Pyrite is a mineral that is commonly found in shale gas reservoirs. Its genetic mechanism and impact on pore and organic matter in shale gas reservoirs are critical for shale gas exploration. This study selects the Longmaxi shale (Lower Silurian) in the Changning area of the southern Sichuan Basin by comprehensively using a scanning electron microscope (SEM), X-ray diffraction (XRD), total organic carbon (TOC), and image processing technology. The type and characteristics of pyrite in shale reservoirs are studied, the sedimentary environment and genetic mechanism of pyrite are analyzed, and the influence of pyrite formation on organic matter enrichment and reservoir formation is evaluated. The results showed that pyrite in shale primarily forms framboidal pyrite, euhedral pyrite, and subhedral pyrite, with particle sizes ranging from 1 to 15 μm. The maximum framboid diameter (MFD) is less than 20 μm, with the average particle size distribution of 3–5 μm. These parameters indicate the vulcanization and blocking environment. The reducing environment promotes organic matter enrichment and preservation. Framboidal pyrite has two genetic sequences: rich organic matter and poor organic matter. The development of organic matter will limit the continuous radial growth of pyrite and is conducive to the protection of pores, and the formation of pyrite can reduce the activation energy of kerogen reaction and catalyze the hydrocarbon generation of organic matter, resulting in higher gas content. The framboidal pyrite content can be used to predict high-quality shale gas reservoirs.

Comprehensive the analysis of cores, thin section, physical properties, mercury injection, and X-CT, genesis of heterogeneity of conglomerate reservoirs in Baikouquan formation of Mahu sag is studied. The Baikouquan formation belongs to fan delta glutenite deposit, typical conglomerate reservoir is developed. The dissolution pores are the dominated reservoir space, and its development degree has strong heterogeneity among different intervals, sedimentary facies, and rock types. Seven lithofacies are divided according to composition, structure, and sedimentary characteristics, among these; underwater distributary channel lithofacies is the most favorable for reservoir development followed by braided dam lithofacies and braided branch channel lithofacies. The strong hydrodynamic underwater deposition is favorable for the formation of high-quality reservoirs. Sedimentation determined the original difference and the difference of diagenesis promotes the formation of conglomerate reservoir heterogeneity.

The characteristics of lithofacies in the shale reservoir of the Wufeng and Longmaxi formations located in the Luzhou gas field were studied using the three-end-member method (mineralogical components). In addition, the microscopic characteristics and pore genesis of different lithofacies were studied and compared using TOC, FESEM, and digital core images. The results of the present study showed the development of five lithofacies: 1) quartz-rich argillaceous shale; 2) quartz/clay-mixed shale; 3) calcareous/clay-mixed shale; 4) calcium-rich argillaceous shale; and 5) clay-rich siliceous shale. The degree of pore development between lithofacies was highly heterogeneous. In addition, data indicated that total shale porosity increased with the increase in TOC. Thus, the pores were mainly related to organic matter. The enrichment of framboidal pyrite in quartz-rich shale (S1, S-3) and mixed shale facies (M-2, M-3) is essential during the formation of high-quality reservoirs. On the other hand, the content of organic matter in clayey shale was low, which does not favor the development of high-quality reservoirs. The differences in lithofacies pore structures are controlled by the sedimentary environment. The strong retention and reduction environment are the most favorable features for the formation of organic-rich siliceous shale lithofacies, which promote the preservation of organic matter and the development of reservoirs.

He, J.; Lian, Z.H.; Yu, H.J.; Huang, X.B.; Zhao, Z.J.; Yao, J.; Tang, X.J., and Li, X., 2020. Reservoir control mechanism of dolomite and sulfate rock compound palaeokarst in weathering crust. In: Guido Aldana, P.A. and Kantamaneni, K. (eds.), Advances in Water Resources, Coastal Management, and Marine Science Technology. Journal of Coastal Research, Special Issue No. 104, pp. 768–774. Coconut Creek (Florida), ISSN 0749-0208.In the diagenetic environment of meteoric freshwater, the compound palaeokarst of dolomite and sulfate rock is conducive to the formation of a storage and seepage network in the rock, which is key to the development of most evaporative marine facies reservoirs. Taking the Majiagou Formation in the central Ordos Basin as the study object, this research used a detailed description of drilling cores, realistic writing of typical karst outcrops, and sample analysis to determine the coupling relation between two kinds of rock weathering crust paleokarst. The salinity of seawater changed frequently in the sedimentary period of Ma 5; dolomitic flat, gypsodolomite flat, and anhydrite flat occurred alternately in the basin margins; and a set of compound rocks of dolomite and sulfate rocks deposited. After deposition, Ma 5 member uplifted with the Huabei platform, experiencing weathering and erosion, causing various types of dissolution pores and vugs to appear in the residual weathering crust formation. Dolomite with anhydrite concretion deposited in a gypsum-bearing dolomite flat environment laid the foundation for reservoir development, and the preservation degree of this kind of formation and karstification intensity of weathering crust karst complex are key to the formation of high-quality reservoirs. High-quality reservoirs are mainly located in the overlapping area of the gypsodolomite flat and weathering crust palaeokarst platform on the plane, and in the vertical vadose zone and part of the horizontal phreatic zone longitudinally.