Analysis of main controlling factors of hydrocarbon accumulation in Qinnan Depression, Bohai Bay

The Qinnan Depression has experienced 46 years of exploration and only the Qinhuangdao 29-2 oil and gas field was discovered. In the past 9 years, no oil and gas fields have been found, and the exploration work runs into difficulties. To clarify the main controlling factors of hydrocarbon accumulation for clear direction of oil and gas exploration, we used 3D seismic, and drilling and laboratory data to sort out the basic petroleum geological conditions. By analyzing the success or failure of drilling and by case analogizing, the main controlling factors of each reservoir and reservoir-caprock assemblage were determined, and the favorable exploration direction was predicted. Results show that the source rocks of the third and first members of the Paleogene Shahejie Formation (Fm) (Es₃ and Es₁, respectively) of Qinnan Depression are developed with TOC of 1.5%～4% in the types of Ⅰ-Ⅱ₁, which have great resource potential. Three reservoir-caprock assemblages (1. burial hill; 2. Shahejie Fm to Oligocene Dongying Fm; and 3. Miocene Guantao Fm to the lower member of Miocene-Pliocene Minghuazhen Fm) are developed. The main controlling factor on reservoir formation for the first assemblage is reservoir heterogeneity, for the second assemblage is trap effectiveness, and for the third assemblage is the migration capacity of oil-source faults. We suggested that the favorable exploration zones for future direction should be: the low uplift between the eastern and southeastern depressions in the lower place, the western slope of the eastern depression in the middle place, and the downslope of big fault in the eastern and southeastern depressions in the upper place.