Study on the Ultra-Late Hydrocarbon Accumulation and Migration Lagging Effect in Laizhouwan Sag, Bohai Sea

Abstract

The lagging effect of oil and gas migration refers to the phenomenon that the oil and gas generated and discharged from the source rock too early but accumulated too late, so the oil and gas experienced long⁃term and long⁃distance migration, resulting in the phase state and maturity of the accumulated reservoir being lower than that from the current source rock. This lagging effect of hydrocarbon migration is common in Cenozoic continental rift basins in eastern China, especially in slope zone. By means of geochemical analysis, balanced section technique, basin simulation and inclusion analysis, this paper studies the spatiotemporal coupling relationship of various reservoir forming factors in Laizhouwan Sag, Bohai Sea, It is show that there are obvious oil and gas migration lagging effect and ultra⁃late hydrocarbon accumulation in the study area. The source rocks of the Sha4 formation have entered the hydrocarbon expulsion period during the middle⁃late Dongying Formation, while the shallow traps were formed in the Neotectonic movement period, and the main hydrocarbon filling period is from 2 Ma to now; the oil and gas accumulation time that near northern depression is early, while the structures that far away from northern depression is late, the deep strata is early, while the shallow strata is late. In addition, this paper also discusses the causes of the oil and gas migration's lagging effect. It is considered that tectonic movement controls the process of hydrocarbon generation and expulsion and the vertical migration of oil and gas, the principle of oil and gas differential accumulation controls the sequence of oil and gas charging, and the movement of overpressure blocking fluid developed in the source rock section is the main factors for the lagging effect of oil and gas migration.