Characteristics And Its Controlling Impacts On The Hydrocarbon Accumulation Of Mudstone Caprock Of The Xiayang Formation In Wushi Sag

The Xiayang formation in Wushi sag is one of the most targeted strata for hydrocarbon exploration in the middle-shallow reservoirs of the Beibu Gulf Basin in recent years.This formation has been interpreted as being deposition in a shallow marine environment with abundant reservoirs and sufficient organic sources.Whether the presence of caprock has been the key factor for controlling hydrocarbon accumulation.This thesis took advantage of plentiful drilling,rock analysis,logging,and seismic data to evaluate the effectiveness of the caprocks in the Xiayang formation.Firstly,well logging and coring data were used to characterize the caprocks in the Xiayang formation.The data reveal that the caprock of the Xiayang formation in Wushi sag mainly are grey to grey black mudstone.The caprock of the Xiayang formation were divided into eight grounds along the vertical direction.The top caprock shows the widest geographical distribution in the sag with a great thickness and high sealing quality,and it serves as the most important regional caprock for the middle-shallow hydrocarbon reservoirs.Secondly,the development characteristics and the sealing ability of the capwere analyzed by applying the following five parameters,including thickness,mud-land ratio,porosity,permeability,and breakthrough pressure.The sealing ability of the caprock was comprehensively evaluated by the grey correlation method.The results show that the influence degree of each parameter on sealing performance is as follows: breakthrough pressure,porosity,permeability,caprock thickness,and mud-land ratio.In the west of the Xiayang formation of Wushi sag,class I mudstone caprock is presence.In the East,class III caprock are more common,with limited class I mudstone caprock.According to the known hydrocarbon reservoir anatomy,the mudstone caprock comprehensive evaluation index of the blocks is relatively strong and above can be used as favorable zones for sealing hydrocarbon in the Xiayang formation.The analysis results obtained by this evaluation method are consistent with the exploration results,which could provide effective prediction and reasonable reference for future hydrocarbon exploration.Thirdly,based on the spatial relationship between faults and caprocks,the influence of faults on the effectiveness of caprocks was analyzed.The data shows that the mudstone caprock of the Xiayang formation deposited during the Neogene.During this time,the fault activity has stop,resulting in deposition of pure and thick mudstone as an idea regional caprock.The deep fault has not caused great damage to it.In addition,in terms of the relationship between the effective sealing time of cap formation and the hydrocarbon expulsion period of source rock,the data indicate that the source rock is still expelling many hydrocarbons,which was prior to the time that the effective sealing ability of mudstone cap in each area of Wushi sag has archived.Thus,the cap plays an effective role in controlling the forming time of hydrocarbon.Finally,the effective traps formed in the Xiayang formation were evaluated based on the characteristics of caprock development,comprehensive sealing ability of caprock,sealing ability of faults,and hydrocarbon expulsion history of source rocks.Two high-quality traps are selected to provide reasonable guidance and reference for the future hydrocarbon exploration in Wushi sag.