Grain assemblages and diagenesis in Ordovician-Silurian transition shale deposits of the Upper Yangtze Platform, South China

Abstract

• Grain assemblages in shale establish a key starting condition for diagenetic pathway. • There are four origins of quartz and two origins of organic matter in marine shale. • Cementation and mechanical compaction make a significant reduction of porosity. • Shale with different lithfacies has differences in pore types. • Authigenic quartz cementation made porosity reduction but enhanced the brittleness . This paper considers possible impacts of mineralogy and organic matter contents and assemblages on the diagenetic history, pore structure, and brittleness of the Ordovician-Silurian Wufeng Formation and Longmaxi Formation (Northwestern Hunan, upper Yangtze Platform) of South China. Multiple analytical approaches, including X-ray diffraction, scanning electron microscopy, cathodoluminescence, and X-ray mapping, were utilized to identify favorable shale gas intervals. Analyzed shale samples of the Wufeng-Longmaxi succession are dominated by such extrabasinal components as quartz, K-feldspar, albite, lithic fragments, and K-rich and Mg-rich clay minerals. Subordinate components include siliceous tests and organic particles. Authigenic minerals present as grain replacements and pore-fillings include quartz, illite, pyrite, and carbonate minerals. Depositional and diagenetic organic matter, which are discriminated petrographically, are present in the studied deposits. Wufeng Formation samples are enriched in organic matter (mainly of diagenetic origin) and authigenic quartz. The dominant types of porosity of the Wufeng samples include pores hosted by organic particles and interparticle pores between authigenic quartz crystals. The precipitation of authigenic quartz appears to have been the principal means of porosity reduction and enhanced the brittleness of the shale. Analyzed samples of the Longmaxi Formation are dominated by extrabasinal grains and lesser authigenic minerals. Reduction of depositional porosity of these deposits appears to have been largely the result of physical compaction. Thus, extensive authigenic quartz cementation and thermal maturation of organic matter of the Wufeng Formation likely enhanced the reservoir capacity and fracturing ability of these deposits relative to the overlying Longmaxi Formation. This understanding is especially relevant to shale gas exploration and production considerations of organic-rich fine-grained sediments of similar sedimentological background.