Evidences of Overpressure Caused by Oil Generation in Dongying Depression
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摘要: 东营凹陷沙四段和沙三段普遍存在异常高压, 泥岩声波时差对超压具有很好的响应关系, 超压段均对应异常高的声波时差值, 而且泥岩声波时差随着岩石颗粒垂直有效应力的减小而增加.利用东营凹陷丰富的测井、测试和地质资料综合分析超压成因, 在确定超压泥岩和砂岩均属于正常压实的基础之上, 提出生油作用为超压的主要成因机制.主要证据有: (1)超压泥岩不具有异常低密度特征; (2)超压段泥岩密度与岩石颗粒垂直有效应力缺少线形关系; (3)东营凹陷超压砂岩不具有异常高的原生孔隙度和地温梯度; (4)超压顶界面深度范围为2000~3000m, 随着烃源岩埋深的增加而增大, 所对应的成熟度Ro(%)为0.50%~0.75%, 温度大约为87~123℃; (5)超压烃源岩现今仍具有很强的生油能力; (6)超压带内钙质泥岩的存在使超压具有很好的压封闭条件; (7)烃源岩中发育大量的裂缝; (8)超压储层主要为油层或者油水同层, 水层很少.东营凹陷砂岩超压主要是由烃源岩中排出的高压流体运移至储层中而发生超压传递造成的.Abstract: Overpressures are common in the third member and the fourth member of Shahejie (Es3 and Es4) Formations of Dongying depression. The sonic log displays an obvious response to the overpressure from which the overpressured mudstones have higher acoustic travel time and the acoustic travel time of the normally pressured and overpressured mudstones is reduced with increasing vertical effective stress. In this paper, large numbers of well-log, tests and geological data in Dongying depression are used to determine the origin of overpressure. We put forward that the oil generation is the major overpressure mechanism based on the confirmation of that the overpressured sediments are normal compaction. It is supported by the following evidences: (1) The overpressured mudstones exhibit no anomalous low density; (2) There is apparent lack of correlation between mudstone densities and effective vertical stress; (3) The overpressured reservoir sandstones show neither anomalous high matrix porosities nor anomalous geothermal gradient; (4) The depths to the top of the overpressure range from 2000m to 3000m, increasing with the increased source rocks depths and the temperatures for the top of overpressure zones are approximately 87-123℃ corresponding to an estimated vitrinite reflectance (Ro) of 0.5%-0.75%; (5) The overpressured source rocks are still capable of oil generation; (6) There are good conditions for overpressure seals because of the calcareous mudstones; (7) There are large numbers of microfractures in the source rocks; (8) The overpressured reservoirs are predominantly oil saturated or oil-bearing and overpressured water reservoirs are rare. It is concluded that overpressures in the reservoir rocks are generated by the fluid transmission from the overpressured source rocks.
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图 1 东营凹陷构造单元划分及油田分布
Fig. 1. Map showing the tectonic subdivisions and the distribution of the oil fields in Dongying depression
图 2 东营凹陷沙三段和沙四段砂岩实测地层压力和压力系数与深度关系
Fig. 2. Plots of measured pore pressures and pressure coefficients versus depth in the Es3 and Es4 Formations of Dongying depresion
图 3 东营凹陷典型井泥岩声波时差、电阻率、密度和井径与深度关系
Fig. 3. Profiles of sonic transit times, resistivity, density and caliper of mudstones versus depth for the representative wells in Dongying depression
图 4 东营凹陷泥岩声波时差(a)和密度(b)与有效应力关系
Fig. 4. Plots of sonic transit times (a) and density versus depth (b) in Dongying depression
图 5 东营凹陷砂岩原生孔隙度与深度关系
Fig. 5. Relationship between the genetic porosity of the sandstones and depth in Dongying depression
图 6 东营凹陷实测地层温度与深度关系
Fig. 6. Plots of measured temperatures versus depthin in Dongying depression
图 7 东营凹陷超压顶面等值线
Fig. 7. Contour map of the depth to the top of the overpressure zone from 352 wells in the Dongying depression
图 8 东营凹陷剖面AB地层展布、超压顶界面和超压段岩性分布特征
Fig. 8. Cross Section AB in Dongying depression showing the top of the overpressure zone and the distributions of the mudstones, calcareous mudstones and oil shales
图 9 东营凹陷烃源岩成熟度Ro与深度关系
注：等效Ro-1和Ro-2分别据蒋启贵等(2008)和郭汝泰等(2003)数据
Fig. 9. Plots of maturity of the source rocks versus depth in Dongying depression
图 10 东营凹陷胜科1井模拟结果
Fig. 10. Modelling results for well Shengke1 in the Dongying depression
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