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An estimated trillions of cubic meters of natural gas have recently been discovered in the Sinian dolomite reservoirs in the Sichuan Basin, China. The 20–26 m thick reservoirs were found in deep structural–lithologic traps of 5000–5500 m deep. The Sinian gas is thought to have been derived mainly from the Sinian source rocks, and partly from the Cambrian source rocks. The gas is thermally matured, with a dryness ratio (C 1 /∑C 1–4 ) of 0.997–0.9998, C 2 H 6 content <0.2%, and H 2 S content of 0.5–2.75%. The stable carbon isotope compositions (δ 13 C) of CH 4 and C 2 H 6 are similar in all the reservoirs, with values of −33.5‰ to −31.5‰ and −36.0‰ to −27.0‰ (PDB), respectively. The Sinian gas reservoirs are believed to have been accumulated by the following processes: initial paleo-oil accumulated during the Silurian–Devonian; gas accumulation after oil cracking of the paleo-oil accumulations during the Jurassic–Early Cretaceous; and the current gas accumulations formed by paleo-gas remigration and adjustment since the Late Cretaceous. This new gas accumulation model is consistent with large-scale gas accumulations for deeply buried paleocratonic basins worldwide, in which the source rocks initially underwent thermal oil generation from kerogen, followed by gas generation via oil cracking. The gas discovery in the Sinian dolomite reservoirs has led to a new wave of hydrocarbon gas exploration in the ancient cratonic basin containing the mostly thermally matured source rocks in China. We conducted a series of geochemical analyses of the H 2 S in the gases. The sulfur isotope compositions (δ 34 S) of gypsum, H 2 S, and secondary pyrite sampled from the Sinian Dengying Formation showed that H 2 S was generated by a moderate thermochemical sulfate reduction (TSR) during the Jurassic–Early Cretaceous periods. TSR was terminated by the depletion of SO 4 2− in the formation water, resulting in a modest H 2 S content of 5–10% in the natural gases. Because the dolomites are rich in cations, a large quantity of H 2 S may have been bound to form metal sulfides. We infer that H 2 S contents of 0.8–10% are present widely in the deeply buried Sinian reservoirs. Therefore, measures should be taken to prevent H 2 S-related accidents during gas exploration in the deep strata. [ABSTRACT FROM AUTHOR]

Mild oxidative degradation was used to release the aliphatic hydrocarbons occluded inside the macromolecular structure of solid bitumens collected from Sinian carbonate reservoir, Sichuan basin, SW China. In contrast to the adsorbed aliphatic hydrocarbons extracted from the solid bitumens, the mild oxidative products showed unaltered n -alkanes distributions, which had been trapped within the macromolecular structure of the bitumens and protected from biodegradation processes. These observations are quite informative and suggest that at least, two phases of hydrocarbon charging to the Sinian reservoirs might have occurred. The occluded hydrocarbons seem to represent the first phase of hydrocarbon charge while the adsorbed and biodegraded hydrocarbons correspond to the second phase of hydrocarbon charge. However, terpane and sterane distributions in both fractions were preserved and similar, and the carbon isotope analysis results obtained in this work along with the previous geological information, corroborates the idea that Sinian reservoirs hydrocarbons have been sourced from the black shale of Lower Cambrian Qiongzhusi Formation. This study reveals the geochemical significance of adsorbed/occluded aliphatic hydrocarbons in understanding the charging history of oil into reservoirs. [ABSTRACT FROM AUTHOR]

Reef and oolitic gas reservoirs in the Permian Changxing and Triassic Feixianguan formations have been discovered in both the Luojiazhai-Puguang and Longgang areas of the Sichuan Basin, but the accumulation scale, abundance and distribution patterns differ greatly. In fact, the two areas have more differences than they have similarities. Similarities include: (1) the lithology, properties and types of porosity of the reef and oolitic reservoirs are primarily consistent in the two areas; (2) well-developed regional caprocks that seal the fluid system; and (3) accumulation process histories, with liquid hydrocarbon charging at an early stage, cracking into gas at a high to over-mature stage, adjustment of gas reservoirs at an uplifting stage, and final reservoir generation at a late stage. Differences include: (1) gas accumulations in the Luojiazhai-Puguang area that are dominated by oil-type cracked gas, with multiple and sufficient gas sources, contrasting with gas accumulations in the Longgang area that are dominated by coal-sourced gas with insufficient gas sources; (2) heterogeneity and horizontal diversity of reservoirs in the Longgang area that are stronger than in the Luojiazhai-Puguang area; and (3) the development of deep and large faults in the Luojiazhai-Puguang area that link source, reservoir and migration pathways, contrasting with poorly developed faults in Longgang area, where laterally variable fractures are the major migration pathway. Because of these differences, abundant large-scale structurally-controlled gas accumulations are developed in the Luojiazhai-Puguang area, whereas lithology plays a more important role in gas accumulation in the Longgang area where accumulations are smaller-scale and have lower gas-bearing abundance. We suggest that future exploration for reef and oolitic gas accumulations should be strengthened in NE Sichuan and NW Sichuan Jiulongshan. In particular, the transitional zone between Longgang and Kaijiang-Liangping Bay has the best potential for new discoveries. [ABSTRACT FROM AUTHOR]

In order to evaluate rare earth elements (REEs) as a potential proxy for solid bitumen classification, we employed traditional correlation approaches, such as carbon isotopes and V/(V + Ni) ratios, to infer the source rocks of Sinian-Cambrian reservoir solid bitumens in the Sichuan Basin and analyse solid bitumens for their REE compositions by inductively coupled plasma-mass spectrometry. Our data suggested that Sinian-Cambrian reservoir solid bitumens were primarily sourced from the lower Cambrian shales deposited under anoxic-euxinic conditions. REE and their associated parameters seemed to be more sensitive to classify solid bitumens than traditional correlation approaches. REE concentrations played a dominant role and fractionation degree between light and heavy REE and played a secondary role in solid bitumen classification. REE concentration and pattern in solid bitumens might be controlled by two processes, including inheritance from source rocks and water-rock interaction. The major factor controlling REE concentrations in solid bitumens was the type of organic matter of source rocks. Authigenic minerals in solid bitumens formed by water-rock interaction appeared to have little influence on REE compositions of solid bitumens because of their trace contents. Overall, REE could be used as a novel complementary approach to solid bitumen classification in complicated petroleum systems. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Reef-bank of Sichuan (四川) basin have been explored along the platform margin belt on both sides of the Kaijiang (开江)-Liangping (梁平) trough for a long time. Correspondingly, a series of major breakthroughs have been obtained and a number of large oil and gas fields have been found. After the breakthroughs of Kaijiang-Liangping trough, how reef-bank exploration will proceed? Is there another 'Kaijiang-Liangping trough' in Sichuan basin? Previous research shows that Sichuan basin underwent intracratonic rift and basement fault activities in Late Permian, developing a paleogeography framework of 'three uplifts and three depressions' under the background of the regional large ramp, including Exi (鄂西)-Chengkou (城口) trough, Kaijiang-Liangping trough and Yanting (盐 亭)-Tongnan (潼南) trough. Based on sedimentology and tectonics dynamics analysis, data of drillings, outcrops and seismic, Yanting-Tongnan trough has been discovered in central Sichuan basin in our study which distributes from Mianzhu (绵竹), Jiangyou (江油) in western Sichuan basin through Nanchong (南充) to Guang'an (广安), as long as about 210 km, extending in NW-SE nearly paralleling to Kaijiang-Liangping trough, opening to the sea north-westward, and gradually pinchout to the east platform. Similar to Kaijiang-Liangping trough, another platform margin reef belt distributes along a U-shape belt around Yanting-Tongnan trough, which is expected to become another natural gas accumulation zone after the platform margin around Kaijiang-Liangping trough. The discovery of Yanting-Tongnan trough expands favorable reef exploration area from original 2×10 km on both sides of Kaijiang-Liangping trough to more than 6×10 km in Yanting-Tongnan trough platform margin belt and Kaijiang-Liangping platform margin belt, greatly enlarging the reef exploration field. But compared to Kaijiang-Liangping trough, Yanting-Tongnan trough is relatively shallower and smaller, and the scale and quality of reef-bank reservoirs in its margin belt is a little poor relatively. Therefore, much research should be done timely such as comparative study of two troughs, detailed research of formation and evolution of the trough, regularity of reef migration, meticulous depiction of reef-bank reservoir size and distribution in order to obtain major breakthroughs around the Yanting-Tongnan trough. [ABSTRACT FROM AUTHOR]

• Three lamination types are distinguished in the Upper Ediacaran botryoidal dolostones in Western Yangtze Platform. • EPS and filaments are documented in botryoidal laminae which were previously believed to be abiogenic origin. • Different microbial mineralization types are recognized in different botryoidal laminae. Botryoidal dolostones are abundant in the Second Member of the Upper Ediacaran Dengying Formation in the Western Yangtze Platform. Although they have been extensively discussed as important host rocks for giant gas reservoirs, there is no consensus on the microbial impact on mineralization processes in the botryoidal laminations. Typical botryoidal laminations were studied under optical microscopes and scanning electron microscopes (SEMs) to investigate their origin. There are three macroscopic morphological types of botryoidal lamination: stromatoid lamination, cavity-lining lamination, and spherical-encrusting lamination. These three types of lamination are composed of fibrous and bladed crystals growing perpendicular to the botryoidal laminae, which have hitherto been implicitly considered to be from abiogenic chemical precipitation. There are many lines of new evidence that demonstrate microbial activities in the interbedded dark and bright laminae constituting the laminations. In the dark laminae, euhedral dolomite rhombs are observed sticking to linear and tubular filaments, or wrapped in sheets of extracellular polymeric substances (EPS) with dolomite crystal sizes of generally <5 μm. The bright laminae are dominated by coarser dolomites and connected by radial sheaths and trichomes with dolomite crystal sizes of primarily around 10 μm. Different species of bacteria, which are closely related to the macroscopic morphology of the laminations, are present in both the bright and dark laminae. The results are useful in understanding the origin and formation mechanisms of the Neoproterozoic botryoidal dolostones. The new findings may also shed new light on the occurrence and evolution of early life on Earth in the Precambrian Era. [ABSTRACT FROM AUTHOR]