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Objective Fracture network analysis plays an important role in oil and gas exploration and development. However, complexity analysis of tight sandstone fracture networks and their control factors is relatively lagging. Based on an experimental study of the dynamic evolution of the complex fracture network in tight sandstone, the fractal and multifractal spectral characteristics of the fracture network were defined, and the complexity and main controlling factors of the fracture network were analyzed. Fracture network complexity analysis of tight sandstone plays an important role in hydraulic fracturing optimization, fracture network prediction, and fracture modeling. Methods Rock mechanics and X-ray computed tomography scan experiments determined the characteristics of rock mechanics and fracture networks . The microstructure and fracture network fractal characteristics of tight sandstone were quantitatively characterized by SEM and fracture network fractal analysis. Results The results showed that the quartz content of tight sandstone ranges from 28.08 to 52.88%, clay content ranges from 11.54 to 25.45%, particle size ranges from 61.18 to 184.55 μm, and porosity ranges from 8.125 to 10.296%. Uniaxial compressive strength ranges from 69.09 to 188.33 MPa, and the elastic modulus ranges from 31.69 to 92.76 GPa. The fractal dimension (DB) ranges from 1.28 to 2.35 and average spectral width (Δα) ranges from 1.0851 to 1.3638. Conclusion The initiation and propagation of fractures extend through the entire stress–strain process. The complexity of the fracture network of tight sandstone is mainly controlled by microscopic fabric characteristics, and has obvious confining pressure as well as scale effects. The DB of the three-dimensional fracture network and average Δα of the multifractal spectrum represents the complexity and heterogeneity of the fracture spatial distribution, respectively, and are relatively independent. As the content of quartz, feldspar, and other brittle minerals in sandstone increases, the porosity of the reservoir increases, particle size of the sandstone decreases, DB of the fracture network increases, and average Δα decreases. In the absence of confining pressure, the complexity of the sample fracture network is mainly controlled by the microscopic fabric characteristics, and the complexity increases with increase of axial pressure. When present confining pressure plays a leading role; the higher it is, the lower the DB value, and the higher the mean Δα value. Clay minerals are unconducive to complex fractures formation. The mean values of DB and Δα of small-scale samples are greater than those of large-scale samples. The elastic modulus and compressive strength of sandstone are positively correlated with DB and mean Δα.

The multi-stage tectonism of compression and extension occurred in the southern part of the Lower Yangtze region in the Mesozoic, which is one of the important factors restricting the breakthrough of oil and gas exploration in this area. This paper aims to reveal the influence of Mesozoic tectonisim on oil and gas preservation in the Lower Yangtze area. Based on the seismic profile interpretation data, field outcrop survey, regional tectonic evolution history analysis and the drilling core data and oil-gas display of the Well Wanjingdi-1, we comprehensively discussed the favorable structural preservation conditions in the southern part of the Lower Yangtze region. The results of seismic interpretation profile and surface survey show that the southern part of the lower Yangtze River has experienced two important tectonic processes since the Late Triassic. The first stage is fold deformation, thrust fault and uplift erosion caused by Indosinian compression, and the second stage is extension since the Late Cretaceous, resulting in the formation of fault basin and normal fault activity. Shale gas display was obtained from the Permian shale in the Well Wanjingdi-1, and light oil was found in the drilling of the Lower Triassic Yinkeng Formation, indicating that the southern Lower Yangtze region has good enrichment conditions and resource potential, which points out a new direction for oil and gas exploration in the region. The analysis of structural preservation conditions shows that the Well Wanjingdi-1 is located in the syncline of the fold-depression belt. The amount of uplift and denudation was small in the Indosinian period. Since the late Cretaceous, the depression has a small amplitude of extension and has good sealing and reservoir forming conditions. It is a favorable structural position for oil and gas preservation in this region. Based on the comprehensive analysis of regional structure and sedimentary basin evolution, it is considered that in the southern Lower Yangtze region, the syncline structural belt in the southern margin of the basin with small erosion amount of Triassic strata or the central zone of fault basin overlying thick Cretaceous-Cenozoic strata should be taken as the key target to search for favorable oil and gas zones.

In order to investigate the effect of primary productivity, organic matter dilution, and preservation on the accumulation of organic matter, geochemical data, and proxies of primary productivity, clastic influx, and redox conditions were obtained for organic-rich shales in the Cambrian Niutitang Formation. The primary productivity (total organic carbon [TOC], Mo, P, Ba, and Babio) and redox (Ni/Co, V/Cr, U/Al, and Th/U) proxies suggest the organic-rich shales were deposited in anoxic-euxinic conditions during periods of high primary productivity. Pyrite in the Niutitang Formation comprises spherical framboids, which also indicate that anoxic bottom waters were present during organic matter deposition. High primary productivity enhanced the organic C flux into the thermocline layer and bottom waters, which lead to the development of anoxic bottom waters owing to O2 consumption by microorganisms and organic matter degradation. The anoxic bottom waters were beneficial for the preservation of organic matter. In addition, Ti/Al ratios correlate well with TOC contents throughout the Niutitang Formation, indicating that clastic input increased the burial rate and prevented organic matter degradation during deposition. Therefore, the accumulation of organic matter in the Niutitang Formation was controlled mainly by primary productivity rather than bottom-water redox conditions. [ABSTRACT FROM AUTHOR]

To investigate the influence of organic matter conversion on the evolution of organic matter pores, fractional conversion (TRHI) and loss of TOC (TOCL) from the organic matter conversion of Middle Jurassic Dameigou Formation shale samples were calculated using petrographic analysis. The TRHI of organic matter varies from 0.30 to 0.88 and TOCL content ranges from 0.62% and 4.09%. Relative to samples of Type III organic matter in shales, type II samples exhibit higher TRHI and TOCL values. Petrographic calculations of TRHI reveal that the fractional conversion of different kerogens differs for the same thermal maturity level. The specific surface area (SBET) ranges between 1.25 and 6.63 m2/g and micropore surface area (Smic) ranges between 4.16 and 21.27 m2/g. Correlations between pore structure parameters and TOCL content are higher than those between pore structure parameters and TOC content. The original TOC content decreases with increasing maturity level owing to hydrocarbon generation from organic matter conversion. The development of organic matter pores depends mainly on organic matter conversion, which is influenced by the richness, organic maceral compositions, and thermal maturity of the organic matter. The contents of kaolinite, illite, and mixed-layer illite/smectite (I/S) in the studied shales are 17.83%–37.05%, 5.36%–11.31%, and 5.27%–14.36%, respectively. Pore structure parameters (SBET and Smic) exhibit moderate positive correlations with illite content and I/S content, and moderate negative correlations with kaolinite content, indicating that different clay minerals have differential effects on pore structure. [ABSTRACT FROM AUTHOR]

The Tarim Craton, China, records several significant events in the history of the Rodinia supercontinent, including Neoproterozoic glaciations and rift evolution. In this study, we use geochronological, geochemical, and geophysical data from the Aksu region, northwest Tarim Craton, to investigate the rifting history and three discrete Cryogenian glaciations in the Tarim Craton. A total of 268 concordant zircon U–Pb ages were obtained from three samples in the lowermost unit of the Cryogenian Xifangshan Formation. These ages define four Neoproterozoic populations of ca. 975–900, 885–810, 795–755, and 740–720 Ma, which reflect magmatic events in the Rodinia supercontinent. The age of the Xifangshan Formation is constrained by two maximum depositional ages of 739 ± 9 Ma and 719 ± 7 Ma, suggesting a stratigraphic correlation with the Cryogenian Baiyisi Formation in the Kuruktag region, northeast Tarim Craton. Field and geochemical evidence identify three discrete Cryogenian glaciations in the Aksu region, which are the Xifangshan, Qiaoenbrak, and Youermeinark glaciations that can be chronologically correlated with the Baiyisi, Altungal, and Tereeken glaciations in the Kuruktag region, respectively. The northern rift basin in the Aksu region formed at ca. 740 Ma and is filled by a thick Cryogenian volcanic–sedimentary succession. This rift basin gradually expanded to entire northern Tarim Craton, with sediments being unconformably deposited on underlying formations. Although large-scale deformation occurred in the late Ediacaran, corresponding to the Pan-African orogeny, the Aksu rift depocentre was covered with a series of deep-water rocks in the Early Cambrian and Ordovician. [ABSTRACT FROM AUTHOR]