Mineral composition and seal condition implicated in pore structure development of organic-rich Longmaxi shales, Sichuan Basin, China

Abstract

Abstract Shale pore structure (morphology, type, size distribution, and porosity) is important in controlling the storage and movement of oil and gas. Characterization of pore structure is one of the main research foci for unconventional resources; however, factors influencing its heterogeneity are poorly understood. With total organic carbon ranging at 0.52–4.23%, 23 core samples from three wells (JY2, WY1, and YZ1) in different tectonic units of the Longmaxi shale in Sichuan Basin were studied, using a combination of petrographic, geochemical, and pore-structure characterization techniques. The shale porosity shows positive relationships with quartz and total organic carbon contents in JY2 and WY1, but not in YZ1, because of its different pore structure. Pore-structure differences are primarily attributed to the different contributions of organic matter (OM)-hosted nanopores, which are controlled by preservation conditions. Due to the differences in the duration and intensity of tectonic activities, the JY2 and WY1 areas are characterized by over-pressured conditions with respective pressure coefficients of about 1.5 and 1.4. Conversely, the YZ1 area has a normal pressure condition with a pressure coefficient of about 1.0, indicating a poor seal condition. Under good seal conditions, generated hydrocarbons cannot escape easily. Thus, the accumulation and expansion of shale gas trapped within organic pores increase the pore pressure to abnormally high-levels, which in turn protect the shale reservoir from compaction and support OM-hosted pore development. The OM pore evolution of the Longmaxi shale in the Sichuan Basin during burial was generally uniform; thus, the differences in pore structure reflect subsequent tectonic uplift. The siliceous shales of YZ1 did not develop many OM-hosted pores because of the lack of seal conditions during the high-maturity stage before uplift; most of the pores shrink or close due to an escape of hydrocarbons. Mineral composition, thermal maturity, and seal conditions are identified to influence pore development and preservation in fine-grained sedimentary rocks.