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Natural fractures have an important relationship with shale gas production. This study selected the reservoir of the Wufeng‐Longmaxi Formation in Luzhou in southern Sichuan as an example. Based on the seismic reflection map of the formation, using the mechanical parameters obtained from the core sample experiment and using the finite element numerical simulation method, a three‐dimensional geological model is established, the tectonic stress field is inverted and the fracture distribution is predicted according to the Mohr‐Coulomb fracture criterion of the rock fracture, which provides theoretical support for the subsequent well pattern layout and is conducive to improving the efficiency of shale gas exploration and development. The analysis shows that the study area is dominated by NE reverse faults, with an inclination of more than 40°. The core observation results show that the filling degree of fractures is relatively high, and the width is mostly less than 1 mm, mainly caused by tectonic shear fractures. The tectonic stress in the sedimentary belt where the syncline structure is deeply buried is higher, the stress in the northwest and southeast tectonic ridges is relatively low, and the differential stress in the northeast tectonic ridge and near the fault is high. The NE fault and the surrounding rock mass have the highest fracture degree; the fracture proximity coefficient is >1.5 and the fractures are the most developed. The degree of fracture development in the area with a gentle structure is relatively low, but it is far from large faults, which is conducive to preserving shale gas.

With the Upper Cambrian Xixiangchi Formation in the eastern Sichuan Basin as the target, this study investigates various diagenetic events during different diagenetic stages in deep dolomite reservoirs, accompanied by evaluations of their effects on the formation and evolution of the reservoir rock. A series of experiments are implemented on core and outcrop samples, including petrologic analysis, fluid inclusion analysis, rare earth and minor element investigation, and carbon and oxygen isotope test. During the syngenetic (syndepositional and penecontemporaneous) diagenesis stage, dolomitization is closely related to evaporation concentration and seepage reflux of high-salinity seawater, which facilitates the reservoir rock development by greatly enhancing the permeability of the reservoir. Meanwhile, a small number of secondary pores are generated in the sediments subjected to episodic atmospheric exposure and thus affected by meteoric water. During the early diagenesis stage, recrystallization transforms part of the granular dolomite into the crystalline dolomite with or without the phantom of the grain texture. It also alters the original rock’s pore structure and improves the effective primary porosity. Thus, recrystallization is key in forming the crystalline dolomite reservoir rock. However, compaction, cementation, and filling lead to the loss of massive early-formed primary pores and some secondary pores. During the mesodiagenesis-late diagenesis stage, the burial karstification, related to organic matter maturation, is the most direct control factor of the effective reservoirs space formation, and its alteration effect on the reservoir rock is related to the early process. This research helps to better identify the impact of various diagenetic processes during different diagenetic stages upon the formation and evolution of the deep dolomite reservoir rock, and it also helps analyze the relationships among these diagenetic processes. The findings of this research provide valuable references for investigating the formation mechanism of the deep dolomite reservoir rock in the Sichuan Basin.

Natural fractures are critical factors that should be considered in shale reservoir evaluation, storage condition analysis, horizontal well design, and fracturing stimulation, which also play a non-negligible role in the occurrence state of shale gas in the reservoir. This paper discussed the influence of fracture development on gas-bearing properties based on the analysis results of core observation, scanning electron microscopy, mineral composition, and gas-bearing properties after the development characteristics of fractures and their longitudinal variation law were clarified. In this way, the development characteristics of organic-rich marine shale fractures in the Longmaxi Formation in the 203 well area of the Luzhou member of the Sichuan Basin and their effects on the gas-bearing properties can be analyzed. The results show that the Longmaxi Formation shale develops shear fractures, extensional fractures of tectonic origin, bedding fractures, dissolution fractures, and abnormally high-pressure fractures of nonstructural origin. Specifically, interlayer fractures, intercrystalline fractures, organic matter contraction fractures, and fractures between clay layers are microfractures. Fracture development is characterized by short longitudinal extension, small opening, high degrees of composite filling, and large density changes, with calcite and pyrite as the filling materials. The fracture density has a “three-stage” variation pattern longitudinally, and the bottom is dominated by thin siliceous shale development, together with a small amount of shale mixed with calcareous and calcareous materials. Moreover, the fracture is dominated by “splitting” and “shearing” failure, crossing stratification with the fracture density. The highest fracture density was found in the 2 sub-layer, featuring the joint development of horizontal and vertical fractures, which form the mesh fracture system through mutual cutting and restriction. The lithofacies in the 4 sub-layer are dominated by clay siliceous shale with a small amount of mixed shale of calcareous and siliceous materials. The formation of fractures always expands along the lamellation direction, which has concentrated development members of top and bottom fractures, with the development of horizontal fractures dominated and vertical fractures less developed. Furthermore, a synergistic effect can be found among the total organic carbon (TOC) content, fracture density, and gas-bearing property of the shale in Longmaxi Formation. It is worth noting that a high TOC content and siliceous content are conducive to the formation of microfractures, while the development of fracture contributes to the total gas-bearing property, especially to the increase in free gas content. To be concrete, the free gas content in the fracture development member accounts for more than 55% of the total gas content, thanks to a channel provided by fractures for the desorption of shale gas.

This study addresses the poorly understood physical properties of the Shixi Oilfield reservoir, which consists of a weathered Carboniferous volcanic rocks with strong heterogeneity and in which logging identification and evaluation are difficult. Using the lithology, lithofacies, and reservoir space characteristics of volcanic materials, this comprehensive study uses core, well logging, mud logging, and production testing data to analyze the relationship among the lithology, physical properties, electrical properties, and oil-bearing properties (referred to as the “four properties”) of weathered Carboniferous volcanic crust in addition to fluid identification. 1) The lithology of Carboniferous volcanic crust is dominated by breccia lava, agglomerate, banded lava, and compact tuff, and the lithofacies are mainly effusive facies. Secondary pores and tectonic fissures are important reservoir spaces, and the corrosion-fracture pores are significant for reservoir properties. 2) The “four properties” of volcanic reservoirs in the study area have clear relationships. On this basis, data on the electrical properties of the material, such as interval transit time, density, and neutron, were used to establish a logging interpretation model of the properties and oil saturation of the volcanic rock. 3) Using the resistivity-porosity cross-plot method, normal probability distribution method, and Rt/Rxo-Rt cross-plot method, volcanic reservoir fluids were identified with coincidence rates of 80%, 63.63%, and 63.63%, respectively. The cross-plot method determines lower limits of the reservoir’s physical properties and oil saturation, yielding porosity>9%, permeability>0.2 mD, and oil saturation>45%.