Characteristics and Impact Factors of Nanoporosity in Marlite Reservoir of First Member of Maokou Formation in Central Sichuan Basin

Abstract

AbstractUnconventional marlite reservoirs in the first member of Maokou Formation (P1m1) in Sichuan Basin have important exploration practical significance. This study focuses on the marls of the first Member of Maokou Formation in Well HS3 in central Sichuan basin. The comprehensive measurements including TOC, porosity, permeability, X-ray diffraction, carbon dioxide and nitrogen adsorptions, nuclear magnetic resonance and FE-SEM were conducted on marl samples to depict the nanoporosity within the marlite reservoir of the first member of Maokou Formation and hence to clarify the regulating elements of the occurrence of nanopores. The results show that P1m1a and P1m1c sub-members of Maokou Formation are mainly marls which are enriched in carbonates,followed by a little bit of quartz and clay minerals. The organic matter content varies greatly, and the overall TOC content is not high (0.17%–2.11%). The nanopore radius of P1m1 marls ranges from 2 nm to 160 nm, and the overall development degree is mesopore > macropore > micropore, which is worse than shale. The interlayer pores of talc aggregate and organic pores are the main ones, followed by intragranular dissolution pores. The nano-pore development is mainly controlled by the content of organic carbon and clay minerals, which shows that the pore volume is positively correlated with TOC and clay mineral content. During the deposition of the first member of Maokou Formation in central Sichuan, a large area of transgression was developed and marlstones developed, especially P1m1a and P1m1c, with high TOC and clay mineral content and well-developed nanopores, which can be used as the main storage space for natural gas.KeywordsNanoporosityReservoirsNatural gasMarlMaokou formationSichuan basin