Quartz types in the Wufeng-Longmaxi Formations in southern China: Implications for porosity evolution and shale brittleness

Abstract

Quartz is an important component of shale and has a significant influence on shale physical properties. However, the specific effects that different quartz types exert on shale porosity evolution and brittleness remain unclear. A series of experiments including petrographic observations, geochemical analyses, and triaxial compress experiments were conducted on shale samples from the Wufeng-Longmaxi Formations in southern China. The Wufeng-Longmaxi samples have been classified into siliceous shale, argillaceous shale, and mixed shale. Several quartz types were identified in the shale samples, including extrabasinal detrital quartz and authigenic quartz (microcrystalline quartz, quartz overgrowths, and bioclastic quartz). Based on the different origins, the authigenic quartz was divided into biogenic quartz derived from the dissolution of siliceous organisms and S–I quartz derived from the conversion of smectite to illite. Bioclastic quartz, most microcrystalline quartz, and most quartz overgrowths are biogenic quartz. A small number of quartz overgrowths and microcrystalline quartz are S–I quartz. The quartz content of different genesis processes was quantitatively calculated using major and trace elements data collected via XRF. Quartz types in siliceous shale samples are dominated by biogenic quartz (avg. 49%). Extrabasinal detrital quartz is the primary quartz type in argillaceous shale samples (avg. 68%) and mixed shale samples (avg. 50%). Porosity reduction mechanisms in most of the Wufeng-Longmaxi shale samples are primarily compaction-dominated. Biogenic quartz acts as a pore protector rather than destroyer, while S–I quartz derived from the conversion of smectite to illite acts as a pore destroyer rather than protector. The brittleness index (BI) of siliceous shale samples is the highest of all the shale samples since the abundant biogenic microcrystalline quartz particles dispersed within the shale matrix connect rigid grains to form an interconnected force framework.