Brittleness features and controlling factors of continental shale from Da’anzhai Member of the Lower Jurassic Ziliujing Formation, northeastern Sichuan Basin

The brittleness features and their controlling factors of shale are clarified based on the systematic study of lithofacies and mineral composition, dynamic and static rock mechanical properties, and diagenesis of the Da’anzhai Member of the Lower Jurassic Ziliujing Formation in the northeastern Sichuan Basin. The following results are obtained. First, the continental shale sequence of the Da’anzhai Member is mainly composed of clayey shale, silty shale, shell limy clayey shale and shell limestone, featuring high content of clayey minerals and local enrichment of carbonate minerals. The source of felsic minerals is mainly terrigenous detritus, and the correlations between the TOC, clay, felsic and carbonate minerals are complex. Second, the brittleness of shale strata is under a joint effect of mineral content, mineral architecture and their interactions. The Da’anzhai Member is at the middle diagenetic stage, with a low degree of concretion, and its felsic mineral content has a weaker effect on brittleness compared with carbonate minerals. The increasing content of calcareous shells leads to differences in dynamic and static rock mechanical parameters and reduces the energy consumed for rock fracturing and fracture propagation as required. Third, the interbeds in the Da’anzhai Member are diverse in type and complicated in mineral composition and architecture. The dynamic and static mechanical parameters of shale with interbeds of homogeneous architecture are in good positive correlation, while those of shale with interbeds of heterogeneous architecture (i. e. shell interbed) are in negative correlation in terms of Young’s modulus. In all, factors such as lithology, interbed type, differences in rock mechanical properties, and diagenesis should be fully considered to establish an evaluation system for the brittleness and fracability of continental shale in the Jurassic. The conclusions achieved above are of important referential value to deepening the understanding of the brittleness and the evaluation of geological-engineering sweet spots of continental shale reservoirs with interbeds of complex types.