The Characteristics Of Depositional Reservoirs And Their Effects On Hydrocarbon Accumulation In The Upper Part Of The Fourth Member Of Shahejie Formation In Bonan Sag

Located on the middle of Zhanhua depression,Bonan sag,covering an area of 800km2,is the mainly target area of deep exploration in Jiyang depression,Multi-type reservoirs were developed during the deposition period of the upper part of fourth member of Shahejie Formation,reservoir characteristics and forming-mechanism of which were very complicated due to different presentation on depositional environment and the evolution diagenetic environment,thus influenced exploration process of the oil and gas.In order to solve this problem,based on comprehensive theory and technology of sequence stratigraphy,sedimentology,reservoir geology and petroleum geology,deposition mechanism,diagenetic evolution,reservoir characteristics of multi-type reservoirs and its relationship with reservoir forming were studied systematically.The results show that: the paleoclimate is warm and humid during the deposition period of the upper part of fourth member of Shahejie Formation in Bonan sag,and under its influence,three depositional facies including fan-delta,nearshore subaqueous and fan lake were developed in research area.Three depositional facies can be further divided into nine subfacies and more than thirty microfacies.The fan delta and sandstone beach bar distributed in lowstand system tract and nearshore subaqueous fans,fan delta,limestone beach bar distributed in transgressive and highstand system tract.Summarizes the genetic mechanism that depositional background and provenance system control vertical evolution and water environment control transverse distribution of multi-type sedimentary bodies,and use water environment quantified the formation conditions of multi-type sedimentary bodies is realized.At last,sedimentary models of lowstand system tract and transgressive-highstand system tract are established.The clastic rock reservoir is mainly debris arkose and the accumulation spaces are mainly secondary pore of upper part of Fourth Member of Shahejie Formation in Bonan sag.The rock types of carbonate reservoirs are divided into limestone,dolomite,mix rock and the accumulation spaces are mainly primary pore.Reservoir property have two secondary pore zones in vertical and present a feature that the property was high in north and east and low in south and west in horizon.In general,the reservoir property of the upper part of fourth member of Shahejie Formation in Bonan sag is low porosity and low permeability or extra low permeability.The reservoir of the upper part of fourth member of Shahejie Formation in Bonan sag is almost in the middle of diagenesis A stage and a few in the middle diagenesis B stage.And mainly have three types of diagentic phenomenon that include compaction,cementation and dissolution.Under the influence of alkali fluid from gypsum and organic acid,the alternately diagenetic environment of acid and alkali is formed Because of this,every types of reservoir follow the diagenetic evolution mode that the reservoir have middling alkali diagenetic environment at early stage diagenesis will caused later dissolution and if have weak alkali diagenetic environment at early stage diagenesis will caused early dissolution.And based on this,the 2800~3200m section that gypsum development is defined “diagenetic dense zone” under the controlling of diagenetic evolution mode.According to the model of sedimentation control primary pore and diagensis control secondary pore,the favorable reservoir of clastic rocks and carbonate rocks were predicted.The upper part of fourth member of Shahejie Formation in Bonan sag has the feature of sag-wide oil bearing.And the oil reservoir type presented distribution rule of structure,lithologic,stratigraphic reservoirs from northern steep slope to southern gentle slope belt.The characteristics of depositional reservoirs effecting on hydrocarbon accumulation is shown on two respects below.First of all,multi-type of sedimentary and diagenesis influenced the differences about reservoir forming factors.Secondly,the “diagenetic dense zone” controlled the differences about hydrocarbon enrichment,and formed two kinds of exploration methods about two kinds of oil and gas accumulation area that existed up and down the “diagenetic dense zone”.The exploration method effectively guide the exploration deployment.