Study On Hydrocarbon Accumulation Process Of Typical Oil And Gas Reservoirs In Western Sub-sag Of Xihu Depression

The south central part of west sub-sag of Xihu Depression is rich in natural gas resource,the overall burial depth of the formation is deep,and the hydrocarbon accumulation process is complex,which is a new area for exploration and development of the Xihu Depression.In order to figure out the process of hydrocarbon accumulation in the west sub-sag,this paper mainly selects two typical oil and gas structures,structure A and structure B,and analyzes the static geological elements of oil and gas accumulation in the study area,such as source rock,reservoir,reservoir-cap combination,fault and fault-sand configuration relationship by using seismic,drilling and logging,and geochemical experiment analysis data,and uses fluid inclusion analytical technology to determine oil and gas accumulation periods,combines with basin simulation technology,and carries out a comprehensive study of the dynamic matching relationship of the "four histories",mainly including tectonic evolution history,stratigraphic burial history and thermal history,source rock generation and excretion hydrocarbons history,reservoir formation and evolution history.The results show that the study area is located in the hydrocarbon generation sag of Shaoxing 36 sag.The conditions of the coal-bearing source rock are good,the organic matter type is type II to type III,and now it is in a mature and highly mature evolutionary stage,and oil and gas are mainly derived from the source rock of the local Pinghu Formation.The reservoir is now in the stage of mesodiagenetic evolution,and the upper reservoir of the Huagang Formation is basically not compact,the lower reservoir of the Huagang Formation is basically completely compacted,the main causes of reservoir densification are compaction and cementation,the minerals that cause the reservoir to become dense are mainly carbonate cement,clay mineral illite and secondary enlarged quartz,and the dissolution function is the main factor that improves the physical property of the reservoir.The early Longjing movement compressional and reverse structural activity adds the braided channel sand body to form a low-amplitude anticlinal structure-lithology trap.A series of "Y" shaped source-connecting positive faults transport oil and gas vertically,which are formed hydrocarbon reservoirs under the plugging of the regional cap of the H1 and H2 sections at the top of the reservoir in Huagang Formation.The fault is the key factor for oil and gas accumulation,and the hydrocarbon reservoirs are distributed where the faults reach.The oil and gas accumulation periods are respectively in the Longjing Movement and Okinawa Trough Movement.During the early Longjing movement,crude oil was mainly filled,and during the late Okinawa Trough movement,natural gas was mainly filled,and the late large amount of natural gas accumulation is the most critical.The Okinawa Trough Movement adds the hydrocarbon generation of the Pinghu Formation source rock resulting in the pressure increase as the main driving force of the late natural gas accumulation.Therefore,the key factors restricting hydrocarbon accumulation in the study area are trap condition and fault transport condition,forming early oil and late gas,and the late reservoir-forming model of structure-lithology trap controlled by source-connecting faults.The ideas and methods of comprehensive research on the accumulation process in this paper can provide theoretical and technical support for finding oil and gas accumulation areas of similar scale in the Xihu Depression.