Abstract:

The mineral composition and brittleness characteristics of core samples from two sets of the Lower Paleozoic marine shale gas reservoirs, i.e.the Lower Cambrian Qiongzhusi and Lower Silurian Longmaxi formations in the southern Sichuan Basin, were studied by X-ray diffraction (XRD) in this paper. The results showed that the Qiongzhusi Formation and Longmaxi Formation marine shales in the southern Sichuan Basin have their different mineral contents and compositions. In the Qiongzhusi Formation shale, there are the most abundant quartz content (28.4% to 42.8%, average of 35.7%), higher clay mineral content (25% to 35.3%, average of 28.7%), and relatively lower carbonate mineral content (1.4% to 21.7%, average of 11.2%). In the Longmaxi Formation shale, there are high clay mineral content (13.4% to 66.1%, average of 34.7%) and carbonate mineral content (14.6% to 80.0%, average of 35.9%), and relatively lower quartz content (5.2% to 41.4%，average of 21.0%). In addition, other mineral components, including pyrite, feldspar and dolomite, also exist in these shales. Generally, the mineral composition characteristics of the Lower Paleozoic marine shale samples in the southern Sichuan Basin are similar to those of the North America. Both the Qiongzhusi Formation shale and the Longmaxi Formation shale in the southern Sichuan Basin have high contents of brittle minerals. The former ranges from 55.6% to 73.9%, with an average of 64.1%. The latter ranges from 26.9% to 86.6%, with an average of 62.5%. It is suggested that the Lower Paleozoic marine shale in the southern Sichuan Basin have good brittleness and compressibility, and are beneficial to fracturing treatment for shale gas.