Migration And Accumulation Patterns Of Natural Gases In The Qiongdongnan Basin

The Qiongdong Basin is characterized by high temperatures and strong pressures, and complicated types of natural gas pools. For these reasons, there has been no deep understanding on the migration and accumulation patterns of gases for a long period, and lack of scientific ideas for further exploration in this basin. In this study, a combination method of fluid inclusion technique with hydrocarbon generation kinetics modeling has been applied to investigate the migration and accumulation pattern of gases in the basin. The following main results have been obtained:(1). The Ya13-4 and Ya13-6 gas pools formed with the same period of the Ya 13-1 gas pool. They were originated from a set of coal measure source rock in the Yacheng Formation from the slope area of the Yanan Depression. The gas pools were mainly filled after 2 Ma. The gas migrated from the Ya13-1/Ya13-6 structure to the Ya13-4 structure.(2). The Baodao gas pools experienced two phases of gas charging. The first phase occurred before 10-8 Ma with a lower mature hydrocarbon gas, and the second after 10-8 Ma with a large amount of CO₂ gas. The reservoirs were fully filled by the CO₂ gas during a short period, which prevented the hydrocarbon gas from further migration into the reservoirs to form the CO2 gas pools.(3) . The super-pressure system of The Ya21-1 structure was formed during late Miocene, obviously earlier than the main stage of gas generation and expulsion of the source rock. The super-pressure obstructed natural gases from a regional migration into the reservoir, which became the main reason for the Ya21-1 trap without a commercial gas.(4). The perspective area for natural gas in the Yacheng Uplift is believed in the north- west region of the Yanan Depression. The exploration for hydrocarbon gases should look for the later-formed trap , or be away from the faults in the Baodao area. The gas exploration in the super-pressure area should address the earlier-formed traps.These results not only give a guide to the gas evaluation and exploration in the Qiongddongnan Basin, but also provide a case study of gas migration and accumulation for the petroleum-bearing basins with a similar geological background.