Formation and Evolution of Concealed Fault Zone in Sedimentary Basins and Its Significance in Hydrocarbon Accumulation

Concealed fault zones are a type of weak deformation fault zones that form in sedimentary covers and have expressive forms of faults that show in the early-middle stage of their formation and evolution processes. It can be found by reservoirs, sedimentary facies, sags (depressions) and ridges ranged regularly. Concealed fault zones are supposed to be potential direction for future exploration since reservoirs are usually distributed in concealed fault zones, displaying en-echelon, string or belt patterns. On the vacant area of concealed fault zones should be worthy of oil and gas exploration area in the future According to the scale, concealed fault zones are classified into 5 types in this paper: (1) the trap scale concealed fault zone consists of a series of en echelon fault blocks, curved blocks or compound blocks in line; (2) the sub-sag scale concealed fault zone composed of a lot of faults and folds, with the distribution of echelons and string of many small faults; (3) the sag scale concealed fault zone showing that there is an intermittent string of sub-sags, ridges, sedimentary facies or traps; (4) the depression scale concealed fault zone identified by the mutation of basement depth, the thickness of the sedimentary cover and sedimentary facies, belonging to regional-concealed fault; (5) the basin scale concealed fault zone characterized by large-scale nosing folds or low-amplitude ridges ranged in defined rows. In addition, the evolution model of concealed fault is established under the evolution of transtensional regional stress field and basement fault activities combined with the structural analogue experiments. It is found that concealed fault zones have controlled the accumulation of the oil and gas in the following six patterns: en echelon fault block accumulation pattern, banded fault block accumulation pattern, en echelon fold accumulation pattern, an accumulation pattern of parallel combination traps, an accumulationpattern of grid combination traps, an accumulation pattern of circularity combination traps.