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Geofluids
Volume 2017 (2017), Article ID 6740892, 16 pages
https://doi.org/10.1155/2017/6740892
Research Article

The Early Cambrian Mianyang-Changning Intracratonic Sag and Its Control on Petroleum Accumulation in the Sichuan Basin, China

1State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China
2Geological Survey of Canada-Atlantic, Dartmouth, NS, Canada
3Sichuan Geophysical Company of CNPC Chuanqing Drilling Engineering Company Limited, Chengdu 610213, China

Correspondence should be addressed to Shugen Liu; nc.ude.tudc@gsl and Bin Deng; moc.361@0003nibgned

Received 2 March 2017; Accepted 1 June 2017; Published 16 August 2017

Academic Editor: Xiaorong Luo

Copyright © 2017 Shugen Liu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The older and deeper hydrocarbon accumulations receive increasing attention across the world, providing more technical and commercial challenges to hydrocarbon exploration. We present a study of an asymmetrical, N-S striking intracratonic sag which developed across the Sichuan basin, south China, from Late Ediacaran to Early Cambrian times. The Mianyang-Changning intracratonic sag is ~50 km wide, with its steepest part in the basin center. In particular the eastern margin shows its greatest steepness. Five episodes in the evolutions of the sag can be recognized. It begins in the Late Ediacaran with an uplift and erosion correlated to Tongwan movement. Initial extension occurred during the Early Cambrian Maidiping period, when more strata of the Maidiping Formation were deposited across the sag. Subsequently, maximum extension occurred during the Early Cambrian Qiongzhusi period that resulted in 450–1700 m thick Maidiping-Canglangpu Formations being deposited in the sag. Then, the sag disappeared at the Longwangmiao period, as it was infilled by the sediments. The intracratonic sag has significant influence on the development of high-quality reservoirs in the Dengying and Longwangmiao Formations and source-rock of the Niutitang Formation. It thus indicates that a high probability for oil/gas accumulation exists along the intracratonic sag, across the central Sichuan basin.

1. Introduction

Continental rift basins are widespread extensional structures on the Earth’s surface and are known from the Archean up to the present. They account for ~30% of global hydrocarbon discoveries [1]. In particular, the Neoproterozoic assembly and break-up of two supercontinents, that is, Rodinia and Greater Gondwana, were marked by the development of a long series of rift valleys, half-graben, pull-apart basins, and intramontane molasse basins, along the Peri-Gondwanan Margin, from Australia through Pakistan, Oman and North Africa, and South China block (Figure 1(a)) [2, 3]. A series of studies was undertaken to unravel the generation and accumulation of hydrocarbons in this unique geological time [47]. The area which was particularly intensively studied is the South Oman Salt Basin [8, 9]. In most of these areas, the Neoproterozoic-Early Cambrian organic-rich strata are widespread, forming the Neoproterozoic-Early Cambrian (“Infra-Cambrian” [10]) hydrocarbon plays. The occurrence of prolific source-rocks was controlled by strong postglacial sea-level rise, following the major Neoproterozoic glaciations [2]. Furthermore, deposits of evaporites and black shale are widespread in rift-related basins, providing effective seal and source-rock across from Oman, Pakistan, and India to South China [3, 11]. This generated an interest in exploring for the Infra-Cambrian plays along the Peri-Gondwanan Margin. Despite the existence of proven Infra-Cambrian hydrocarbon plays in many parts of the world, the petroleum prospective is associated with much higher exploration risks than in conventional Phanerozoic petroleum systems, as the strata are usually much more deeply buried and relatively poorly known.

Figure 1: (a) Generalized Late Neoproterozoic-Early Cambrian palaeogeography of the “Peri-Gondwanan Margin” and its petroleum system (modified from Craig et al. [2]). (b) Late Neoproterozoic-Middle Cambrian Stratigraphic column for the South China block. (c) The Digital Elevation Map (DEM) showing the topography and intracratonic sag in the Sichuan basin, the red lines indicating 2D seismic profiles to unravel the sag structure, and the black dotted line showing 3D seismic data of the Gaoshiti-Moxi area. (d) Insert: map of the basic tectonic framework of China.

The Sichuan basin located in the South China continental block has generated interest for brine/natural gas exploration since the ~1100 AD [12] and is one of the largest sedimentary basins rich in petroleum resources in China (Figure 1). It is noteworthy to mention that workers in the old salt industry have used bamboo casing and piping to drill for gas near Zigong, in the southern Sichuan basin. This idea inspired the first significant gas pools discovery in the Shiyougou and Shengdengshan structures in the 1930s, followed later by a series of oil/gas discoveries in the Sichuan basin (Figure 1), for example, Well Nv-2 in 1958 and the Ediacaran Weiyuan gas fields in 1964 (with a proven in-place gas volume of 408.6 × 108 m3 (1.5 tcf), etc.) [1315]. In particular, recent major discoveries are genetically related to Neoproterozoic-Paleozoic extensional tectonics across the basin, for example, Puguang gas field with a proven in-place gas volume of 3500 × 108 m3 (12.4 tcf) [16] and the Anyue gas field with a volume of 4403.8 × 108 m3 (15.6 tcf) [17]. The later Ediacaran-Cambrian carbonate gas field is also the biggest gas field presently in China. Based on seismic data, structural features, hydrocarbon condition, and so on, we reconstruct the Early Cambrian extensional structure, that is, the Mianyang-Changning intracratonic sag located in the basin center, and discuss its influence on petroleum accumulation. It may stimulate further exploration in these hydrocarbon rich provinces.

2. Geological Setting

The Sichuan basin is located at the western margin of the South China block, which is comprised of the Yangtze platform in the northwest and the Cathaysian Block in the southeast (Figure 1). To the north, it is separated from the North China block by the Late Paleozoic to Middle Mesozoic Qinling Orogen [19], and to the west from the Mesozoic-Cenozoic Eastern Tibetan Plateau by the Longmen-Daliang thrust fold belt [20, 21]. Within the South China block, the Sichuan basin is separated from a 1300-km wide Mesozoic intracontinental orogenic belt by the Qiyueshan-Daloushan structure [22, 23]. Thus, the Sichuan basin is a superimposed basin influenced by its peripheral orogens and dominated by four stages of basin evolution: (a) a rift at the cratonic margin in the Neoproterozoic, (b) a marine carbonate platform from the Early Cambrian to Middle Triassic, (c) a foreland basin from Late Triassic to Late Cretaceous, and (d) subsequent exhumation and structural modification. The sedimentary cover is comprised mainly of Paleozoic and Middle Mesozoic strata of shallow-marine deposits, and post-Late Triassic terrestrial strata, of a total thickness ~8 km to 12 km.

The South China block is generally considered being a part of the Precambrian Rodinia supercontinent [2426]. Along its western and northern margins, the Proterozoic basement of the craton, that is, the Kongling complex or Banxi group, is characterized by the presence of migmatitic granites and gneiss, a metamorphosed greywacke-slate succession, unconformably overlain by the Sinian/Ediacaran sedimentary rocks. The Sinian/Ediacaran Doushantuo and Dengying formations deposited during a major marine transgression are predominantly comprised of shallow water dolomitic carbonates. Due to the Tongwan tectonic movement, Sinian (Ediacaran) dolomites were exposed to freshwater karstification, resulting in widespread development of caverns and dissolution porosity in the upper of Dengying Formation and at unconformities (Figure 1(b)) [27, 28]. That becomes a favorable niche for petroleum accumulation in the Proterozoic Dengying Formation, as demonstrated by the Weiyuan gas field with proven gas reserves of 408.61 × 108 m3 (1.5 tcf) [29]. Furthermore, the major extensional movements in the Sichuan basin were the Xinkai taphrogenesis or “extensional movement,” which is coeval with assembly and break-up of the Rodinia supercontinent [3033]. It resulted in the formation of deep extensional faults representing favorable conduits for thermal fluids migration from the deep basin. The evidence is widespread dissolution, precipitation of silica, occurrence of zebra structures, and saddle dolomite in the Late Proterozoic strata, improving the reservoir quality of the Dengying Formation, which widely host the lead-zinc deposits of MVT type and barite-fluorite and hydrothermal chert along the western margin of the South China block [3436]. The Early Cambrian strata paraconformably overlie the Sinian (Ediacaran) rocks. In particular, the Qiongzhushi Formation (or Niutitang Formation) is one of the most important source-rocks (dominated by Type I-kerogen) in the basin, which generated hydrocarbons found in the Upper Sinian and Early Cambrian formations reservoirs [3739]. Recently, another giant gas field has been discovered in coarsely crystalline dolomite facies, in the Cambrian Longwangmiao Formation, with proven gas reserves of 4403.8 × 108 m3 (16.2 tcf).

During Mesozoic to Cenozoic times, the area was affected by several phases of tectonic movements, including the Caledonian movement and Indosinian and Yanshannian movements [40, 41]. This resulted in most of the upper Silurian, Devonian, and Carboniferous strata being absent across the South China block, in the development of several regional unconformities and an E-W trending Leshan-Longnvshi paleouplift in the Sichuan basin center. These structures significantly influenced the formation of the carbonate reservoirs and hydrocarbon accumulations across the Sichuan basin and its periphery [7, 29, 38]. Furthermore, those multiphase tectonic movements gave rise to a complicated tectonic evolution and sedimentary history of the basin [42, 43].

3. The Early Cambrian Mianyang-Changning Intracratonic Sag

3.1. Precambrian Extensional Faults across the Sichuan Basin

Stratigraphic horizons were mapped in the Gaoshiti-Moxi area, located in the center of the Sichuan basin, using the three-dimensional seismic data and well control (Figure 1(c)). We observed that extensional faults are extremely common in most of the Precambrian strata (Figure 2) (see Supplementary Material, available online at https://doi.org/10.1155/2017/6740892). There are at least two extensional faults present with an opposite dip, indicating a rift. The continuity of reflectors indicates presence of syn-rift deposits. Furthermore, an offset of extensional faults is well imaged in the seismic data. This structural model was used as the basis for our 2D and 3D interpretation across the Sichuan basin. The density of seismic profiles is 5–30 km across most of the basin. The data shows that there are many extensional faults and related structures in the Precambrian strata, across the Sichuan basin. Most of these structures have NE-strike; some of extensional structures further show the influence of the deformation of the overlain strata. In particular, some of the strata overlying the extensional structures (i.e., Upper Sinian Dengying and Lower Cambrian sequences) are characterized by distinctly increased thickness (Figure 2).

Figure 2: 3D seismic profile indicating Precambrian extensional structure in the center of the Sichuan basin (please find the original data in Supplementary Material). The red dotted line and blue arrow show the pre-Sinian extensional faults and eastern boundary of extensional structure; the inset box shows the location (: base of Lower Jurassic Zhiliujing Formation, : base of Upper Triassic Xujiahe Formation, : base of the second member of Lower Triassic Jialingjiang Formation, P: base of Permian, Є: base of Cambrian, : base of the third member of Sinian Dengying Formation).
3.2. Boundary of the Mianyang-Changning Intracratonic Sag

As borehole data for pre-Sinian strata are lacking, therefore there is much of uncertainty about the rifts. However, based on regional geology studies [3033], we argue that such extensional tectonics extended across the Ediacaran (Sinian) to Early Cambrian periods. Occurrence of extensional tectonics is supported by the borehole data of the Sinian strata and related seismic data, which indicate presence of Early Cambrian extensional structures across the Sichuan basin. The borehole data indicates that there is distinct increased thickness of the Early Cambrian Qiongzhusi Fm in the GS-17 and Z-4 wells, located at the hanging walls of the extensional faults mentioned above, in comparison with W-28 and GS-1 wells located at the footwalls. In particular, there is an additional strata, the Maidiping Fm, in the center of the extensional structure, compared to other places (Table 1, Figure 3). Thus, we argue that there is an extensional structure across the center of Sichuan basin, named the Mianyang-Changning intracratonic sag. We use the term “intracratonic sag” according to P. A. Allen and J. R. Allen [44], as it is difficult to identify how large the offset of extensional faults was during the Early Cambrian periods.

Table 1: The thickness of Sinian (Ediacaran)-Early Cambrian strata in wells across the intracratonic sag.
Figure 3: The key seismic profiles indicating presence of Early Cambrian extensional intracratonic sag across the Sichuan basin. (B-B′: NE-striking Weiyuan-Gaoshiti cross-section across the intracratonic sag. The yellow lines show log-data for well picks of the horizon; based on GS-1 and GS-17 wells. Grey arrows show western and eastern sag boundaries. and are bases of Lower Cambrian Longwangmiao and Canglangpu Fm. C-C′: NE-striking Ziyang-Shehong section. All the seismic profiles are rotated back with the Middle Cambrian Gaotai Fm. The inset map shows locations of cross-sections).

Most of the seismic profiles across the Mianyang-Changning intracratonic sag show significant changes in the thickness of Early Cambrian strata; those stara westwards and eastwards onlap onto the Sinian strata, with decreasing thickness from the center of the sag to its sides. It should be noted that the underlain Late Sinian strata show decreasing thickness towards the boundary of the sag. The continuity of reflection horizons suggests asymmetric geometry of the Mianyang-Changning intracratonic sag, with a steep boundary at the eastern margin, where the discontinuous reflections indicate that it probably accommodated some extensional faults to result in a dustpan-like geometry.

From north to south, the seismic data shows an increasing steepness along the eastern margin of the sag, for example, from the Langzhong to the Shehong sections (Figure 4 D-D′ section), where the orientation of eastern margin changes from NE-striking to NS-striking in the Shehong area. At the center of the Sichuan basin the increased thickness of Early Cambrian strata is consistent with overlain extensional faults in pre-Cambrian strata in the Gaoshiti-Moxi area (Figure 2). The discontinuous reflections at the base of the Early Cambrian strata suggest a steeper margin of the Mianyang-Changning intracratonic sag than at its northern segment (Figure 3). Further to the south, the steepness of eastern margin substantially decreases, as can be observed at, for example, the Dazhu and Hejiang areas (Figure 4 E-E′ section).

Figure 4: 2D seismic profile across eastern margin of the Mianyang-Changning intracratonic sag. D-D′: SE-striking Langzhong section in northern segment of the sag; E-E′: SE-striking Hejiang section in southern segment. It should be noted that there is a distinct decrease in the steepness of the eastern boundary from the north to the south.

The western margin of the sag as indicated by seismic profiles is characterized by less steep beds and more complicated strata orientation than that of the eastern margin. The western margin can be subdivided into two segments, one is NW-striking and the other NE-striking, separated by the Weiyuan-Zigong area. In the south, both the south segment of western margin and south segment of the eastern margin are characterized by low steepness (Figure 5 F-F′ section and Figure 4 E-E′ section). Although the western margin of the Mianyang-Changning intracratonic sag shows generally less steepness than the eastern margin (Figure 3), the northern segment of western margin of the sag is much steeper than its southern segment (Figure 5 G-G′ section). It should be noted that the underlying Dengying Formation is thinner along the western margin than at the eastern margin of the sag. That is a result of the uplift and erosion which occurred during the end of the Ediacaran epoch [3032].

Figure 5: 2D seismic profile across western margin of the Mianyang-Changning intracratonic sag. F-F′: SE-striking Qianwei section in southern segment of the sag; G-G′: NW-striking Zigong-Ziyang section, showing a distinct decrease in steepness of the western sag boundary from the north to the south.
3.3. Geometry of the Mianyang-Changning Intracratonic Sag

We have used seismic lineups at the base of the Early Cambrian Maidiping and Canglangpu formations to unravel spatial thicknesses of the deposits during Early Cambrian times. It demonstrates syn-extension sedimentation during that time and shows geometry of the Mianyang-Changning intracratonic sag across the Sichuan basin (Figure 6). The isopach map (Figure 6(a)) shows thickest Maidiping-Canglangpu Formations to be located in the center of the Sichuan basin, where the thickness is about 450–1700 m higher than 0–600 m at any other place. It suggests that a geometry of the Early Cambrian Mianyang-Changning intracratonic sag is with its eastern margin along the Bazhong-Shehong-Dazhu-Hejiang zone, and its western margin along the Xinjing-Weiyuan-Qianwei zone. To the east of the sag, the thickness of the Maidiping-Canglangpu Formations is about 300–600 m, with a thickness center at the Guan’an-Hechuan area. To the west, the thickness is about 0–500 m, with a westward decrease in thickness due to the erosion.

Figure 6: Cross-sections showing Early Paleozoic strata geometries in the intracratonic sag. ((a) spatial distribution of the sag indicated by the isopach map of the Maidiping-Canglangpu Formations; the sag is characterized by asymmetrical, N-S striking geometry. The sag can be subdivided into three segments; in particular, the eastern margin is much steeper than the western margin. (b) E-W striking cross-section of the sag in Leshan-Yibin-Zigong area. (c) N-S striking cross-section of the sag in Mianzhu-Changning area.)

The Mianyang-Changning intracratonic sag can be separated into three segments, roughly separated by the Weiyuan-Gaoshiti area. The northern segment located at the north of the Shehong-Xinjing area has thicker Maidiping-Canglangpu Formations than the other two segments. Also its width increases northwards. In particular, some extensional faults developed in the center of the sag during the Early Cambrian times. The middle segment with a N-S striking strata is characterized by the greatest steepness of both margins of the sag. The narrowest place in the middle segment has about 50 km width. Furthermore, more strata of Maidiping Formation was deposited in the center, which resulted in the thickness of Maidiping-Canglangpu Formations being significantly larger than that outside the sag. The southern segment located at south of the Dazhu-Qianwei area is characterized by the least steepness of the margins and thinnest Maidiping-Canglangpu Formations than in other segments. Due to erosion, the western margin of the southern segment is not very distinct, resulting in a roughly southwestward increase in its width.

3.4. Evolution of the Mianyang-Changning Intracratonic Sag

Based on E-W-striking seismic sections of the Mianyang-Cangning sag, the balanced cross-sections were constructed considering the template line as flat (e.g., the top of Longwangmiao Formation, Canglangpu Formation, and Qiongzhusi Formation), to unravel the multistage history of the Mianyang-Changning intracratonic sag. Thus, the evolution of the intracratonic sag can be divided into five episodes: presag period (i.e., uplifting and erosion) at the end of Late Sinian Dengying period, an early stage at the Early Cambrian Maidiping period, a main stage of extension during the Early Cambrian Qiongzhusi period, and a decay and dispersal stages of extension at the Early Cambrian Canglangpu and Longwangmiao periods, respectively (Figure 7).

Figure 7: Evolution of the Early Cambrian Mianyang-Changning intracratonic sag (it should be noted that there is more strata of Maidiping Formation and much thicker Niutitang Formation in the center of the Mianyang-Changning sag, indicating that the initial extension as well as maximum of extension occurred during the Early Cambrian Maidiping to Niutitang periods).

Widespread uplift and erosion across the Sichuan basin occurred at the end of Late Sinian Dengying period, named the “Tongwan movement” [3032]. It is associated with different magnitudes of erosion across the sag, for example, the Z-4 and GS-17 wells located in the center of the sag lack the third and fourth members of the Dengying Formation (Table 1). This indicates that uplift and erosion across the sag were stronger than at other outside places during this time.

Early in the Early Cambrian Maidiping period, the Maidiping Formation was deposited where in presag time there was maximum erosion. This unit is characterized by deposits of black phosphorus silicic shale and phosphorus dolomite, interbedded with cellophane beds. Their occurrence is related to submarine volcanism and hydrothermal processes in a deep oceanic environment [45, 46], which indicates rapid subsidence and extensional tectonics. The Qiongzhusi Formation is characterized by the occurrence of black shale and mudstone at its base, and by grey-to-black muddy siltstone in the upper part of the formation, of which the geochemistry suggests it is dominated with I-type source-rocks with high TOC content [17, 18]. The latter indicate some shallowing, or progressive basin filing during the final stage of extension. The thickness of black shale in the center of the sag is up to 180 m, for example, in the GS-17 well, which is greatly important for petroleum occurrences in the center of the Sichuan basin. The extension and sedimentation across the intracratonic sag decreased distinctly during the decay and dispersal stages at the Canglangpu and Longwangmiao periods. The Longwangmiao Formation is comprised of carbonate rocks of roughly consistent thickness across the center of Sichuan basin, indicating that the Mianyang-Changning intracratonic sag already disappeared.

4. Control of the Intracratonic Sag on Oil/Gas Occurrences

The development of the Mianyang-Changning intracratonic sag significantly influenced the Late Proterozoic to Early Cambrian oil/gas occurrences in the Sichuan basin, that is, chiefly through its influence on the distribution of high-quality reservoirs in the Late Proterozoic Dengying and Early Cambrian Longwangmiao formations and of source-rocks in the Niutitang Formation.

4.1. High-Quality Reservoir of Late Proterozoic Dengying Formation

The reservoir characteristics of the Late Proterozoic Dengying Formation in the boreholes across the intracratonic sag indicate distinct differences in karst development, burial and hydrothermal dissolution, porosity and permeability, and so on (Table 2). There were variable magnitudes of erosion across the Mianyang-Changning intracratonic sag (Table 1, Figure 7); therefore there are variable occurrences of karst caves impacting the reservoir. The Z-1 and GS-1 wells located around the edges of the intracratonic sag have much higher density of karst caves than the JS-1 and W-113 wells, located further away from the western sag boundary. Furthermore, there are different volumes of bitumen in the reservoir of the Dengying Formation, indicating different magnitudes of burial dissolution. Westward of the intracratonic sag, no bitumen is present in the 2nd and 4th member of the Dengying Formation at the JS-1 well. Eastward, near the sag boundary, the W-113 well shows widespread bitumen in the 2nd and 4th members of the Dengying Formation, extending for 96.42 m in a 110.42 m long core in the 2nd member and for 14.82 m in a 23.6 m long core of the 4th member, respectively. The bitumen impregnated 1%~6% in porosity, in both the 2nd and 4th members of the Dengying Formation, as well as in the Z-1 well, located at the western margin of the sag. In particular, a core 31.05 m long in the GS-1 well, located at the eastern margin of the sag, is totally impregnated by bitumen in the 4th member of the Dengying Formation, indicating much more burial dissolution along the eastern margin.

Table 2: Reservoir features of the Ediacaran Dengying Formations across the intracratonic sag.

It should be noted that the porosity and permeability across the sag show significant differences. The strata in JS-1 well show poor porosity and permeability. The reservoir is comprised of dissolution pores and fractures, with most of them totally or partially filled with dolomite. To the east along the intracratonic sag, the Z-1 and GS-1 wells Dengying Formation strata have much better porosity and permeability than the JS-1 and W-113 wells, in which the reservoir is represented by dissolution pores and caverns. In particular, the thickness of reservoir in the GS-1 well is up to 184.05 m, with a gas production of 106 m3 per day. It indicates that a better porosity and permeability in the reservoir is located along the border of the intracratonic sag. That is consistent with a distinct increase in the karst weathering, dissolution pores occurrence, presence of bitumen in the reservoir, and intensity of burial and hydrothermal dissolution towards the sag.

4.2. High-Quality Source-Rock of Early Cambrian Niutitang Formation

During development of the Early Cambrian Mianyang-Cangning intracratonic sag, the 50–450 m thick Qiongzhushi/Niutitang Formation comprised of black shale was deposited (Figure 8). It should be noted that the geometry and thickness of Qiongzhushi Formation show much difference than those of the Early Cambrian Maidiping-Canglangpu Formations (shown in Figure 6), which could be attributed to the interpolation of different thickness between them (i.e., the 50–450 m thick Qiongzhushi Formation and the 450–1700 m thick Maidiping-Canglangpu Formations). However, the thickness isopach of the Qiongzhushi Formation roughly parallels the orientation of the sag, indicating that the maximum thickness of the Qiongzhushi Formation is controlled by the morphology of the sag. Strata located around the intracratonic sag provided most of the hydrocarbons to charge its Late Proterozoic to Lower Cambrian petroleum systems [7, 18, 38, 39].

Figure 8: The thickness and hydrocarbon-generating intensity of the Lower Cambrian mudstone across the Sichuan basin ((a) Thickness isopach map of the Qiongzhushi/Niutitang Formation; (b) isomap of hydrocarbon-generation of the Qiongzhushi/Niutitang Formation, modified from Huang et al., 2002 [18]).
4.3. High-Quality Reservoir of Early Cambrian Longwangmiao Formation

Based on thin-section analyses from three wells across the intracratonic sag, different diagenesis regimes were noted in the Early Cambrian Longwanmiao Formation. A 20–60 m thick high-quality reservoir in the Anyue supergiant gas field shows dolomitization, cementation, compaction, and pressure solution. Dolomitization, selective freshwater dissolution, and liquid hydrocarbon filling had significant roles in the reservoir development, in particular the last two processes. The Longwangmiao Formation reservoir in the MX-12 well located at the eastern margin of the sag shows strong selective freshwater karstification and hydrocarbon charging, indicated by multiple episodes of bitumen filling. Also it has indications of moderate epigenic karstification and hydrothermal activity (Table 3). However, the reservoir in the boreholes located in the center of the sag shows strong epigenic karstification and hydrothermal dolomite, for example, the GS-17 well. To the west of the sag, magnitude of hydrocarbon charging, epigenic karstification, and hydrothermal dolomite decrease in the Longwangmiao Formation. For example, the JS-1 well demonstrates only weak epigenic karstification and no hydrocarbon charging. Thus, it suggests that the high-quality reservoir in the Longwangmiao Formation is associated with the intracratonic sag development. Most importantly, the best reservoirs of Longwangmiao Formation are located at the eastern margin of the sag.

Table 3: Differential diagenesis of Early Cambrian Longwangmiao Formation across the intracratonic sag.

There are two reasons which may explain the occurrence of a high-quality reservoir at the eastern margin of the sag in the Early Cambrian Longwanmiao Formation. During extensional tectonics, the footwall of the intracratonic sag may have formed a paleohigh along its margins. Although the difference in elevation between the paleohigh of the footwall and hanging wall may not be substantial, it had significant influence on reservoir development. Our studies of seismic data support existence of such a paleohigh during deposition of the Longwanmiao Formation. The Moxi-Gaoshiti area (in Figure 1 the MX-GS area) had maximum paleoelevation during that time period. Thus, we suggest that the paleohigh formed at the sag margin was conducive for a grainstone-bank facies deposition during Longwangmiao Formation time. The thickness of this facies is 20–70 m at the Moxi-Gaoshiti area. Subsequent surface exposure resulted in freshwater karstification and development of intergranular and intragranular dissolution porosity.

The lower part of the Lower Cambrian had significant capacity to generate oil/gas, which together with hydrocarbons from the Ediacaran Sinian strata migrated to Early Paleozoic traps [7, 37, 38]. The critical time periods for hydrocarbons generation, migration, and transformation were the Silurian, which was the initial period for liquid hydrocarbons generation, Late Permian to Late Triassic period for peak hydrocarbon-generation, and the Middle Jurassic to Late Cretaceous interval for hydrocarbon-cracking into gas [7, 47]. Furthermore, the paleouplift of Leshan-Longnvshi formed during Late Carboniferous-Early Permian period and is roughly prior to the period of peak hydrocarbon-generation [48, 49]; we argue that the paleohigh along the margins of the intracratonic sag has the highest probability to be charged by hydrocarbons, as maturity of the source-rocks was increasing with increased burial.

Study of thin-sections indicates that there are two periods of hydrocarbon charging: the first one during shallow-moderate burial of the strata and the second one during moderate-to-deep burial. The first one followed a period of uplift and freshwater karstification of exposed carbonate strata, which occurred at the Sinian-Cambrian boundary, and is marked by the unconformity. Occurrence of trace amounts of bitumen suggests that limited hydrocarbon-generation may have occurred due to the presence of hot hydrothermal fluids, indicated by Pb-Zn mineralization near the boundary. The increased burial resulted in increased thermal maturity of the source-rocks and the main peak of hydrocarbon-generation during the Permian. The Gaoshiti-Moxi and Weiyuan-Ziyang areas located at the eastern and western margins of the sag represent the best area for hydrocarbon charging and migration (Figure 9). Both of those areas contain high-quality reservoirs in the Longwangmiao Formation. Also there is a steeper margin at the eastern margin of the sag than its west, which would contribute to a better efficiency of hydrocarbon migration. Furthermore, the Gaoshiti-Moxi and Weiyuan-Ziyang areas were located at the paleohigh and upper slope during the deposition of Longwangmiao Formation. Both of them account for most of high-quality reservoirs in the Gaoshiti-Moxi area. Therefore, we suggest that the most prospective areas characterized by occurrence of high-quality reservoirs and hydrocarbons accumulation in Longwangmiao Formation are at the eastern part of the overlapping areas of the Mianyang-Changning intracratonic sag and the paleouplift, represented by the Gaoshiti-Moxi area.

Figure 9: Conceptual model of high-quality reservoir rocks of Early Cambrian Longwangmiao Fm in the central Sichuan basin. ((a) Paleo-structure of Longwangmiao Fm at the end of Permian, (b) hydrocarbon charging model of Qiongzhushi Fm).

5. Conclusions

The Neoproterozoic break-up of the Rodinia supercontinent, that is, Xinkai taphrogenesis in the South China block, had significant influence on the Neoproterozoic-Early Cambrian hydrocarbon occurrences in the Sichuan basin. Our study shows an asymmetrical, S-N striking intracratonic sag, that is, the Mianyang-Changning intracratonic sag, which developed across the Sichuan basin from the Late Ediacaran to Early Cambrian time. The sag can be divided into three segments, with margins roughly along the Bazhong-Shehong-Dazhu-Hejiang area at the eastern margin and the Xinjing-Weiyuan-Qianwei area at the western margin. In particular, the eastern margin shows much greater steepness than the western margin. The narrowest part of the sag is ~50 km across, and its greatest steepness is in the basin center. Five episodes of evolutions of the sag can be established. It begins in the Late Ediacaran with an uplift and erosion correlated to Tongwan movement preceding extension. Initial extension occurred in the Early Cambrian Maidiping period, with strata of the Maidiping Formation deposited across the Mianyang-Changning sag. Subsequently, maximum of extension took place at the Early Cambrian Qiongzhusi period that resulted in deposition of the 450–1700 m thick Maidiping-Canglangpu Formations in the center of the sag. Finally, the sag was infilled by sediments at the end of the Early Cambrian (i.e., the Longwangmiao period) and as structural entity disappeared.

The Mianyang-Changning intracratonic sag had significant influence on the Late Proterozoic to Early Cambrian hydrocarbon occurrences in the Sichuan basin. It is chiefly through its influence on the presence of high-quality reservoirs rocks in the Late Proterozoic Dengying and Early Cambrian Longwangmiao formations and distribution of source-rocks of the Niutitang Formation. During the evolution of the Mianyang-Changning sag, the paleohighs and erosion account for the development of high-quality reservoirs in the Late Proterozoic Dengying and Early Cambrian Longwangmiao formations. Furthermore, it was the sag development which provided conditions favorable for the deposition of I-type source-rock of the Niutitang Formation. Considering all of these factors there is a high probability of oil/gas accumulation along the Mianyang-Cangning intracratonic sag, particularly along its eastern margin across the center of the Sichuan basin.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work was supported by the National Basic Research Program of China (no. 2012CB214805) and the Natural Science Foundation (nos. 41230313, 41402119, and 41472017). The authors are thankful to Frank Thomas for review of the English in the manuscript.

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