鄂尔多斯盆地东部上古生界现今地层压力分布特征及成因

Distribution characteristics and genesis of present formation pressure of the Upper Paleozoic in the eastern Ordos Basin

现今地层压力分布是地史过程中盆地构造演化、沉积环境、烃源岩有机质热演化等多种地质因素在孔隙流体中综合作用的最终结果。抽水实验和试井获得的地层压力资料显示，鄂尔多斯盆地白垩系、三叠系均具有低压特征。盆地东部上古生界正常压力、异常低压和异常高压并存，正常压力类型比例达56.76％，异常低压类型比例为30.63％，高压最少；下石盒子组、山西组、太原组和本溪组随地层深度增加，压力系数总体呈现降低趋势。以子洲气田为例，采用压力梯度曲线法，将山2段划分为多个压力系统，单个压力系统之间互相分隔、互不连通。流体包裹体实验表明，早白垩世末期地层压力为超压，地层压力系数为1.14～1.66。现今盆地东部正常压力是盆地压力演化过程中的最后一幕。地层抬升剥蚀和构造热事件消退引起地层温度下降，从而导致压力下降了8.6～11.1 MPa，占整个压力降幅的32％～40％；天然气散失引起压力降低占整个压力降幅的20％～30％。盆地东部压力系数较高的主要原因是，东部现今地层埋藏深度浅；而沟壑纵横的地形和天然气富集程度的差异，导致了气田内压力系数各异。

Distribution of present formation pressure is the final result of joint action of many factors on pore fluid ,such as basin tectonic evolution ,sedimentary environment and thermal evolution of organic matter in source rock in geologic histo-ry.Analysis of pressure data obtained from pumping tests and well tests shows that both the Cretaceous and Triassic in the Ordos basin feature in lower pressure .In contrast ,the Upper Paleozoic in the eastern Ordos Basin shows normal pressure , abnormal low pressure and abnormal high pressure in different places and intervals .The normal pressure type is domina-ted,accounting for 56.76%,followed by the abnormal low pressure type which accounts for 30.63%,while the abnormal high pressure type is the most rare .The pressure coefficients of the Lower Shihezi Formation ,Shanxi Formation,Taiyuan Formation and Benxi Formation in the eastern Ordos Basin drop with increasing burial depth .Taken the Zizhou gas field as an example ,several separated pressure systems are identified in the 2 nd member of Shanxi Formation base on the pres-sure gradient method .These pressure systems are separated and disconnected .According to fluid inclusion analysis , the formation was over pressured with pressure coefficients ranging from 1.14 to 1.66 at the end of Early Cretaceous .Current normal pressure in the eastern Ordos Basin is the result of the last stage of pressure evolution .The formation temperature decreases due to basement uplift and subsequent erosion as well as regression of the tectonic -thermal event ,leading to for-mation pressure reduction by 8.6-11.1 MPa,which accounts for 32%~40% of the total pressure drop .Pressure drop caused by gas dissipation accounts for 20%~30%of the total pressure drop .The higher pressure coefficients in the stud-y area are mainly resulted from its shallower burial depth .The high-relief landform and difference of gas enrichment cause different pressure coefficients among different gas fields in the study area .