Characteristics of concealed fault zone and its significance in hydrocarbon accumulation in Qikou Sag

Key words:  concealed fault zone  basement fault activity  flower structure  accommodation structure  decollement structure  hydrocarbon accumulation

Abstract:

Concealed fault zones or fault trend zones formed in the cap rocks of sedimentary basin, which is influenced by the regional or local stress field, and activities in the basement rift system. A concealed fault zone is characterized by strong concealment and is associated with dominant fault zone. Based on the previous research of Subei and Bohai bay basins, using basement structure, structural framework, small dominant structure, seismic section features and coherent slices of Huanghua Sag, we recognized a NNE concealed fault zone and analyzed its characteristics, formation mechanism, and hydrocarbon accumulation potentials. The results show that the newly found concealed fault can be divided into three different tectonic deformation sections, the flower structure linked by the basement rift and the cap rock rift system in the south part; the Paleogene flower structure ended in the second member of Shahejie Formation in the middle part where bending deformations only occurred in the Neogene stratum and became the balance point of different structural units; and the flower structure developed in the Neogene stratum of north part deeply converged to gravitational detachment fault of Paleogene, where basement rift controlled the cap rock rift system by affecting gravitational detachment fault balance point. Studies show that former Paleogene basement rift twist controlled and affected the formation of concealed fault zone in the cap rock in three forms:direct transmission, regulatory transmission and indirect transmission. Deep buried faults cut into the basement, facilitating heat transfer to accelerate the maturity of source rock. Concealed fault zones in the cap rock controlled the formation and the distribution of the sedimentary system and disguised traps. The viewpoint of concealed fault zone enriches structural geology theory and provides a new understanding for oil and gas exploration.