Abstract:

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 history.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 dominated,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 pressure 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 formation 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 study 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.

Key words: pressure coefficient, pressure system, formation pressure, Upper Paleozoic, Ordos Basin