Hydrocarbon Accumulation Models And Main Controlling Factors Of Qingdong Sag In Jiyang Depression

Despite a number of commercial oil reservoirs have been revealed in Qingdong Sag,a sub-sag of Jiyang Depression,the hydrocarbon potential in Qingdong Sag is still unknown due to the low degree of development.The characteristics of crude oils,the oil/gas sources and the accumulation processes as well as the enrichment pattern are still not understood comprehensively.On the basis of geochemical studies on source rocks and crude oils of Es₃^x and Es₄^s members of Shahejie Formation,a series of analyses are completed in this paper,including evaluation of hydrocarbon generation potential,the generation mechanism of immature oil,the oil-source correlation and the analysis of accumulation stage.The hydrocarbon accumulation models are established and the main controlling factors of hydrocarbon enrichment are discussed as well.The major source rocks of Qingdong Sag are dark mudstones of Es₃^x and Es₄^s members of Paleogene Shahejie Formation.Source rocks of Es₃^x member are distributed mainly in the middle-northern part of the sag,while source rocks of Es₄^s member are widely distributed in the whole sag.The maturity of the source rocks are low and the mature source rocks are limited to the deep zone of the middle-northern sag.The immature oils is proved to exist in Qingdong Sag,which mainly distribute in Es₄^s reservoirs in the western slope,with Es₄^s member as the source rocks.This Es₄^s source rocks are rich of sporinite,liptodetrinite,alginite and mineral-bitumineous groundmass andbacteria,algae and higher plants are the basic materials for immature oils.The source rocks,co-existed with evaporates,are proved to be deposited in saline environment,and high salinity of formation water are all favorable for the generation of immature oils.What's more,the source rocks have undergone two hydrocarbon generation stages comprised by immature and mature oils,indicating immature oils are generated based on chemical reaction,bacterial activities and catalytic actions under low temperature.The immature oils are believed to exist by the comprehensive analysis of geochemical markers and a new indicator composed by 4,4,8,9,9-pentamethyl decahydronaphthalene/8??H?decahydronaphthalene is proposed to distinguish immature?value less than 0.34?and mature oils?value greater than 0.34?.Characteristics of crude oils of Es₃^x and Es₄^s reservoirs in Qingdong Sag are complex,with co-existence of conventional and heavy oil.The heavy oils are composed of immature oils and biodegraded oils,and the highest biodegradation level is eight.Conventional oils ares mainly distributed in the deep reservoirs in middle-northern part of the sag,while the heavy oils are mainly distributed in the shallow reservoirs at edges of the sag.Some indicators with high anti-biodegradation including gammacerane,?-carrotanes and tricyclic terpanes together with stable isotopic analysis are used for oil-source correlation.Three new oil-source correlation indicators are proposed,including?-carrotanes/(2×C₂₉ hopone)?(2×C₂₄ tetracyclic terpane)/C₂₆ tricyclic terpane and(2×C₁₉)/(C₂₈+C₂₉)tricyclic terpanes.Besides,the ratio of chrysene/phenanthrene is also innovatively proposed in oil-source correlation of biodegraded oils which proves to be effective.Oils in the middle-northern and northwest part of the sag are derived from source rocks of Es₃^x and Es₄^s members,biodegraded oils in Qingdong-Qingnan transitional belt are mainly sourced from Es₄^s source rocks of Qingnan sag in Dongying depression,with some oils generated by Qingdong source rocks,immature oils in southwestern part of the sag are proved to be generated by source rocks of Es₄^s member.The Qingdong Sag has undergone two oil filling events?Ed and Ng-Q?,with dominating of the later stage.Due to tectonic events,oils show characteristics of biodegradation while getting entraped.Five oil accumulation models are established including self-source lateral/vertical migration accumulation model?immature oil of Qingdong 5?,near-source accumulation model?central horst?,accumulation model of vertical migration along steep fault?steep slope belt?,accumulation model of long distance migration along fault strike in gentle slope?western slope?,self generation and storage accumulation model?central sag?.The hydrocarbon enrichment is mainly controlled by source rocks,physical property of reservoirs,fault systems and sealing conditions.The size of oil reservoirs is controlled by effective source rocks and oils mainly distribute in intervals near the source rocks.Heavy oils in Qingdong Sag show characteristic of biodegradation during migration,so the accumulation is strictly controlled by physical property of reservoirs.The fault systems serve as the main migrated pathways,but are also factors causing destruction to the oil reservoirs.The local cap rocks can effectively preserve the hydrocarbon,and high salinity of formation water also reveals that the formation has high sealing capacity.