Development Characteristics And Genetic Mechanism Of The Taiyuan Formation In The Kaijiang-Liangping Trough In The Eastern Sichuan Basin

The development of platform-edge reef/shoal is a character around Kaijiang-Liangping trough in Changxing formation. Based on the analysis of outcrops, cores, slices and other information, we found that at the end of Changxingian age in Late Permian epoch, East Sichuan was exposed in large area and the development of reservoir was controlled by both sedimentary facies and karstification. Though theoretical guidance of reservoir geology and based on geological background analysis of Changxingian reservoir development, in association with using outcrops, cores, slices and analytical laboratory datas, this article systematically studied the characteristics and genetic mechanism of platform-edge reef reservoir of Changxing formation in research area. The main understanding is as follows:1. Changxing formation of upper Permian around Kaijiang-Liangping trough is a series of platform-edge sedimentary. Under the effect of both basement faults activities and rise-falls of ancient ocean, Changxing formation developed polycyclic superimposed platform-edge reef-shoal complex with good horizon stability and lateral continuity.2. Reef-flat rocks and their karsts are main reservoirs. Reservoir space are mostly residual intergranular pores, intragranular dissolved pores, cavity holes, skeleton holes and residual karst caves. Neck throats, tubular throats and lamellar throats are developed and are well configured with pores. Pore (cave)-crack reservoir and crack-cave reservoir are major reserving types.3. Penecontemporaneous evaporation-concentration and reflux-infiltration dolomitization in association with buried hydrothermal dolomitization exists in research area. However, evaporation-concentration and reflux-infiltration dolomitization is the fundamental cause of Changxingian dolomite that makes a main contribution to reservoir forming anti-pressure solution lattice, which benefits early pore layer preserving.4. The world sea level fall event in end Changxingian age caused Changxing formation lifted and exposed after shallow buried diagenesis, which just experienced eogenetic period. We call it "eogenetic weathered crust karst" and this kind of karst is the major force developing Changxing reservoir. Karst character is as follows:(1) The top of Changxing formation has ancient karst unconformities and develops weather crust, layer karst, cross layer karst, vertical grike/cave and karst breccia. (2) Reef root/cap develop spotted karst which is combined with offwhite base rock and dark gray karst system. Pores of base rock are filled with dolomitized seepage silt, forming geopetal structures, which illustrates supergene karst took place in base rocks which was under half consolidation in early shallow burial period. (3) Karst are well developed in granular rock. Surfaces between fillings and substrate in karst system are transitive and there is no obvious boundary. We call this area "friable halo".5. The development of reservoir was effected by combined actions of depositional environment, diagenesis and tectonic fracturing.