--> Controls on Hydrocarbon Accumulation Model for the Neogene Traps, Bohai Bay Basin, China

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Controls on Hydrocarbon Accumulation Model for the Neogene Traps, Bohai Bay Basin, China

Abstract

Chengdao, an offshore area of the Bohai Bay Basin in China, has approximately 3.68×108 tons oil and gas resources within Neogene sandstone reservoirs. The failure rate of drilling for hydrocarbons in the Chengdao area is up to 50% due to inaccurate predictions of hydrocarbon accumulation in the Neogene traps. Therefore, an improved exploration model for the Neogene traps could reduce the exploration risk. In this study, we selected ninety-two traps from Neogene strata in the Chengdao area to quantify volumetric filling degrees, an indicator for hydrocarbon accumulation efficiency. The quantified filling degrees are based on actual geological and exploration data and differ significantly among different types of traps. The anticlinal-lithologic traps have the highest average volumetric filling; whereas the lithologic-fault traps have the lowest average filling. The volumetric filling of traps also varies significantly with their structural locations and decreases generally from the northwest to the southeast along the Chengbei fault zone. Vertically, the volumetric filling degrees are highly heterogeneous, increasing first from the bottom to the middle of the Neogene strata and then decreasing toward the top of the strata. Hydrocarbon accumulation and filling degrees of the Neogene traps are influenced differently by different geological factors in the Chengdao area. The Neogene reservoirs are far away from Paleogene source rocks, so their enrichment of hydrocarbons is constrained largely by hydrocarbon-generation of source rocks, hydrocarbon migration distances, and vertical migration pathways. Sedimentary facies affect the physical characters of reservoir rocks, thus their ability to accumulate hydrocarbons. Sealing ability of faults and cap-rocks as the preservation conditions determine the maximum height of the oil and gas column, which in turn affects the largest accumulation amount of hydrocarbons in the traps. For each individual structural unit in the Chengdao area, the traps with the same type are grouped together for the purpose of constructing hydrocarbon accumulation model. For each type of traps, the dominant controls on hydrocarbon accumulation were found based on the scatter plot analysis between individual controls and volumetric filling. A equation as the prediction model of hydrocarbon accumulation was constructed for lithologic-fault traps using the multivariate linear regression analysis.