Controls of Late Jurassic-Early Cretaceous tectonic event on source ...

SCIENCE CHINA Earth Sciences • RESEARCH PAPER •

February 2013 Vol.56 No.2: 228–239 doi: 10.1007/s11430-012-4494-0

Controls of Late Jurassic-Early Cretaceous tectonic event on source rocks and seals in marine sequences, South China JIN ZhiJun*, YUAN YuSong, LIU QuanYou & WO YuJin Petroleum Exploration and Production Research Institute, SINOPEC, Beijing 100083, China Received December 27, 2011; accepted May 11, 2012; published online October 22, 2012

Thermal evolution of source rocks and dynamic sealing evolution of cap rocks are both subjected to tectonic evolution. The marine sequences in South China have experienced superposed structural deformation from multiple tectonic events. To investigate the effectiveness of preservation conditions, it is of great importance to understand the controls of key tectonic events on the dynamic evolution of cap rocks. This paper discusses the controls of Late Jurassic-Early Cretaceous (J3-K1) tectonic event on source and cap rocks in marine sequences in South China based on the relationships between J3-K1 tectonic event and the burial history types of the marine sequences, the hydrocarbon generation processes of marine source rocks, the sealing evolution of cap rocks, the preservation of hydrocarbon accumulations, and the destruction of paleo-oil pools. The study has the following findings. In the continuously subsiding and deeply buried areas during the J3-K1 period, marine source rocks had been generating hydrocarbons for over a long period of time and hydrocarbon generation ended relatively late. At the same time, the sealing capacity of the overburden cap rocks had been constantly strengthened so that hydrocarbons could be preserved. In the areas which suffered compressional deformation, folding and thrusting, uplifting and denudation in J3-K1, the burial history was characterized by an early uplifting and the hydrocarbon generation by marine source rocks ended (or suspended) during the J3-K1 period. The sealing capacity of the cap rocks was weakened or even vanished. Thus the conditions for preserving the hydrocarbon accumulations were destroyed. The continuously subsiding and deeply buried areas during the J3-K1 period are the strategic precincts of the petroleum exploration in marine sequences in South China. South China, J3-K1 tectonic event, marine petroleum, hydrocarbon generation, cap rock sealing history Citation:

Jin Z J, Yuan Y S, Liu Q Y, et al. Controls of Late Jurassic-Early Cretaceous tectonic event on source rocks and seals in marine sequences, South China. Science China: Earth Sciences, 2013, 56: 228–239, doi: 10.1007/s11430-012-4494-0

The polytectonic events have occurred since the Phanerozoic in South China, and the regional tectonic events include Caledonian (Yunnan, Duyun and Guangxi Movements), Hercynian (Liukiang, Qiangui and Dongwu Movements), Indosinian, Yanshanian, and Himalayan Orogenies [1]. Tectonic events in different periods are different from one to another both on their expression and efficacy and their controls on the hydrocarbons of marine origin. Wang and Cai [2] pointed out the main tectonic events that had

*Corresponding author (email: [email protected])

© Science China Press and Springer-Verlag Berlin Heidelberg 2012

significant impacts on the paleogeographic evolution in South China: the events resulting in continental rifting and development of passive margins during the late Neoproterozoic to early Paleozoic, the orogenic events in South China during the middle Paleozoic, the extensional rifting during the late Paleozoic, the orogenic event leading to the closure of the Paleo-Tethys during the Mesozoic, the Pacific Plate subduction event since Jurassic, and the collision event between Indian and Eurasian plates during Cenozoic. All the above stated events lasted for a limited period of time and the intensity of these structural deformations varied in space. The impacts of these events on the formation of earth.scichina.com

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hydrocarbon accumulations and later adjustments differed from one time to another and from one place to another. Therefore, this paper focuses on the tectonic events that control the thermal evolution of the marine source rocks and the sealing dynamic evolution of the cap rocks in South China. It is beyond the scope of this paper to discuss in detail the control of every tectonic event on the hydrocarbons of marine origin. Readers can refer to numerous publications discussing the relationships between tectonic evolution and hydrocarbons [2–5]. Therefore, this paper only discusses the tectonic event during the Late Jurassic (J3) to Early Cretaceous (K1), namely the J3-K1 tectonic event. To avoid the confusion of time limit, here we adopt the “J3-K1 tectonic event” instead of “Yanshanian Orogeny”, proposed initially by Wong [6]. It indicates the important events taking place in the Yanshan range in North China during the Middle and Late Jurassic. As time goes on and with further studies, there are obvious differences regarding the division and time limit of the Yanshanian Orogeny [7–9]. The Yanshanian Orogeny is the important tectonic event during the Jurassic-Cretaceous period. It happened not only in eastern China but also in central and western China, and even in eastern Asia. It is also a global tectonic event during the late Mesozoic. The transition from Jurassic to Cretaceous is an important historical period for the earth evolution, during which the structure, geomorphology, climate, and ecosystem of the Earth had all undergone enormous changes. A series of important events, such as plate accretion, intracontinental orogeny, and plateau uplifting occurred in eastern China and eastern Asia [10], and the J3-K1 tectonic event would certainly have a profound impact on the hydrocarbon accumulations in marine sequences in South China. The marine sequences in South China contain multiple sets of source-reservoir-seal assemblages. They possess good initial elements for formation of hydrocarbon accumulations and experienced the processes of abundant hydrocarbon generation, migration, accumulation, and entrapment [11]. However, after exploration for half a century, the present discovery of the hydrocarbons of marine origin is still limited to the Sichuan Basin. Although the Yancheng gas accumulation in the Lower Yangtze region and the Kaixiantai oil accumulation in the Middle Yangtze region are

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related to marine hydrocarbon source, the pay zones are all in the Mesozoic and they are very small in terms of reserves. Therefore, the exploration of hydrocarbons of marine origin outside the Sichuan basin has not yet made any significant breakthroughs. The reason for that is that the effectiveness of preservation is the key. The effectiveness of preservation involves the effectiveness of the hydrocarbon generation of source rock [12–15], the effectiveness of the dynamic sealing of cap rock [16–20], and the effectiveness of dynamic matching between the marine cap rock and hydrocarbon generation [21]. In both the Sichuan basin and the MiddleLower Yangtze region, multiple sets of good marine source rocks were developed, which shows similar thermal history and hydrocarbon generation history. Fairly good reservoir conditions also exist in these areas, but the evolution history varied greatly after the formation of the paleo-oil accumulation. What is the reason? By analyzing and comparing the elements influencing the marine source and cap rocks, such as the tectonic deformation styles, the burial history types, the process of hydrocarbon generation of the marine source rocks, the sealing evolution of the cap rocks, the hydrocarbon accumulations, and the damage of paleo-oil accusations between the Sichuan Basin and the Middle-Lower Yangtze region, we found that the J3-K1 tectonic event controlled the formation and evolution of the marine cap rocks and the hydrocarbon generation history in South China. By discussing the controls of J3-K1 tectonic event on the marine source and cap rocks in South China, this paper aims at discovering the strategic precinct of the exploration of hydrocarbons of marine origins in South China.

1 Characteristics of J3-K1 tectonic event In the Yangtze areas, the J3-K1 tectonic event was characterized by obvious differences (Table 1) between the eastern and western parts, which were separated by the Qiyue Mountain fault. During J3-K1 times, the area west of the Qiyue Mountain fault subsided and received deposition. In contrast, the area east of the fault was uplifted continuously, subject to compressional deformation, folding, and thrusting. Furthermore, the folding and thrusting took place in the

Contrasting characteristics of J3-K1 tectonic event in different parts in South China

Subsiding/uplifting in J3-K1 Onset of J3-K1 tectonic event Deformation type Contact relationship

West of Qiyue Mountain Fault Western and central Eastern Sichuan Basin Sichuan Basin subsiding subsiding ~97 Ma undeformed folding conformity or parallel conformity or parallel uncomformity between uncomformity between J3-K1 and underlying J3-K1 and underlying sequence sequence

East of Qiyue Mountain Fault Western Hunan and Jianghan Basin Subei and Sunan basins Hubei provinces uplift and denudation uplift and denudation uplift and denudation ~137 Ma ~157 Ma folding and thrusting folding and thrusting folding and thrusting overlapped unconformunconformities unconformities ities resulting from between the K2 and between the K2 and Yanshanian and underlying sequences underlying sequences Himalayan Orogenies

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order of time sequence and was characterized by propagation. During the Late Jurassic to Early Cretaceous, part of the Sichuan Basin west of the Qiyue Mountain fault subsided continuously and received deposition dominated by red clastics. The stratigraphic successions are of conformity or parallel unconformity contacts. In J3-K1, the Mesozoic and Paleozoic sequences had not suffered significant folding deformation yet [4]. In the central Sichuan Basin, the Paleozoic-Middle Triassic and the Upper Triassic-Middle Eocene tectonosequences were folded synchronically, and no folding was recorded during the Yanshanian [22]. In the Western Depression of the Sichuan Basin, the Paleocene and pre-Paleocene sequences were also folded synchronically, and the Neogene Liangshuijing Formation overlies the Paleogene unconformably [23]. In the areas east of the Qiyue Mountain fault, however, the Lower-Middle Jurassic sequences was preserved only in the core of the syncline structure and generally eroded in the wings. The LowerMiddle Jurassic overlies the pre-Jurassic sequences conformably or disconformably. The Middle Jurassic is the youngest sequence involved in the folding deformation. However, the upper Cretaceous and the Tertiary overlie the pre-Cretaceous unconformably. In the areas east of the Qiyue Mountain fault, the J3-K1 tectonic event led to a large scale of folding and thrusting of the Mesozoic and Paleozoic sequences. The Xuefeng Uplift Trend and the area south of it were deformed to a basement detached uplift orogenic belt, and the interior part of the Yangtze Plate was characterized by the slipping deformation of the sedimentary cover [4]. In the Middle and Lower Yangtze region, a north-verging and south-dipping tectonic framework was formed [24, 25]. With the reinforcement of the convergent interaction between the south and north blocks of the Jianghan Basin in the Middle Yangtze region, the Lower-Middle Jurassic and pre-Jurassic sequences were folded and thrusted in a large scale, which led to the formation of nappe structure styles and a series of secondary tectonic zones dominated by fault bend folds [26]. The foreland basin in the Middle Yangtze region was transformed considerably, and the eastern part was uplifted to form high mountains, speeding up the progress of the westward regression of sea waters. Both the eastern part and the Huangling uplift emerged out of the sea water and were eroded. As a result, the Upper JurassicLower Cretaceous sediments are absent in the Huangling uplift and most of the areas east of it. The Lower Yangtze region was tectonically stabilized and kept subsiding prior to the Indosinian Orogeny, which was characterized by the closure of the Paleo-Tethys in Late Triassic. The orogney led to the steady uplifting of the Lower Yangtze region and the development of large scale uplifts and downwarps within the crust. Thus, it controlled the formation and development of the basins during the Early and Middle Jurassic, but its impact on the destruction of hydrocarbons of


marine origin was insignificant. After the Middle Jurassic, the intensive collision between the Yangtze Plate and the North China Plate resulted in the decoupling between the basement and sedimentary cover, decoupling within the cover, as well as thrusting and napping in the Lower Yangtze region. Thereafter the Yangtze region entered the period for the formation of sheet sliding structures, which reinforced the folding that resulted from the Indosinian Orogeny. The wings of anticlines were further deformed and thrusting and napping structures with a significant size were formed in specific localities. The thrust faults detached into the base of the Silurian system and disappeared [27]. The Paleozoic and Mesozoic sequences were subject to intensive compressional deformation, position changes, and sequential order alterations. During the Late Jurassic-Early Cretaceous, large scales of neutral-acidic volcanic eruptions and magma intrusions occurred synchronically with the orogeny in the southeastern coastal areas. The Zhejiang-Fujian-Guangdong region had experienced a large scale of thrusting and folding deformation, resulting in the formation of intensive thrusting, large left-lateral strike-slip faults, as well as the development of pull-apart basins locally. The J3-K1 tectonic event showed an obvious time sequence and its onset became younger and younger from the east to the west. The age data and length simulating results of the apatite fission tracks consistently indicate that the marine sequences in the Xuefeng Uplift zone and the Jianghan Basin started the uplifting and erosion at 157 Ma. The starting time was 137 Ma for the western part of Hunan and Hubei provinces and Wuling fold zone. It was 97 Ma for the eastern and southeastern Sichuan, western Hubei, eastern Chongqing, and Central Guizhou; 56 Ma for the central and northeastern Sichuan; 23 Ma for the western Sichuan and western Yunnan [28]. The time sequence indicates that the J3-K1 tectonic event was confined to the areas east of the Qiyue Mountain fault. The uplifting and erosion in the west areas of the Qiyue Mountain fault started after 97 Ma, i.e., the end of Late Cretaceous. The J3-K1 tectonic event led to the basement decoupling and multilevel sliding within the sedimentary cover in the Middle Paleozoic Yangtze basin. The resultant structural deformation was characterized by a progressional decreasing attenuation [29]. Since the frontal fault of the buried Xuefeng basement decoupling system rushed out of the earth’s surface from the Huaying Mountain fault, most of the thrusting stress and displacement was released so that the tectonic deformation of the areas west of the Huaying Mountain fault became weak [30]. The marine sequences in the areas west of the Qiyue Mountain fault within the Sichuan Basin were folded for the first time during the Himalayan Orogeny. The marine sequences in the areas east of the Qiyue Mountain fault were initially folded during the J3-K1 and were further deformed by the subsequent Himalayan Orogeny. As a result, the Qiyue Mountain fault constitutes a dividing line. In the areas west of it, typical wide

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spaced anticlines were developed and they can be found in the eastern Sichuan. In the western part of Hunan and Hubei regions east of it, battlement-shaped folds consisting of equilateral anticlines and synclines were formed and they were resulted from the mutual interference and stacking of both the Himalayan folding and the J3-K1 folding.

2 J3-K1 tectonic event and burial history of the marine sequences and hydrocarbon generation history 2.1

J3-K1 tectonic event and the burial history

The burial history type of the marine sequences in South China was determined by the J3-K1 tectonic event. In the areas east of the Qiyue Mountain fault, the burial history type was an early uplifting type (early uplifting and then keeping uplifting, or early uplifting and then subsiding again), and the one in the areas west of the fault was a late uplifting and then keeping uplifting type. The Jianghan and Subei Basins, located east of the Qiyue Mountain fault, underwent uplifting and erosion during the J3-K1, and subsided again during the K2-E. Following the Indosinian Orogeny which terminated the marine deposition, the marine sequences experienced subsidence and uplifting twice. Having undergone the subsidence during T3-J1-2 and uplifting and erosion during J3-K1, the marine sequences experienced subsidence during K2-E and uplifting and erosion at the end of Paleogene. The burial history of the marine sequences was characterized by an early uplifting and then subsiding (Figure 1(a)). The Wuling folds zone and the western Hunan and Hubei, located east of the Qiyue Mountain fault, underwent uplifting and erosion during the J3-K1. Following the Indosinian Orogeny, the structure was characterized by a single cycle of subsidence and uplifting process, namely, once uplifted and eroded, there would be no subsidence and burial. Therefore, the burial history was an early uplifting and then keeping uplifting type (Figure 1(b)). In the areas west of the Qiyue Mountain fault, there was subsidence in J3-K1, and uplifting and erosion began after J3-K1. Following the Indosinian Orogeny, there was a single cycle of subsidence and uplifting process, namely, once uplifted and eroded, there would be no subsidence and burial. Therefore, the burial history was a late uplifting and then keeping uplifting type (Figure 1(c)). 2.2 J3-K1 tectonic event and hydrocarbon generation history The hydrocarbon generation history of source rocks was reconstructed on the basis of the burial history and thermal history reconstruction [31, 32]. By taking the source rock of the Lower Silurian Longmaxi Formation and the Lower Permian as an example, we discuss the controls of the J3-K1 tectonic event on the marine hydrocarbon generation pro-


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cess in South China. The source rocks in the Longmaxi Formation began to generate oil in the Dachigan gas field in the eastern Sichuan from the Late Permian and reached the peak in the Middle Triassic. They began to generate gas from the Late Triassic, and entered the stage of over mature dry gas generation in the Jurassic-Cretaceous. The hydrocarbon generation ended at the end of the Early Cretaceous. In the areas with paleo-oil accumulations such as Majiang, Kaili, Nanshanping, and Wangcun, the hydrocarbon generation by the source rocks in the Longmaxi Formation ended earlier. Currently they are of low thermal maturity. The hydrocarbon generation ended without reaching the stage of over mature dry gas generation (Figure 2). In the above areas, the Lower Permian source rocks were eroded away completely. In areas with gas fields such as Puguang, Yuanba, and Heba in the northeastern Sichuan, the hydrocarbon generation history was similar. Hydrocarbon generation began in the Triassic and ended at the end of Eocene. In the eastern and southeastern Sichuan, hydrocarbon generation also began in the Triassic, but ended at the end of the Early Cretaceous (Figure 3). The J3-K1 tectonic event controlled the hydrocarbon generation of the marine source rocks in South China. In the continuously buried areas in J3-K1, the marine source rocks continued to generate hydrocarbons and hydrocarbon generation ended late. The thermal evolution reached the stage of over mature dry gas generation and the thermal cracking of crude oil was a source for gas. In the areas that were uplifted and eroded in J3-K1, thermal maturity was relatively low, and the hydrocarbon generation ended early without reaching the stage of over mature dry gas generation, which is unfavorable for late gas charging and formation of gas accumulations.

3 J3-K1 tectonic event and cap rock sealing evolution 3.1 J3-K1 tectonic event and cap rock preservation condition The preservation conditions of the overlying regional cap rocks in the marine sequences were determined by the J3-K1 tectonic event. During J3-K1, the areas east of the Qiyue Mountain fault subsided continuously and received continuous deposition. In the western Sichuan Depression, the Late Triassic-Paleogene sediments were continuously deposited and are all preserved. In the central Sichuan, the Upper Jurassic is widely distributed. In the northeastern Sichuan, there exist relict Lower Cretaceous sediments. The Middle Jurassic was well preserved in the syncline of the folding zone in the eastern Sichuan. However, outcrops are dominated by the Middle Triassic rocks in the western Hunan and Hubei east of the Qiyue Mountain fault. The Upper Triassic-Jurassic sequences were eroded away completely. The Paleozoic crops out in the Wuling fold zone. In the

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Figure 1 Burial history types of the marine sequences in South China. (a) Well Hai-4 in Jianghan Basin, early uplifting and then subsiding again; (b) Well Xi-1 in the western part of Hunan and Hubei provinces, early uplifting and then keeping uplifting; (c) Well Nüji in Sichuan Basin, late uplifting and then keeping uplifting.

Middle Yangtze region, the Upper Triassic-Middle Jurassic sequences were largely eroded away except in the Dangyang Depression and the southern Chenhu-Tuditang area. In the Lower Yangtze region, the Upper TriassicMiddle Jurassic sequences were not preserved anywhere except along the Yangtze River. It is obvious that the regional differences of the performance and efficacy of the J3-K1 tectonic event caused the obvious regional differences

of eroded strata volume. Thus, the event controlled the present day preservation of the overlying cap rocks. 3.2 J3-K1 tectonic event and sealing evolution of cap rocks Some scholars thought that when the mudstone came into the “incompressible” phase, the brittleness increased and

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Figure 2

Diagram showing the hydrocarbon generation history of the Longmaxi Formation source rocks in South China.

Figure 3

Diagram showing the hydrocarbon generation history of the Permian source rocks in South China.

could be cracked easily, resulting in weakening of the sealing capacity [33]. The results of the triaxial shear and compression experiment suggest that with the increasing burial depth and the confining pressure, the sealing capacity is enhanced [34]. The data of the rock mechanics experiment show that without the confining pressure, rocks are fractured by tensional stresses and the fractures are tensional ones. Under a medium confining pressure, fractures are shearing ones. Under a high confining pressure, mylonitization plastic deformation occurs. With the increase of the confining pressure, the collapsing strength and yield stress increase accordingly, and the brittle break of the rock turns into a plastic deformation. Therefore, the increasing burial depth and temperature is favorable for plastic transformation and it enhances the sealing capacity of the deeply buried mudstones [34]. If the mudstone with a high degree of diagenesis underwent uplifting and the overburden pres-

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sure was released, cracks would be easily formed. Thus, the sealing capacity was weakened or even vanished. In other words, the sealing of the mudstone with a high degree of diagenesis is closely related to the stratigraphic conditions under which it is placed. Under the original stratigraphic conditions, the sealing was superior, but with the subsequent uplifting and erosion, formation pressure was released and fracturing might occur which would weaken the sealing capacity. 3.2.1 Reconstruction of displacement pressure for argillaceous cap rock during construction phase Displacement pressure was mainly regarded as the evaluation parameter during the process of cap rock sealing evaluation. The experimental data indicate that the porosity is obviously related to displacement pressure (Figure 4). The smaller the gross porosity, the higher the degree of compac-

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tion; the smaller the throat radius of pores, the larger the displacement pressure of the mudstone; the lower the permeability, the higher the displacement pressure and the stronger the sealing capacity. Figure 4 shows the relationship between the displacement pressure and the measured gross porosity of the mudstone cap rocks. When the gross porosity is 25%, the displacement pressure approaches to the minimum constant value, and when the gross porosity is 0.5%, the displacement pressure is close to the maximum constant value. The range of gross porosity from 2% to 7% is regarded as the sensitivity interval that influences the cap rock sealing quality, during which the displacement pressure vary from 14 to 5 MPa and is influenced most obviously.

Figure 4 Relationship between displacement pressure and porosity under simulated stratigraphic conditions. The data are from refs. [35–38].

Through function fitting, the nonlinear function between the gross porosity and the displacement pressure of mudstone is derived as the following: P = 24.799×0.8189, where P refers to the displacement pressure (in MPa);  re-

Figure 5


fers to the gross porosity of rocks (in %). If the porosity evolution history is known, according to the above formula, the displacement pressure evolution history of the argillaceous cap rock during the construction phase can be derived, and the porosity evolution history can be constructed from the reconstruction process of the burial history. 3.2.2 Impacts of J3-K1 tectonic event on sealing capacity of cap rocks By taking the argillaceous cap rocks in the Cambrian and Silurian in the western Hunan and Hubei as an example, we discuss the impacts of the J3-K1 tectonic event on the sealing capacity of the cap rocks. Based on the method introduced above, the displacement pressure of the mudstone cap rocks at the base of the Cambrian in Well Xi-1 in the western Hubei was greater than 5 MPa at the end of the Cambrian, and therefore they had possessed the capacity to seal natural gases. The displacement pressure was greater than 10 MPa in the Permian, and the mudstone could make up the seal for the gas reservoir with a high pressure. The displacement pressure was greater than 15 MPa at the end of the Triassic, and the mudstone could seal overpressured gas reservoirs. The displacement pressure reached the maximum value of 19.7 MPa at the end of the Jurassic. The mudstone at the bottom part of the Silurian could seal oil in the Permian and natural gas to a certain degree after the Middle Jurassic. The displacement pressure reached its maximum of 7.1 MPa at the end of the Jurassic (Figure 5). It can be seen that prior to the J3-K1 tectonic event, both the Cambrian mudstone and Silurian mudstone could seal natural gas to a certain extent. But the data of chemical analysis of the formation waters in the western Hunan and Hubei region show that the preservation conditions do not exist currently and the sealing capacity of cap rocks has been damaged. The salinities of the lower Paleozoic formation

Sealing evolution of cap rocks in Well Xi-1 in western Hubei during construction phase.

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waters from 16 exploratory wells are quite low. They fall in the range of 1.1–14 g/L with an average of 5.5±3.8 g/L. The metamorphic coefficients (rNa+/rCl) are all greater than 0.87. They are in the range of 1.0–40 with an average of 6.8±10.9, well above the metamorphic coefficient of the present day sea waters (Figure 6). The water types are NaHCO3 and Na2 SO4. It is concluded that the sealing capacity of cap rocks was drastically destroyed by the J3-K1 tectonic event in the areas that had undergone uplifting and denudation.

Figure 6 Relationship between total dissolved solids of formation water and metamorphic coefficient in western Hunan and Hubei.

4 J3-K1 tectonic event and preservation of typical reservoirs and destruction of paleo-oil pools 4.1 J3-K1 tectonic event and preservation of hydrocarbon accumulations The marine oil and gas distribution in South China is obviously controlled by structural deformation, uplifting, and erosion caused by the J3-K1 tectonic event and preservation conditions of regional cap rocks. The marine oil and gas fields discovered in South China are all distributed in the Sichuan Basin. The exploration has not yielded any significant breakthroughs outside the Sichuan Basin. The main reason is that the Sichuan Basin was affected weakly by the J3-K1 tectonic event. The Sichuan Basin is characterized by relatively weak tectonic deformation, late uplifting, and relatively small amounts of erosion. The source rocks continuously generated hydrocarbons over a long period of time and the hydrocarbon generation ended fairly late. The regional seals consisting of the Lower-Middle Triassic gypsum and the Upper Triassic-Jurassic terrestrial clastic were well developed in the basin. The infiltration of meteoric water was suppressed. As a result, the preservation conditions are not destroyed. 4.2

J3-K1 tectonic event and paleo-oil pool destruction

Most marine paleo-oil pools in South China are the consequence of destruction by the J3-K1 tectonic event. Over 40 large and medium-sized paleo-oil pools have been found in


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marine sequences in South China so far. Among them, the Weng’an, Majiang, Danzhai, Wanshan, Nanshanping, and Bankeng paleo-oil pools or remnant oil reservoirs and the recently found Wangcun paleo-oil pool are confined mainly to the Caledonian paleo-highs and their surrounding areas in the Middle-Upper Yangtze region [39]. The formation and distribution of these pools are controlled by the Qianzhong Paleo-uplifts and Jiangnan-Xuefeng Paleo-uplifts while their destruction is related mainly to the J3-K1 tectonic event. The Nanshanping paleo-oil pool is located in Cili County of Zhangjiajie, Hunan Province. The Niutitang Formation source rocks entered into the oil window in the EarlyMiddle Ordovician. Oil generation peaked in the Late Permian to the Late Triassic. The source rocks began to generate gas in the Late Triassic and reached over mature dry gas generation stage by the mid Middle Jurassic. At the end of the Late Jurassic, the maximum burial depth of 5656 m was reached. The temperature was 203°C and Ro up to 2.4%, i.e., the stage of over mature dry gas generation. The subsequent uplifting decreased the temperature and ended the hydrocarbon generation. The Silurian source rock began to generate oil in Late Permian and peaked at the end of Late Triassic. The hydrocarbon generation ended just when it started to generate gas at the end of the late Jurassic. Its current Ro is 1.3% (Figure 7). The previous studies show that the Nanshanping Anticline (trap) was the structure to trap oils in the Nanshanping paleo-oil pool. The formation and destruction of the paleo-oil pools are closely related to the formation and evolution of the anticline (trap). The Late Caledonian-Early Hercynian tectonic movements created the embryonic form of the Nanshanping Anticline, which is time equivalent to the formation of the initial oil accumulation. The anticline was enlarged by the Indosinian Orogeny and constituted a structural trap, which is time equivalent to the formation of the gas accumulation. During the second episode of Yanshanian movement, the anticline (trap) was complicated by folding and faulting. Northwest thrusting faults were formed. The overlying seals were uplifted and denudated. The original oil accumulations were completely destroyed, and eventually became paleo-oil pools. Previous studies proposed that the main factors responsible for the destruction of the Nanshanping paleo-oil pools are the thermal cracking during Indosinian Orogeny and tectonic uplifting and erosion during the second episode of Yanshanian movement with the latter being the dominant factor [40]. The hydrocarbon accumulation process for the Nanshanping paleo-oil pool is similar to that for the Weiyuan Gas Field in the Sichuan Basin [41]. The main reason for the destruction of the Nanshanping paleo-oil pool and preservation of the Weiyuan Gas Field is that the Late Jurassic-Early Cretaceous Yanshanian Movement had an insignificant impact on the Weiyuan Gas Field but strong influence on Nanshanping area (Figure 8).

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Figure 7

Source rock maturity and hydrocarbon generation history in Nanshanping, Cili area.

Figure 8

Geological map of Nanshanping, Cili area (a) and the evolution profiles of paleo-oil pools (b) [42].

The oils in the Weng’an paleo-oil pools are sourced from black carbonaceous mudstone of the Lower Cambrian Niutitang Formation, reservoired in the Mingxinsi Formation and sealed by the Jindingshan Formation mudstone. At the end of the Caledonian Orogeny, the Weng’an

paleo-oil pool was affected by the uplifting of the Central Guizhou, which led to the erosion of the overlying Silurian but only a limited erosion of the Lower Cambrian cap rocks. As a result, the Weng’an paleo-oil pools were not destroyed [43]. During the Early and Middle Hercynian Orogeny, the

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paleo-oil pool was subject to a long period of uplifting and adjustment until the Early Permian. During the Permian and Triassic, rapid deposition occurred. The paleo-oil pool was buried to a depth of more than 5000 m [24]. Intensive folding and regional uplifting during the main episode of Yanshanian movement exposed the reservoir intervals and led to their destruction [44]. Oil in the Taishan paleo-oil pool is mainly sourced from the black carbonaceous mudstones in the lower part of the Upper Sinian Xifengsi Formation and the Lower Cambrian Hetang Formation. The basinal area west of the paleo-oil pool is the main source kitchen. Source rocks matured in Ordovician and oil was accumulated in the Taishan paleo-oil pool in Ordovician-Silurian [45]. The hydrocarbon was buried to a depth up to 6000 m at the end of Caledonian Orogeny after it got migrated into the reservoir. The deep burial improved the thermal evolution of hydrocarbon and almost complete the transformation of oil into gas and pyrobitumen prior to Triassic [45]. Due to the intensive folding and complex faulting during the the Indo-Yanshanian movements, the reservoir intervals were exposed and the structure was also destroyed [44]. The Upper Triassic Zhonghe paleo-oil pool, which lies in the Zhonghe area of Yonghe County in the Chuxiong Basin of Yunnan Province, was formed during the Yanshanian Orogeny and destroyed by the uplifting and erosion at the end of the Early Cretaceous, which led to exposure of the reservoir intervals [44]. In the Qianzhong uplift and surrounding areas, there are a number of Lower Paleozoic paleo-oil pools and residual oil-gas pools, which include Yankong, Jinsha, Yangshui Kaiyang, Baidoushan Weng’an, Majiang, Huzhuang Kaili, Wanshan Tongren, Sandu Danzhai, Benzhuang Shiqian, Taijiang Gedong, Wuhe, and Duoding Weng’an. Those pools were destroyed by the Yanshanian-Himalayan tectonic movements [46]. Since the crude oils in the paleo-oil reservoirs had cracked into methane and solid metamorphic asphalt prior to the Yanshanian Orogeny, the orogeny not only destroyed the paleo-oil pools but also caused the leakage of natural gases [47]. Although the times for the destruction of the paleo-oil pools from previous research discussed above are defined to be the “Second episode of Yanshanian movement”, “Main episode of Yanshanian movement”, “Yanshanian movement in Late Jurassic-Early Cretaceous”, “Stage of Yanshanian”, and “Yanshanian movement”, it is not difficult to determine that the timing is equivalent to the J3-K1 tectonic event discussed in this paper.

5 Strategic directions for petroleum exploration in marine sequences in South China The controls of the J3-K1 tectonic event on source and cap rocks indicate that the strategic areas for petroleum explora-


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tion in marine sequences in South China should be the areas with continuous burial and subsidence in J3-K1. In those areas, the source rocks generated hydrocarbon over a long period of time and the hydrocarbon generation ended fairly late, which is favorable for the late gas charging and the formation of gas accumulations. Meanwhile, the sealing capacity of the top seal was continuously strengthened, and the “source rock” and “cap rock” were in good match so that the oil and gas could be preserved. For the areas that underwent compressional deformation, folding and thrusting, and uplifting and erosion in J3-K1, the hydrocarbon generation ended (or was suspend) in J3-K1. The sealing capacity of the cap rock was weakened or the cap rock was denudated completely, and the preservation conditions were destroyed. After the J3-K1 tectonic event, the overall marine sequences in the Sichuan Basin were slightly destroyed and they are in a stable setting. The marine sequences are characterized by a single-layer stable structure and double-layer continuous pattern. They have the features of “long lasting subsidence, short periods of uplifting, continuous hydrocarbon generation by multiple source rocks, and late petroleum charging and accumulation”. Hydrocarbon accumulations are largely of primary origin. The chemical features of formation waters show that the present preservation conditions are still favorable in the depression areas in the northeast, central, and west Sichuan Basin, which indicates that the late reformation is weak. For instance, the total dissolved solids in the formation water in the northeast Sichuan Basin fall in the range of 22–265 g/L with an average of 66.7 g/L. The deterioration coefficient is low and is in the range of 0.1–3.3 with an average of 1.08. The main water type is dominated by CaCl2. There is a thick Cretaceous succession in the western Sichuan Basin which has considerable sealing capacity to the underlying sequences. The total dissolved solids are up to 10–20 g/L in the formation water in the Upper Jurassic Penglaizhen Formation, and those in the Middle Jurassic are even higher. Besides, the natural gas shows are abundant and a number of gas fields including Xiaoquan, Xinchang and Hexingchang have been discovered. The formations waters in the Upper Triassic Xujiahe Formation have a high concentration of dissolved solids and a low deterioration coefficient. Over-pressure is common and widespread. All these features indicate that the preservation conditions are superb. The hydrocarbon generation ended soon after the J3-K1 tectonic event in the western Hubei and Hunan areas. The strong uplifting, denudation, and faulting caused dramatic desalting of formation waters and therefore the preservation conditions were destroyed. The total dissolved solids are lower than 14 g/L in the formation waters in the SinianSilurian successions. The formation waters show higher deterioration coefficients of greater than 0.87 and they are of NaHCO3 or Na2SO4-type water. Formation waters could move across stratigraphic boundaries, which suggests that

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the preservation system was reformed and destroyed, and the overall preservation conditions are not good. The hydrocarbon generation was once suspended following the J3-K1 tectonic event in Middle Yangtze (Jianghan Basin) and Lower Yangtze regions. The earlier-formed marine hydrocarbon accumulations were destroyed, but the extension and rifting events during the K2-E times led to the re-burial of the marine hydrocarbon source rocks with low thermal maturity prior to the J3-K1 tectonic event. The deep burial increased the maturity of source rocks and made it possible for source rocks to generate hydrocarbons again. However, areas with high thermal maturity prior to the J3-K1 tectonic event could only generate little hydrocarbons after further burial. Furthermore, the thick overlying K2-E succession could improve the preservation conditions which had been destroyed during J3-K1 times. The overall sealing capacity is increased and shows fairly good exploration potential.

6 Conclusions (1) The marine sequences experienced reformations from multiple tectonic events in South China, of which, the J3-K1 tectonic event is the key tectonic event controlling the dynamic evolution of the marine source and cap rocks in South China. (2) The features and intensity of the J3-K1 tectonic event controlled the burial history type of the marine sequences, the hydrocarbon generation process of source rocks, and the sealing capacity of cap rocks, and therefore determined the preservation and destruction of hydrocarbons of marine origin. (3) In those areas with continuous subsidence and deposition during the J3-K1, hydrocarbon generation ended fairly late and the sealing ability of cap rocks was constantly strengthened. The marine sequences were only slightly deformed with few major faults. Therefore, the conditions are favorable for the preservation hydrocarbons of marine origin in those areas. In the areas that underwent extrusion and deformation, folding and thrusting, and uplifting and denudation during the J3-K1, the burial history is characterized by an early uplifting so that hydrocarbon generation ended (was suspended) early. The cap rocks were denudated considerably and their sealing capacity was weakened greatly. Therefore, hydrocarbon accumulations were destroyed. (4) The strategic areas for petroleum exploration in marine sequences in South China should be the areas with continuous burial and subsidence during J3-K1. Although the intensive subsidence in K2-E and elevated temperature made it possible for source rocks to generate hydrocarbons for the second time, the amount of hydrocarbons generated is very limited in these areas (such as Jianghan and Subei Basins) either because the marine sequences were mostly


denudated in J3-K1, or the marine source rocks were highly or over matured prior to K1. Therefore, the exploration potential in these areas is significantly inferior to areas with continuous subsidence and burial during the J3-K1 times. And this is the key reason why no significant breakthroughs have been made in petroleum exploration of marine sequences in those areas. We thank Prof. Bai Guoping for improving the English version and the anonymous reviewers for their constructive comments and suggestions that have greatly improved the manuscript. This study was supported by National Natural Science Foundation of China (Grant No. 40974048), National Basic Research Program of China (Grant No. 2005CB422108) and National Science & Technology Special Project (Grant No. 2008ZX05005). 1

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