CHARACTERISTICS, DISTRIBUTION AND EVALUATION OF REEF-SHOAL RESERVOIRS: A CASE STUDY OF PERMIAN-CARBONIFEROUS IN THE NORTHWEST OF TARIM BASIN, CHINA

It is widely recognized that reef-shoal reservoirs accounting for about 40% of global carbonate oil and gas reserves. Numerous studies show that Permian-Carboniferous reef-shoal reservoirs widely develop in Tarim basin and industrial oil and gas have already been obtained in reef-shoal reservoirs (e. g., Tahe1 well, Tazhong 4 well). However, the oil and gas exploration for Permian-Carboniferous in the northwest of Tarim basin is still at preliminary stage with limited data, and there are still some critical petroleum geoscience problems unsolved, such as the characteristics and distribution of reef-shoal reservoirs, the relationship between the time for hydrocarbon migration and the time for the dissolution and cementation, and what are the key factors affecting microbial reef reservoirs quality. To solve these problems, we did a lot of work as follows. On the basis of observing and measuring outcrop sections in detail, combing with known outcrop data points on stratum and lithology, we restored the distribution of reef-shoal facies and built the sedimentary evolution model of reef-shoal facies in the study area. Through the observation of the hand specimens and thin sections from outcrop samples, we find that the main type of microbial reef is algal reef, and that the sedimentary environment is closed to algal breccia beach related to growth fault. Thin sections show that few primary pores develop in reef-shoal reservoirs as a result of intense cementation, and that a certain amount of secondary pores (e. g., dissolved pores and tectonic fractures) may be the main reservoir space. Test results of the homogeneous temperature of fluid inclusions suggest that cementation filling takes place at the early stage of late diagenesis which is earlier than the stage of hydrocarbon generation and expulsion. In other words, primary pores as hydrocarbon reservoir space were difficult to be preserved. Comprehensive studies indicate that the key factors affecting reef-shoal reservoirs quality are tectonism, dissolution of acidic fluid and weathering leaching process and that high quality reservoirs mainly develop in both sides of fault zone. The research results provide several new insights about Permian-Carboniferous reef-shoal reservoirs and could be significant for exploration and development in the northwest of Tarim basin.