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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 southern Junggar Basin in NW China is an important tectonic unit in the region of the Tibetan Plateau and has been the focus of considerable research into its tectonic processes and geodynamic setting. However, the relationship between deep structural deformation and stress in this region remains unclear. This study investigates the Gaoquan and Hutubi anticlines in the southern Junggar Basin using three-dimensional geophysical data and a finite-element numerical simulation to examine the crustal stress distribution and stress regime at depths of up to 7 km. Numerical simulation results indicate that the stress regime in the southern Junggar Basin changes from west to east. In the western part of the region, including the Gaoquan anticline at depths of 4900–6100 m, the maximum horizontal principal stress shows a peak of 140–200 MPa, the minimum horizontal principal stress is 110–170 MPa, and the vertical principal stress is 115–175 MPa, indicating a mixed stress regime incorporating both compression and strike-slip components. In the eastern part of the region, including the Hutubi anticline at depths of 5400–7800 m, the maximum horizontal principal stress shows a peak of 160–280 MPa, the minimum horizontal principal stress is 155–250 MPa, and the vertical principal stress is 125–215 MPa, indicating a compressive stress regime. The stress magnitude and orientation are affected by the presence of faults and depth in the crust. Combining these results with the regional tectonic setting, it is considered that the geometrical relationship between pre-existing faults and the current stress field is the main control on the west–east differentiation in the stress regime, with spatial variations in the mechanical parameters of the crust and the pressure coefficient being secondary factors. These results provide insights into the relationship between stress and deformation, and support the updated version of the World Stress Map database. • Stress regime changes from west to east of the southern Junggar Basin. • The deep stress is closely related to the structures and faults. • Spatially varying stress regime is mainly determined by the relationship between pre-existing faults and present-day stress. [ABSTRACT FROM AUTHOR]

• Two phases of Proterozoic orogen are proposed in the northern Tarim. • The Paleoproterozoic orogen was initiated at ca. 1.9–1.8 Ga and terminated at ca. 1.8 Ga, whereas the Neoproterozoic orogen was initiated at ca. 900–740 Ma and terminated at ca. 600 Ma. • Proterozoic orogen provides provenance for its surrounding basins in the northern Tarim. • Neoproterzoic successions are firstly reported and dated in the Sawapuqi and Yuli regions. The Tabei Uplift is the oldest unit in the Tarim Basin, but its initiation and evolution are unclear, which restricts our understanding of the Neoproterozoic basin–orogen framework of the northern Tarim Craton, and the breakup process at the margin of the Rodinia supercontinent. In this study, we analyzed U–Pb age spectra of detrital zircons from sedimentary rocks of the Tabei Uplift and surrounding regions. By integrating these data of outcrops, wells, and seismic investigation, we identify spatial–temporal changes in Neoproterozoic sedimentary provenances, establish a geological framework of basin–orogen structure, and elucidate the long-term evolution of the Tabei Uplift. Five sedimentary rock samples were dated by zircon U–Pb geochronology to constrain their depositional ages and regional stratigraphic correlations. Precambrian strata of Well YL1 in the Yuli region belong to the upper Cryogenian–lower Ediacaran (ca. 660 Ma); Precambrian strata of the Sawapuqi region belong to the Ediacaran (ca. 595 Ma); and strata of the Wikiritag region belong to the middle Paleozoic (ca. 410 Ma). Analysis of 3962 zircons from these 5 new samples and 54 published Neoproterozoic samples reveals temporal and spatial variations in their U–Pb age spectra. From the periphery of the northern Tarim Craton to the Tabei Uplift for a given period (such as the Tonian Aksu Group, Cryogenian, and Ediacaran), the proportion of Neoproterozoic zircons decreases, whereas the proportion of Neoarchean–Paleoproterozoic zircons increases. From the Tonian to the Ediacaran for a given location, the proportion of Neoproterozoic zircons decreases, whereas the proportion of Neoarchean–Paleoproterozoic zircons increases. Combining this information with a Neoproterozoic wedge structure in the northern Tarim Craton observed in three geological/seismic profiles, the North Tarim Orogen and adjacent rift basin are inferred to have formed simultaneously during the Neoproterozoic. The Proterozoic North Tarim Orogen served as the provenance for surrounding regions in the northern Tarim Craton. We propose two main phases of Proterozoic orogenesis in the northern Tarim Craton, during the Paleoproterozoic and Neoproterozoic. The Paleoproterozoic orogen was initiated at ca. 1.9–1.8 Ga and terminated at ca. 1.8 Ga, whereas the Neoproterozoic orogen was initiated at ca. 900–740 Ma and terminated at ca. 600 Ma. The temporal–spatial evolution of the Neoproterozoic basin–orogen framework in the northern Tarim Craton was closely related to the breakup process at the margin of the Rodinia supercontinent. [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]

Neoproterozoic–Cambrian black shales are one of the most important hydrocarbon source rocks worldwide, including in the Shunbei and Fuman regions in the Tarim Basin, China. However, the hydrocarbon generation from these shales is complex, with liquid petroleum found in reservoirs at depths of 6000–8000 m. Based on drilling, seismic, and well testing data, the ultra-deep tectonic evolution of the northern Tarim Basin, and its effects on hydrocarbon generation from black shales were investigated. During the Neoproterozoic, the northern Tarim Basin was filled with rift and passive continental margin sedimentary rocks between the north and central Tarim paleo-uplifts. A sequence of lower Cambrian black shales was developed on the slope around the paleo-uplifts over a wide area, which are thick and have high contents of total organic carbon. Even after multiple stages of tectonism, including the Caledonian, Hercynian, Indosinian, Yanshanian, and Himalayan orogenies, the north (Tabei) and central paleo-uplifts were preserved. The Caledonian, Hercynian, and Himalayan orogenies were the main periods of hydrocarbon generation. Four hydrocarbon-generation models were established, which are the Tabei-, Bachu-, Tazhong-, and Tadong-types. The current R o values of the Tabei-type black shales are 1.0%–1.5%, whereas those of the Bachu-, Tazhong-, and Tadong-type black shales are >2.0%. Due to the long-lived paleo-uplifts and short-lived deep burial during the Himalayan Orogeny, the Tabei-type black shale is in the oil and early gas generation stage, which obviously differs from the over-mature Cambrian and Ordovician black shales in South China. Therefore, the long-lived paleo-uplifts not only affected the deposition and distribution of black shale, but also hydrocarbon generation processes and the depth limit of active source rock. The thermal evolution of black shale in the ultra-deep Tarim Basin requires further investigation. • Neoproterozoic–Cambrian black shales were deposited in the passive continental margin setting. • The Tabei-type black shales represent the oil and early gas generation stage. • Long-lived paleo-uplifts in the northern Tarim affected the distribution and evolution of black shales. [ABSTRACT FROM AUTHOR]

The development characteristics of organic matter pores (OMPs) in shales is fundamental to the gas-bearing property of the shales. Detachment is one of the most common types of structural deformation in shale formations. To investigate the characteristics of OMPs in shear-deformed shale and the associated effects on the pore structure of shale gas reservoir, two sets of detachment zones and the intercalated non-detachment zone of shale formations in the Niutitang Formation of the Well XAD1 are studied. Based on scanning electron microscope observations and low-pressure gas adsorption tests, the impacts of shear on the shale pore structure are evaluated quantitively and the deformation mechanism for OMPs in shale is established. Results indicate that the dominant pore type of shale in the Niutitang Formation of the Well XAD1 is OMPs. With the mineral rheology, organic matters are enriched in the clay gouge, which are more prone to shear deformation compared with those outside the gouge. Under shear, pores developed within the organic matter experienced directional elongation and compression, decreasing the pore sizes and lowering the volumes of the corresponding pores <30 nm; and the contact surface between organic matter and minerals becomes dislocated and open, increasing the volumes of the corresponding pores >30 nm. The effect of shear on OMPs is dominated by pore reduction. The volume loss ratio of shale micropores in detachment zones is the highest which can reach over 90%. The impact of shear on pore structures of non-detachment zone cannot be neglected within the thickness range of 5–20 m from the detachment surface, and the degree of pore reduction gradually decreases as the distance from the detachment zones increases. It is indicated that the detachment and its adjacent horizons are not favorable for shale gas exploration because of the low porosity and adsorption capacity. • The deformation of organic matter pore is controlled by the micron-wide clay gouge in the shale matrix. • Shear deformation can result in a significant reduction of pore volume, especially for organic matter pores <30 nm. • The impact of shear on pore structures of non-detachment zone cannot be neglected. • The competency of shale formations and their distance from detachment zones can affect the pore deformation degree. [ABSTRACT FROM AUTHOR]

Marine shales in southern China experiences multiple complex tectonic movements, which highly affects shale pore characteristics and pore evolution. The effects of tectonic deformation on nanopore structure and petrophysical properties of organic shales remain unclear. Three ductile deformed shale samples were collected and analyzed through an integrated experiment procedure for a comparative research against undeformed shale samples. Field emission-Scanning electron microscopy (FE-SEM), low pressure N 2 and CO 2 adsorption and mercury injection porosimetry (MIP) analyses were performed to characterize pore structure of the shales. The results reveal that the mineral compositions of these shale samples are controlled by sedimentary environment and source and exhibits no obvious relationship with tectonic deformation. With increasing deformation degree, the micropore proportion gradually decreases (from 17% to 6%), and the macropore proportion increases from 16% to more than 20%. Micropore volumes and proportion in the fold core samples are the smallest (4.2 × 10−3 cm3/g and 6.3%) and account for only one-third of the undeformed shale values, while the meso- and macropore values are larger-1.2 times those of the undeformed shale values. The evolution characteristics and deformation modes of organic matter (OM), interparticle (interP) and intraparticle (intraP) pores and microfractures under ductile deformation were examined. All pore types become interconnected during deformation and form microfractures ranging from a few nanometers to tens of microns, which is the main reason for the enhanced shale connectivity caused by deformation. Under the same porosity conditions, the deformed shale permeability is much higher than the undeformed shale permeability, and stronger deformation corresponds to higher permeability. The fold core permeability is generally higher than the limb permeability and exceeds 30 times. The different ductile deformation positions were divided into five levels, and the corresponding reservoir quality was evaluated. This study is of great significance to better understand the pore structure evolution process for shale exploration and development under tectonic deformation. • Pore structure deformation develops widely during tectonic deformation. • Ductile deformation significantly affects the micro- and macropores of shale. • The fold core permeability is generally much higher than the limb permeability. • The pore interconnected during deformation promotes the development of microfractures and increases the permeability. [ABSTRACT FROM AUTHOR]