A Study On Paleostress Inversion Of Hydrocarbon Accumulation Period

Paleostress state is an important controlling factor for the slip behavior of faults and fractures in the earth's crust and hence for the migration and accumulation of hydrocarbon.Current estimation methods of stress tensor mainly rely on outcrop data or directional core data.However,in the field of oil and gas industry,the study of underground fault structures is mostly based on seismic interpretation data,thus it is impossible to directly observe and measure fault slip information.In view of the limitations of current paleostress inversion methods,this paper presents two paleostress inversion methods suitable for oil and gas industry:slip tendency inversion and slip direction inversion,both of which are based on seismic interpretation data.Slip tendency is a function of stress direction,magnitude and fault orientation.It represents the difficulty and possibility of fault activity.The larger the slip tendency,the easier the friction slip occurs in the fault and.the change of stress will cause the change of slip tendency value,which is the basis of slip tendency inversion.The presupposition of this method is that the slip trend of faults is positively correlated with the fault displacement,that is,the larger the slip tendency,the easier the fault slips,and the larger the accumulated fault displacement.The cyclic search method is used to bring different stress states into the formula of slip tendency.When the slip tendency of each fault calculated under a stress state is in the highest agreement with its fault spacing distribution,the stress tensor at this time represents the stress state of these faults when they were active.This process is quantitatively realized by calculating the deviation between the slip trend and the fault spacing distribution by the least square method.The second inversion method used in this paper is slip direction inversion method,which is based on seismic quantitative interpretation and uses the undulations of the fault surface to obtain the slip direction of the fault and then calculate the paleostress.In this paper,the slip tendency inversion and the slip direction inversion are used to invert the paleostress of the strata in the Xicaogu structure belt of Shulu sag during the hydrocarbon accumulation period?the late Dongying Formation-the Guantao Formation stratigraphic sedimentary period?.The result of slip tendency inversion is that ?₁ vertical, ?₂ points to N50°, ?₃ points to N140°,and the shape ratio is R=0.38.The result of slip tendency inversion is that ?₁ vertical, ?₂ points to N56°, ?₃ points to N146°,and the shape ratio is R=0.47.By comparison,the inversion results of the two methods are coincidence in principle.Therefore,the slip tendency inversion and the slip direction inversion proposed in this paper have a large application and population value in the research of paleostress inversion in oil and gas industry.