The Preferential Pathways Of Hydrocarbon Migration And Its Effect On Hydrocarbon Accumulation In The Slope Belt Of Qikou Sag

The Qinan and Qibei slope belts are located in the south of the Qikou Sag.The great difference of tectonic activity intensity and sedimentary phases in various regions lead to the complexity of hydrocarbon transport system in the slope area.For a long time,there has been a lack of research on the controls of the preferential migration pathways on the inhomogeneous accumulation of hydrocarbons.This paper aims to enhance oil-source correlation and to reveal the oil charging history in the Qinan and Qibei slope belts.And the geological,geophysical,geochemical methods and numerical simulation were employed to investigate the mechanism and process of hydrocarbon differential accumulation controlled by the preferential migration pathways.The hydrocarbon accumulation in the Qinan and Qibei slope belts can be divided into the deep hydrocarbon accumulation system(Es₃～Es_1z)and the middle-shallow hydrocarbon accumulation system(Es_1s～Nm).The source-reservoir relationship in the deep hydrocarbon accumulation system is“self-generating and self-storing”or“side-generating and side-storing”.The oils in the deep hydrocarbon accumulation system mainly originated from the Es₃ and Es_1x source rocks,with part from the Es_1z source rock,and mainly migrates laterally.For the middle-shallow hydrocarbon accumulation system,the source-reservoir relationship is“lower-generating and upper-storing”,the oils mainly originated from the Es₃ and Es_1x source rocks,and the accumulation of oils was mainly controlled by the faults connecting the source rock and reservoirs.The formation and distribution of hydrocarbon preferential migration pathways in the deep hydrocarbon accumulation system are jointly controlled by the heterogeneity of the transport layer,the morphology of the transport layer and the migration power.And fault activity,fault surface morphology and fault sealing affected the distribution of the fault preferential migration pathways in the middle-shallow hydrocarbon accumulation system.The deep hydrocarbon accumulation system experienced two oil charging stages,i.e.,the late of Ed and the late of Nm-present.The oil accumulation events at the end of Ed were mainly the filling process of Es₃ sourced oils in the inner source-reservoirs in the low slope.The scale of hydrocarbon accumulation is large at the late of Nm-present.During this period,under the control of the preferential migration pathways,the oils contributed by Es₃ and Es_1x source rock mainly migrated laterally into the middle and high slope area within the slope.And the oils originated from Es_1z source rock were mainly limited in inner source-layers of the low slope.The middle-shallow hydrocarbon accumulation system only experienced one oil charging stage that occurred in the late of Nm-present.The oil accumulation events were mainly that the oils contributed by Es₃ and Es_1x source rocks migrated and accumulated in multi reservoirs of middle-shallow layers along the fault in the Beidagang and high slope area,respectively.On the whole,the configuration of source rocks and the preferential migration pathways controlled the inhomogeneous enrichment of oils in the deep hydrocarbon accumulation system.The configuration of the preferential migration pathways of the deep system and the fault preferential migration pathways controlled the oil enrichment location in the middle-shallow hydrocarbon accumulation system.The oil resources within 2km from the preferential migration pathways in the deep and middle-shallow hydrocarbon accumulation system are over 65%and 75%,respectively.According to the inhomogeneous accumulation process of oils in different hydrocarbon accumulation systems,it proposed two hydrocarbon differential accumulation models,i.e.,supplied by inner source rocks and“source-ridge”configuration controlled model in the deep hydrocarbon accumulation system and supplied by outer source rocks and“ridge-fault”configuration controlled model in the middle-shallow hydrocarbon accumulation system.