Chemical compositions of Longmaxi shales and implications for fault stability during the extraction and storage of fuels and energy

Abstract

Shales of the Longmaxi formation are currently the most important target zone for gas recovery in the Sichuan Basin, southwest China. Considering the frequent occurrence of hydraulic fracturing implicated induced earthquakes, it is important to understand the mineralogical controls on fault stability – across this transitional basin. We recover a full stratigraphic sequence of the Longmaxi and complete XRF and TOC analyses on these shales. Major components include SiO2, CaO, Fe2O3, Al2O3, K2O, MgO, TiO2 and TOC, and a transition from Ca-to Si-dominant contents from the top to the base of the stratigraphic column. This variation in chemical composition reflects a change in sea level during deposition with both biogenic and hydrothermal sediments contributing to the sedimentary record. Chemical compositions are closely related to shale fault stability with high tectosilicate and/or carbonate contents promoting the potential for both instability and thus triggered earthquakes. The highest Si- and Ca-contents in sub-section 1-1 (S1l1-1) and section 2 (S1l2) of the Longmaxi contribute the highest contents of tectosilicates and carbonates, with these minerals potentially enabling and promoting fault instability during hydraulic fracturing. Our results have important implications for understanding the relationship of shale composition and fault stability during hydraulic fracturing for shale gas exploitation.