Mechanism Of Deep Hydrocarbon Migration And Accumulation In The Bozhong Depression

The geological process of its temporal and spatial evolution could be best reflected by the migration and accumulation of oil and gas as a fluid mineral,which is an important part of oil and gas accumulation theory and exploration target optimization.The effect of the deep oil and gas exploration is outstanding in the Bozhong depression,but the research on hydrocarbon migration and accumulation process was weak.The source of oil and gas under the condition of multiple differential hydrocarbon sources,and the process of hydrocarbon migration and accumulation under the joint action of transport system and fluid dynamics of the Bozhong depression were attempted to study in this paper,so as to explore the mechanism of deep oil and gas migration and accumulation,as well as the optimal selection of exploration and target area.The deep hydrocarbon reservoir of the southwest of the Bozhong depression was selected as the main anatomical area on the basis of the basic geological data of more than30 deep exploration wells and evaluation wells.Source of oil and gas and deep fluids tracing,migration system developing characteristics,and the fluid dynamics recovering and its evolution,and dominant migration direction were carried out through detailed data such as core observation,microsection,oil/gas properties,inorganic and organic geochemistry.The process of hydrocarbon migration and accumulation was analyzed by means of the physical experiment of fluid displacement and Petro Mod^?numerical simulation.The following main conclusions have been drawn:The deep oil and gas in the southwest of the Bozhong depression are mainly from the source rocks of the main depression,the south depression and the southwest depression in the hydrocarbon rich deep sub sag.And the contribution of the Shahejie Formation is the main part on the interval,and followed by the Dongying Formation.The tectonic background of the thermal expansion at the bottom of lithosphere and the crust mantle material gushed upward under diapirism,and the characteristics of petrographic assemblages and organic/inorganic geochemistry indicate that comprehensively reflect that the deep fluids mainly comes from the deep part of the upper mantle,slightly contaminated by the crust source material,and that the fluid activity pattern was aided by the deep fracture-fault systems in addition to central and fissure-type wide-area eruption activities.The transporting system of the Bozhong depression were consisted of highly permeable rock mass,fault,unconformity and fractures.The NNE and nearly NS trending positive translation faults and NW and NE trending conjugate strike slip faults formed in multiple stages were activated and redeveloped during the neotectonic movement,which provided a dominant migration channel for the deeply fluid.The fracture types mainly include nearly vertical fracture,oblique fracture and horizontal fracture.The formation time of the horizontal fracture is earlier than that of the near-vertical fracture,and the near-vertical fracture is earlier than that of the oblique fracture.The coupling of multi-stage tectonic evolution and hydrocarbon generation and expulsion from source rocks contributes to the formation of fracture networks.The causes of overpressure mainly include sedimentary overpressure,hydrocarbon-generating superpressure and pressure transfer caused by faults,in which sedimentary overpressure and hydrocarbon generation pressurization are the main contributors of the deep overpressure in the Bozhong depression.Overall evolution of the fluid dynamic is that the oil potential gradient is increasing gradually,and the oil potential gradient has reached the maximum since about 5.3Ma.The comprehensive results show that the deep oil and gas in the Bozhong depression has experienced the multi-stage episodic mixed charging process of early crude oil charging,late natural gas charging,and the large-scale migration and accumulation of natural gas have taken place since about 5.3 Ma.Driven by the fluid potential gradient,oil and gas were migrated vertically and laterally for a long distance along the fault-highly permeable rock mass-unconformity surface-fractures.The hydrocarbon accumulation model of"multi-source hydrocarbon gathering and supplying,early oil and late gas,long-distance vertical and lateral differential migration and accumulation"has been formed.