Shiyou shiyan dizhi (Jul 2022)
Pore characteristics of marl reservoir in Maokou Formation of Middle Permian, southeastern Sichuan Basin
Abstract
Breakthroughs of oil and gas exploration have recently been achieved via several wells in marl reservoirs of the first member of the middle Permian Maokou Formation in the southeastern Sichuan Basin. As a new type of unconventional gas reservoir, a lot of attention was attracted. In order to understand lithological characte-ristics and pore characteristics of marl reservoirs, lithological, pore types and pore structure characterization, works were carried out based on field and core samples. Results show that: (1) The TOC content of the marl reservoirs, which is mainly in outer gently sloping facies, is 0.76%-1.1%, with an increasing trend from northwest to southeast, similar to porosity changes. (2) Reservoir spaces of the marls mainly include organic pores, inorganic pores and microfractures. The organic pores can be classified into independent organic pores, organic pores with talc and intergranular organic pores of pyrite. The inorganic pores can be classified into intergranular pores, intercrystalline pores, and crystal dissolved pores. The microfractures can be classified into talc fractures, grain boundary fractures, and stress fractures. Organic pores are not dominant in the marl reservoirs, while microfractures are the most important reservoir space. (3) Combined characterization of cryogenic nitrogen adsorption, cryogenic CO2 adsorption, high-pressure Hg injection experiments and CT scan show that pores of the marls are mainly ink-bottle shaped with narrow neck and wide body. Micropores, mesopores and macropores are developed, with pore diameters of about 0.4-100 nm. The marls have obvious lithologic heterogeneity and poor connectivity. Fractures have an important influence on the connected pores. The marl in the first member of the Maokou Formation is a new type of unconventional reservoir, which has complex mineral composition and special pore characteristics. Targeted acid fracturing technology should be strengthen to promote the exploration and production of the marl reservoir.
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