Lithologic characteristics and diagenesis of the Upper Triassic Xujiahe Formation, Yuanba area, northeastern Sichuan Basin

Abstract

The Upper Triassic Xujiahe sandstone in the Yuanba area, northeastern Sichuan Basin, is a typical coal-bearing tight gas reservoir. Whether or not the commercial production of gas from these tight sandstones can be achieved largely depends on the quality of the reservoirs. In this study, a petrographic analysis using a thin section and a scanning electron microscope (SEM), combined with reservoir property data, was carried out to discuss the effects of primary mineral compositions on diagenetic processes and reservoir quality.The Xujiahe sandstones are mainly litharenite and feldspathic litharenite with minor sublitharenite, quartzarenite and calcarenaceous sandstone. Reservoir quality is poor, with porosity ranging mainly from 1% to 4% and permeability mainly from 0.01 md to 0.1 md, among which the relatively high values are limited to litharenite and feldspathic litharenite. The anomalously poor quality of sandstones is attributed to the high compaction and calcite cementation during the early diagenetic stage while the relatively high quality is associated with the precipitation of chlorite rims. These diagenetic processes are functions of the sandstones composition and the primary sandstones composition is intrinsic to the quality of the Xujiahe sandstones. The sandstones with high contents of matrix and ductile rock fragments (DRFs) and/or carbonate rock fragments (CRFs) commonly experience intense compaction or cementation, which are key factors for the poor quality of reservoirs. Although the contents of matrix, DRFs and CRFs are low in quartzarenite, intense compaction also occurs due to the finer grain size. Only litharenite and feldspathic litharenite with minor contents of matrix, DRFs, CRFs and volcanic rock fragments (VRFs) commonly experience the precipitation of chlorite rims, and are therefore recommended to be the favourable tight gas reservoirs in the Yuanba area.