Research On Development Geology Of Fracture Cavity Gas Reservoir In The Middle Permian, South Sichuan

Since industrial mining started in 1957, the Middle Permian gas reservoir which located in south of Sichuan basin has experienced three stages: production, stable production and declining. The output of natural gas reached a peak in 1979 with a volume of 49.46×10~8m~3. However this number decreased to 11.42×10~8m~3 in 2002.Karstification and multiple tectogenesis are two important factors to control gas reservoir in middle Permian. After the formation of hydrocarbon in research area, small faults grow and the fracture-cavity reservoir is with strong heterogeneity complex gas-water relation. Now, the Middle Permian gas reservoir is in its late-middle exploitation period. There is a large scale of water breakthrough, the production is declining sharply and the reserve recovery is quite low. In the same time, limited by reservoir conditions and exploration techniques, the exploration wells success rate and oil test layer success rate is not high. So, strengthening the study of fracture-cavity gas reservoirs development geology of Middle Permian in south of Sichuan basin is of great importance for increasing exploration wells success rate, enhance the recovery ratio, correctly dealing with water output problems and reasonably exploiting and adjusting the gas field. Based on study about gas reservoir geological characteristics and in combination with other related techniques and methods, this article has a detailed research in facies distribution characteristic, the combination of fracture-cavity reservoir pore space, reevaluation of fracture-cavity reservoir formation and evolution history, the gas field formation and evolution process, gas reservoir type and detailed description about water-gas distribution. All these studies have provided feasible basis for the next phase of exploration and exploration of the gas reservoir, and the main conclusions are:(1) The Middle Permian formation is set of open sea carbonate platform facies with a thickness of 300 to 500m; it could be divided into Qixia Formation and Maokou Formation, the average limestone thickness is 280m which has provided favorable conditions for Karstification. The research area could be divided into three sedimentary areas including platform, platform margin and basin depositional area; the seven facies include restricted platform facies, open-sea platform facies, inner-platform shoal facies, biological mud mound facies, platform marginal shoal facies, crinoids mud mound and wide continental shelf facies, and the facies are strictly controlled by transgression and regression and paleogeomorphic.(2) The reservoir in studying area is of typical fracture-cave reservoir,which matrix porosity and permeability is quite low, could not store or infiltrate fluids and the main infiltration-storage space is made up of caves and fractures. Cave is the infiltration-storage space and the fracture mainly works as a connector as its storage ability is limited. Also, the karstification during Dongwu period is the key factor in forming big caves; though big scale of caves could not be formed during Himalayan period, the original caves and fractures could still be reformed at this time. A reservoir geological model could be used to describe the combination of caves system formed in Dongwu period and fractures network formed in Himalaya period.(3) The Middle Permian gas reservoir has a rich gas source. The Middle Permian and the underlying Silurian formation have great hydrocarbon generation capability; the accumulative total output is far less than total available resources and has a great potential for development. The current gas reservoir has experienced multiple tectonic movements, division of gas reservoir density matrix and limitation in fracture development, est. Gas-water coexistence is very common in fracture-cave systems; the water-gas relation is complex; the water-gas contact is usually different in the same structure or the same fracture-cave system; higher position producing water and lower position producing gas phenomenon is quite common. The recovery rate in the gas reservoir could be increased gradually by effective drainage measures.