Studies On Oil-gas Character And The Forming History Of Deep Reservoir In Duqiaobai Area, Dongpu Depression

Based on the research for the deeply buried source rocks and the geo-chemical properties of the oil and gas within Duqiaobai area in Dongpu depression, the formation principle of the deeply buried gas and crude oil are systemically analyzed in this paper. The properties of the fluids in different period of the injection of the oil and gas are studied by analyzing geo-chemical properties of the free hydrocarbon in pores in oil-bearing sandstone, of the hydrocarbon adsorbed on grains and of the oil and gas in inclusion enclaves. The injection periods and time of the oil and gas are also studied by testing reservoir saturation pressure, authigene K/Ar isotope and homogeneous temperature of oil and gas in inclusion enclaves. Major controlling factors were also obtained by studying hydrocarbon distribution and accumulation so as to get a further understanding of reservoir forming principle.Composition of natural gas is different in Baimiao and Qiaokou, though they are both wet gas. In Qiaokou area, the content of methane CH4%<80%, the value of the ratio between methane CH4 and heavy hydrocarbon is about 5, while in Baimiao area, the value of methane CH4>88%, the value of the ratio between methane CH4 and heavy hydrocarbon is more than 10. This difference is caused not only by maturity but also by source rock. Natural gas in Baimiao area is mainly derived from Tertiary source rock and Carboniferous Permian formation in Qianliyuan sag, which is near the high point on Lanliao fracture belt. The ES2-ES32 gas net pay Shallowly buried has a high value of Ar40/Ar36, which generates coal-type gas or mixed gas; while in the lower part off the Lanliao fracture belt, ES3'-ES34 as net pay has a low value of Ar40/Ar36, which generates oil-type gas, so does ES33, ES34 gas net pay in Qiaokou structure and ES33 in Duzhai area.The correlation between the oil and source rock shows that in Duzhai area, condensate in well Pushenl5 is from formation ES33-ES34, and crude oil in well Qian8 is from ES32. In Qiaokou area, deep parts of well Qiaol4, 20, 23, 25, 60 are related to source rock in Gegangji sag, while crude oil in well Qiao38 is related to source rock in Qianliyuan sag; in Baimiao structure, crude oil in well Bail2 is related to source rock of ES33-ES34 in Qianliyuan sag, while condensate in well Bai54, 17, 9 is related to source rock of ES32 in Qianliyuan sag and crude oil in well Bai6 is from oil kitchen Gegangji.The study on illite age testing, inclusion enclave temperature testing and saturation pressure testing has been used to know the time of the deeply buried gas net pay formed. The time of the deeply buried gas net pay formed can be divided into two stages. The first is early period of Dongying-Guantao, which is the main stage for natural gas forming with erosion and decrease of pressure. The second is from the beginning of Quaternary to update, during which gas reservoir is reforming and the reforming gas is refilling into the reservoirs. According to the value K/Ar in Illite, the time to form the gas reservoirs can be determined as the period of Dongying -Guantao, among which Duzhai is 43.0-38.0Ma and 25.4Ma, Qiaokou area 37.8-25.4Ma and 24.4-18.4Ma, Baimiao 38.0-25.4Ma and 23.4-20.4Ma. Natural gas in Duzhai area was formed the earliest. Inclusion enclave temperature testing not only determined the first stage for natural gas formed to be 28-20Ma but also the second stage to be from Quaternary (2Ma).