Integrated Study On Reef And Shoal Facies Reservoir Of The Changxing Formation In Eastern Sichuan-Northern Chongqing Area

Reef-beach is a special type of carbonate sediments, which has peculiar hydrocarbon pore volume and is an excellent carbonate reservoir. Reef-beach reservoirs are widely distributed all over the world, and they are also common in our country. For example, a series of reef gas reservoirs have been founded in Changxing Formation of Upper Permian, eastern Sichuan Basin. However, the prediction of reef-beach reservoirs has always been the difficulty of hydrocarbon exploration. The most reef gas reservoirs so far have been founded by random drilling, while the successful prediction has been few. Therefore, the research on the origin and distribution rule of reef-beach reservoirs in Changxing Formation is of theoretical and practical significance to improving the prediction successs rate.The article was based on the latest achievements home and abroad and guided theoretically by sedimentology, petroleum geology, geophysics and cabonate reservoir geology. Combined with the geologic background of the area, and using rock-mineral identification, stable carbon, oxygen and strontium isotope, trace element of Fe, Mn and Sr, and fluid inclusion, a systematic research was made on the origin, diagenetic system, prediction and evaluateon of reef-beach facies reserveoirs. The conclusions are as follows:1.The characteristics of sedimentary facies of Changxing Formation in eastern Sichuan-northern Chongqing were studied sysmatically by drilling, field profile, thin section, logging and seismic data. Five depositional facies were developed in Changxing Formation, including foreslope, platform-margin reef, platform-margin shoal, open platform and restricted platform. The depositional model was established. The platform-margin reef and shoal facies are the most favorable reservoirs.2.Based on core description and rock-mineral analysis, the dolomite types were identified, including dolomicrite, micro-powder crystal reef dolomite, micro-powder crystal bioclastic dolomite, powder- fine crystal bioclastic dolomite, fine crystal dolomite, middle-macro crystal dolomite. According to the texture-genetic characteristics, they could be divided mainly into penecontemporaneous replacive dolomitization and buried dolomitization, which can be classified further as syngenesis dolomitization, early-diagenesis buried dolomitization, middle-diagenesis buried dolomitization, late-diagenesis buried dolomitization and structure crack dolomitization. With the increasing diagenetic degree, the degree of order of dolomites from different phases and genesis become higher and the ratio of MgCO3/CaCO3 is closer to 1.3.Through the comprehensive study of the texture-genetic classification, diagenetic manners and evolution stages, trace element characteristics of Fe,Mn,Sr,and isotopic geochemistry characteristics of carbon,oxygen,strontium and fluid inclusion of dolomites in Changxing Formation,four diagenetic systems of different stages and environments were divided in the dolostone of Changxing Formation according to"Hydrological system", which were pore brine during penecontemporaneous stage, canning brine during early diagenetic stage, mixed hot brine during middle-late diagenetic stage and structure hot brine during tectonic uplift stage.4.The characteristics of reservoirs in Changxing Formation were studied intensively and sysmatically, it could be determined that the most effective pore space are intergranular pores and intergranular dissolution pores, the minor are outsized solution pores and intragranular dissolution pores, and the partial pore space are cracks. Combined with reservoir properties, pore structure parameters and other indicators, the reef-beach reservoirs in Changxing Formation were classified and evaluated, and the controlling factors for the reservoir development and distributions were discussed intensively. Such new acquaintances can be concluded as follows:①the reef-beach facies controlled regional distributions and development scales of the reservoirs;②burial dolomitization in sealed formation brine diagenetic systems was the foundation of reservoir development;③burial dolomitization of mixed hydrothermal diagenetic system expanded the reservoir distributions and improved the reservoir quality; and④the fracture and dissolution of structurally controlled hydrothermal diagenetic system were the key to improving the quality of the reservoirs.5.Synthesizing geology and drilling data, the reservoir types of Changxing Formation in eastern Sichuan-northern Chongqing were discussed, including Pinhole type, micro-crack- Pinhole type,pore-cave-crack type and crack type. The study area is dominated by the first two types, and then the logging response models for different reservoir types were established.6.On the basis of the concluded basic geological conditions for hydrocarbon accumulation, such factors were chosen as comprehensive quantitative evaluation indexes, including the sedimentary facies types, accumulative grain thickness, distribution area, effective reservoir thickness, property, hydrocarbon shows and hydrocarbon migration path. The favorable reef-beach reservoir development area of all structural belts were quantificationally evaluated in Changxing Formation, Eastern Sichuan– Northern Chongqing, and the favorable exploration areas within the study region were shown. They could provide a geological foundation for further study and exploration and development.