Control Function Of Caprock And Its Post-destruction On Oil And Gas Distribution In Qinkou Sag

In order to study the preservation conditions of oil and gas in Qikou Sag,on the basis of geology,seismic and logging data,reservoir caprock development and distribution characteristics in different layer was studied,firstly.Two excellent regional cap rocks were identified.They are Es1z and Ed2.They developed with large thickness,high ratio of mud-strata ratio,wide distribution and less through-going faults.Then micro sealing capacity of two regional seal was analysed in three aspects.There are the capillary sealing,pressure sealing and hydrocarbon concentration sealing characteristics.For capillary sealing,through the experimental analysis,the phenomenon that shale caprock sealing ability mainly restricted by its purity and Compaction degree of diagenesis.Then,the law on the basis,combined with the data of measured mudstone sample displacement pressure,a relationship between measured mudstone displacement pressure data and the product of its depth and clay content was established.By fitting the formula,displacement pressure distribution in this two sets of regional seal,Es1z and Ed2,was studied.It showed that displacement pressure has volatility trend as high-low-high,from south to north.The maximum displacement pressures are 15MPa and 7MPa.According to the cover layer pressure sealing and hydrocarbon concentration sealing mechanism,two sets of regional cap rock overpressure distribution characteristics and abnormal gas concentration distribution characteristics were analyzed respectively.The result is that in the deep sag area,cover layer with high pressure sealing and hydrocarbon concentration sealing ability.According to the study of fault developmental characteristics in Qikou Sag,an obvious relationship between that the regional cap rock sealing ability and fault damage to cover layer thickness and the sealing ability.Based on the shale caprock in fault with internal plastic deformation mechanism as the foundation,through the correlation of a large number of fault SSF value in the cover layer section and shallow oil and gas distribution,the SSF minima that can destroy the continuity of Es1z and Ed2,were found being 3.6 and 1.5,respectively.And on this basis,the establishment of effective cover thickness evaluation formula was used to analyze the cover layer thickness distribution characteristics along faults.The effective cover is thick in the northern study area and internal fault zone of deep sag zone,while it is thin or destroyed completely in internal fault zone of buried hill tectonic belt.Aiming at the degree of damage to sealing ability of cap rock by fault,fault rock displacement pressure prediction method was established,allowing for the effect of both fault rock compaction pressure and compaction time.After the study of fault rocks sealing ability in the two sets of regional cap rocks,the following was found.fault rock displacement pressure characteristic in Es1z was similar to it in Ed2,where displacement pressure in Depression is higher than that in the buried hill tectonic belt.And displacement pressure is mainly in the range of 2.5 MPa to 5 MPa and 1 MPa to 3MPa.respectively.By integrating all aspects of cap rock sealing characteristics in Qikou sag,caprock evaluation parameters were screened.Then using fuzzy comprehensive evaluation method,based on established caprock sealing ability comprehensive evaluation standard which is fitted for this area,using the grid method,the two sets of regional caprock sealing ability was studied.Comprehensive sealing ability of Es1z caprock is generally in good level;and that of Ed2 cap rock is in Medium level,worse than that in Es1z;and that of buried hill tectonic belt Cap rock which is linked to belt,is in poor level.At present,two sets of regional caprock comprehensive sealing ability evaluation results and the oil and gas distribution are in good correlation.Final we select typical oil and gas reservoirs Gangdong oilfield and Gangxi oilfield,through analysis of cap rock sealing ability evolution,cap rock damage evolution by fault and the matching relationship between fault activity and hydrocarbon source rock evolution,the conclusion was gotten as the followings.The shallow oil reservoirs formed during Nm sediment deposition,and deep oil and gas reservoirs formed in early stage and adjusted in late stage.This indicates the formation period of hydrocarbon reservoir and enrichment degree of hydrocarbon is the result of comprehensive effect by cap rock and fault.