Pore characterization of isolated organic matter from high matured gas shale reservoir

Abstract

Knowledge of pore structure of isolated organic matter (OM) provides guidance for better understanding the origin, distribution and characterization of pore system in gas shale reservoir, and can be used to better comprehend the storage and transport mechanism of natural gas in shale reservoir. Pore structure characteristics of isolated organic matters and its corresponding bulk shales were investigated for five high matured marine shale samples from the Lower Silurian Longmaxi formation in south China using X-ray diffraction (XRD), total organic carbon content (TOC) tests, field emission scanning electron microscope (FE-SEM) observation and ultra-low pressure nitrogen gas physisorption. The results indicate that OM hosted pores are abundant in bulk shale samples under FE-SEM observation. The BET Surface area and the BJH pore volume of isolated OM are approximately three times greater than that of bulk shale samples. Pore size distribution (PSD) of isolated OM from five shale samples show a consistent variation pattern. Additionally, with increasing TOC content, the PSD curves of isolated OM gradually grow down as result of the ductility of OM. Bimodal PSD in surface area and unimodal PSD in pore volume exist in both isolated OM and bulk shale samples, indicating surface area is mainly associated with micropores and fine mesopores (<10nm) and larger pores are the dominate contributor to pore volume. Paralleled variation trends in PSD, especially in the pore size smaller than 4nm, are observed between isolated OM and its corresponding bulk shale samples. However, the PSD divergences at the pore size larger than 10nm between different samples are obvious. Therefore the pore network in gas shale reservoir is predominantly associated with the organic matter, especially small pores, and the mineral compositions are expected to be responsible for lager pores.