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The tight sandstone of Lower Shihezi Formation of Permian in Northwestern Ordos Basin is taken as the research object, based on the observations of cast thin slice, scanning electron microscopy, nuclear magnetic resonance and nitrogen adsorption porosity permeability test, the characteristics of tight sandstone reservoirs in Lower Permian Shihezi Formation and the factors affecting reservoir quality were analyzed. The findings indicate that the Lower Shihezi Formation reservoir of Permian in the study area predominantly comprises feldspar lithic sandstone and lithic sandstone, representing a typical example of tight sandstone. Rapid burial is the primary cause of reservoir densification, while compaction plays a crucial role in reducing reservoir quality, resuming in a significant loss of 58% in primary porosity. Cementation further compacts the sandstone reservoir, resulting in a secondary reduction of reservoir physical properties and loss of 24% in primary porosity. Other factors diminishing reservoir quality include the occlusion of intergranular pores through diagenetic processes such as calcite and iron calcite formation. The factors influencing the improvement of reservoir quality include extensive intergranular porosity provided by the dissolution of feldspar and the protection of some primary pores by the resistance of rigid quartz particles to compaction. These phenomena can serve as a valuable reference for reservoir research in other fluvial tight sandstone basins. [ABSTRACT FROM AUTHOR]

The Caledonian Palaeohigh is an important gas exploration domain of Sinian and Lower Paleozoic in western Central Sichuan Basin where gas discoveries have been made successively in recent years. In order to sort out the relationship between the tectonic evolution of this Palaeohigh and hydrocarbon accumulation there, we carried out a new round of research based on previous study results. The evolution history of this Palaeohigh can be divided into seven episodes: the Late Sinian overall tilting and the youth form development of this Palaeohigh, the Cambrian-Ordovician syndepositional uplifting, the Silurian joint uplifting, the Devonian-Carboniferous uplifting and erosion, the Permian overall subsidence and deposition of regional caprock, the Triassic-Jurassic migration of structural high of the eastern segment of this Palaeohigh, and Cretaceous-Neogene strong deformation of the western segment of this Palaeohigh. The hydrocarbon accumulation in the Sinian-Lower Paleozoic experienced three evolution stages, namely the formation of ancient oil reservoirs in the Silurian-Triassic period, the development of the paleo-gas reservoirs in the Jurassic-Oligocene period, and differential evolution of gas reservoirs since the Miocene era. The Sinian-Lower Paleozoic paleo-reservoirs mainly occur in the Leshan-Ziyang-Gaoshiti-Longnǚsi zone and in the area to its north. The tectonic movement of the western segment of the Palaeohigh was strong, while its eastern segment was relatively stable since the later Himalayan epoch, which is favorable for gas accumulation and preservation. It is believed that the Gaoshiti-Moxi-Longnǚsi structural belt and its northern flank are not only favorable for the development of structural gas reservoirs in the Lower Paleozoic but also for gas reservoirs of karstic-lithologic type on the top of the Cambrian and Ordovician. The later type will be the major exploration target of this area in the future.

In order to evaluate reservoir characteristics and gas potential of organic-rich shale in Bottom-Lower Cambrian Hetang Formation and Upper Ordovician Xinkailing Formation, Xiuwu Basin, Jiangxi Province, Lower Yangtze Plate of China, researches regarding reservoir characteristics of organic-rich shale thickness, moderate burial depth, abundance and maturity of organic matter, fracturability, organic-matter pores and absorption performance have been conducted to Hetang Formation and Xinkailing Formation through field survey, shallow drilling, analysis to hydrocarbon-generation potential, scanning electron microscopy, measuring specific surface area and isothermal adsorption experiments. The results show that (1) Hetang Formation reservoir, although with extremely rich material basis, is of unfavorable hydrocarbon generation conditions due to late-stage thermal maturity, besides of undeveloped micropore structure, reservoir space development mainly by macroscopic crack, so further exploration should focus on locating favorable areas of low maturity and developed cracks, which is considered distributing in the core of Yanshan-Lingangshan-Shiziyan Mountains syncline; (2) Xinkailing Formation reservoir is of better organic matter abundance, pore development and absorption performance than Xinkailing Formation and hence further exploration should focus on surveying favorable areas of better sedimentary facies belt and stable structure for major breakthrough in shale-gas exploration, which is considered distributing in Wuning-Lixi and Jinkou-Tuolin areas. [ABSTRACT FROM AUTHOR]

3-D seismic data obtained from Joint Development Zone (JDZ) Licensing Round, 2004, was used to investigate the structural and stratigraphic characteristics on distal parts of the outer fold and thrust belt of southern Niger Delta, Nigeria. Five main seismic reflectivity units (I, II, III, IV and V) were identified and interpreted based on seismic reflection patterns to represent the prograding cyclic sequence of sedimentation in the study area. These units record the episodic gravity spreading that is responsible for thrusting in the toe of the delta. Faults in the central parts of the study area exhibit hard linkage by the gradational propagation of segments into an overlap that developed into soft-linked relay structures. Thrust faults in the study area can be classified into two groups based on the width of the individual thrust faults and their extension and the geometry of the hanging wall folds. These are E-W south-verging thrusts and NE-SW SE-verging thrusts. In the proximal parts of the JDZ, thrust faults are widely spaced with a large structural closure of the hanging wall folds. Faults in the eastern part are complex and typically verge in the opposite direction. In the distal parts of the study area, thrust faults are regularly spaced with hanging wall folds that occasionally created bathymetric expressions on the seabed. The high concentration of pockmarks in the proximal JDZ and the large structural closure of the hanging wall folds characterized by stacked amplitude anomalies and discernable flat spots imply the possible existence of an active hydrocarbon system in the proximal JDZ. Several lenses of sills observed in unit III may impact on hydrocarbon generation and expulsion in the study area. If hydrocarbon migration into traps is occurring after the intrusion of magma, then the sills could act as a barrier to fluid flow, depending on post-emplacement tectonics. Seismic cross sections show that faults in the study area have variable cross-sectional geometries but possess scaling properties typical of thrust faults. [ABSTRACT FROM AUTHOR]