Modelling of Pressure Evolution and Hydrocarbon Migration in the Baiyun Depression, Pearl River Mouth Basin, China
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摘要: 珠江口盆地位于中国南海北部, 是中国近海含油气盆地中一个重要的盆地.珠江口盆地勘探逐渐由浅水区向深水区推进, 浅水区钻井揭示地层为正常压力, 但对深水区的油气运移指向和地层压力分布认识不清楚.应用钻井测试资料, 地震资料和盆地模拟技术对珠江口盆地白云凹陷深水区的地层压力演化和油气运移进行了研究.模拟结果显示地层压力的聚集与释放以及流体运移与构造运动的发生有密切的关系.从模拟结果的分析中可以得出如下结论: (1) 珠二坳陷经历了3次幕式地层压力的聚集与释放.这3次压力的释放与晚始新世珠琼运动二幕、渐新世中期南海运动、中中新世末至晚中新世末东沙运动有关, 其中东沙运动期间是本区油气运移的主要期次, 现在浅水区的常压是由于东沙运动时超压释放后的结果. (2) 钻井测试与模拟结果显示, 现今地层压力在浅水区为常压, 在深水区有弱超压存在.引起地层压力分布的差异主要是由于浅水区和深水区沉积的岩相和岩性的不同. (3) 油气运移有2个指向, 一个运移指向浅水区域, 另一个指向深水区域.由于浅水区砂岩百分比含量高, 砂体连通性好, 油气聚集比较分散, 比较而言深水区油气在扇体里富集程度较高, 更有利于形成大油气田.Abstract: The Pearl River Mouth basin is one of the most important offshore basins in China. Petroleum exploration is being carried out in the deep water area of the basin. Well tests indicate that the pore pressure is normal in the shallow water area. The hydrocarbon migration and geopressure distribution in the deep water area are poorly understood at present. These issues have been considered using geological data and basin modelling. The modeled results indicate that pressure accumulation and release, and fluids migration have a close relationship with structural movement. On the basis of our investigation, the following conclusions have been drawn: (1) At least three episodic accumulations and release of pressure occurred in the Cenozoic. The pressure release and associated hydrocarbon migration occurred primarily during the Dongsha Movement, when most of the overpressure was released to normal pore pressure except for some overpressure in the deep water area. (2) Both the measured pressure and the modeling results indicate that pressure is normal in the shallow water area and is abnormally high in the deep water area at present stratigraphy. Difference of pressure distribution is caused by lithologies and facies, because of the high percentage of mudstone deposited in the deep water and the high percentage of sandstone in shallow water. (3) The hydrocarbon migrated to the shallow and deep water areas along faults. Because of the high ratio of sandstone in the shallow water and high ratio of mudstone in the deep water area, the hydrocarbon scattered into the sandstone more easily in the shallow water area, so large oil & gas fields appear to form more easily in the deep water area.
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图 1 珠江口盆地构造单元(据代一丁和庞雄修改, 1999)
Fig. 1. Classification of structural units and study area in the Pearl River Mouth basin
图 2 珠江口盆地地层综合柱状图(据陈长民修改, 2003)
Fig. 2. Generalized stratigraphic column of the Pearl River Mouth basin
图 4 A井地温和VR实测值与模拟值对比
Fig. 4. Correlation between observed geotemperature and VR modeled geotemerature in Well A
图 5 A-B剖面地层超压模拟
a.47 Ma时的地层压力剖面(珠琼运动二幕前); b.39.4 Ma时的地层压力剖面(珠琼运动二幕后); c.32 Ma时的地层压力剖面(南海运动前); d.30 Ma时的地层压力剖面(南海运动后); e.13.8 Ma时的地层压力剖面(东沙运动前); f.现今的地层压力剖面(东沙运动后)
Fig. 5. Modeled pressure section of A—B
图 6 地层压力演化
a.陆架区文昌组地层压力演化; b.陆坡区文昌组地层压力演化
Fig. 6. Evolution of pressure. (a) is in offshore; (b) is in slope of seabed
表 1 A井砂泥岩百分含量统计表
Table 1. Percentage of sandstone and mudstone of Well A
表 2 珠江口盆地不同沉积相的有机碳含量统计(据陈长民等, 2003修改)
Table 2. Percentage of TOC in different depositional facies, Pearl River Mouth basin
表 3 A井地层压力RFT测试数据
Table 3. RFT data of Well A in Pearl River Mouth basin
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