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Failures and Successes Along With the Emergence of Gas-Rich Deep Bozhong Depression in Cenozoic Lacustrine Rift Bohai Bay Basin

Abstract

Bozhong depression is the sedimentary center of Bohai Bay Basin since Oligocene with Cenozoic sediments thickness over 11000m and large area over 9000km2. 23 oilfields, 1.8 billion cubic meter oil reserves and 147 billion cubic meter gas reserves are discovered. Bozhong depression was considered as oil-rich gas-poor before Bozhong 19-6 large condensate gas field discovered in deep Eocene and Archean in 2018. 2 failures and 3 successes are listed to tell how the natural gas exploration frontier of deep (below 3500m) Bozhong depression emerges. Penglai 7-1 is the type of seal failure due to early hydrocarbon generation and intense Neo-tectoic activity (fault activity up to 25m/Ma). Source rocks become mature from Oligocene and over-mature (Ro up to 3.5%) now. The Mesozoic buried hill gas reservoir have abundant light oil inclusions, GOI up to 13%, bitumen, and 25-norhopanes indicating early oil accumulation and late reservoir destruction. Bozhong 21-22 is the type of gas resources failure due to high CO2 content (35%~49%), though the daily gas yield of Paleozoic carbonate gas reservoir is 400000 cubic meters. CO2 is from mantle based on δ13C‰PDB (-0.4) and 3He/4He (6.4). CFD18-2 is a small condensate gas field with oil rim and the production layer is Eocene conglomerate and Archean condensate gas reservoir. Source rocks are high and over mature (Ro=1.3%~2.5%) and the trap is charged by oil and gas successively based on hydrocarbon inclusion biomarker analysis. The conglomerate reservoir with porosity as 6.7% is poor for oil but medium for gas. Bozhong 13-1 locates in the lower part of CFD18-2 and is a small oilfield with excellent bioclastic dolomite reservoir. The bioclastic dolomite develops on Mesozoic buried hill, which can supply nutrients for biology and is favorable for the reservoir development. The reservoir with porosity as 19.8% is critical when the hydrocarbon is still liquid at the reservoir condition of 168 oC and 40.5MPa. Bozhong 19-6 is discovered in 2018 with gas reserves over 100 billion cubic meters and daily gas/condensate yield of DST being 310000/305 cubic meters. The favorable conditions include multiple high mature humic-sapropelic source rocks, large-scale buried hill traps resulted from multi-stage tectonic evolution, high quality natural gas reservoirs resulted from the coupling of Tanlu strike-slip fault activity and multi-type lithology and good preservation conditions due to weak neo-tectonic movement.