Mechanism of dolomitization and reservoirs in the second Member of Permian Maokou Formation in Hechuan-Tongnan Block, central Sichuan Basin

Key words: 
• Sichuan Basin /
• Permian /
• Maokou Formation /
• Dolomitization /
• Reservoir-forming mechanism

Abstract: [Objective]The Hechuan-Tongnan block in the central Sichuan Basin has drilled multiple high-yield wells in the second member of the Permian Maokou Formation dolostones, revealing significant exploration prospects. However, the characteristics and genesis of dolomite reservoirs remain unclear, hindering the prediction of favorable reservoir distribution. [Methods]Based on observations of cores and thin sections, combined with geochemical methods such as carbon-oxygen isotopes, rare earth elements, and carbonate U-Pb dating, a systematic study was conducted on the genesis and controlling factors of the second member of the Maokou dolomite reservoir. [Results]bioclastic-bearing micrite limestone, micrite bioclastic limestone, dolomitic limestone, and grain-bearing dolomite. Among these, grain-bearing dolomite is the primary reservoir rock type. There are three types of reservoir spaces: intergranular dissolution pores, dissolution pores, and dissolution fractures, with dissolution pores being the most developed. The formation of dolomite occurred during the quasi-syngenetic shallow burial period, with early matrix dolomite and late saddle dolomite formed at 262 ± 16 Ma and 259.1 ± 4.7 Ma, when high Mg/Ca ratio seawater infiltrated and flowed back into the porous beach sediments in the confined environment between the shoals, leading to dolomitization.[Conclusions]The reservoir is controlled by sedimentary facies, early dolomitization, and burial hydrothermal dissolution related to structural fractures. The high-energy particle beach facies in the elevated areas of paloe-morphology provide the basis for reservoir development. Dolomitization during the quasi-syngenetic shallow burial period created conditions for the preservation of early pores and their transformation during the mid-deep burial period. The hydrothermal dissolution process related to the Emei taphrogenesis leads to the heterogeneous development of dissolution pores, which is key to reservoir development.