Gravitational Detachment Fault Controls Hydrocarbon's Accumulation in the Slope Area: A Case Study of Laizhouwan Depression, Bohai Bay Basin

Abstract

Bohai Bay Basin is a Cenzoic rifting basin affected by strike-slip motion in eastern China. Many studies about the control of extensional and strike-slip faults on hydrocarbon accumulation have been done in this basin. But the latest research indicates that another structural deformation, which are gravitational detachment faults, also play key roles in the process of hydrocarbon accumulation. Taking the gravitational detachment faults found recently as an example, with variance slices, 3D visualization, seismic attributes, and quantitative basin analysis methods, we investigated their geometry, kinematics and dynamics in detail, on which based a model that how gravitational detachment fault zone controls hydrocarbon accumulation was established. The gravitational detachment fault zone found in the slope area of Laizhouwan sag are controlled by slope gradient, plastic strata, and differential compaction under strike-slip and extension complex stress. Vertically, the major gravity detachment fault's roots extend and fade away gradually into the deep plastic strata such as halite or gypsum rocks. Opposite compensating faults and Major detachment faults appear in pairs showing a flower structure. In plane, the gravity detachment faults extend more than 30km and display an east-west arc distribution along the slope and sag area. In addition, Influenced by the translation of stress field from extension to compression, this gravitational detachment fault experienced an evolution from synsedimentary to gravitational detachment. Research indicates that the gravitational detachment faults control the hydrocarbon migration and accumulation in the slope of Laizhouwan depression: (1)Gravitational detachment faults controlled the formation of big scale traps, which provide a good trap condition for hydrocarbon accumulation. (2)The stronger the fault's activity is, the bigger the stress release is, the more active the hydrocarbon accumulation will be. (3)The main fault has a good vertical sealing property, which is favorable to the petroleum preservation at the downthrown side of faults. Oil mainly accumulate in the deep traps and no oil shows in shallow traps. Reversed compensation faults have a good opening property in vertical, which is beneficial for the migrating of oil vertically, but bad for the preservation of oil in deep formation. At present, by using this theory we have found three oil and gas structures with a petroleum geologic preserve of 200 million barrels.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015