Sinian Doushantuo Formation Research Articles

Provenance and Tectonic Setting Discrimination and Their Effects on the Pore-Scale Fluid Flow of Black Shale: Take the Micangshan Tectonic Belt, Central China, as an Example

The Lower Paleozoic reservoir in the Micangshan tectonic belt is a new shale gas exploration area with excellent potential. However, the black shale’s sedimentary environment and tectonic setting and their effects on pore-scale fluid flow have not been thoroughly elucidated and urgently require further research. A large amount of trace and rare earth element data of black shale in this area was tested to analyze the sediment’s provenance, tectonic setting, and coupling process, and nuclear magnetic resonance was applied for fluid flow ability determination. The characteristics of trace elements and rare earth elements in shale samples were drilled from the Upper Sinian Doushantuo Formation, the Lower Cambrian Niutitang Formation, and the Lower Silurian Longmaxi Formation in the Micangshan tectonic belt. The results reveal that the provenance of the shales was dominated by island arc volcanic rocks and some ancient fine-grained sediments. The provenances of shale in the Micangshan, Huijunba, and Zhenba areas may have a strong genetic relationship with the igneous rocks from the nearest Hannan massif. The provenance of the Niutitang shale in the Ningqiang area may comprise sediments derived from the Longmenshan continental rift system. The provenance of the shale tends to transition from the island arc to the continental margin from the depositional stage of the Doushantuo Formation to the Longmaxi Formation. The Micangshan tectonic belt inherited the tectonic evolution of the South China block, North China block, and Qinling, and those blocks provided the igneous provenance of the shale by island arcs, which were formed due to weathering caused by the process of subduction and collision orogeny. The movable fluid distribution of the Longmaxi Formation and Niutitang Formation is relatively uniform, and excessive pursuit of large amount of movable fluid will be counterproductive to the shale reservoir. The proportion of the cracks was generally less than 3% except for one sample from the Doushantuo Formation with 12.06%. Based on the centrifugation force in this research, the low fluid flow limits were less than 1.47, and there are strong positive relationships between movable fluid saturation and saturated porosity. In this study, the coupling relationship between the material source and pore scale flow is discussed for the first time, and the results can provide theoretical references for local provenance analysis and tectonic movement.

Studies on geochemical characteristics and biomineralization of Cambrian phosphorites, Zhijin, Guizhou Province, China.

Phosphate rocks, an important ore resource in Guizhou Province, China, are mainly hosted within the Sinian Doushantuo Formation and the Cambrian Meishucun Formation. In addition, the phosphate rocks of the Cambrian Meishucun Formation are rich in biological fossils. Although numerous studies investigating the genesis of phosphate deposits have been performed, the relationship between biological activity and the formation of phosphate deposits in the lower Cambrian Meishucun Formation has not been convincingly explained. This study focuses on the biological fossil assemblage, the characteristics of phosphorus, and the relationship between biological and phosphorus enrichment of the lower Cambrian phosphorites. The primary objectives of our study are to analyze the role of organisms in the formation of phosphorites, restore the phosphorus-formation environment of the Cambrian Meishucun Formation, and construct a sedimentary model of the phosphorites in the Meishucun Formation. The results indicate that there is a significant positive correlation between biological activity and the deposition of phosphorites, that is, the higher the degree of biological enrichment and differentiation, the stronger the deposition. The geochemical analysis of several profiles in the Zhijin phosphorite block shows that the phosphorite block was deposited in an oxygen-rich environment and was affected by a high-temperature hydrothermal fluid. Upwelling ocean currents supplied abundant phosphorus and other nutrients, which provided the conditions for small shells and algae to flourish. Biochemical activity was a crucial factor in the deposition of the phosphorite.

Hydrocarbon accumulation conditions and exploration targets of the Sinian Dengying Formation in the Southeastern Sichuan Basin

No breakthrough has been made in petroleum exploration of the Sinian Dengying Formation in the southeastern Sichuan Basin as the major factors controlling the hydrocarbon accumulation of the Dengying Formation have not been well established, including the development period and distribution of the platform margin, the effectiveness of the source-reservoir combination and the reliability of the Cambrian pre-salt structural trap. Based on the interpretation of seismic data and hydrocarbon-source correlation, the distribution of the platform margin of the Sinian Dengying Formation in the southeastern Sichuan Basin was mapped. Moreover, the gas source rocks of the Dengying Formation were clarified according to geochemical characteristics of fluid inclusions, and the pre-salt structures were further confirmed by seismic forward modeling and experiments of prestack depth migration. Results showed that the sedimentary facies of the 4th member of Dengying Formation in the southeastern Sichuan Basin was platform margin of gentle slope type. The platform margin reservoir of the Dengying Formation and the Lower Cambrian source rocks constitute a favorable source - reservoir combination. The carbon isotope characteristics of gas in fluid inclusions indicate that some of the oil/gas in the Dengying Formation were contributed by the Doushantuo Formation source rocks. As the difference in seismic velocities between carbonate rocks and gypsum/salt beds may cause a reduction in the amplitude of pre-salt structures, the low-amplitude structures identified on current seismic profiles are underestimated instead of overestimated. Accordingly, the Sinian structural traps are confirmed to develop in the deep pre-salt sequences in the low-steep anticline belt, southeastern Sichuan Basin. On the basis of the above new understanding, we propose two types of hydrocarbon migration modes for the Sinian Dengying Formation in southeastern Sichuan Basin, including lateral migration of hydrocarbons generated from the Cambrian Qiongzusi Formation and vertical migration of hydrocarbons derived from the Sinian Doushantuo Formation. Lithologic traps at the platform margin and pre-salt structural traps within the platform are favorable targets for natural gas exploration.

Characteristics and significance of the solid bitumen in the second member of Dengying Formation of Sinian system in the north of central Sichuan Basin, China

With ongoing advancements in natural gas exploration, the second member of the Sinian Dengying Formation (Deng 2 Member) has emerged as a crucial region for providing natural gas reserves in the north of the central Sichuan Basin. The Deng 2 Member has a significant volume of reservoir solid bitumen. Geochemical characteristics and development mechanism of the solid bitumen were determined through measurements and analyses of the Deng 2 Member samples collected from primary exploration wells by using optical microscopy, SEM, gas chromatograph-mass spectrometer, and laser Raman spectrometer. The results show that the Deng 2 Member has a generally high content of solid bitumen, ranging from 2.96% to 5.13% on average in single wells. The solid bitumen mainly occurs as the fillings of dissolved pores (caves) and fractures, followed by intergranular pores, in the shape of spots, balls, plates, and veins dominantly. Diasteranes content and laser Raman spectrograms indicate that the solid bitumen is in the high maturity stage. The bitumen reflectance calculated by laser Raman spectroscopy is distributed between 2.49% and 4.09%, indicating the major source of the thermal cracking of crude oil. Solid bitumen in the Deng 2 Member has different contents of 21α(H)–C29 norhopane, C35 hopane, and C34 hopane, and Ts/Tm values from the Deng 4 Member in the Gaoshiti Moxi area. It is inferred that some solid bitumen is from the source rocks of the Lower Cambrian Maidiping Formation and the Sinian Doushantuo Formation. Two stages of bitumen were developed in the Deng 2 Member, indicating possible two stages of oil filling and thermochemical sulfate reduction (TSR) in the geologic history.

Characterization of marine shale in Western Hubei Province based on unmanned aerial vehicle oblique photographic data

The marine shale in the Sinian Doushantuo Formation of Qinglinkou outcrop section is well developed, but the current characterization methods for outcrops are unsatisfactory. In this paper, the data of outcrop in the field study area were collected by Unmanned Aerial Vehicle, then processed and interpreted by oblique photography technology combined with manual investigation. Subsequently, we established a quantitative geological knowledge database of the shale formations and carried out the typical section of anatomy analysis. The results showed that the high-precision image information captured by unmanned aerial vehicle oblique photography technology can be well coupled with a three-dimensional coordinate system. The three-dimensional digital model was used to characterize the lithologic assemblage, thickness and distribution characteristics of the target reservoir. Based on this digital model, we established the three-dimensional lithology and the total organic carbon models of the outcrop area. The spatial distribution characteristics of interbedding between marine dolomite and shale in the outcrop area were displayed, and the distribution of total organic carbon was revealed under lithological constraints. The models are beneficial for the analysis and prediction of the lithology and total organic carbon, which is of great significance to the understanding of shale gas sweet spots. Cited as: Yin, S., Feng, K., Nie, X., Chen, Q., Liu, Y., Wang, P. Characterization of marine shale in Western Hubei Province based on unmanned aerial vehicle oblique photographic data. Advances in Geo-Energy Research, 2022, 6(3): 252-263. https://doi.org/10.46690/ager.2022.03.08

Fields and directions for shale gas exploration in China

China is highly dependent on foreign oil and gas, and the exploration and exploitation of shale gas is an important way to alleviate the energy and environmental pressure of China. Although a series of significant progress has been made in a short time, China is still at the crossroads of shale gas exploration direction. Therefore, this paper systematically reviews the particularity of shale gas geological conditions in China, and points out China's exploration direction of shale gas in different fields combined with the research achievements of shale gas accumulation mechanism. The following results were obtained. (1) China's shale gas exploration fields can be divided into three major parts, i.e., marine, continental and marine–continental transitional facies. The geological characteristics of shale gas in North China, Yangtze and Tarim plates are different. The interaction of multiple factors, including tectonic and sedimentary evolution, shale gas accumulation, late-stage transformation and etc., has led the complexity of the geological distribution and the diversity of exploration fields of shale gas in China. (2) The organic pores and fractures of marine shale are developed. The Ordovician Wufeng-Silurian Longmaxi Formation will still be the primary target of shale gas exploration in future. Meanwhile, the Cambrian Niutitang and Sinian Doushantuo Formation in the Yangtze region, the Devonian Luofu and Carboniferous Jiusi Formation in the Yunnan-Guizhou-Guangxi region, the Jixianian Hongshuizhuang and Mesoproterozoic Xiamaling Formation will be the targets of strategic breakthrough in shale gas exploration. (3) The structural deep and slope of middle-large basins and the sedimentation centers of small-middle basins are the main directions of continental shale gas exploration. Inter-laminar cracks and intergranular pores are well developed in continental shale. The coupling of various organic matter types and thermal evolution results in the symbiosis of shale oil and shale gas, which will become an important field for further exploration of continental shale gas. (4) The marine–continental transitional shale, which mainly formed in Late Carboniferous-Permian, is characterized by cyclic association with sandstone, mudstone, coal and carbonate. Diagenetic fractures are well developed in transitional shale. The favorable structural and diagenetic preservation are the two major factors for the enrichment of this type of shale gas. The flat-lagoon and delta sedimentary systems, middle-large superimposed and small-middle residual basins, and deep layers with quality caprock and good structural preservation are the favorable directions for shale gas exploration in transitional facies.

The geochemical characteristics, distribution of marine source rocks and gas exploration potential in the northwestern Sichuan Basin, China

In recent years, the northwestern part of the Sichuan Basin has become a popular location for natural gas exploration. However, up to this point, the geochemical characteristics and distribution of marine source rocks in this area have not been thoroughly investigated. In the northwestern Sichuan Basin, there are eight sets of marine source rock, including the Lower Sinian Doushantuo Formation, the third Member of Upper Sinian Dengying Formation, the Lower Cambrian Qiongzhusi Formation, the Lower Silurian Longmaxi Formation, the Middle Devonian, the Middle Permian Maokou Formation, the Upper Permian Longtan Formation/Wujiaping Formation, and the Upper Permian Dalong Formation, from which eight outcrop profiles were selected for systematic organic geochemical analysis. The Qiongzhusi Formation has the thickest high-quality source rocks, up to 80 m thick, followed by the Upper Permian Longtan/Wujiaping and Dalong formations, which have up to 20–30 m thick high-quality source rocks, whereas the thickness in other layers is relatively thin. By the abundance of organic matter, the Doushantuo, Qiongzhusi, Longmaxi, Longtan/Wujiaping, and Dalong formations developed high-quality source rocks with total organic carbon content (TOC) greater than 3%, while the third Member of Upper Sinian Dengying Formation, Middle Devonian and Maokou formations had low TOC. The kerogen of source rocks gradually enriched 13C from the type Ⅰ organic matter of the Lower Cambrian Qiongzhusi Formation to the Upper Permian Dalong Formation, and the δ13C values of Doushantuo Formation, the third Member of Dengying Formation, and Qiongzhusi Formation are less than −30.0‰. The δ13C values of Longtan Formation or Wujiaping Formation and Dalong Formation with type Ⅱ2 organic matter ranges from −28.0‰ and −26.0‰. Others lie somewhere in between type I and type II2. Except for Permian source rocks in Guangyuan and Chaotian areas, which are in the mature stage, source rocks in the northwestern Sichuan Basin are in the over-mature stage and largely in the thermal stage of dry gas. High abundance source rocks have provided rich marine natural gas resources. As a result, future gas exploration in the northwestern Sichuan Basin will focus on the gas-bearing system associated with the Qiongzhusi Formation and Upper Permian source rocks.

Paleo-Temperature and -Pressure Characteristics of Fluid Inclusions in Composite Veins of the Doushantuo Shale (Yichang Area, South China): Implications for the Preservation and Enrichment of Shale Gas

The Sinian Doushantuo Formation has been identified as a promising shale gas reservoir in South China and potentially worldwide. This study aims to restore the paleo-temperature and paleo-pressure ...

Porosity characteristics of different lithofacies in marine shale: A case study of Neoproterozoic Sinian Doushantuo formation in Yichang area, China

This study evaluates the pore network systems in the Neoproterozoic Sinian Doushantuo (Z1d) shale from the Yichang area in the Middle Yangtze region. All samples were classified based on their lithologies and examined by geochemical and petrographic analysis, assisted with carbon dioxide/nitrogen (CO2/N2) adsorption, mercury injection capillary pressure (MICP) and focused ion beam-scanning electron microscopy (FIB-SEM) to fully characterize the nanoscale porosity of the different shale lithofacies. The calcareous shale lithofacies group (I-C), mixed shale lithofacies group (II-M) and siliceous shale lithofacies group (III-S) were identified via ternary mineralogy diagrams. Clearly, different lithofacies had different porosities, pore volume, surface area and pore-size distribution (ranging from 0.3 nm to 36 μm). The mixed shale lithofacies group (II-M) had the highest total pore volumes, followed by the calcareous shale lithofacies group (I-C) and the siliceous shale lithofacies group (III-S). Peak values (2–20 nm, 20–50 nm, 50–60 nm and 90–200 nm) obtained from N2 adsorption were identified as organic matter (OM) pores, dissolution intraparticle (intraP) pores and intercrystalline pores (using SEM images analysis). Porosity shows a non-monotonic trend with TOC content and a maximum at 4.5–5.5 wt% TOC, similar relationships between TOC and mesopore/macropore volumes, due to pore collapse and compaction. The FIB-SEM images revealed solid bitumen and two types of kerogen in the examined shale samples; these different organic matter types had clearly distinct pore characteristics. While the smaller OM particles mobilized and converted into solid bitumen in which OM pores were larger and more abundant, the large-scale OM particles were most likely non-porous inert maceral and sponge-like porous amorphous kerogen. Overall TOC content and organic types are the key controlling factors to the nanoscale porosity development in the Doushantuo Shale, and the Sinian Doushantuo shale (in the Yichang area) has great potential for shale gas exploration and exploitation in South China.

Lithofacies paleogeography and exploration significance of Sinian Doushantuo depositional stage in the middle-upper Yangtze region, Sichuan Basin, SW China

Abstract In recent years, natural gas exploration in the Sinian Dengying Formation and shale gas exploration in Doushantuo Formation have made major breakthroughs in the Sichuan Basin and its adjacent areas. However, the sedimentary background of the Doushantuo Formation hasn't been studied systematically. The lithofacies paleogeographic pattern, sedimentary environment, sedimentary evolution and distribution of source rocks during the depositional stage of Doushantuo Formation were systematically analyzed by using a large amount of outcrop data, and a small amount of drilling and seismic data. (1) The sedimentary sequence and stratigraphic distribution of the Sinian Doushantuo Formation in the middle-upper Yangtze region were controlled by paleouplifts and marginal sags. The Doushantuo Formation in the paleouplift region was overlayed with thin thickness, including shore facies, mixed continental shelf facies and atypical carbonate platform facies. The marginal sag had complete strata and large thickness, and developed deep water shelf facies and restricted basin facies. (2) The Doushantuo Formation is divided into four members from bottom to top, and the sedimentary sequence is a complete sedimentary cycle of transgression–high position–regression. The first member is atypical carbonate gentle slope deposit in the early stage of the transgression, the second member is shore-mixed shelf deposit in the extensive transgression period, and the third member is atypical restricted–open sea platform deposit of the high position of the transgression. (3) The second member has organic-rich black shale developed with stable distribution and large thickness, which is an important source rock interval and major shale gas interval. The third member is characterized by microbial carbonate rock and has good storage conditions which is conducive to the accumulation of natural gas, phosphate and other mineral resources, so it is a new area worthy of attention. The Qinling trough and western Hubei trough are favorable areas for exploration of natural gas (including shale gas) and mineral resources such as phosphate and manganese ore.

Evaluation of shale gas resources of the Sinian Doushantuo Formation in the southern Huangling anticline, western Hubei Province

Evaluation of shale gas resources of the Sinian Doushantuo Formation in the southern Huangling anticline, western Hubei Province

Sedimentary lithofacies characteristics and sweet-spot interval characterization of the Sinian Doushantuo Formation in Upper Yangtze Platform, South China

ABSTRACT The Sinian Doushantuo Formation is the oldest shale gas reservoir discovered in the world, which contains good shale gas shows as the Eyangye-1 and Zidi-1 wells in the Yichang Region of western Hubei province, China. The shales in the Doushantuo Formation feature considerable thickness (135 m), high gas content (4.83 m3/t) and high fragile mineral content (up to 75%). Due to the influences of the sedimentary environment, the shale reservoirs here have high dolomitic content (54%) and abruptly changes in vertical lithofacies. Moreover, the characteristics of the shale sweet-spot differ significantly from that in the Wufeng Formation and Longmaxi Formation in southeast Sichuan Basin. A high-resolution sequence stratigraphic correlation was performed on the shales of the Doushantuo Formation at the Eyangye-1 well, Zidi-1 well, Zidi-2 well and some outcrop profiles in Yichang area for the identification of their sedimentary microfacies characteristics. A comprehensive comparative analysis was made by incorporating the elemental geochemistry, wireline and mud logging data of the well to further identify the sweet-spot interval therein. With the analysis of Eyangye-1 well, the sweet-spot interval of the Doushantuo Formation is 3360−3408 m, of which sedimentary microfacies is deepwater reduced environment with many pyrite laminaes, and the TOC is high to 3.42%, the Ro is 3.3%, the organic and matrix pore are well developed and the aperture more than 50 nm, the porosity is 2.7%, the desorption gas content average is 2.16%, and the main mineral is dolomitic (54%) and siliceous quartz (21%). Additionally, these parameters combined with previous studies will shed light on evaluating and characterizing the layers therein, also provide referential geological data for the following exploration and development activities of this shale system.

The evolution of the Huangling uplift and its control on the accumulation and preservation of shale gas

ABSTRACT There are plenty of Sinian and Cambrian potential shale gas resources in South China, which is characterized by high thermal evolution degrees, poor drilling performances and only occurs in local areas. Taking the principle “high to find low” is the key issue to achieving a breakthrough in older shale. China Geological Survey drilled in the periphery of the Proterozoic basement, i.e. the Huangling anticline, in the western Hubei, and Hannan paleocontinent in the southern Shanxi. It received high-quality gas-bearing shale with relatively low Ro in the in Lower Cambrian Niutitang formation and Sinian Doushantuo formation. Based on geological conditions of shale gas reservoirs in the Huangling anticline, this paper puts forward the new model named “Control over reservoirs by periphery of basement” about shale gas accumulation, suggesting that the shale deposited in a deepwater continental shelf in the periphery of the basement is characterized by shallow burial, a short burial time, stable tectonics, relatively low thermal evolution degrees, and shale gas reservoirs in a good condition. The shale of the Sinian-Cambrian strata deposited in deepwater continental shelves in the periphery of Chuanzhong paleo-uplift in Sichuan, Hannan paleocontinent in the southern Shanxi, Huangling anticline in western Hubei and Jiangnang-Xuefeng paleo-uplift in Hunan and Guizhou province have good shale gas exploration potential.

The source of Fengjia and Langxi barite–fluorite deposits in southeastern Sichuan, China: evidence from rare earth elements and S, Sr, and Sm–Nd isotopic data

The barite–fluorite metallogenic belt in southeastern Sichuan is one of the most important barite–fluorite ore‐concentrated areas in China. In order to find the ore‐forming source and the mineralization age of the Fengjia and Langxi barite–fluorite deposit, we systematically evaluated the rare earth element (REE) contents and isotopic characteristics of S, Sr, and Sm–Nd. The results show that the δ34S values of barites and pyrites are high and similar to S isotopic values of gypsum in the Cambrian Qingxudong Formation (∈1q), suggesting that the sulphur source was from Cambrian evaporite strata. The initial 87Sr/86Sr ratios of fluorites and barites range from 0.708800 to 0.712999, and these values are similar to Cambrian carbonate rocks and Lower Ordovician limestone. Except for black shales of the Lower Cambrian Niutitang Formation, the Ba content of other strata was generally low. The REE characteristics of barite and fluorite have close relationships, in regard to the source of ore‐forming materials, to the Lower Cambrian Niutitang Formation black shales. Hence, we have concluded that the Cambrian carbonate rocks and Lower Ordovician limestone provide the source of calcium for mineralization and that the source of barium comes from the Lower Cambrian Niutitang Formation black shales. The fluorine contents of the Upper Sinian Doushantuo Formation and the Lower Cambrian Mingxinshi Formation are much higher than other strata, and these formations could provide the source for fluorine. The Sm–Nd isochron age of fluorites is 104 ± 11 Ma, which demonstrates that the mineralization mainly occurred during the late Yanshanian. Copyright © 2016 John Wiley & Sons, Ltd.

Shale Gas in China: Reality and Dream

China extracted shale gas of 23, 500∼24, 600 m3/d from the Lower Cambrian Qiongzhusi Shale Formation in Weiyuan gas field of Sichuan Basin in 1966. This shale interval, however, was considered less prospective because the Sinian Doushantuo Formation dolomite reservoir was defined as the target layer for drilling at that time. Forty years later, this black shale interval became attractive as triggered by the “Shale Gas Evolution” in North America. In 2005, by learning from shale gas mode of the United States, PetroChina Research Institute of Petroleum Exploration and Development (RIPED) reviewed the existing well data of the region where the Weiyuan and Longchang gas fields and the Changning structure were located within the Southern Sichuan Basin. In order to re-examine the gas shows and geochemical property and to evaluate the gas potential of shale strata within the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation and the Lower Cambrian Qiongzhusi Formation. On that basis, for the entire Sichuan Basin and adjoining regions, PetroChina has carried out a geologic survey on organic-rich shales, drilled shallow shale gas appraisal wells, evaluated the shale gas resource potential, and defined some favorable targets for shale gas exploration and development by 2008. W201, the first shale gas appraisal well of China, drilled by PetroChina in 2009, initially produced 17, 000 m3/day from the Wufeng-Longmaxi Formation shale and 10, 800 m3/day from the Qiongzhusi Formation shale, marking the commencement of shale gas exploration and development activities in China. Thus far, China has received shale gas flow from marine, transitional-limnetic coaly and lacustrine shales in many regions, such as Sichuan, Chongqing, Guizhou, etc. The commercialization of marine shale gas has been achieved in Sichuan and Chongqing totaling 0.2 billion cubic meters of shale gas production in 2013. Through an overall review on the progress of shale gas exploration and development in China, we concluded that the geological conditions for shale gas in China are extremely complex. First, the most sedimentary basins in China belong to superimposed basins, all of them have experienced the depositional evolution from marine to terrestrial facies, and three types of shales were well developed including marine, transitional, and terrestrial organic-rich shales. Second, the conditions of shale gas formation and accumulation in China are favorable and shale gas resource has a great potential for further development in China. Third, like North America, the marine organic-rich shales of China have the best and most realistic shale gas exploration and development prospects. But it faces technical, environmental, regulatory, infrastructural and other challenges that are likely to delay rapid shale gas development. Fourth, until now, although the exploration and exploitation of shale gas in China are relatively low, very good signs have been emerged to prove that China has good prospects in the future.

Geology, resource potentials, and properties of emerging and potential China shale gas and shale oil plays

This paper describes the geology of organic-rich shales in China, their resource potentials, and properties of emerging and potential China shale gas and shale oil plays. Marine, lacustrine, and coastal swamp transitional shales were estimated to have the largest technically recoverable shale gas resource (25.08 trillion cubic meters or 886 trillion cubic feet) and 25 to 50 billion barrels of technically recoverable shale oil resource. The Precambrian Sinian Doushantuo Formation to Silurian Longmaxi black marine shales mainly accumulated in the intrashelf low to slope environments in the Yangtze Platform in South China and in the Tarim Platform in northwest China. The marine shales in the Yangtze Platform have high maturity (Ro of 1.3%–5%), high total organic carbon (mainly [Formula: see text]), high brittle-mineral content, and have been identified as emerging shale gas plays. The Lower Paleozoic marine shales in the Upper Yangtze area have the largest shale gas potential and currently top the list as exploration targets. The Carboniferous to Permian shales associated with coal and sandstones were mainly formed in transitional depositional settings in north China, northwest China, and the Yangtze Platform in south China. These transitional shales are generally rich in clay with a medium level of shale gas potential. The Middle Permian to Cenozoic organic-rich lacustrine shales interbedded with thin sandstone and carbonate beds are sporadically distributed in rifted basins across China. Their main potentials are as hybrid plays (tight and shale oil). China shales are heterogeneous across time and space, and high-quality shale reservoirs are usually positioned within transgressive systems tract to early highstand systems tract intervals that were deposited in an anoxic depositional setting. For China’s shale plays, tectonic movements have affected and disrupted the early oil and gas accumulation, making tectonically stable areas more favorable prospects for the exploration and development of shale plays.

Accumulation conditions and exploration directions of natural gas in deep subsalt Sinian-Cambrian System in the eastern Sichuan Basin, SW China

Abstract Based on comprehensive analysis of outcrops and drilling data, and interpretation of more than 6×10 4 km 2 seismic data, this paper delineates the regional distribution of the gypsum-salt layers in Middle-Lower Cambrian in eastern Sichuan Basin, and discusses accumulation conditions for natural gas in deep subsalt Sinian-Cambrian System in eastern Sichuan Basin, and points out favorable exploration directions. There are two sets of regional gypsum-salt layers in the Middle-Lower Cambrian in eastern Sichuan Basin, which are regional detachment layers, with fault-related folds occurring above and broad anticlines occurring below. There are 6-7 rows of NNE and NE trending large traps developed under the salt. Good space configuration of source rocks, reservoirs and cap rocks provide favorable accumulation conditions for natural gas in the deep subsalt in this region: three sets of hydrocarbon source rocks including Sinian Doushantuo Formation, the third Member of Sinian Dengying Formation, and dominant Cambrian Shuijingtuo Formation; three sets of weathering crust karst reservoirs and grain beach reservoirs including the second and fourth Members of Sinian Dengying Formation, and Cambrian Shilongdong Formation; and the regional cap of gypsum-salt in the Middle-Lower Cambrian and shale in Shuijingtuo Formation. The top target in deep subsalt exploration in eastern Sichuan Basin should be the fourth Member of Dengying Formation, followed by Shilongdong Formation and the second Member of Dengying Formation, and areas on migration direction with appropriate burial depth and being far away from the edge of the basin should be the first choices in exploration. Therefore, the favorable hydrocarbon exploration areas in the near future are the Honghuadian-North Huaying Mountain, Liangshuijing-Pubao Mountain-Leiyinpu, and Datianchi structural belts.

Microborings in Early Cambrian phosphatic and phosphatized fossils

A small proportion of phosphatic and phosphatized fossils, mostly microfossils, extracted from Lower Cambrian (Qiongzhusian) lime mudstones of deeper water aspect, belonging to the Yu'anshan and Shuijingtuo formations of South China, is perforated by microborings. These microborings are present in bradoriid and phosphatocopine arthropod carapaces, lingulid brachiopod valves, Microdictyon plates and tommotiid sclerites. Their absence in co-occurring phosphatized eodiscid trilobite exoskeletons and palaeoscolecid worm trunk fragments indicates a substrate specificity that suggests the boring microorganism was heterotrophic, extracting particular residual organic compounds present in only certain bioclast types. Microborings occur as meandering tunnels and grooves on bioclast surfaces, and most are 10–20 µm wide and circular in cross-section. The euendoliths were thus able to live partly exposed as well as within the substrate. Tunnel and groove walls bear a distinct longitudinal sculpture of straight striae separated by cuspate ridges 1–2 µm apart that parallel the microboring, never twisting. The microborings accord with the ichnotaxon Microptychoites fuquanensis Dong in Dong et al. (Dong, Y., Ji, Q., Wang, S., 1984. The discovery of microscopic trace fossils in the Sinian Doushantuo Formation in southwestern China. Scientia Geologica Sinica 346–347 [in Chinese]). The fluted sculpture appears to have no modern analogue, but may have arisen by localized secretions at the tip of a bundle of filaments. This kind of microboring had been previously reported from phosphatic substrates of Ediacaran, earliest Cambrian, Middle Cambrian and probably Early Devonian age, but appears to have no Recent counterpart.

High δ 13C Carbonates from the Songpan-Garzê Orogenic Belt: Implications for Correlation of Neoproterozoic Carbon Isotope Anomalies Across the Yangtze Platform, China

A detailed stable isotopic profile measured systematically across a thick Upper Neoproterozoic (Sinian) carbonate succession at Danba, western Yangtze Platform, China, records an overall positive δ 13C Carb excursion. Even having experienced amphibolite facies metamorphism, most of carbonates in the interior of the marble layer preserve pre-metamorphic δ 13C and δ 18O values, which vary with stratigraphic level. The marbles can be divided into lower and upper units. The lower and upper units have distinctive isotopic composition ranges, δ 13C Carb=∼+5 to +7.3% and δ 18O Carb=∼20 to 25%, and δ 13C Carb=∼+1.5 to +3% and δ 18O Carb=∼28 to 30%, respectively. This pattern of variation, especially in δ 13C, is similar to that reported for the unmetamorphosed upper Sinian Doushantuo and Dengying Formations in the eastern Yangtze Platform. This suggests that the positive Neoproterozoic δ 13C Carb excursion can be correlated on a basinal scale within the Yangtze Platform. The data presented here establish that high δ 13C values of carbonates from the upper Sinian Doushantuo Formation are a regional feature across the Yangtze Platform.

New microfossils from the neoproterozoic (Sinian) Doushantuo Formation, Wengan, Guizhou Province, southwestern China

Well-preserved, three-dimensional microfossils are reported from a 3.7 m thick black phosphorite bed in the Sinian Doushantuo Formation at Wengan Phosphate Mine, Guizhou Province, southwest China. The diverse assemblage, observed in thin sections, comprises planktonic acritarchs, benthic cyanobacteria, bacteria, and thallophytes. Twenty one taxa are recognized, including 1 new genus and 4 new species. Acritarchs include Baltisphaeridium rigidum sp. nov., Vulcanisphaera phacelosa sp. nov., Comasphaeridium magnum Zhang 1984, Cymatiosphaeroides yinii sp. nov., Dicrospinasphaera zhangii gen. et sp. nov., Echinosphaeridium maximum (Yin, 1987) Knoll, 1992, Ericiasphaera sp., ?Ericiasphaera sp., Hocosphaeridium scaberfacium Zang & Walter, 1992, Meghystrichosphaeridium sp., Meghystrichosphaeridium wenganensis Chen & Liu, 1986, Polyhedrosphaeridium sp. The cyanobacteria and bacteria comprise Paratetraphycus giganteus Zhang, 1984, Gloeodiniopsis lamellosa (Schopf, 1968) Knoll & Golubic, 1979, Globophycus sp., Myxococcoides sp., type A, Siphonophycus rugosum (Maithy, 1975) Hofmann & Jackson, 1994, Siphonophycus robustum (Schopf, 1968) Knoll, Swett & Mark, 1991, Gunflintia sp., and a rod-like bacterium (type B). At least 6 distinctive acritarch taxa from the black phosphorite bed are also represented in the Pertatataka assemblage reported by Zang & Walter (1992) in the Amadeus Basin of central Australia, suggesting a biostratigraphic correlation with the Pertatataka Formation, which underlies strata containing Ediacaran metazoans and overlies Varangerage glacial sediments. Abundant bacteria-like coccoids and filaments preserved in the vesicles of many acritarch specimens attest to the presence of a thriving microbiota of decomposers within dead acritarchs, and a specimen of Thallophyca ramosa Zhang 1989 has well-preserved reproductive structures that comparable with those of the modern rhodophyte Bangia and the phaeophyte Fucus.