Effect of silica diagenesis on porosity evolution of deep gas shale reservoir of the Lower Paleozoic Wufeng-Longmaxi formations, Sichuan Basin

Abstract

Silica diagenesis is critical to reservoir properties of shale, including porosity and rock mechanical properties, thus exerting a significant effect on shale gas production, but the role it plays on porosity evolution of deeply-buried shale remains unclear. In order to address this issue, a suite of deep gas shale samples (5910–5965 m depth) recovered from the Lower Paleozoic Wufeng-Longmaxi formations (WF-LMX) of the Sichuan Basin was selected to study silica diagenesis in these deposits and its effect on porosity evolution by the use of optical, SEM and SEM-CL observations, geochemical analysis, and low-pressure gas adsorption experiments. TOC of the WF-LMX shale sample suite that was altered to the thermally overmature stage ranges from 0.63 to 6.70 wt% (average = 3.26 wt%). Quartz hosted by the studied shale samples is present as six forms, including detrital quartz, quartz overgrowth, siliceous skeletal fragments, aggregates of amorphous to euhedral microquartz, silica nanospheres, and matrix-dispersed microquartz. Silica involved in the diagenetic alteration of the WF-LMX samples appears to have been chiefly sourced from the dissolution of siliceous skeletal fragments (mostly radiolarians) and secondarily from the transformation of clay minerals and alteration of detrital K-feldspar. Detrital quartz played a minimal role in porosity preservation, whereas authigenic quartz influenced porosity evolution in two ways. Siliceous skeletal fragments, aggregates of microquartz, silica nanospheres, and early diagenetic matrix-dispersed microquartz formed a rigid framework that shielded organic pores from compaction. However, quartz overgrowth and the precipitation of late stage diagenetic microquartz reduced porosity by filling voids. We estimate that a moderate content of authigenic quartz (Sixs = 10–30%) is optimal for the development and preservation of porosity (especially organic porosity). Organic pores, the dominant and pervasive pore type in the deep shale gas play, appear to have been best preserved in samples containing less than 5.5 wt% TOC. It is noteworthy, however, that some of the larger organic pores in samples containing >5.5 wt% TOC have collapsed in response to compaction. It is apparent, then, that organic porosity of the WF-LMX deep shale has been largely preserved showing great potential for shale gas exploration.