Study On Dynamic Processes Of Petroleum Pool Formation In The Southwest Of Jiangling Depression

At present, the eastern exploration region in China is still the major area of oil production and reserve. Jiangling depression, 6500 km², the largest structural unit of Jianghan Basin, is a fault basin formed on the Yanshan folded basement of Late Jurassic under the depression setting of Late Cretaceous-Paleogene.The early discovered oils in the depression mostly originate from the Huayuan, Balinshan, Jingxi and Shashi oilfields which are located in the Jingsha anticline zone, and exploration targets are centered primarily on the Paleogen Xingouzui Fm. Due to newly discovered oil pools in the Paleogene Xingouzui Fm and Cretaceous Yuyang Fm of Songzi oilfield in the southwest of Jiangling Depression, major breakthrough about new zone,new reservoir and new pool type in Jiangling depression and even in Jianghan Basin, target reservoirs have changed from single to multiple,exploration field have been expended from the Jingsha anticline zone to Wancheng fault zone which is not far from the Meiguiqiao source kitchen. In the dissertaton, on the basis of integrated exertion of geological, seismic, well-logging and geochemical data and analyzing geologic conditions of petroleum accumulation, tracing hydrocarbon migration directions realized by fine oil-source correlations, nitrogen compounds analysis and fluid potential modelling, petroleum distribution characterization and accumulation are then determined by analyzing the entrapment formation, hydrocarbon generation and expulsion and fluid inclusions data, and finally petroleum accumulation modules and profitable exploration fields are pointed out in the studied area.Based on fine structural interpretation of seismic profiles, faulted horst and trough,reverse block, sliding structure, dip reversal,fault nose mainly occur in the southwest of Jiangling Depression.The major entrapment type drilled is fault blocks, and then is the fault noses, there is nearly no anticline traps. Results of tectonic evolution histroy show that Caixue-Fuxingchang Block were seperated at the early stage of shashi period and changed into two individual blocks at the early stage of Jingsha period, while Xiefengqiao-Babao and Nangang-Mijitai nose-like structure zones were formed initially at the early stage of Jingsha period and at the late stage of Jingsha period they decomposed respectively induced by the Jingsha movement. Because of few active faults in the southwestern of Jiangling Depression, tectonic units were generally shaped at the early stage of the Qianjiang period.The reservoirs of Yuyang and Xingouzui Fms are mainly composed of packsandstone and siltsandstone that can be clsssified as feldspathic quartz arenite and lithic quartz arenite with high component and textural maturity.There are several different types of cements including calcite, dolomite and gypsum which usually present in the form of pore-cementation in the reservoirs,.They are generally low porosity and permeability reservoirs with values of porosity 1.6%～17.9% and permeability in the range of 0.1×10^-3μm²ï½ž12×10^-3μm², respectively.The dominating pore type is secondary pore regulated mainly by granular solution pore. Pore structure is characterized by medium to large pore size and fine to medium pore throat. Contaction between sandstone grains is regulated by point-line contaction and then is line-seam contaction.The content of carbonate cements is usually more than 30%, gypsum and pyrite are the dominated components of sulfate cements, with a thin quart overgrowth, metasomatism often occur in our studied areas, clay minerals in reservoirs is regulated by Illite. All of the above indicates that reservoir sandstone is now at the late diagenetic phase A.Two secondary pores zones take place at the depth of 2800～3100m and 3300～3600m repectively. The first one is induced by solution of feldspar and carbonates that is attributed to organic acids dismission during the course of organic matter maturation and the second deeper one may be related to thermo-chemical sulfate reduction.Results of subsidence history modeling indicate that in the southwest of Jiangling Depression, the depocenter is located at the Meiguiqiao and Niutougang sags. Because of much larger subsidence rate in the Meiguiqiao main sag, it is the major depocenter and thus becomes the most preferential area for source rock sedimentation. Source rock well-logging evaluation demonstrates that the domain source rocks bearing excellent source rocks locally are the Lower Xinggouzui Fm and the Upper Shashi Fm containing as much as <1% organic carbon and type II₂ and III kerogens with maturity of 0.6%～1.3%.Reuslts of petroleum generation modelling show that the Upper Shashi and the Lower Xingouzui Fms entered the oil threshold (Ro=0.5%) was at 47～45Ma duing the depositional period of upper Jingsha and lower Qiangjiang Fms, and reached the middle maturation stage (Ro=0.7～1 .0%) was at 44～29Ma duing the depositional period of lower Qiangjiang and lower Jinghezhen Fms.The maximum maturity of source rocks have been received before the Late Paleogen reginal uplifting appeared, and the latter deposited overlay formations have a minor impact on source rocks' maturity. The intensity of petroleum generation and expulsion in the Meiguiqiao and Niutougang sags are obviously different due to different burial depth resulting in different maturity of source rocks.Combining with burial and thermal history modeling results, homogenization temperatures of hydrocarbon inclusions and coeval-aqueous inclusions suggest continous petroleum-charging in our studied area.The timing of petroleum filling in the different reserviors is same,generally at about 45～26Ma duing the depositional period of Qiangjiang and Upper Jinghezhen Fms. Crude oils sourcing from the Upper Shashi and Lower Xingouzui Fms were trapped in the reservoirs which are not far from the source ketchen, indicated by the oil-source correlations and distribution trends of nitrogenous compounds and fluid potential modeling.The high maturity oils in the north, such as Caixue, Fuxingchang, Xiefengqiao, Nangang, are primarily generated from the high maturity source rocks in the Meiguiqiao main Sag and the low maturity oils in the south primarily come from the local source rock, and the oils in the medium are mixed. Fault growth index and fault throw indicate that the Xiefengqiao fault activity was strong during the periods of Shashi and Jingsha Fms deposition, and was weak during the period of Xingouzui Fm deposition.Along the fault strike, activity strength of Xiefengqiao fault enchanced gradually from the south to the north. The cross-fault lithology juxtaposition is influenced by fault activity and it acts as an important role in regulating petroleum migration and reservoir-forming. Fuxingchang No.I tectonic unit and Xiefengqiao faulted nose can be considered as petroleum preferential maigration pathways and Fuxingchang No.II tectonic unit is not suitable for petroleum accumulation suggested by fluid potential of fault section.Crude oil entrapments discovered are located along the Xiefengqiao and Fuxiangchang faults, fault-nose and fault-block traps are the major traps in the studied area. Petroluem accumulation is controlled mainly by faults and source rocks leading to a short petroleum migration distance. The factors which controlled petroleum accumulation are as follow: the timing of entrapment-forming and hydrocarbon-yeilding, the reservoir beds in uplifted wall being connected with the source beds in throw wall controlled by fault, hydrocarbon generation center controlling the oil pools distribution. According to the spatial distribution of source rocks and reserviovrs and petroleum migration and accumulation characterization, crude oil reservoir formation modules can be dividied into three types known as self-sourced and reservoired, new-sourced and old-reservoired and old-sourced and new-reservoired. Much more attention, therefore, should be given to entrapments around the Meiguiqiao source kitchen.By analyzing petroleum accumulation elements and present petroleum exploration status, the most favourable field of petroluem explortion should be concentrated on the Crateceous Yuyang Fm in Caixue and Fuxingchang No.I tectonic unit, Xingouzui and Jingsha Fms are the sweet points in the area of Xiefengqiao and Nangang structure, Xingguozui Fm in the area of Dingjiahu tectonic unit can be assumed as a secondary petroleum exploration target.