Pore size distributions contributed by OM, clay and other minerals in over-mature marine shale: A case study of the Longmaxi shale from Southeast Chongqing, China

Abstract

Nano-scale pores play an important role in determining the adsorption capacity of shale gas. To quantitatively evaluate the pore size distributions contributed by organic matter (OM), clay and other minerals in an over-mature shale reservoir, the Lower Silurian Longmaxi shale from Southeast Chongqing was selected as a case study and was analyzed by the pore size distributions contributed by OM, clay and other minerals (PSDCOCO) method. Based on the results of total organic carbon (TOC), carbon element content in OM, X-ray diffraction (XRD) and low temperature nitrogen adsorption/desorption (LTNA) experiments, the pore size distributions contributed by a unit mass of OM, clay and other minerals were calculated and the differences between the results of the LTNA and PSDCOCO methods were determined for the four verified samples. The average value of the difference between the experimental and calculated pore volumes is 0.154 × 10−3 cm3/g. For the Longmaxi shale from the Py1 well, the contributions of OM and other minerals to the pore volume (pore width <200 nm) are primarily distributed in the pore width ranges from 2 to 20 nm and <2 nm, respectively. The nano-scale pore size distributions (<200 nm) contributed by the clay minerals have two relatively high peak values in the pore width ranges of 20–100 nm and 2–5 nm. The pore volumes for pore widths less than 200 nm contributed by a unit mass of OM, clay and other minerals were 115.776 × 10−3 cm3/g, 10.489 × 10−3 cm3/g and 0.006 × 10−3 cm3/g, respectively. In the shale reservoir, the contributions of OM, clay and other minerals to the pore volume with pore widths less than 200 nm were 45.75%, 54.17% and 0.08%, respectively, based on the limited data available.