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Brittle minerals, especially quartz, play an important role in the quality of shale reservoir. The precipitation of authigenic quartz caused a significant change in the rock stiffness and porosity. However, the formation mechanism of authigenic quartz in shale is still unclear. Lower Cambrian Niutitang shale from Guchengcun outcrop has been examined with respect to quartz cement. Four different types of quartz have been identified using SEM/CL/EDS. Type 1 occurs as angular to sub-rounded silt-size quartz grains with uniform luminescence, indicative of detrital origin. Type 2 appears as relatively large quartz aggregates approximately 30–150 μm in size, which is composed of small quartz grains (1–3 μm) and abundant in siliceous shale. The petrological evidence and consideration of potential diagenetic process suggest that Type 2 quartz was formed by local re-crystallization of biogenic silica in syndiagenesis and preferentially precipitated in the axial cavities of sponge spicules. Type 3 occurs as quartz overgrowths that are essentially non-luminescent at the periphery of high luminescence detrital quartz grains. This type quartz is present in most investigated samples. Type 4 represents microcrystalline quartz embedded in the illitized clay matrix as discrete, short chains or small clusters/nests and they are most commonly observed in argillaceous shale. The petrological evidence and CL responses indicate that both Type 3 and Type 4 quartz are derived from precipitation of silica released during the smectite to illite dissolution–precipitation reaction. S-I transition can be driven by microbial metabolic processes in shallow buried diagenesis or by heat and pressure in late diagenesis. Some of Type 4 quartz may be originated from low-temperature microbial pathway in shallow buried diagenesis, while Type 3 quartz and the remaining Type 4 quartz were formed in late diagenesis. Siliceous shale contains less abundant detrital quartz than argillaceous shale, indicating that siliceous shale was deposited in deeper water and far from terrigenous input than argillaceous shale. Although identifying the exact cause for diagenetic silica deposition in shale is not a trivial matter, its recognition is important for the reconstruction of the depositional history of shale sequences and understanding the role of diagenesis in shale reservoirs development, moreover, it also has great potential for enhancing prediction of mechanical rock properties in shales. [ABSTRACT FROM AUTHOR]

The paper investigates lateral variation of gas saturation and its geological controls of the No.2 coal seam in the Permian Shanxi Formation based on coal samples from 21 exploration wells in the Yanchuannan Coalbed Methane (CBM) Field, Southeastern Ordos Basin, China. The data set reveals that gas saturation of the No.2 coal seam shows a high lateral variation from 32.44% to 89.69%, and that there is an overall trend in gas saturation with depth overprinted by four other factors: 1) influence of faults, 2) secondary biogenic gas generation, 3) local top seal conditions and 4) variations in coal properties. The reverse faults F1 and F2 are effective barriers horizontally for the hydraulic communication and divide our study area into relatively closed and recharge zones. Gas saturation of the relatively closed zone is generally higher than that in the recharge zone. In the recharge, gas saturation increases generally along the flow pathway of groundwater with the exceptions of the areas affected by secondary biogenic gas. The stable isotope compositions suggest that late-stage biogenic methane was generated via CO 2 reduction associated with meteoric water recharge, which significantly contributes to higher gas saturation than expected. Meanwhile, the biogenic gas generation was/is restricted to shallow depths. The apparent variations in local areas of lithology of the overlying stratum next to the coal seam suggest that the top seal conditions are deteriorative, which can cause gas dissipation and reduce gas saturation remarkably. Integrally, for the variations in coal properties, gas saturation has a general positive relationship with coalification, and weak negative correlations with moisture content and ash yield but the relationships lacks statistical significances. Gas saturation is not associated with coal maceral in the study area. [ABSTRACT FROM AUTHOR]