Genesis of ultra-deep dolostone and controlling factors of largescale reservoir: A case study of the Sinian Dengying Formation and the Cambrian Longwangmiao Formation in the Sichuan Basin

This paper investigates the origin of ultra-deep dolostone and the factors influencing large-scale dolostone reservoirs,focusing on the Sinian Dengying Formation and the Cambrian Longwangmiao Formation in the Sichuan Basin.The study involves petrology,microscale X-ray diffraction,trace element analysis,and C-O-Sr-Mg isotope experiments to provide a detailed analysis.The research findings indicate that the Dengying and Longwangmiao formations comprise six types of matrix dolostone and four types of cement.The Dengying Formation,which developed under a sedimentary background of a restricted platform,contains special microbial and microcrystalline dolostones.The dolomite grains are small （<30μm） and have a low order degree （Min=0.55）,with large unit cell parameters and an extremely high Na content （Max=788 ppm）.The87Sr/86Sr value of the dolostone is consistent with contemporaneous seawater,while the δ13C and δ18O values are lower than those of the contemporaneous seawater.The δ26Mg value is small （Min=-2.31‰）.Powder crystal,fine-crystalline,and calcite dolostones with coarser and more ordered crystals exhibit similar δ13C and87Sr/86Sr values to microbial and microcrystalline dolostone.During the sedimentary period of the Dengying Formation,ancient marine conditions were favorable for microbial survival.Microorganisms induced the direct precipitation of primary dolomite in seawater,forming microbial and microcrystalline dolostones during the seawater diagenesis period.During the subsequent diagenesis period,dolostones underwent the effects of dissolution-recrystallization,structures,and hydrothermal fluids.This resulted in the formation of dolostone with coarser crystals,a higher degree of order,and various types of cement.The Longwangmiao Formation was developed in an interplatform beach characterized by special particle dolostone.The particle dolostone has a large grain size （>30μm）,high order degree （Min=0.7）,small unit cell parameters,high Na content （Max=432 ppm）,and low Fe and Mn content.The δ26Mg and δ13C values are consistent with the contemporaneous seawater,while the δ18O and87Sr/86Sr values are higher than those of the contemporaneous seawater.There is mutual coupling between multiple-period varying δ26Mg values and sedimentary cycles.The dolostone in the Longwangmiao Formation resulted from the metasomatism of limestone by evaporated seawater.The thickness and scale of the dolostone in the Longwangmiao Formation are controlled by the periodic changes in sea level.The period of dolostone development from the Sinian to the Cambrian coincides with the transition from Rodinia’s breakup to Gondwana’s convergence.These events have resulted in vastly different marine properties,microbial activities,and sedimentary climate backgrounds between the Sinian and the Cambrian.These differences may be the fundamental factors leading to the distinct origins of dolostone formed in the two periods.The distribution of sedimentary facies and deep tectonic activities in the Sichuan Basin from the Sinian to the Cambrian is influenced by the breakup and convergence of the supercontinent.This process plays a key role in determining the distribution,pore formation,preservation,and adjustment mechanisms of ultra-deep dolostone reservoirs.To effectively analyze the genesis and reservoir mechanisms of ultra-deep dolostone in other regions or layers,especially during the specific period of supercontinent breakup and convergence,it is crucial to consider the comprehensive characteristics of seawater properties,microbial activities,sedimentary environment,and fault systems driven by tectonic activities.This can help predict the distribution of high-quality and large-scale ultra-deep dolostone reservoirs.