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Genetic Types And Migration-accumulation Dynamics Of Natural Gases In The Ying-Qiong Basins, The South China Sea

Posted on:2003-07-21Degree:DoctorType:Dissertation
Country:ChinaCandidate:B J HuangFull Text:PDF
GTID:1100360092970963Subject:Institute of Geochemistry
Abstract/Summary:PDF Full Text Request
The Ying- Qiong Basins in the north continental shelf of the South China Sea are in fact two separate Cenozoic basins with close geographic proximity and genetic linkage. Great attention has been paid to these basins in recent years because of their large sedimentary volumes and huge gas potentials. Geochemical and isotopic data of the natural gases from these basins have indicated the presence of at least four genetic types of gases in these basins. They are biogenic gas, thermogenic gas, gase with a mixed origin and inorganic CC^ gas. Biogas was generated from coarse fiber, hemicelluloses and protein that can be degraded by bacteria in Quaternary and Pliocene marine mudstone. The data from simulation test of source rocks, combined with the gas-measurement log and geothermal gradient data of the basins, has shown that the main stage of the bio-gas generation is between 35℃ and 65 ℃ and the death line of bio-gas is about 85 ℃. Using the methods of comparative analyses and gas-source direct correlation, it was suggested that thermogenic gases in the Ying-Qiong Basins were derived from two geographically separate petroleum systems. The gases in the Yinggehai Basin were likely from the marine shales in the Meishan-Sanya Formations within this basin, whereas gases in the Yacheng Field were mainly from the coal-bearing source rocks in the Yacheng Formation in the Qiongdongnan Basin, with only small amounts of gases with high inorganic CC>2 from the Yinggehai Basin. By studying the inorganic CO2 and helium isotopes in Yinggehai Basin, and investigating the Tertiary lithology and geothermal condition, combined with the results of thermal simulation test, it was suggested that inorganic CO2 gas principally came from thermal decomposition of Tertiary calcareous shales and Pre-Tertiary carbonate under the burial depth of 6000m, approximately at a strata temperature of about 250℃.The hydrocarbon source rocks in Yacheng Formation in Qiongdongnan Basin and Meishan and Shanya Formation in Yinggehai basin contain a dominant amount of terrestrial organic matter. This study has indicated that the overpressure cannot significantly delay the organic thermal maturation of the source-rock. However, the high temperature conditions have had strong influence on the thermal evolution and hydrocarbon generation of the source rocks. In places where the source rocks were intensely influenced by deep sourced hot fluids, the rapid heating resulted in an advanced maturation of organic matter deposited in shallower depth. It is the hightemperature, combined with relatively abundant humic organic matter in the Miocene and Yacheng Formation source rocks, that led to predominantly abundant natural gases in the Ying-Qiong basins. Moreover, the focused-episodic gas migration significantly increased the hydrocarbon expulsion efficiencies of the source rocks, and thus provided favorable conditions for the accumulation of large amounts of gases in the diapric structures (such as Dongfangl-1 gas field) in very short geological time. PVT simulation results of fluid inclusions have revealed that an overpressure formed in middle or late Miocene in Yacheng21-l structure, earlier than the phase of enormous hydrocarbon generation and migration of the Yacheng source rock. This overpressure has stopped migration of the later-generating gases into the reservoir, having led to only small amount of aqueous gas in the reservoir.
Keywords/Search Tags:natural gas, genetic type, palaeopressure simulation, migration-accumulation dynamics, Ying and Qiong basins
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