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The main target for hydrocarbon exploration of terrestrial strata in Sichuan Basin is the tight gas sandstone reservoir of the Upper Triassic Xujiahe Formation. Xujiahe Formation in YB area is a major discovery of hydrocarbon exploration in recent years, which is characterized as tight with ultralow porosity, ultralow permeability, and intensive heterogeneity. The tectonic fractures have important influence on accumulation and productivity of gas in the area. In the paper, analysis data of outcrop, core, imaging logging and other experiments were used to determine the characteristics, formation periods and genetic mechanisms of tectonic fractures. There are mainly high-angle and low-angle shear fractures in the area, which caused by tectonic stress. The length of the fractures is mainly 0–4 m from the outcrop and 10–20 cm from the cores and fracture frequencies (number per meter) is mostly 2-7 m−1, 2-4 m−1 respectively. The fracture fillings are mainly calcite, argillaceous and carbonaceous, about 60% of the fractures is unfilled or incompletely filled. Without considering the diagenetic fractures, there are three periods of the fractures: the middle-late Yanshanian tectonic movement, the early-middle Himalaya tectonic movement and the late Himalaya tectonic movement. The first period of the fractures are mainly filled with fine calcite, homogenization temperature of inclusion is 75–85 °C and the maximum effective principal stress of paleostress is 71.64 Mpa. The second period of the fractures are half-filled (or unfilled) with coarse calcite, homogenization temperature of inclusion is 150–175 °C and the maximum effective principal stress of paleostress is 74.91 Mpa. The third period of the fractures are low filling degree and cuts formed fractures clearly, the maximum effective principal stress of paleostress is 86.52 Mpa. For these tectonic movement, there are four sets of the plane "X" shear fractures in the area: NE strike fractures, NW strike fractures, NEE strike fractures, S N strike fractures. There are one set of the profile shear fractures of NWW strike. Finally, the complex fault system of nearly N S, NW and NE strike are formed in YB area. • A new method for comprehensive study of the fracture formation period is provided. • The main fracture formation dating of Xujiahe Formation in YB area as follows: 36–176 Ma and 0.2–36.4 Ma. • There are 3 periods tectonic fractures of Xujiahe Formation and formation period of different fracture types is identified. [ABSTRACT FROM AUTHOR]

The Yingshan Formation in Lower Ordovician is an important hydrocarbon reservoir of the Tarim Basin, northwest China; however, the factors controlling reservoir development in the formation are not fully understood. Data from thin-section observations, scanning electron microscopy, and gamma-ray logging were applied to produce a fourth-order sequence stratigraphic framework for the Yingshan Formation in the Yubei area of the Tarim Basin. The framework was assessed in terms of the carbonate diagenesis of the reservoir rock. The influence of diagenesis on reservoir porosity, the factors restricting reservoir quality, and the significance of the results for exploration are discussed. Diagenesis, dolomitization, and cementation by atmospheric fresh water occurred in the lowstand systems tracts. The sediments in the transgressive systems tract were consolidated by seawater, or dolomitization occurred in the landward mixed zone, or the sediments quickly entered the shallow burial marine diagenetic environment and became compacted. The highstand systems tract experienced significant diagenesis, with seafloor cementation, atmospheric water dissolution and dolomitization. These factors were related to sea-level fluctuation and sedimentary features. The porosity of some high-frequency sequences in the lower part of the Yingshan Formation was improved significantly. In the Yubei area, cementation of the middle part of the fourth-order sequence during burial reduced the porosity and permeability of the reservoir. This was the main cause of reservoir deterioration. During oil and gas exploration, attention should be paid to the higher degree of dissolution in the middle part of the fourth-order sequence and the superposition of atmospheric fresh-water dissolution in the upper part of the sequence. • Gamma-ray logging (Th/K ratio) were applied to produce a high-frequency sequence stratigraphic framework. • In Yingshan Formation, burial dolomitization, seafloor cementation and burial dissolution occurs in TST, seafloor cementation and burial dissolution occurs in HST, atmospheric fresh-water dissolution and atmospheric fresh-water cementation occurs in LST. • The superposition of the burial dissolution and the atmospheric fresh-water dissolution is the key to the formation of the high-quality reservoir in the middle of the Yingshan Formation. [ABSTRACT FROM AUTHOR]

Residual water significantly influences the permeability and recovery of low-permeability reservoirs. In this study, we evaluated the effect of different residual water clusters on gas permeability in low-permeability sandstone by using the X-ray computed tomography technique, digital core analysis and different numerical methods (i.e., the finite difference method (FD), lattice Boltzmann method (LBM), and Kozeny-Carman method (KC)). The residual water distribution patterns are classified into water film, isolated water and branch water by the sphericity index and Euler number, and these two parameters can respectively characterize the shape and the connectivity of the residual water clusters. The water film significantly influences the gas permeability, while the influence of branch water and isolated water on permeability is observed to be insignificant. The difference in gas permeability values influenced by branch water and isolated water is minimal. The FD and KC equation express the average properties of the study space, which will result in the permeability to be insensitive to the space structure affected by the different residual water clusters. The LBM is appropriate to simulate the microscopic flow with complex boundary conditions, and is recommended to evaluate the permeability affected by the different residual water clusters in microscale, because of its sensitivity to the change of space structure. [ABSTRACT FROM AUTHOR]

An accurate estimation of the magnitudes of three in-situ stresses can usefully improve the production performance of hydraulic fracturing and reduce the risk of wellbore instability. Considering two types of linear elastic stress around the non-perforated arbitrarily inclined borehole (plane strain and plane stress conditions), the theory of hydraulic fracturing, and the maximum tensile criterion, the generalized governing formulas are derived to explain the mechanism of hydraulically or drilling-induced longitudinal, oblique, and transverse tensile wall-fractures in arbitrarily inclined boreholes. These formulas provide useful implications for constraints on the magnitudes of in-situ stresses, which determine the initiation of different types of tensile wall-fractures under different faulting stress regimes. The present paper extends the initiating conditions of the common transverse tensile wall-fractures related to the works of Hossain et al. (2000) and Nelson et al. (2005). The prerequisite for the special transverse tensile wall-fractures that tensile fractures simultaneously initiated around a whole borehole wall (Huang et al., 2012) is generalized by introducing the concept of the turning locations. The formulas corresponding to the turning locations are provided under different faulting stress regimes, revealing that the work proposed by Huang et al. (2012) is only a particular case. It is found that the turning locations are related to the critical borehole inclination or azimuth angles. Overall, all the formulas in this study for longitudinal, oblique, and transverse tensile wall-fractures can be used to design hydraulic fracturing experiments and to determine the in-situ stress by using the information of drilling-induced tensile wall-fractures. Moreover, the governing formulas of the turning locations can also be used to optimize the borehole trajectory. • Potential candidates of transverse hydraulically induced or drilling-induced tensile wall-fractures are generally formulated. • The same initiating sequences for longitudinal tensile wall-fractures occur between the vertical and horizontal boreholes. • The governing formulas of transverse and longitudinal tensile wall-fractures constrain the magnitudes of in-situ stresses. [ABSTRACT FROM AUTHOR]