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 structures, controlled by various microbial textures and diagenesis imprint, are poorly understood, which constrains hydrocarbon production in microbial dolomite. The Middle Triassic Leikoupo Formation microbial dolomite in the 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 imaging of rock samples from outcrops and cores. The sedimentary environment, and the hydrodynamic conditions in particular, plays a significant role in determining the multiscale textures of microbial dolomite. The pore structures are affected by the different micro-textures that influence 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 the 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, the pore-throat distribution is typically unimodal, with micro- or meso-pore throats influenced by the intensity of cementation. Stromatolites commonly show a bimodal pore-throat distribution, with size variations dependent on cementation and the intensity of dissolution. Strongly cemented agglutinated thrombolitic stromatolites and spongiostromate stomatolites exhibit micro-pore throat dominance, whereas laminated fine-grained agglutinated stromatolites and skeletal stromatolites display a mixture of micro- and meso-pore throats. Moderate cementation or dissolution may result in a mix of micro- and meso-pore throats in agglutinated thrombolitic stromatolites, while spongiostromate stomatolites and skeletal stromatolites may show meso- and macro-pore throats.