Quartz types and origins of the Upper Permian Dalong Formation shale of the Sichuan Basin: Implications for pore preservation in deep shale reservoirs

Abstract

Quartz is critical to reservoir properties (e.g., rock mechanical behavior and porosity) of organic mudstones, exerting an important effect on shale gas production. However, the types and origins of quartz in deeply-buried Upper Permian mudstones, a potential target of shale gas exploration in South China, remain poorly understood. A suite of deeply buried (5636–5645 m) mudstone samples of the Upper Permian Dalong Formation (DL) in the Sichuan Basin (South China) was selected to characterize quartz minerals using petrological and geochemical approaches, including SEM-CL observation. The studied mudstone samples are relatively enriched in organic matter (OM) (total organic carbon (TOC) = 1.88–18.46 wt%) and have been thermally over-matured. The DL organic mudstones accumulated in a failed rift basin, far removed from provenance areas, are chiefly composed of intrabasinal quartz and carbonate (mostly calcite). Quartz is represented by six major forms, including detrital quartz grains, siliceous skeletal fragments, silica nanospheres, microquartz aggregates, grain-replacive quartz, and matrix-dispersed microquartz, of which the latter five are of authigenic origin. Detrital quartz grains are not common in the DL mudstones. Silica reprecipitated as authigenic quartz was largely sourced from dissolved siliceous skeletal fragments (mostly radiolarians), and some silica appeared to have been released from the alteration of volcanic debris (mainly plagioclase) to form matrix-dispersed microquartz grains. Authigenic quartz influenced pore development and preservation of shale reservoirs in two ways. A rigid framework formed from numerous authigenic quartz grains (e.g., microquartz aggregates, silica nanospheres, and siliceous skeletal fragments) shields organic pores during mechanical compaction, whereas authigenic quartz precipitation (e.g., cavity-filling quartz, grain-replacive quartz) reduces porosity by filling voids. A moderate abundance of authigenic quartz (Sixs = ∼20%) is most favorable for development and preservation of pores (especially organic pores) in deep mudstone. Organic pores, widely developed in the studied DL deep mudstones, seem to be largely preserved in the samples with TOC values as much as 18.46 wt%. In summary, the DL mudstone deposits enriched in OM and contained moderate contents of authigenic quartz are especially favorable targets of deep shale gas exploration.