Differences of carbon and oxygen isotopes in coarse-grained calcite from the Permian Maokou Formation in different regions of Sichuan Basin and their significance in paleokarst environment
Oil and gas fields in karst carbonate reservoirs are widely distributed in China.A number of karst fracture-vuggy reservoirs have been found in the Tarim Basin,the Ordos Basin and the Sichuan Basin.Therefore,research of reservoirs in karst fractured caves has become one of the focuses in recent years,which indicates a promising future and huge potential for exploring karst carbonate reservoirs in China.Reservoirs in fractured caves of the Maokou Formation have always been one of the key areas for oil and gas exploration in the Sichuan Basin.Since the last century when reservoirs in fractured caves in the southern Sichuan region were discovered,breakthroughs have been made in this field in the Yuanba area of northern Sichuan,Yunjin area of southern Sichuan,and the Penglaiba area of central Sichuan,demonstrating good exploration prospects.However,there has always been controversy over the origin of fractured caves.One view suggests that the exposure of the Maokou Formation caused by the Dongwu Movement and the occurrence of freshwater karst are the main reasons for the development of fractured caves in the Maokou Formation,while another view suggests that fractured caves in the Maokou Formation are related to the tectonic fluid activity during the burial period.these two views are mostly based on research in the southern Sichuan region,and further consideration of the entire Sichuan Basin is needed to search for relevant evidence from geological phenomena to geochemistry.To understand the causes and differences of these fractured caves,we systematically sampled calcite for data of carbon and oxygen isotopes from outcrop profiles or from drilling fractured caves in different areas of the Sichuan Basin,in order to determine the paleokarst environment in which fractured caves were developed.Study results show that fractured caves of the Maokou Formation in this basin are mainly composed of small-and-medium-sized dissolution pores,fractures,and dissolution fractures.The carbon and oxygen isotopes of calcite in the Maokou Formation can be divided into four types,indicating four types of paleokarst environment with different fluid properties.Type Ⅰ is the quasi syngenetic karst environment in which the oxygen isotope values of calcite in fractured caves are basically consistent with the values of limestone or of Middle Permian seawater in the Maokou Formation,with δ13C ranging from 1.80‰ to 3.83‰ and δ18O ranging from-8.95‰ to-4.11‰.Type Ⅱ is the freshwater karst environment in which high negative δ13C and δ18O values are present in calcite of fractured caves,with δ13C＜0‰ andδ18O ranging from-12.00 ‰ to-9.00 ‰.Type Ⅲ is the buried karst environment in which lower δ18O values are significantly negative compared to the values of bedrock.However,δ13C values are basically consistent with those of bedrock,with δ13C ranging from 0‰ to 3.83 ‰ and δ18O ranging from-12.00‰ to-8.50‰.Type Ⅳ is the high-temperature hydrothermal environment in which δ18O values are significantly negative(＜-12.00‰).Regional comparison shows that the oxygen isotopes in central and southern Sichuan are more negative than those in northern Sichuan,while the carbon isotopes in northern and central Sichuan are more negative than those in southern Sichuan.Research suggests that buried karst is the main type in southern Sichuan,followed by freshwater karst and quasi syngenetic karst,all together forming reservoirs dominated by fractures.In addition to quasi syngenetic karst and freshwater karst,high-temperature hydrothermal karst is also developed in central Sichuan.The high-temperature hydrothermal karst has formed dissolution pore reservoirs related to dolomite.The northern Sichuan is mainly characterized by quasi syngenetic karst and freshwater karst,forming reservoirs mainly composed of dissolution pores.
the Maokou Formation in Sichuan Basincarbon and oxygen isotopespaleokarstkarst reservoirhydrothermal karst
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