| The slope belt is an important zone for oil and gas accumulation in graben basin,the prospect of exploration is extremely wide.It is of great strategic significance and practical value to strengthen the study on the theory of hydrocarbon accumulation in slope zone.Both consequent fault and antithetic fault are the two types of faults that widely distributed in slope belt,which play an important role in controlling oil and gas migration and preservation.This paper is based on the three-dimensional seismic data of Wen’an slope,analyzing the development characteristics of consequent and antithetic fault as well as their differences.Then combining with the distribution law of oil and water and the accumulation conditions,the fault controlling differences between consequent and antithetic faults are determined.All these are aimed at enriching the fault controlling theory and providing theoretical basis for oil and gas exploration deployment in slope area.The main conclusions of this study are as follows:The tectonic framework of Wen’an slope is divided into blocks in north-south and belt in east-west.By analyzing of the accumulation factors,it is concluded that the Paleogene in central and southern of Wen’an slope has excellent accumulation conditions:(1)The source rocks in Es3 and Es1 of Baxian Sag and Maxi-Mozhou trough in west side of the study area provide sufficient material bases.(2)Good reservoir conditions,sealing conditions and their mutual configuration provide favorable reservoir space for hydrocarbon accumulation.(3)The fault-sand transport systems composed of fault and sandstone converters provide favorable channels for oil and gas migration.the development characteristics of consequent and antithetic faults in different tectonic units and periods are different.In the aspect of fault strike,the consequent faults show the transition from NE to NEE,and then to EW.The reverse faults show a NEE direction on the basement and gradually upward to EW.In the aspect of fault density,the density of consequent fault is obviously larger than that of reverse fault in fault depression tectonic layer,but in rift depressiontectonic layer the two values are basically the same.In the aspect of fault scale,the scale of consequent fault is obviously larger than that of reverse fault in fault depression tectonic layer,and there is little difference between above two in rift depressiontectonic layer.In the aspect of fault kinematics characteristics,both the consequent and antithetic faults are formed in each period.The faults formed in faulted period are mainly the consequent faults,especially under the extension in Ek~Es3.To the fault-rift period,some of the faults are resurrected and continue to move under strike-slip extension,at the same time associated with a series of small-scale reverse faults.To Depression stage,A series of minor faults formed by continuous tensional torsion in the late period which display ―flower‖ pattern with main faults.Based on the analysis of favorable accumulation conditions in the study area,guided by fault controlling theory,combined with the detailed anatomy of reservoir and oil-water distribution law,finally summarized five kinds of fault controlling differences between consequent and antithetic faults in slope belt:(1)The reservoir development sites which associated with consequent and antithetic fault are different,the traps of consequent fault are usually developed at the segments of the growth sites,while the traps of antithetic fault are usually developed at the maximum displacement areas of footwall.(2)Hydrocarbon accumulation period which associated with consequent and antithetic fault traps are different,the trap of consequent fault is formed in the stage of ―hard-linkage‖,while the trap of antithetic fault is formed during the beginning of fault activity and terminated at fault setting.(3)Adjustment migration of oil and gas by consequent and antithetic faults are different,the oil and gas adjustment by consequent faults have multi-layer or single-layer distribution characteristics,while antithetic fault only have single-layer distribution characteristics.The damage extent to caprock between consequent and antithetic fault are different,,specific performance as the faulted thickness of consequent fault is larger than that of antithetic fault.(4)Oil and gas preservation ability controlled by consequent and antithetic fault is different,on the extent of damage to the caprock,the damage extent of consequent fault is greater than that of antithetic fault,on the extent of lateral sealing of faults,when the faults scales are small,the consequent faults are difficult to form lateral seals while the antithetic faults are easy to form juxtaposition seals.when the faults scales are large,both the consequent and antithetic faults could seal,and the sealing ability of antithetic faults is better than that of consequent faults. |
