Pore heterogeneity and evolution of the Lower Silurian Longmaxi shale reservoir in the Southern Sichuan Basin: Responses to sedimentary environment

Abstract

As the cornerstone for evaluating the storage capacity of shale reservoirs and assessing the viability of shale gas exploitation, shale pores play a very important role in shale reservoirs. Based on drilling well and test data, deep learning, and artificial intelligence for image analysis, the types, characteristics, structures, and vertical heterogeneity of shale pores were studied. Additionally, we conducted a comprehensive investigation into the sedimentary environment characteristics and their influence on the heterogeneity of shale reservoir pores in the Longmaxi Formation using associated geochemical analyses. The results revealed that the sedimentary environment underwent significant changes in the Longmaxi Formation, characterized by a sea level fall, accelerated sedimentation rates, weakened reducing conditions, decreased palaeoproductivity, and increased terrigenous influx, which led to a decrease in the frequency of organic matter (OM) pores, paralleled by a gradual increase in inorganic pores. In the S₁l₁¹⁻¹, S₁l₁¹⁻², S₁l₁¹⁻³ sections of the Long11 sub-member, OM pores constitute the primary pore type. Conversely, in the S₁l₁¹⁻⁴ section and the Long12 sub-member, inorganic pores prevail as the primary pore type. Furthermore, the strongly anoxic sections are also pay zones that are not only enriched in OM but also concentrated with OM pores. The S₁l₁¹⁻¹ and S₁l₁¹⁻² sections are characterized by anoxic conditions, with total organic carbon (TOC) content exceeding 4% and abundant biogenic silica. Notably, an increased terrigenous influx adversely impacts the development and preservation of pores in the Long11 sub-member while exerting a dual effect in the Long12 sub-member. The increased terrigenous influx dilutes the OM and decreases the probability of intergranular pores and intragranular pores filled by OM. Moreover, more brittle mineral particles were deposited, which promotes the development and preservation of intergranular and intragranular pores.