Characteristics Of Formation Pressure And Its Relationship With The Oil And Gas Distribution In The Bozhong Area

Bozhong sag, located in the center of the Bohai Bay Basin,is the largesthydrocarbon-rich sag in the Bohai Sea. In addition, overpressure commonly developed in theDongying Formation (E3d) and Shahejie Formation (E2-3s) of the deep Paleogene strata in thisarea, which made the Bozhog sag one of the largest overpressured area in the Bohai Sea Basin.In this study, the formation pressure of Bozhong Area is calculated the Bowers method, thebalancing depth method and the Eaton method. All the calculated results are calibrated by themeasured formation-pressure data. The effectiveness of the three methods for differentmechanisms of overpressure are evaluated in this study as well. Based on all these studies, weestablished formation pressure profile for each well in which overpressure developed. Takinginto account the lithology, sedimentation rate, and organic matter maturity, the single-welloverpressure mechanisms and the lateral and horizontal distribution characteristics offormation pressure are analyzed. Finally, we discussed the relationship between thedistribution of the hydrocarbon and the formation pressure. The main findings in this studyare as follows:(1) The Bowers method and the Eaton method are relatively accurate for predictingformation pressure of the study area, whereas the balancing depth method is more suitable forcalculating formation pressure at a shallow depth, which has low accuracy for calculatingdeep formation pressure. The calculation results of formation pressure shows that the Eatonindex of2.3–2.6fits the actual situation of formation pressure in the Bozhong Area.(2) The vertical pressure distribution series in the study area is generally as follows:normal pressure zoneâ†'slight overpressure zoneâ†'overpressure zone. Normal pressuregenerally exists between the Minghuazhen Formation and the lower segment of the2ndmember of Dongying Formation (E3d2l)(~3000m). Belowe a transition zone with slightoverpressure (several to several tens of meters), overpressure widely developed in the E3d3 and the deeper formations.(3) The main overpressure mechanisms in the study area are undercompaction,hydrocarbon generation, and fluid conduction, or two of the main mechanisms superimposed.The overpressure in mudstone sections of the2nd and3rd member of Dongying Formation(E3d2and E3d3, respectively) was mainly formed by undercompaction, whereas theoverpressure in the1st,2nd and3rd member of Shahejie formation (E3s1, E3s2, and E2s3,respectively) was mainly formed by hydrocarbon+undercompaction, fluid conduction,hydrocarbon generation+undercompaction, respectively.(4) In the Bozhong Area, oil and gas bearing strata are mainly distributed in the lowerMinghuazheng Formation (N2mL), Guantao Formation (N1g), and E3d. Hydrocarbons wereseldom discovered in the E2-3s. The oil and gas reservoirs in N2mLand N1g are dominated bynormal pressure, whereas those in E3d and E2-3s are associated with overpressure. At present,the oil and gas reservoirs in Bozhong sag are mainly concentrated in the surrounding buglesand the slope zone between the bulges and the sag, showing a scattered pattern.(5) In the middle to med-depth formations of the study area, at least three types of oil andgas reservoir were developed, i.e., closed slight-overpressure-type oil and gas reservoir(IB-type), closed normal-pressure-type oil and gas reservoir (IC-type), and closedoverpressure-type oil and gas reservoir (IA-type). Obviously, the closed normal-pressure typeand closed slight-overpressure-type oil and gas reservoirs are mainly distributed in the slopezone between the sag and the bulges.(6) In the study area, normal pressure generally occurs in the center of the bulges whileoverpressure primarily exists in the sag and the adjacent slope zones. The pressure coefficientgradually decreases from the center to the margin of the sag and eventually approachesnormal hydrostatic pressure. In addition, the pressure coefficient changes rapidly in the slopezone between the sag and the bulges. The distribution patterns of overpressure in the studyarea results in the migration trend of oil and gas which charged into the reservoirs into themarginal areas associated with slight overpressure or normal pressure from the central sagassociated with overpressure. The distribution pattern of the abnormal pressure is also thereflection of oil and gas migration from the hydrocarbon-generating sag to the surroundingstructural highs and shallower formations which provides excellent dynamic conditions forhydrocarbon migration and accumulation.