The impacts of sedimentary micro-textures and diagenesis on the pore structure of microbial dolomite in the Middle Triassic Leikoupo Formation, Western Sichuan Basin, China

Abundant microbial dolomite reservoirs have been discovered worldwide. However, their complex pore structure, as controlled by various microbial textures and diagenesis imprint, are poorly understood, which constrain the hydrocarbon production in microbial dolomite. The Middle Triassic Leikoupo Formation microbial dolomite, in Western Sichuan Basin, provides a typical example to decipher the controls of pore structure in microbial dolomite through integrated studies of petrography, high-pressure mercury injection and micro-CT on rock samples from outcrops and cores. The sedimentary environment, particularly the hydrodynamic conditions, play a significant role in determining the multi-scale textures of microbial dolomite. The structures of pores are affected by the different micro-textures by determining the overall pore throat characteristics, including morphology, stratification and original connectivity. Diagenetic processes play a crucial role in altering pore structures, with differences observed in pore distribution due to cementation or dissolution. In the thrombolites, densed micrite thrombolites, porphyritic micrite thrombolites, and peloidal-aggutinated thrombolites typically exhibit micro pore-throat dominance with a bimodal distribution, but may display meso and macro pore-throats under dissolution or weak-moderate cementation. For the foam laminated thrombolites, pore-throat distribution is typically unimodal, with micro or meso pore-throat influenced by the intensity of cementation. Stromatolites commonly show a bimodal pore-throat distribution, with size variations dependent on cementation and dissolution intensity. Strongly cemented agglutinated thrombolitic stromatolites and spongiostromate stomatolites exhibit micro pore-throat dominance, whereas laminated fine-grained agglutinated stromatolites and skeletal stromatolites display a mix of micro and meso pore-throat. Moderate cementation or dissolution may result in a mix of micro and meso pore-throat in agglutinated thrombolitic stromatolites, while spongiostromate stomatolites and skeletal stromatolites may show meso and macro pore-throats.