Detrital quartz and quartz cement in Upper Triassic reservoir sandstones of the Sichuan basin: Characteristics and mechanisms of formation based on cathodoluminescence and electron backscatter diffraction analysis

摘要

The relationship between detrital quartz grains and silica cement is significant because cement is often a major cause of porosity reduction in oil and gas reservoirs, thus degrading hydrocarbon reservoir quality. On the other hand, the attributes of grain surfaces may inhibit cementation, thus preserving porosity in the reservoirs. In this study we focus on the microstructures in quartz grains and secondary overgrowths (quartz cement) in the heavily cemented sandstones from uplifted Upper Triassic siliciclastic rocks in the southern Sichuan basin, China. Electron backscatter diffraction (EBSD), optical, and cathodoluminescence (CL) imaging methods have been used to address the microstructural and crystallographic attributes of the detrital grains and their overgrowths. This study confirms that: (1) the overgrowth zones of quartz are clearly visible in optical and CL images; (2) all silica overgrowths are perfectly crystalline quartz; (3) quartz cements have a syn-taxial relationship with the host detrital grain, indicating crystallographic continuity between the detrital quartz and its overgrowth; and (4) crystallographic maps confirm that Dauphine twins commonly occur both in the detrital grains and in the quartz cement. Most Dauphine twins are developed at grain-grain contacts, including the margins of detrital grains where they are in contact with quartz cement. Some twins are limited to a single grain, whereas others cross grain-cement boundaries. Based on these twin distributions, we can distinguish inherited twins from twins that formed during/after burial compaction. This study documents the grain deformation structures induced by sedimentary compaction, and these structures can be accurately illustrated by EBSD data.