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

Abstract

In China, shale gas has been successfully produced from the Wufeng-Longmaxi shale 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 due to its strong tectonic alteration and weathering erosion. The current work aims to characterize the Wufeng-Longmaxi shale in this belt through detailed field investigation and experiments performed on more than 500 shale samples collected from 3 wells and 21 sections. The experiments include thin section identification, TOC, Ro, mineral contents tested by XRD, pore structure observation by SEM, porosity and permeability by mercury intrusion and Hellion adsorption. Here we report that the 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. The 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 TOC content is normally abundant in the lower part and decreases from the bottom up. It negatively correlates with the contents of brittle minerals in Wufeng but shows strong positive correlations occurring in the Longmaxi shale, implying that the siliceous minerals of the Longmaxi formation are biogenic while those of the Wufeng formation are terrigenous. Organic matter has been over matured, indicated by an equal Ro range between 1.8% and 2.2%. Porosity ranges from 0.67% to 1.76%, with an average value of 1.27%. And a positive correlation exists between the TOC and porosity. Permeability is 0.0049-0.6912mD, with a mean value of 0.1528mD. Pore types include organic pore, intergranular pore, intercrystalline pore, and secondary dissolution pore, in which the organic pores might dominate only in organic-rich shale. 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 8m to 32m. Shales are primarily preserved in synclines, but significantly eroded in anticlines. The burial depth is generally higher than 1500m in or near the axis of a syncline. Five favorable areas can be determined basing on the criteria of shale gas exploration.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017