The Origin And Accumulation Mechanism Of Petroleum On The Shijiutuo Uplift,Bohai Sea

Recent years,a few large oil and gas fields have been proved on the Shijiutuo Uplift,however,the oil origin deserves a further research.The objectives of this paper are to provide a detailed study to estimate the ability of hydrocarbon supplement by source rocks in the Sags around the Shijiutuo Uplift based on source rock geochemical characteristics and pyrolysis experiments,to differentiate oil origins and charging directions by multivariate analysis of the selected saturated biomarker parameters,and to analyze the differences in accumulation conditions of the QHD29-2/2E oil fields and further summarize the petroleum enrichments on the Shijiutuo Uplift.On basis of this limited research,the conclusions can be drawn as follows:To the southern of the Shijiutuo Uplift,the Dongying Formation(Ed),the first member and the third member of the Shahejie Formation(Es1 and Es3)source rock intervals have been buried mature in the Bozhong Sag,moreover,Dongying Formation source rock generally enters the main stage of oil generation,with corresponding Ro being 0.7%~1.0%.In the northeast Qinnan Sag,Shahejie Formation source rock intervals are of good quality and generally stay at the main stage of oil generation,providing the abundant oil source to the traps distributed along the slope of the Shijiutuo Uplift.The distribution of effective Dongying Formation source rock may be limited to the center of the Qinnan Sag.On the basis of hierarchical cluster analysis of ten saturated biomarker parameters indicating original organic matter inputs or sedimentary environments,all oil samples QHD32-6 and QHD33-1/1S oil fields,locating on the middle part of the Shijiutuo Uplift,could be generally differentiated into the Es1 dominates and Es3 dominates,respectively,with minor or no contribution from the Ed source rock.Oils accumulated in the QHD32-6 and QHD33-1/1S traps are originated from the Bozhong Sag,and possible migration pathways may exist between these two large oil fields.In the western part of the Shijiutuo Uplift,the NB35-2 oil field is dominated by Es3-derived oil,with main oil charging from the Shehejie Formation source rocks in the western part of the Bozhong Sag.In the QHD 29-2/2E oil field,Es1-derived oil is mainly contained in Paleogene reservoirs,whereas Es3-derived oil is generally accumulated in Neogene reservoirs,and the oil was charged from the Qinnan Sag.In terms of the oil fields on the middle and western part of the Shijiutuo Uplift,the traced possible migration directions by geochemical parameters approximately agree with the results of modeled preferential petroleum migration pathways.This may indicate that petroleum migrated to the margin of the Shijiutuo Uplift by boundary fault and sand body could continue to laterally migrate to the structural high through the regional Guantao Formation(Ng)sandstone conduit.Oil accumulated in the lower part of Minghuazhen Formation(NmL)can be explained that the juxtaposition of sandstone distributed along the fault-plane sections may be conduits for oil upward migrating from the Ng conduit to Nm L traps.Generative kitchen and conduit system play a controlling role in the distribution of preferential petroleum migration pathways,and large oil fields focused on the local structural highs on the midlle and western part of Shijiutuo Uplift.During the period of neotectonism,the segmented fault activity intensity of boundary fault plays a controlling role in the petroleum accumulation on the eastern margin of the Shijiutuo uplift.The occurrence of abundant mantle-derived CO2 is limited to traps near the fault with relatively high-intensity activity,and the decrease in the activity intensity along the fault trend is accompanied by the decrease in CO2 content,with oil accumulation in the fault of relatively low activity intensity.The high activity intensity segment of boundary fault vertically communicated deep CO2 fluids and transferred deep Es3-derived oil to shallower reservoirs,therefore the Es3-derived oil were identified in the Neogeng and Paleogene reservoirs near to the high activity intensity section of fault.Fault may vertically serve as a barrier in the segment of low activity intensity,and the distribution of sandstone plays a controlling role in petroleum accumulation in Es1 reservoirs.In Es1 layers,sandstones connected the mature source rock and focused the Es1-derived oil in the Eocene reservoirs without extensive migration.Es3-derived oil in Neogene traps mainly originated from the Qinnan Sag and was laterally transported by Neogene sandstones.