Meso-Neoproterozoic Geology and Petroleum Resources in China

It is a long-term objective for Chinese geologists to make the Meso-Neoproterozoic strata in North China Craton (NCC) the international reference section of the Late Precambrian chronostratigraphic stratigraphy. However, the establishment of highly precise and reliable Precambrian chronostratigraphic frame would be a top priority in current chronostratigraphic study and also a basis for Precambrian stratigraphic correlation and tectonic interpretation in each continent of the globe. This chapter will be focused on the achievements of the Chinese Meso-Neoproterozoic chronostratigraphy, especially for the new calibration of Meso-Neoproterozoic stratigraphic division in North China Craton (NCC) as well as for the new isotopic dating of Neoproterozoic chronostratigraphy in Yangtze Craton (YC in southern China) and Tarim Block (northwest China). Those new isotopic dating data would result in a complete change in the Meso-Neoproterozoic chronostratigraphic calibration, division and correlation among above three regions, and update the regional geological cognition and the metallogenic mechanism interpretation. In recent years, the Meso-Neoproterozoic petroleum geological study and exploration put forward the precise chronostratigraphic division and correlation of sedimentary cover and crystalline basement of the petroliferous basins. In view of the achievements in the chronostratigraphic isotopic technique, it is possible to obtain the complete data of precise and reliable isotopic dating for Precambrian cover and basement in NCC, YC and Tarim Block (TB), so that more complete Meso-Neoproterozoic isotopic chronostratigraphic data would be available for the vest number of geologists in this chapter.

The Precambrian sedimentary strata in the Yanliao Faulted-Depression Zone (YFDZ) on the North China Craton is the most-developed Meso-Neoproterozoic sequences in China. The sequences consist of the Pt₂¹ Changchengian, Pt₂² Jixianian, Pt₂³x Xiamaling Formation and Pt₃¹ Qingbaikouan, which are traditionally subdivided into 12 formations and 43 members. This chapter aims to review the history and the state of art of the studies on the stratigraphic sequences in aspects covering the tempo-spatial distribution, lithostratigraphic correlation, geochronology and palaeontology. Some important aspects have been discussed in detail. As the oldest unmetamorphosed sedimentary sequence in China, the Changchengian (1670–1600 Ma) shows a regional micro-angle unconformity with the underlying Dahongyu Formation of Jixianian (1600–1400 Ma) with a local conformity at the Dahongyu depocenter. Therefore, both Changchengian and Jixianian should be referred to a set of basically continuous sedimentary strata in the Jixian stratotype section. The Changchengian may be attributed to the aulacogen clastic deposition in the early stage of the YFDZ related to early breakup of the Supercontinent Columbia.

The Meso-Neoproterozoic sedimentary basin in Yanliao Faulted-Depression Zone (YFDZ) is famous for its complete sedimentary strata, continuous outcrop, well stratigraphic preservation, abundant flora and algae fossils, etc. Based on the field investigation results in the Jibei Depression, YFDZ, including sedimentary petrology, sequence stratigraphy, etc., the Meso-Neoproterozoic sequence can be divided into 13 2-order sequences and 79 3-order sequences. It is believed that the sedimentary facies in Jibei Depression are referred to a carbonate rock system of epicontinental sea and a clastic rock system of pericontinental sea. The carbonate platform facies are mainly distributed in the Jixianian Gaoyuzhuang, Yangzhuang, Wumishan, Hongshuizhuang and Tieling Formations as well as Qingbaikouan Jing’eryu Formation; the organic reef facies mainly limited to the Gao-9 to Gao-10 Members of Gaoyuzhuang Formation at the western margin; The neritic shelf facies and baffle-free coastal facies are mainly distributed in Xiamaling and Luotuoling Formations. The research results show that there are 3 source-reservoir-seal bed assemblages, i.e., the Hongshuizhuang Formation (source)-Tieling Formation (reservoir)-Xiamaling Formation (seal), Hongshuizhuang Formation (source)-Xiamaling sandstone (reservoir)-Xiamaling shale (seal), and Gaoyuzhuang Formation (source)-Wumishan Formation (reservoir)-Hongshuizhuang Formation (seal).

Abundant micro- and macrofossils have been found from the Ediacaran (Sinian) deposits in South China. These fossils offer diverse information for understanding the evolution of early lives before the Cambrian Explosion, and also provide fundamental evidences for the chronological division and correlation of Ediacaran System. Until now all the Ediacaran microfossils were found from the middle and lower part of Doushantuo Formation in South China with two assemblages established, i.e., the lower Tianzhushania spinosa assemblage in the Dou-2 Member of Doushantuo Formation and the upper Hocosphaeridium anozos-H. scaberfacium assemblage in Dou-3 Member. Tianzhushania spinosa assemblage can be correlated with the microfossil assemblage in the Infrakrol Formation in Lesser Himalaya of India, and the Hocosphaeridium anozos-H. scaberfacium assemblage can be correlated with the ECAP (Ediacaran complex acritarch palynoflora) assemblage in South Australia. In other aspects, several exceptionally preserved Ediacaran macrofossil biotas have been reported from South China, including the Lantian biota from the lower part of Doushantuo Formation, Wenghui and Miaohe biotas from the upper part of Doushantuo Formation, as well as Xilingxia (Shibantan), Gaojiashan, Wulingshan and Jiangchuan biotas from the middle and upper part of the Dengying Formation. Among these macrofossil biotas, Miaohe and Wenghui biotas can be correlated with the White Sea biota in Russia and the well-known Ediacara assemblage from the western Flinders Ranges, South Australia; Xilingxia (Shibantan), Gaojiashan, Wulingshan and Jiangchuan biotas can be correlated with the Nama biota in Namibia. According to the biostratigraphy and carbon isotope stratigraphy, the Ediacaran system in South China is suggested to be divided into two series and six stages.

The well-exposed Neoproterozoic sedimentary successions of South China are rich in fossils and mineral resources, and have proven crucial to understanding Neoproterozoic stratigraphy, biological evolution and palaeoenvironmental changes. Based on analysis of the most complete Ediacaran (Sinian) successions in the Yangtze Gorges area and the pre-Ediacaran (Cryogenian and Tonian) succession in the deep water basin of the southeast Yangtze Craton (YC), a Neoproterozoic stratigraphic framework for South China is established and discussed, and recent advances plus remaining problems in the subdivision and correlation of the Neoproterozoic successions in South China are summarized. Also reviewed are the characteristics and spatiotemporal distribution of hydrocarbon source-reservoir-seal beds in the Neoproterozoic (mainly Ediacaran/Sinian) in this region.

Precambrian source beds have been discovered worldwide. The sedimentary ages of these strata are mainly 2.7–2.6 Ga, ca. 2.0 Ga, 1.6–1.4 Ga, ca. 1.0 Ga, 0.7–0.6 Ga, and 0.6–0.5 Ga respectively. The characteristics of these source beds are closely related to the evolution of microbe, from aquatic cyanobacteria to eukaryotes as well as the preservation conditions for organic matter. The occurrence and thriving of eukaryotic algae in ecosystems may increase the possibility of oil-prone source bed formation. Geological processes, including crustal weathering, volcanic activity and glaciations, are also essential for the development of large-scale source beds. Thermal degradation of organic matter has been regarded as one of the most important factors controlling Precambrian source bed quality, and thus detailed and precise research on maturity would be very important for the evaluation of oil and gas potential. As macro fossils are rare in Precambrian source beds, the biomarkers, i.e., molecular fossils, present powerful tools for tracing the Precambrian microbial communities. For example, based on some specific biomarkers, the first appearance of sulfate-reducing bacteria and green sulfur bacteria can be dated to 1.64 Ga. The diversity of biomarkers in Precambrian strata provides a basis for oil-source rock correlation. For example, the source beds of bituminous sandstones in the Xiamaling Formation as well as oils in Oman have been successfully confirmed using both the specific biomarkers, 13α(n-methyl)-tricyclic terpanes and C₁₉ A-norsterane. The researchers also reveal that carbon isotopic composition of organic matter in Precambrian strata remarkably differs from those in the Phanerozoic. Although the tremendous progress has been made in the discovery of Precambrian biomarkers, trace amounts of those compounds in the strata also provoked the debate regarding the indigeneity of Precambrian organic matter. Nevertheless, it is believed that this problem can be solved as analytical technology progresses in the future.

The Mesoproterozoic geochemical record is limited, but critical in revealing the relationships between Precambrian eukaryotic ecosystem evolution and source bed formation. In North China Craton, the Xiamaling Formation (1400–1320 Ma) is a set of tens-million-years-old and well-preserved low-mature to mature sedimentary sequence on a passive continental margin, and records the interaction among climate, ecosystem, and dynamic ocean chemistry. Moreover, the low thermal maturity and excellent preservation of the Xiamaling sediments allow organic and inorganic geochemical analysis. Overall, by means of multidisciplinary investigation, including iron speciation, trace elements, biomarkers, and so on, the palaeo-oceanic geochemistry during the Xiamaling deposition is evaluated. Based on both sedimentological and geochemical criteria, the Xiamaling Formation is tentatively divided into six lithological units in the present study. Of the six units, four units are adequately investigated and each unit contains distinct geochemical features, indicating specific water-column environment respectively with oxic, non-sulfidic anoxic, and sulfidic anoxic sedimentary environments. The variation of the palaeo-oceanic chemistry has significant influence on the primary productivity of organic matter and the preservation of hydrocarbons, which is consistent with the regional palaeo-climatic change, particularly for the place where the Xiamaling Formation was related to the intertropical convergence zone and the corresponding Hadley cell fluctuations.

The North China Craton (NCC) was formed around 2.50 Ga. A 2.50–2.35 Ga period involved a time of tectonic quiescence, and then an important orogenic epoch, i.e., the period of Hutuo Orogeny. The tectonic processes of rifting, subduction and collision characterize this tectonic event, and possibly relate to development of the Supercontinents Nuna or Columbia. Three Palaeoproterozoic orogenic belts have been recognized in the NCC, which are termed as the Jiaoliaoji, Jinyü, and Fengzhen Mobile Belts, respectively. The Jiaoliaoji Mobile Belt is located in the eastern NCC, and made up mainly with the Liaohe and Fenzishan Groups. The Jinyü Mobile Belt occurs in the centro-western NCC, and made up of the Lüliang, Hutuo and Zhongtiao Groups. The Fengzhen Mobile Belt developed in the northwestern NCC, and is composed of the Fengzhen and Erdaowa Groups. The Palaeoproterozoic rocks are mostly basic and acid volcanic and sedimentary rocks, but metamorphosed into low-grade amphibolite facies-greenstone facies. Volcanic rocks show bimodal volcanic features in petrology and geochemistry, indicative of an intraplate setting for their occurrence. These orogenic belts apparently recorded the plate tectonic evolution in the Mesoproterozoic time. The NCC underwent extension in the period from 2.3 to 2.0 Ga, as evidenced by the development of rifts and oceanic basins. There happened subduction, collision, and resulting contraction during 2.01–1.95 Ga. The NCC became stabilized after the Hutuo orogeny or during the “Earth’s middle age”, and remained as a stable platform for more than 1.0 Ga. Meso-Neoproterozoic sedimentary sequences were extensively deposited on the basement. The Xiong’er Rift was formed in the centro-south NCC, and the Yanliao Rift (or called Yanliao Faulted-Depression Zone, YFDZ) occurred in the centro-north NCC. Two other rifts developed in the northwestern and eastern margins of NCC, i.e., the Zha’ertai-Bayan Obo-Huade Rift and Jiao-Liao-Xu-Huai Rift, respectively. The Meso-Neoproterozoic sequences in the Yanliao Rift record sedimentary development of the NCC, and are divided into the Pt₂¹ Changchengian, Pt₂² Jixianian, Xiamaling Formation (Pt₂³x) and Pt₃¹ Qingbaikouan. The Changchengian started from the Changzhougou Formation with the age of ca. 1.67 Ga. The Xiong’er Rift began with volcanic rocks dated at ca. 1.78 Ma, indicating that the rifting developed earlier than the Yanliao Rift. The Jixianian is corresponding to the Calymmian sequences, and followed by the Xiamaling Formation (Ectasian) and then the Neoproterozoic Qingbaikouan (Tonian). Four magmatic events are recognized in the NCC during Late Palaeo-Neoproterozoic times: ① ca. 1800–1780 Ma Xiong’er magmatism; ② ca. 1730–1620 Ma anorogenic magmatism; ③ ca. 1400–1320 Ma diabase/gabbro-diabase sill/vein swarm; and ④ ca. 925 Ma mafic dyke swarm. These magmatic events suggest that the NCC was in an intraplate/intracraton setting from ca. 1800 Ma to ca. 700 Ma. Late Palaeo-Neoproterozoic ore deposits include magmatic iron deposits and REE-Nb-Fe or Pb–Zn-Cu-Fe deposits, while orogenic metal deposits are absent, and no geologic evidence exists for the Grenville or other orogenic events in the NCC. Accordingly, the NCC was probably far from the Nuna Supercontinent in the Proterozoic. The “Earth’s middle age” was a period when the Earth possessed a stable lithosphere and secular warm mantle, which could only result in multiple episodes of magmatism and rifting. It is thus suggested that the Earth experienced an evolution from non-plate tectonics through primitive plate tectonics to modern plate tectonics in the Proterozoic.

Neoproterozoic magmatic rocks are widespread in South China, which can be divided into three major groups based on their formation ages and their genetic relationships with regional tectonics, metamorphism and basin evolution: ① Group 1 formed during the syn-orogenic process (1.0–0.9 Ga); ② Group 2 formed in the early rift phase (0.85–0.80 Ga); and ③ Group 3 formed in the major rift phase (0.79–0.75 Ga). Despite numerous studies during the past two decades, the tectonic settings responsible for the Neoproterozoic magmatism, in particular the intensive granitoid plutonism in South China is still an issue of hot debate. It is noteworthy that the geochemical characteristics of granitoid rocks are reflective of their source compositions as well as the melting and crystallization histories of the melts, thus could not be simplistically used to assess the tectonic regime under which the granitoids were formed. On the other hand, geochemistry of basaltic rocks (including basalts and basaltic dikes) reflects the compositions and thermal structures of their mantle sources that are relevant to different petrotectonic associations. In this chapter we compile the published high-precision isotopic ages and geochemical and isotopic data for the Neoproterozoic basaltic rocks and the pre-Neoproterozoic crystalline basement rocks from South China. These datasets, combined with other aspects of geological records, are used to investigate the petrogenesis of the basaltic rocks, as well as their mantle source compositions and potential temperatures, aiming to shed new lights on the Neoproterozoic petrotectonic evolution in South China. The Group 1 syn-orogenic basaltic rocks occur sporadically along the Yangtze Craton margins, including the ca. 0.9 Ga Yanbian basalts and the ca. 1.0 Ga Huili mafic dykes in western margin, the ca. 0.95–0.89 Ga Xixiang basalts in northwestern margin and the ca. 0.96 Ga Pingshui spilites in southeastern margin. They are closely associated with the calc-alkalinic intermediate-acidic volcanic/granitic rocks; all of the rocks are clearly deformed and metamorphoses to varying degrees. The Xixiang basalts and Pingshui basalts are calc-alkaline in compositions with geochemical characteristics similar to those of island arc basalts (IAB). In Yanbian area, both tholeiitic and calc-alkaline basalts are exposed, and their geochemical features are similar to those of the back-arc basin basalts (BABB). Overall, the Group 1 basaltic rocks surrounding the Yangtze Craton are likely formed in active continental margins. The Group 2 basaltic rocks formed during the early rift phase consist mainly of the tholeiitic and alkali basalts as well as dolerites around the Yangtze Craton margins, including the ca. 0.85 Ga Zhengzhushan bimodal volcanic rocks and the Shenwu dolerites in southeastern margin, ca. 0.83–0.82 Ga Yiyang komatiitic basalts, the Guangfeng alkaline basalts and the Yingyangguan spilites in southern margin of the Yangtze Craton, and ca. 0.82–0.80 Ga Suxiong alkaline basalts, Bikou and Tiechuanshan tholeiitic basalts along the western to northwestern margin of the Yangtze Craton. In the Cathaysia Block, the Mamianshan bimodal volcanic rocks (alkaline basalts and rhyolites) in northwestern Fujian are also dated at ca. 0.82 Ga. The majority of the ca. 0.83–0.82 Ga bimodal volcanic rocks occurred at the bottom of the volcano-sedimentary sequences in a number of mid-Neoproterozoic rift basins. The geochemical compositions of the Group 2 basaltic rocks are either similar to those of typical oceanic island basalts (OIB), or transitional between OIB and IAB due to various degrees of contamination of the lithospheric mantle and/or continental crust materials. The Group 3 basaltic rocks formed in the major rift phase include the ca. 0.79 Ga Shangsu basalts and Daolinshan dolerites in southeastern margin of the Yangtze Craton, the ca. 0.77 Ga spilites in northern Guangxi and dolerites in western Hunan and the numerous 0.78–0.75 Ga mafic dykes occurring in the Kangdian rift in western margin of the Yangtze Craton. All these basaltic rocks belong to tholeiitic series and alkaline series, and their geochemical features are similar to those of OIB. The calculated mantle potential temperatures (T_p) of the Group 1 basaltic rocks and the ca. 0.85 Ga Shenwu dolerites (Group 2) range from 1355 to 1420 ℃, which are consistent with those of the Neoproterozoic mid-oceanic-ridge basalt (MORB)-source mantle (ca. 1350–1450 ℃). On the contrary, the ca. 0.82 Ga Yiyang komatiitic basalts were generated by melting of an anomalously hot mantle source with T_p up to ca. 1618 ℃, ca. 260 ℃ higher than the MORB-source mantle, suggesting derivation from an abnormally hot mantle plume. The T_p for the other Group 2 and 3 basaltic rocks are ca. 25–140 ℃ higher than that of the MORB-sourced mantle, indicating various degrees of contribution from hot mantle plumes. The changes of rock types, geochemical compositions and the mantle potential temperatures are attributed to the regional tectonic transformation of the South China Block (SCB) from ca. 1.0–0.9 Ga Sibao Orogeny to ca. 0.85–0.75 Ga intracontinental rifting. Mantle plume/superplume activities play important roles in the mid-Neoproterozoic rift magmatism. Our interpretations of the geochemical results of basaltic rocks are consistent with other aspects of geological records. The Yangtze Cratob and Cathaysia block eventually amalgamated at ca. 0.9 Ga, forming a united South China continent that links Australia–East Antarctica and Laurentia in the Rodinia supercontinent. Termination of the Sibao Orogeny indicates that the Rodinia might have eventually assembled at ca. 0.9 Ga. The mid-Neoprterozoic (0.83–0.75 Ga) mantle plume/superplume activities resulted in the formation of widespread anorogenic magmatism and rift basins in South China.

Basic intrusions are commonly found within the Xiamaling Formation in an E–W–trending belt of more than 400 km along the north of the Yanliao Faulted-Depression Zone (YFDZ), North China Craton (NCC). And in the Jibei Depression in particular, up to four lit-par-lit intruded gabbro-diabase sills (named as βμ1–βμ4 in ascending order) can be observed with a cumulative thickness of 117.5–312.3 m. By contrast, no basic intrusion has be seen within the Xiamaling Formation along the south of the YFDZ, indicating that the northern YFDZ experienced a period of intensive intrusion of mantle-derived magma. The distribution and scale of intrusive masses indicate that the center of basaltic magma intrusion should be in the Jibei depression and that the activity of basaltic magma was controlled by the discordogenic faults on the northern boundary of YFDZ. The SIMS 207Pb/206Pb dating results of baddeleyite revealed that the emplacement ages of βμ1 and βμ3 gabbro-diabase sills are 1327.5 ± 2.4 Ma and 1327.3 ± 2.3 Ma (equivalent to Mesoproterozoic) respectively. Petrological and geochemical features show that the βμ1–βμ4 gabbro-diabase sills are characteristically similar to those of typical within-plate basalt (WPB). These results suggest that the Xiamaling gabbro-diabase sills in Jibei Depression may be related to rifting of the supercontinent Columbia. Comprehensive analysis of the distribution and age of the bentonitic tuff (1366–1372 Ma) and diabase dike (dike groups; ca. 1345 Ma) in the same region suggests that the ca. 1327 Ma gabbro-diabase sills in the YFDZ likely manifaest the rift development and separation of the NCC from Supercontinent Columbia.

Since 1960s, the significant research progress on the Proterozoic early lives and life diversity as well as on the Meso-Neoproterozoic shales and carbonate source rocks have established a material foundation for the studies of indigenous Meso-Neoproterozoic sedimentary organic matter and petroliferous nature, and providing the prerequisites for the prospectivity of indigenous petroleum resources. So far at least dozens oil and gas fields, some of considerables size, have been discovered in the Meso-Neoproterozoic strata, oil and/or gas of which were sourced from the Infracambrian source beds. Based on uncompleted global statistics, there are four countries, i.e., Lena-Tounguska Petroleum Province (LTPP) in Siberia Craton (Russia), Oman Basins in Arabian Craton (Oman), Baghewala Oilfield in Indian Craton (India), and Anyue Gasfield in western Yangtze Craton (China), in the world, containing proven geological reserves and/or commercial production of indigenous Infracambrian petroleum; nine regions/countries having been confirmed indigenous Meso-Neoproterozoic oil flow, oil-seep and/or asphalt, but so far no commercial production has been obtained yet; five regions/countries being revealed to possess hydrocarbon generation potential in the Infracambrian strata. China is one of the countries where the Meso-Neoproterozoic sequences are most completely developed and preserved, and Chinese Meso-Neoproterozoic geology was earlier studied in the word. However, both geological research and exploration of Meso-Neoproterozoic petroleum resources are faced with a challenging reality of more age-old strata, more complicated geological tectonics and more extensive scientific innovative possibility. Therefore, it is a considerable urgent question to study and evaluate the prospectivity of Meso-Neoproterozoic oil and gas resources in China. In this chapter, the distribution, exploration and development of Meso-Neoproterozoic oil and gas respectively in Russian LTPP, Sultanate of Oman, Pakistan and India basins, East European Craton, African Taoudenni Basin, Australian Centralian Basins, American Midcontinent Rift System as well as Chinese basins have been compiled, and their prospectivity approached.

As for the Mesoproterozoic sequence (1800–1320 Ma) in the Jibei Depression of Yanliao-Faulted-Depression-Zone (YFDZ), there are 2 sets of oil source beds, i.e., black micritic-dolostone in the Gaoyuzhuang Formation and black shale in the Hongshuizhuang Formation, as well as 3 sites of basal bituminous sandstone as fossil-oil-reservoirs in the Xiamaling Formation respectively at Longtangou, Shuangdong and Lujiazhuang. The bituminous sandstone appears as two phases of solid bitumen components (SBCs): the early SBC with 1.68–2.52% in bitumen reflectance R_b and the late one with 0.81–1.01%. Owing to the intrusion of gabbro-diabase sills into Xiamaling Formation, original oil-reservoir had been altered into bituminous sandstone, i.e., so-called fossil-oil-reservoir, whereas viscous oil-seeps are still found within wall-rock alteration zone which provide an evidence for late oil charging after the magma cooling and consolidation. The discovery of bituminous sands indicates an early oil charging and entrapment process at early diagenetic stage of the basal sandstone in Xiamaling Formation (ca. 1400 Ma), while its alteration age could be determined according to the emplacement age of gabbro-diabase sills in Xiamaling Formation (1327 Ma). Based on the analyses of stratigraphic sequence and thickness, it is defined that early oil charging and entrapment in the Xiamaling basal sandstone can only be sourced from the Gaoyuzhuang source bed, and it’s hydrocarbon-generating threshold depth would be around 3600 m. Based on this threshold depth, the oil entrapment age sourced by the Hongshuizhuang source bed should be at the Mesozoic period. Oil-source rock correlation results show that all the liquid oil-seeps found in the Wumishan and Tieling Formations and the soluble bitumen fraction of late SBC in the Xiamaling basal bituminous sandstone would be sourced by Hongshuizhuang source bed in Jibei Depression.

Based on the research results of petroleum geological exploration during the past half century, the Sinian natural gas prospectivity and exploration potential in the western Yangtze Craton (YC) are discussed by means of the analyses of lithofacies and palaeotectonics. The black shale of early Sianian Doushantuo Formation would be a qualified hydrocarbon source bed and the algal dolostone and shoal facies dolorudite of Late Sinian Dengying Formation are high-quality reservoir bed, which can constitute the oldest realistic source-reservoir assemblage in the western Yangtze Craton. Regionally, Dengying Formation can be divided into two subformations: the upper one includes Deng-3 and Deng-4 Members, Deng-3 Member contains bluish-gray shale or clastic rocks, its bottom is disconformably overlaid on the Deng-2 Member; while the lower one consists of Deng-1 and Deng-2 Members. Among which, the Deng-2 and Deng-4 Members are optimal reservoir beds. During the sedimentary period of Lower Cambrian Qiongzhusi (or called Jiulaodong) Formation, black shale was deposited, which could act as a perfect source bed and direct seal bed for the Sinian gas reservoirs. Since the wide-spread Sinian-Lower Cambrian sequence contains superior source beds, well preserved oil/gas reservoirs and successively developed palaeo-uplifts, the geological conditions for large-scale gasfield formation should be available in the western Yangtze Craton. Before or during Triassic, the Chuanzhong palaeo-uplift would be significant for oil entrapment and accumulation of original oil-reservoirs. Before Cenozoic, the gas-reservoir with abnormally high pressure was formed due to the influences of persistently deep-burying, geotemperature rising and the oil-cracking. The oil and gas were redistributed during the compressional uplifting course. Finally, the present gas reservoirs are formed. The key points for the future petroleum exploration of Dengying Formation should be focused on the Chuanzhong, Huayingshan and Tianjingshan palaeo-uplifts.

Anyue Gasfield is located in the western Yangtze Craton (YC), i.e., the current Sichuan Basin. It is a marine carbonate gasfield with the oldest stratigraphic age, the highest thermo-evolutional level and the largest individual gas reserve in China. By the end of 2016, its P1 gas geological reserve is up to 8488 × 10⁸ m³ and 3P reserve 1.2 × 10¹² m³ respectively in Sinian Dengying and Lower Cambrian Longwangmiao Formations. The emplacement of Sinian to Lower Cambrian gas reservoirs have been constrained by the giant successional Chuanzhong palaeo-uplift, especially at Moxi-Gaoshiti Anticlines, which have been always situated at a high position on the uplift and providing favorable geological conditions for the early entrapment of fossil-oil reservoirs and the late enrichment of cracking-gas reservoirs as well as for the formation of Anyue Gasfield. The Deyang-Ziyang Faulted-Sag is just between the Weiyuan Anticline and the Moxi-Gaoshiti Anticlines, where the fine-grained sediments of Lower Cambrian Qiongzhusi (or called Jiulaodong) Formation was filled, and controlling the occurrence of Qiongzhusi black shale/mudstone as the main hydrocarbon source kitchen, while the development of Dengying algal dolostone and Longwangmiao grain-dolostone at both sides of the faulted-sag as the high-quality reservoir beds. There are two sets of main hydrocarbon source beds, i.e., Lower Cambrian Qiongzhusi black-greyish black shale/mudstone and Doushantou black shale in the western YC. However, the Qiongzhusi shale/mudstone would be the effective source bed for both Dengying and Longwangmiao cracking-gas reservoirs, while the Doushantou black shale would be a potential one in Chuanzhong Uplift. In addition, three sets of reservoir beds are developed in Anyue Gasfield, i.e., two algal dolostone reservoir beds of Deng-2 and Deng-4 Members respectively with reservoir bed thickness 5.1–69.1 m and 60–110 m, and one grain-dolostone reservoir bed of Longwangmiao Formation with reservoir bed thickness 10.8–61.1 m. Three types of gas reservoirs have been found in Anyue Gasfield, including the structural-lithologic type of Longwangmiao gas reservoir, the structural-stratigraphic type of Deng-4 gas reservoir as well as the structural type of Deng-2 gas reservoir, all of which can be attributed to medium–low sulfur-contented and medium CO₂-contented dry gas reservoir. Moreover, the Longwangmiao gas reservoir is also a deeply buried reservoir with high temperature and high pressure, while the Deng-2 and Deng-4 gas reservoirs are ultra-deeply buried reservoirs with high temperature and normal pressure.

The Longmenshan Fronthill Belt is the most front anticlinal tectonic belt in the Longmenshan Nappe Zone, it is geohistorically in a low thermo-evolutional region due to a long-term crust uplifting state, where there are numerous asphaltic veins with different scales, the remarkable large-scale ones are directly sourced from the original dolostone oil-reservoir in the Late Sinian Dengying Formation, while the hydrocarbon composition of asphaltic veins is well correlated with that of the early Sinian Doushantuo Formation black shale, both of which appear as unusually predominance of relative abundance of C₂₁–C₂₂ pregnane and homopregnane, C₂₉ 30-norhopane, 13α(n-alkyl)-tricyclic terpanes and C₂₄ tetracyclic terpane biomarkers. Therefore, Doushantuo black shale should be the source bed for these asphaltic veins. The geological occurrence of Erchangliang large asphaltic veins reveal three geological conditions for its genetic mechanism: ① there is an excess of liquid oil as the original material for the asphaltic veins, ② a fault and fracture system resulted from the nappe structure would provide the passageway and reservoir space, ③ an episodic hydraufracturing system driven by abnormal high-pressure in a very short duration. Which is proved by the tectonic stress analysis of Kuangshanliang Anticline and by the investigation of Changjianggou-Kuangshanliang-Xianshuigou Nappe-Overthrust, and there is a genetic relationship between the asphaltic veins and the dual-layer thin-skinned structure of Longmenshan Nappe Zone as the results of the Middle-Late Triassic Indosinian Movement. As a special oil accumulation zone, undoubtedly the nappe zone of Indosinian epoch suffered a larger destruction on the shallow-layer local structures as well as on its inside oil and gas reservoirs, but the deep-layer local structures and associated oil- and gas-reservoirs might still be well preserved, providing geological and geochemical foundation for the exploration of Neoproterozoic indigenous oil and gas reservoirs in the Longmenshan Fronthill Belt.