Formation conditions of deep to ultra-deep large composite buried-hill hydrocarbon reservoirs in offshore Bohai Bay Basin,China
Based on the data of 3D seismic survey,drilling,sidewall coring,thin sections and tests,this paper systematically discusses the necessary conditions for the formation of buried-hills and reservoirs,and accumulations in the large oil and gas fields in deep to ultra-deep composite buried hills in the Bohai Sea through the analysis of the Meso-Cenozoic geotectonic dynamics,buried-hill reservoir characteristics,and differential enrichment patterns of oil and gas in the buried hills,as well as case study of typical reservoirs.The key findings are as follows.First,the double-episode destruction of the North China Craton in the Yanshanian and Himalayan served as the primary tectonic driver for the development of deep to ultra-deep composite buried hills in the offshore Bohai Bay Basin.Jointly controlled by the destruction of the North China Craton and the activity of the Tanlu Fault,the destruction center migrated and converged episodically from the margins of the Bohai Bay Basin towards the Bozhong Sag,resulting in an orderly mountain-building process within the offshore Bohai Bay Basin and subsequently two development zones for composite buried hills,i.e.the middle and inner rim zones within the Bozhong Sag.Second,under the coupling of favorable lithologies and multi-stage structures,the middle and inner rim zones are conducive to the formation of reservoirs in fluid dissolution-pore/fracture zones underlying the weathering crust.Third,during the Episode Ⅱ craton destruction,the middle and inner rim zones subsided intensely,along which massive hydrocarbons were generated,resulting in the overpressure,and then migrated to and accumulated in the composite buried hills.The lower parts and interiors of these buried hills still possess excellent conditions for hydrocarbon accumulation.These findings promote a shift in buried hill exploration to three-dimensional exploration of composite buried hills.It is emphasized that the deep to ultra-deep composite buried hill interiors in the middle rim zone and the multi-stage volcanic edifices in the inner rim zone of the sag represent important successor areas for future exploration in the Bohai Sea.
Bohai Bay BasinBohai Seadeep to ultra-deepcomposite buried-hilldouble-episode craton destructiondissolution-fracture zoneoverpressure
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