Hydrocarbon accumulation conditions and patterns in the southwestern margin of Weixinan Sag, Beibuwan Basin

The hydrocarbon accumulation conditions and patterns in the southwestern margin of the Weixinan Sag of the Beibuwan Basin were studied based on modern reservoir theory, geophysics and geochemistry. Favorable hydrocarbon source rocks of semi-deep or deep facies developed in the Paleogene Liushagang Formation. Four sets of favorable reservoir and cap rock assemblages were deposited in the Paleogene. Faulted nose, faulted block and structure-formation complex traps formed. The good configuration of time and space was favorable for hydrocarbon accumulation. Two accumulation patterns, an upper one and a lower one, were determined, which were separated by an overpressure interface. For the lower system, hydrocarbon was self-generated, self-reservoir and migrated laterally, or upper-generated, lower-reservoir and migrated vertically. For the upper system, hydrocarbon was lower-generated, upper-reserved and migrated vertically. The steep and gentle slopes close to the deep depression were favorable exploration targets.