Gas in place and its controlling factors of the shallow Longmaxi shale in the Xishui area, Guizhou, China

There have been few systematic studies on the potential of the shallow Longmaxi shale in Lower Silurian strata although it develops widely outside the Sichuan Basin in southern China. The Longmaxi shale samples with a burial depth of 362-394 m were taken from the XK2 well in the Xishui area, Guizhou Province and their gas-in-place (GIP) and connate water contents were reported. The relationships of the GIP and connate water contents with the TOC content, mineral composition and porosity were investigated, and the controlling factors of the GIP content were discussed. The results show that the GIP content of the shales varies from 0.83 m(3) /t to 3.04 m(3)/t, with an average of 1.67 m(3)/t, and the connate water content is in the range of 5.01-12.38 mg/g, with an average of 7.46 mg/g. The GIP content of the shales is positively correlated with the TOC, but is not obviously related to the mineral composition, while the connate water content shows a definite positive linear correlation with the clay mineral but shows no correlation with the TOC. Both the organic matter (OM)-hosted and inorganic matter (IM)-hosted pores of the shales are extensively developed, with a total porosity of 3.03-5.50%. Most of the shale gas occurs in the OM pores, while the connate water is mainly distributed in the IM pores. The TOC content controls the porosity and pore structure of the shales, and therefore, TOC content is considered to be the primary controlling factor of the GIP content. The clay-hosted pores are abundant with the connate water, which notably decreases the sorption capacities of clay minerals, making minor contribution to the GIP content.