The Tarim Basin is the largest petroliferous basin in northwestern China and contains the Tahe oilfield, the highest hydrocarbon enrichment in the entire basin. Paleogeomorphology has important controlling effects on carbonate reservoir development. Using core data, thin sections and drilling, logging and seismic data, paleokarst was identified and analyzed. The paleogeomorphology of the top low-middle Ordovician was recovered following a comprehensive utilization of the impression and residual thickness methods in combination with unconformity identification. Based on the filling thickness and residual thickness, the karst paleogeomorphic units were classified, and their controlling effects on reservoirs analyzed. The second-order paleogeomorphic units primarily include four types: karst highland, karst gentle⁃slope, karst steep⁃slope and karst basin. The karst gentle⁃slope and karst steep⁃slope are further classified quantitatively into third-order paleogeomorphic units (monadnock, tableland, karst terrace and trench). The results show that the development of the karst reservoir differed in different geomorphic units. In the karst gentle slope, the favorable areas for karst reservoir development are the karst terrace, tableland, monadnock, and the ends and flanks of trench. In the karst steep⁃slope, the karst terrace and tableland are favorable areas. In the vertical vadose and phreatic zones of the gentle-slope region, a large number of caves and underground river systems developed, but in the steep-slope region, only a small number of shafts and isolated caves are observed. This study plays an important role in oil and gas exploration and development.

Paleocurrent reconstruction is an important aspect of basin research, playing a key role in the reconstruction of sediment development and understanding the evolution of sedimentary environments. The reconstruction of paleowind direction is an important part of the study of paleoatmospheric circulation, playing a vital role in revealing the characteristics and evolution of paleoclimate. Water flow and wind movement present similar interactions with ground objects and leave direction information. This paper summarises the current common reconstruction theories of paleocurrent and paleowind direction, analyses the advantages and disadvantages of the reconstruction theories, and analyses the characteristics of reconstruction results. Based on the technical structure of paleocurrent and paleowind direction reconstruction, the development of reconstruction theories, processes and results are highlighted. The analysis shows that paleocurrent and paleowind directions can be used for paleoatmospheric circulation research, resource exploration, paleogeographic environment reconstruction and planetary geological research.

Summary: Tight oil, present in reservoirs of low porosity and permeability, can be regarded as a kind of unconventional resource. The tightening process in this kind of reservoir is controlled by the lithology and diagenetic history of the host formation. Upper Triassic Yanchang Formation siltstones and sandstones are the main reservoirs for hydrocarbon accumulation in the Heshui Oil Field (HOF), southwestern Ordos Basin. The reservoirs exhibit low porosity, low permeability, and strong heterogeneity. In recent years, numerous drillcores have been recovered from these units, but the porosity-permeability characteristics and burial history of these silty and sandy reservoirs have not yet been reported in detail. In this study, an integrated analysis of the lithofacies, diagenesis, and reservoir characteristics of the siltstones and sandstones was achieved using a combination of core and thin section, grain size, scanning electron microscopy (SEM), X-ray diffraction (XRD), δ13C and δ18O, mercury intrusion capillary pressure (MICP), and porosity and permeability data. Our primary goals were to quantify the porosity-permeability characteristics of these silty and sandy reservoirs, restore their diagenetic histories, and examine the paragenetic relationship of reservoir tightness to hydrocarbon accumulation. The silty and sandy reservoirs represent braided river delta facies consisting of compositionally and texturally immature sediments. In the burial environment, they underwent complex diagenetic processes that reduced porosity from an initial average of ~38% to the present ~8%. Porosity-destructive processes included compaction (~ –12.5%) and cementation (~ –21%), with increases in porosity related to grain dissolution (~ +2.2%) and tectonic fractures (~ +1.1%). The reservoirs underwent four diagenetic stages: (1) Penesyngenetic and Eogenetic A Stage (Late Triassic-Early Jurassic); (2) Eogenetic B Stage (Late Jurassic); (3) Early Mesogenetic A Stage (Early Cretaceous); and (4) Late Mesogenetic A Stage (Late Cretaceous to recent). Hydrocarbon charging of these reservoirs occurred in three pulses. Existing pore space was partly filled by hydrocarbons during the Eogenetic B Stage. A second hydrocarbon charging event occurred during the Early Mesogenetic A Stage, when residual primary intergranular pores and secondary dissolution pores were filled. A third hydrocarbon charging event occurred during the Late Mesogenetic A Stage, when the reservoirs were tight. Siltstone beds deposited in delta front environments are the main future exploration targets in the Chang 6 to 8 members. The results of this study provide a useful reference framework for future exploration of hydrocarbon resources in the Upper Triassic Yanchang Formation of the HOF, as well as potential insights into the evolution of similarly tight reservoirs in other basins.

The Tarim Block is an ancient plate with a basement of ancient continental crust, which has been separated from the Rodinia supercontinent since the Neoproterozoic. During the Neoproterozoic, which lasted nearly 500 Myr, this block experienced significant evolutionary processes, such as proliferation, radioactive decay of elements, and gradual cooling and solidification. The investigation of Neoproterozoic paleogeography may shed light on the evolution of these geological events. In order to realize this potential, this study aimed to infer paleowind directions over the Tarim Block during each epoch of the Cryogenian–Ediacaran and to constrain the paleogeographic location of the Tarim Block. To this end, outcrop magnetic fabric data were employed to analyze the anisotropy of magnetic susceptibility within the Tarim Block. The anisotropy of magnetic susceptibility measurements yielded mean paleowind directions of 308° ± 69°, 277° ± 78°, and 256° ± 76° from the present north for the Early, Middle, and Late Cryogenian, respectively; the corresponding values for the Early and Late Ediacaran were 237° ± 77° and 254° ± 73° from the present north, respectively. Considering the rotation relationship of the Tarim Block from the Neoproterozoic to the present, the paleowind directions during the Early, Middle, and Late Cryogenian were ~55°, ~35°, and ~35° from the paleo-north, respectively. The paleowind directions during the Early and Late Ediacaran were ~35° and ~60° from paleo-north, respectively. By referring to the correspondence between the paleowind directions over the Tarim Block and trade winds in the Northern Hemisphere, this study provides evidence for the location of the Tarim Block during the Cryogenian–Ediacaran. The main contributions of this study can be summarized as follows: (1) paleowind patterns are established through the analysis of the anisotropy of magnetic susceptibility; (2) the paleogeographic location of the Tarim Block during the Cryogenian–Ediacaran is constrained; and (3) a reference for further study of the paleogeography of the Tarim Block during the Cryogenian–Ediacaran is provided.

The Triassic Jiucaiyuan Formation is a vital oil and gas exploration target in the eastern part of the Junggar Basin. In this work, cores, thin sections, particle sizes, and conventional physical properties were analyzed in order to understand the sedimentary characteristics and depositional model of the Jiucaiyuan Formation in the Beisantai Uplift and to clarify the factors controlling reservoir development. The results demonstrate that the Jiucaiyuan Formation can be divided into seven lithofacies, namely massive bedding gravel, massive bedding sandstone, trough bedding sandstone, plane bedding sandstone, wavy bedding sandstone, parallel bedding siltstone, and massive bedding mudstone. Braided river delta facies dominate the Jiucaiyuan Formation. Nine main subtypes of facies were observed: flood plain, braided channel above lake level, natural levees above lake level, braided channel under lake level, interdistributary bays, natural levees under lake level, mouth bars, prodelta mud, and beach bar. The braided channel under lake level, mouth bar, and beach bar reservoirs exhibited the best physical properties, with average porosities of 16.54%, 19.83%, and 20.41%, respectively, and average permeabilities of 3.43 mD, 9.91 mD, and 12.98 mD, respectively. The physical properties of reservoirs in the study area are mainly controlled by sedimentation. Braided channels under lake level, mouth bars, and beach bars are favorable facies for the development of high-quality reservoirs. The results of this study are expected to serve as a theoretical basis for further exploration and development of oil and gas in the study area.

The Sichuan Basin was a part of the Yangtze Carbonate Platform (YCP) during the Cambrian–Ordovician, and marine carbonates were deposited in the basin during this interval. Although previous studies have evaluated the paleogeography, paleoclimate and paleoecology of this basin, they have primarily focused on the paleoecology and biological evolution in the basin; however, analysis of paleogeography and paleoclimate is lacking. This study integrated outcrop sedimentological and magnetic fabric data to document sedimentary differentiation and anisotropy of magnetic susceptibility (AMS) within the YCP. The aims of this study were to infer paleowind directions during each epoch of the Cambrian–Ordovician and to constrain the paleogeographic location of the YCP. The northwestern, central and southeastern sides of the YCP were characterized by high-energy deposition (e.g. sub-angular to rounded intraclasts), medium-energy deposition (e.g. sub-angular to sub-rounded intraclasts) and low-energy deposition (e.g. angular to sub-angular intraclasts), respectively. The centroid D-K_maxvalues for the Early, Middle and Late Cambrian were 116° ± 52°, 145° ± 57° and 159° ± 62° from the present north, respectively; corresponding values for the Early, Middle and Late Ordovician were 169° ± 70°, 139° ± 73° and 91° ± 68° from the present north, respectively. Sedimentary differentiation and AMS results indicated that the prevailing wind directions during the Early Cambrian, Middle Cambrian, Late Cambrian, Early Ordovician, Middle Ordovician and Late Ordovician were 296° ± 52°, 325° ± 57°, 339° ± 62°, 349° ± 70°, 319° ± 73° and 271° ± 68° from the present north, respectively. The present study provides evidence for the location of the YCP during the Cambrian–Ordovician via the correspondence between the paleowind directions over the YCP and the trade winds in the Northern and Southern hemispheres. The novelty of this study lies in the following aspects: (1) it integrates microfacies and AMS analyses to establish paleowind patterns; (2) it constrains the paleo-hemispheric location of the YCP during the Cambrian–Ordovician; and (3) it provides a reference for further studies of the paleoclimate and paleogeography of the YCP during the Cambrian–Ordovician.