Differential oil accumulation and its controlling factors for CFD6-4 oilfield at the steep slope belt on the southern margin of Shijiutuo Uplift in Bohai Sea

Abstract

Biomarkers and stable carbon isotopes suggest that oils accumulated in CFD6-4 oilfield are mixed type by oils derived from source rocks of the third (Es3) and first (Es1) members of Shahejie formation, except the ones from the reservoirs in the third member (Ed3) of Dongying formation of Well A5, which are Es3-derived. Fluid inclusion and basin modeling analyses elaborate two charging periods in CFD6-4 oilfield. The first period initiates at about 7.5 Ma driven by hydrocarbon generating and expelling of source rocks, and another one occurred at about 5.0–2.0 Ma caused by the Neo-tectonic activity in Bohai Sea. The Paleogene fan deltas and their direct contact with source rocks define oil horizontal accumulations in CFD6-4 Oilfield. Due to the poor continuities of fan deltas, oils mainly accumulate near source, leading to oils rich in the east with poor in the west. And oil vertical accumulations in CFD6-4 oilfield depend on fault activity during Neo-tectonic activity in Bohai Sea and sandstone carrier bed of the Guantao (Ng) formation. Oil accumulations in the Paleogene and Neogene strata lies on the activity rate of fault connecting the Paleogene oil favorably accumulating reservoir and the Ng sandstone carrier, because it moved at 5 and 25 m/Ma during Neo-tectonic activity and could be both barrier to and avenue of oils. Furthermore, the oil accumulations in the Neogene strata are restrained by the structural features of the Ng sandstone carrier. Therefore, in order to improve the exploration success rate at the steep slope zones in Bohai Sea, the distribution of fan deltas in the Paleogene strata, thveir contact situations with oil source rocks, and the activity rate of connecting faults during the Neo-tectonic activity should be taken into account.