multi-phase basins. In the Upper Permian Changxing Formation to the Lower Triassic Feixianguan Formation, for example, the sedimentary evolution comprises a complete water transgression-regression cycle. Early Changxing deposits take on retrogradation structure due to the evolution and extinction of the Kaijiang-Liangping Shelf and Pengxi-Wusheng Shelf; middle-late Changxing deposits show as platform marginal reefs and banks pushing forward towards the shelf[18]. During the deposition of Feixianguan Formation,

oolite beaches extended with the extension of the platform and migrated towards and finally leveled up the Kaijiang-Liangping Shelf, ending with wide distribution of oolitic beaches in the platform area. It has been confirmed by exploration findings that gas reservoirs of reef and bank facies overlap vertically and join into large pieces laterally owing to the migration of reefs and banks (Fig. 10). It is noted that different basins, depressions and series of strata have different spatial distribution of golden zones. In general, hydrocarbon accumulations in a superimposed basin developing from a marine craton may have some inheritance

Fig. 10 Schematic of organic reefs and oolitic beaches growth during Changxing-Feixianguan deposition in Longgang area, the Sichuan Basin

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and lateral variation and hydrocarbons of different epochs may share some accumulation elements and zones. For example, Cambrian sub-salt strata, the Ordovician Yingshan-Lianglitage Formations and Carboniferous strata contain oil and gas in a large area in Central Tarim. Hydrocarbon accumulation in a multi-phase foreland (intra-continental depression) inherited superimposed basin have some successive and progressive features, and the golden zones and hydrocarbon distribution in the golden zones have similar pattern, for example the Junggar Basin. For a superimposed basin developing from an early fault depression superimposed with a late depression, different golden zones would have quite different hydrocarbon phase states and distribution pattern due to different source kitchen distribution and accumulation processes caused by differences in the basin generation mechanisms in early and late stages. In the Songliao Basin, for example, volcanic gas reservoirs and sandy conglomerate gas reservoirs mainly occurred during fault subsidence and large oil fields developed in central anticline belt during depression, which may be understood through the delineation of ancient geographical conditions and reservoir distribution as well as the study on accumulation elements and processes. The inherited superimposition and lateral migration of golden zones is important to the research on large oil and gas field distribution and to rapid and orderly multi-stage advancement of hydrocarbon exploration. 3.5 Paleohigh, paleoslope, ancient platform margin and fault zone control hydrocarbon distribution In spite of multiple marine (continental) sedimentary sequences superimposed vertically and multi-phase sedimentary basins of different types overlapping laterally, hydrocarbon distribution in a superimposed basin is dominated by the paleohighs, paleoslopes, ancient platform margins and inherited fault zones[19–20], which are major targets for the exploration golden zones. It is a common rule that large-scale paleohighs and paleoslopes existing long are important places for oil and gas accumulation, besides probable existence of large-scale structural, lithologic or stratigraphic traps as well as migration pathways on a paleohigh or slope setting, the “absorbing”effect to oil and gas migration and control over the formation and distribution of large reservoirs of paleohighs and paleoslopes are all good to oil and gas accumulation in large areas. For example, large carbonate oil and gas fields in North Tarim and Central Tarim uplifts and slopes in the Tarim Basin, large Sinian-Cambrian gas fields in Central Sichuan paleohighs and Upper Triassic Xujiahe gas field in Central Sichuan, and Upper Paleozoic Sulige gas field in the Ordos Basin, are all dominated by paleohigh or paleoslope setting. Ancient platform margins are favorable for the growth of carbonate reef and bank reservoirs, with the evolution and extinction of platform margins, large-scale fluvial, lacustrine and deltaic clastic reservoirs could develop on the platform margin, coupled with faults connecting them, hydrocarbon could ac-

cumulate in multiple series of layers on a large scale. For example, at platform margins in Longgang, the Sichuan Basin, multiple hydrocarbon-bearing strata including Permian-Triassic reefs and banks, Triassic Leikoupo carbonate weathering crusts and the Triassic Xujiahe Formation have been found. Fractures not only act as pathways for hydrocarbon migration, but also would facilitate the action of deep thermal fluid to improve reservoir properties. For example in the Tarim Basin, carbonate reservoirs discovered in Central Tarim and Halahatang in the south of North Tarim are all related to fractures.

4

Significance of multiple golden zones

4.1 Multi-peak reserve increase and long history of discovery Multi-cycle tectonic and sedimentary evolution of a superimposed basin leads to multi-layer hydrocarbon enrichment and occurrence of multiple exploration golden zones. Recent exploration practices have proved when an existing golden zone has been fully explored, a new golden zone would be discovered as a result of deepened understanding and technical progress, making reserves increase in a multi-peak multi-stage pattern. For example in the Sichuan Basin, it was once considered to be impossible to find petroleum in the Sinian-Cambrian in the paleohigh, Central Sichuan because high-mature to post-mature source rocks (with Ro higher than 2.5%) could not generate hydrocarbon on a large scale from the perspective of classical petroleum geology. But in fact, Gaoshiti-Moxi gas field with the reserves scale of a trillion cubic meters has been discovered based on the new understanding of relay generation and double-peak generation as well as re-evaluation of the prospect according to the point of scattered liquid hydrocarbons (inside sources and outside sources) and gas generation from oil cracking; several sets of source rocks of good properties were discovered in the deep zone, the progress in deep drilling technologies have also made big contribution to the discovery of the giant gas field. This case shows the complexity of a superimposed basin itself ordains its exploration would experience a complex long history, and also the potential of the basin. 4.2 Complete hydrocarbon generation history and resources potential beyond expectations A large superimposed basin in China usually has two kinds of source kitchens, i.e. conventional source rocks and gas kitchens of liquid hydrocarbon cracking. The former would generally undergo sufficient thermal evolution and have an oil-generating peak and a gas-generating peak with a large amount of total hydrocarbon generated. The latter is composed of scattered liquid hydrocarbons left in source rocks, semi-gathered semi-scattered liquid hydrocarbons and oil in ancient reservoirs. During the previous resources assessments, the contribution of oil cracking in ancient oil reservoirs was taken into consideration, but semi-gathered semi-scattered

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liquid hydrocarbons and residual scattered liquid hydrocarbons left in source rocks had not been considered. The breakthrough of Well Gucheng6 in Gucheng, the Tarim Basin has proved that this kind of source kitchen can make great contribution to large scale accumulation in the deep zone of superimposed basins. If this part of gas from liquid hydrocarbon cracking is counted in the evaluation, Lower Paleozoic gas resources in the Tarim Basin would be estimated at 4.2×1012 m3, 2.3 times the estimate by the third round of resources assessment, and the total gas quantity generated by Sinian-Cambrian in Central Sichuan would be 12.8×1012 m3, 2.4×1012 m3 more than the estimated 10.4×1012 m3 in the third round of resources assessment. These cases show deep gas resources in China would exceed expectations, especially in the deep zone. 4.3

kitchens and large-scale effective reservoir rocks. Golden zones have the features of vertical superimposition and lateral migration. Hydrocarbon distribution in a golden zone is dominated by the paleohigh, paleoslope, ancient platform margin and inherited fault zone. It has been demonstrated by the exploration of multiple golden zones that deep superimposed petroliferous basins in China have petroleum resources exceeding expectations and hydrocarbon exploration is promising; with deepening geologic understanding and technical progress, more and more oil and gas will be found in the deep zone.

References Tuha Oil and Gas Editorial Department. Norway propose the theory of hydrocarbon exploration golden zones in deep layer.

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In a superimposed basin, both kerogen-type source kitchens and liquid hydrocarbon-type source kitchens in the deep zone could generate hydrocarbon on a large scale, and efficient reservoir rocks dominated by the paleohigh, paleoslope, ancient platform margin and inherited fault zone could also occur in the deep zone, which would make up into multiple golden zones for exploration. In spite of different abundance, the deep zone in a superimposed basin with extensive hydrocarbon distribution and large reserves does have commercial resources, which has already been demonstrated by recent discoveries in Tarim, Sichuan, etc. In particular, the large Cambian Longwangmiao gas field discovered recently in Central Sichuan has single-unit reserves of 4404×108 m3, where ten wells have daily gas production of over a million cubic meters each and open flow capacity of 1035×104 m3 at most. During production test, daily gas output reached 480×104 m3. In the Tarim Basin, the maximum depth of exploration with commercial productivity has exceeded 7000 m both for carbonate rocks in the platform area and for Mesozoic and Cenozoic clastic rocks in Kuqa foreland area. It is anticipated deep to ultra-deep hydrocarbon exploration would be promising in China with the deepening of geologic understanding and technical progress.

5

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Superimposed basins in China have experienced multi-phase tectonic and sedimentary evolutions and they have multiple golden zones for exploration. The occurrence of multiple golden zones depends on three factors: (1) Multi-stage development of source kitchens and double-peak hydrocarbon generation of ancient source rocks are the material basis for multiple golden zones. (2) Multi-stage development of reservoirs is the prerequisite to golden zones. (3) Multi-stage accumulation and effectiveness of late accumulation are essential to multi-layer hydrocarbon enrichment and preservation. The distribution of golden zones is dominated by conventional source kitchens, liquid hydrocarbon-type gas source − 12 −

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