Dolomite karst reservoir characteristics and dissolution evidences of Sinian Dengying Formation, Sichuan Basin

Sinian Dengying Formation was developed extensively as an important natural gas reserving and producing formation in Sichuan Basin. New evidences for micro-petrography and fluid inclusion salinity data in terms of dissolution were obtained through comprehensive research of reservoir petrography and sedimentology, reserving space types, diagenesis and evolutionary sequence, especially thin section identification, SEM identification, and the analysis of element, carbon and oxygen isotope as well as fluid inclusion. It is considered that Dengying Formation has the typical characteristics of facies-controlled dolomite karst reservoirs in terms of reservoir genesis. The main control factors for reservoir development include special sedimentary and diagenetic environment as well as multiphase and multi-type dissolution and failure. Special sedimentary environment is prominently represented as large microbe reef flat and mound flat bodies under the control of dry-hot/arid paleo-climate. These sediments possess frame-building and pore-forming characteristics and early (sea floor) hardening function, thus creating primary matrix porosity reservoir. Special diagenetic environment is featured by poor water-rock interaction, slow mineral stabilization process and excellent preservation of vadose pisolite, aragonite and magnesium-rich calcite under the background of dry-hot/arid paleo-climate, especially beneficial for porosity conservation. Moreover, pore expansion and massive non-texture selective dissolution fissures and caverns are resulted from syngenetic and para-syngenetic dissolution, especially three-phase weathering crust dissolution caused by three-episode Tongwan movement, which fundamentally determines the reservoir formation. Multiphase fissures, especially Late Yanshan-Himalayan tectonic fissures, establish the connection between pores, vugs and dissolution caves to largely enhance reservoir connectivity. © 2016, Science Press. All right reserved.