Geological and Microstructural Characterization of the Wufeng-Longmaxi Shale in the Basin-Orogen Transitional Belt of North Guizhou Province, China

摘要

In China, shale gas has been successfully produced from the Upper Ordovician Wufeng and Lower Silurian Longmaxi shale formation that are located in or adjacent to the Sichuan Basin. However, shale gas exploration in the basin-orogen transitional belt of the Sichuan Basin is fraught with difficulties because of the strong tectonic alteration and weathering erosion. Based on detailed field investigation and experiments performed on more than 500 shale samples collected from 3 wells and 21 sections, here we report on the geological and microstructural characteristics of the Wufeng-Longmaxi shale in the north of the Guizhou Province. Sedimentary facies of the Wufeng-Longmaxi shale formation include the deep marine shelf, shallow marine shelf, offshore, and nearshore, developed from north to south of the study area in Late Ordovician and Early Silurian. Lithology of the lower part is dominated by the black carbonaceous shale, whereas that of the upper part mainly comprises grayish green silty mudstone and muddy siltstone. The total organic carbon (TOC) content is normally abundant in the lower part and decreases from the bottom up. Organic matter has been over matured, indicated by an equal R-o range between 1.8% and 2.2%. Porosity ranges from 0.67% to 1.76%, with an average value of 1.27%. Permeability is between 0.0049 mD and 0.6912 mD, with a mean value of 0.1528 mD. Pore types include organic pore, intergranular pore, intercrystalline pore, and secondary dissolution pore. Mineralogical constituents are significantly dominated by brittle minerals that include quartz, feldspar, and pyrite, ranging from 58% to 94%, indicating a high brittleness index. From south to north, the shale thickness gradually increases from 8 m to 32 m. Shales are primarily preserved in synclines, but significantly eroded in anticlines. The burial depth is generally higher than 1500 m in or near the axis of a syncline. Five favorable areas can be determined based on the basic criteria of shale gas exploration.