Controlling Factors And Models Of Hydrocarbon Accumulation In The Shallow Formations Of Zhu Ⅰ Depression

Pearl River Mouth Basin is one of the important sedimentary basin in China,which is the largest Mesozoic-Cenozoic petroliferous basin in the northern South China Sea.Zhu I depression is an important region in Pearl River Mouth Basin whose exploration prospect is good.Based on research of formers,starting from the seismic,well logging,geochemistry and oilfield production data,under the guidance of petroleum accumulation theories,this paper analyzes source rocks,passage system,reservoir-seal assemblage and traps in the shallow layers and then makes a further study of the influencing factors of shallow reservoir formation.After that,through anatomy of typical reservoirs,models of hydrocarbon accumulation is established which aims to provide guidance for further hydrocarbon exploration in this area.The results show that there are two suits of source rocks which namely Wenchang and Enping formations.Wenchang formation is the most important source rock which is stable distributed in the region.It has high abundance of organic matter type,I-II1 kerogen type and mature-high mature stage.While Enping formation is the secondary source rock which shows small distribution.At the same time,shallow reservoirs in Zhu I depression mainly circle around the hydrocarbon generation center while favorable area of hydrocarbon accumulation is less than 15~20km.Oil source fault is the key factor of hydrocarbon accumulation in shallow reservoir formation.The match between oil source fault and cap rock,reservoir affects the vertical distribution of hydrocarbon.While cap rock thickness is less than 50 m and the ratio of cap rock thickness and fault throw is less than 0.5,hydrocarbon is likely to breaking through deep plugging and accumulating in shallow reservoir formation.The lower the difference of fault and reservoir displacement pressure are in the deep while higher in the shallow,the more likely for hydrocarbon to migrate vertically in the deep and laterally in the shallow.At the same time,matching relationship between source and fault controls oil and gas migrating to the shallow formations,tructure ridges control far distance lateral migration so the traps far from hydrocarbon generation center can accumulate hydrocarbons too.Delta sedimentary system is the most important sedimentary system in shallow reservoir formation.The lower sequence and transgressive system tract is more favorable to become a reservoir while connecting combined mouth bar,branch channel and combined mouth bar are the favorable reservoir facies belts.There are two sets of regional combination of reservoir and seal in the shallow,one is upper-Hanjiang sandstone and MFS10 mudstone,the other is lower-Hanjiang sandstone and MFS13.5 mudstone.The hydrocarbon is likely to accumulate in combination of reservoir and seal of type Ⅱ,Ⅲ and Ⅳ.Structural traps in the shallow layer of Zhu I depression are mainly controlled by faults whose trend is NW and near EW.The traps can be divided into fault block trap,fault nose trap,anticline-fault trap,rollover anticline trap and drape anticline trap.Crossrange blockage of fault determine trap effectiveness while comprehensive evaluation result shows that the traps are effective when the the factor of fault sealing is in 0.5~0.7.By the study of differences of typical reservoir,the hydrocarbon accumulation models of Zhu I depression can be divided into four types including “forward-source” type,“back-source” type,“follow-source” type and “far-source” type.And the distinctions between different positions are clear.Oil and gas transports along hanging wall in “forward-source” type,which mainly exists in Huizhou sag and the anticline reservoir or fault reservoir can be found in hanging wall.The induced fracture zone in the footwall of fault is main migration pathway in “back-source” type.This type mainly exists in Enping sag and then divide into anticline reservoir and fault reservoir too.“Follow-source” type exists in PY4 sub-sag,Enping sag and Huizhou sag.The induced fracture zone both in hanging wall and footwall of fault is effective migration pathway.However,the transporting ability of hanging wall is better than footwall.At the same time,the distance between reservoir and hydrocarbon generation center is relatively far at “far-source” type in which structure ridges play a key role in lateral migration of oil and gas.The differences between sags and between regions in the same sag are apparent.“Follow-source” type is the uppermost model in the study area.