Characteristic Of Concealed Fault Zone And Its Significance In Hydrocarbon Accumulation In Dongying Depression

Concealed fault zones are a type of weak deformation fault zone that form in sedimentary covers in the early-middle stage of their formation and evolution processes.They are consisted of regular secondary faults,reservoirs,facies,depressions and buried hills.They are difficult to be recognized because they are lack of discernible characteristics.Research shows that concealed fault zones are very common in sedimentary basins and are closely related to hydrocarbon accumulation.Nine concealed fault zones of NE-trending,NW-trending and SN-trending can be identified by basement rift,small dominant structure and sedimentary facies in Dongying depression.There are characterized by en-echelon secondary faults in the plane.Parts of them separate the sags and bulges,or different sags.In the section,concealed fault zones are characterized by flower structure,unobvious flower structure or ?Y?-shaped structure.Regional structural background and evolution history of Dongying depression showed that the NE-trending concealed fault zones were formed by transtentional activities of NE-trending basement faults,while NW-trending concealed fault zones were formed by transpressional activities of NW-trending basement faults and the SN-trending concealed fault zone was formed by strike-slip activities of SN-trending basement fault.Set concealed fault zone Bamianhe and Wangjiagang as examples,studying their differences in formation mechanism by physical simulation experiments.Results showed that the concealed fault zone Wangjiagang was formed by 30°~50°transtentional activities of basement faults,while concealed fault zone Bamianhe was formed by transtentional activities of basement faults less than 30°.The quantitative functional relationship between strike-slip displacement of basement fault and development degree of secondary faults was set up by analyzed the experimental parameters.Based on the function,the strike-slip displacement of basement fault of concealed fault zone Wangjiagang and Bamianhe was calculated.The significance in hydrocarbon accumulation of concealed fault zones in Dongying depression represented in 3 aspects: First,they affected the distribution of sedimentary facies,which means they also affected the distribution of reservoir.Second,because of them,plenty of structural traps and lithologic traps are formed,which provided sufficient space for hydrocarbon accumulation.Third,the intensive fractures in concealed fault zones improved the porosity and permeability of reservoir.The planar patterns of hydrocarbon accumulation in concealed fault zones in Dongying depression are grid-shape composite pattern,en-echelon-shape fault block pattern and ribbon-shape fault block pattern.Based on the structural styles and hydrocarbon accumulation rules,section patterns of hydrocarbon accumulation in NE-trending concealed fault zones,NW-trending concealed fault zones and SN-trending concealed fault zone can be set up.