Re-recognition of hydrocarbon accumulation regularity based on oilfield development data:A case study of the Guantao Formation in NB35-2 oilfield, Bohai Bay Basin

According to the contradiction between production and geological understanding of the NB35-2 Oilfield, Bohai Bay Basin, we re-analyzed paleostructure, depositional features and the reservoir forming model as well as resource potential based on drilling, logging, seismic and production performance data. Paleostructures controlled sedimentary facies and hydrocarbon accumulation in the Guantao Formation. (1) Petroleum migration and accumulation were controlled by palaeogeomorphology of the buried hill area in NB35-2 block, which is different from the previous research results that reservoirs were restricted by lithology rather than structure. (2) Braided river sedimentary facies are predominant in the Guantao Formation. Reservoirs mainly distribute over superimposed, linked areas of main stream channels, and the space distribution of sand bodies is controlled by the secondary accumulation and distribution of oil and gas to some extent. (3) In the NB35-2 oilfield, faults controlled hydrocarbon accumulation, and hydrocarbon mainly accumulated in deep strata and migrated to shallow strata. Boundary faults and unconformities worked as a "resource network" and allowed hydrocarbons to accumulate in the Guantao Formation. Secondary faults were active in the shallow formations during late stage, and made crude oil migrate to the Minghuazhen Formation and form secondary oil pools. The Guantao Formation in the NB35-2 oilfield has a great petroleum potential, and may be 3 times larger than the previous estimation.