Characteristics Of Siliciclastic-carbonate Sediments And Forming Mechanism Of High-quality Reservoirs In The Member 1?2 Of The Shahejie Formation,Circum-Bozhong Depression,Bohai Bay Basin

The traditional sedimentology theory believes that the systems of siliciclastic rock and carbonate rock are two separated and independent depositional systems.However,there are generally siliciclastic and carbonate particles(including allochem and micrite)that are composed of textural mixtures or are interbedded in nature,which can be defined as the mixed siliciclastic-carbonate sediments.In the past ten years,the Paleogene mixed lacustrine siliciclastic-carbonate sediments has been widely found in the Member 1?2 of the Shahejie Formation in the Bohai Bay area(China).The achievement revealed two main important facts: 1)mixed sediments can be existed in the continental lake basin;2)the mixed sediment reservoirs obtained giant reserves of hydrocarbons with a common buried depth of more than 3000 m.However,compared to marine mixed sediments,continental mixed sediments were still paied less attention.Besides,it is not very clear about the formation mechanism of medium-deep buried mixed sediment reservoirs.Thoese problems might restrict the understanding about mixed sediment in Bohai Bay Basin and further exploration project.Therefore,as one of the typical mixed-sediment domains,the circum-Bozhong Depression is selected as the study area in this paper.A systematic combination of petrographic,sedimentological and reservoir geological study,by using the analysis of drilling data,core observation,petrophysical and fluid geochemistrytest,has been conducted on the medium-deep buried mixed siliciclastic-carbonate reservoirs.On the basis of petrology study,a multi-disciplinary work,including analysis ofdepositional environment,investigations for diagenesis and their evolution,and the study of excellent reservoir characteristics has been focused on.Finally,this study presented a comprehensive look at the depositional process of mixed sediments and the formation mechanism of high-quality reservoirs.The main conclusions are as following:1.A new petrological triangleclassification system is proposed here that is applicable to most mixed sediments,which is consisted of three end-members: exogenous clasts,allochemsand carbonate mud or micrite.The system produces four general classes and sixteen subclasses of mixed sediments.The classification system can not only solve the puzzle of classification for mixed sediments,but also enlighten interpretations for mixed sediment lithofacies.In addition,two kinds of interbedded mixed sediments are identified: one is large-scale interbedded mixed sediments,which is represented by mixtures of siliciclastic-carbonate series within one parasequence unit.The other one is small-scale interbedded mixed sediments,which is defined as mixtures of siliciclastic-carbonate series within one lamina set unit.2.On the foundation of analysis on different types of mixed sediments and their depositional environment,a depositional system of mixed sediment rocks hasbeen built up.According to the typical facies characteristics observedfromcore analysis and thin section petrography,the system is divided into two main parts: fan-delta and lacustrine mixed sediment system.Of them,fan-delta mixed sediment system can be further subdivided into fan-delta front and distal fan-delta mixed sediment facies associations.The former is composed of two genetic facies,including debris flows and mouth bar mixed sediments;The latter develops distal fan-delta mixed sediments.The main mixed facies association in the lacustrine setting is shore-shallow lake whose genetic facies is beach-bar complex mixed sediments.3.Based on detailed investigations of the inner structures for each sand types,three kinds of main sand types can be identified,including debris flows mixed sediments,fan-delta front interbedded mixed sediments and beach-bar complex mixed sediments.The debris flows mixed sediments are associated with gravity flow formed by secondary slump processes in the fan-delta front.The fan-delta front interbedded mixed sediments are related with sedimentary processes that fan-delta spread out and/or abandon periodically.Beach-bar complex mixed sediments are controlled by joint action of lake-level fluctuations and lake current.Thus,three depositional models can be conceptualized as:(1)fan-delta mixed model driven by gravity-flow;(2)interbedded mixture model formed by constructive-abandoned fan-delta;(3)beach-bar mixed model controlled by hydrodynamic conditions4.Main diagenesis are evaluated and a paragenetic sequence of the successionare further developed.Detailed petrographic observation and geochemical tests indicate that micritization,cementation,dolomitization and dissolution are the main kinds of diagenesis.In addition,having been combined geochemical tests and different features of diagenesis products,it is considered that current reservoirs could be laid in the medium-deep burial stage and have been undergone contemporaneous,penecontemporaneous,and burial stages.5.Multiple parameters,such as lithifacies,sedimentary facie,main diagenesis,reservoir space and pore structures of reservoirs,are used to depict reservoir characteristics,pointing out three kinds of genetic types in high quality reservoir of mixed sediments:(1)primary intergranular-secondary dissolution pore,mixed beach-bar reservoirs;(2)primary intergranular pore,mixed sandy debris flows reservoirs;(3)residual primary pore,mixed gravel debris flows reservoirs.The good quality of mixed sediment reservoirs is featured by well-preservation of primary porosities and large number of secondary fabric dissolution pores.Therefore,good quality of reservoirs are derived from joint contribution of primary and secondary porosities.6.High-quality mixed sediment reservoirs are strongly affected by multiple factors,including sedimentology,diagenesis,tectonic activity and fluid activities.Paleoclimate is the basis for the generation of reservoirs;paleogeomorphology and mostly lake-level fluctuationscontrol the reservoirs distribution and pattern.In one word,the combined contribution of the above three elements determines the reservoir formation.In the view of the reservoir burial history,the Member 1?2 undergone a long-term shallow burial and rapid burial process.The(pene-)contemporaneous diagenesis and the long-term shallow burial are two keys for the preservation of primary porosities.Furthermore,multi-period and various properties of fluid activities play a role in altering the reservoir characteristics,resulting that the reservoirs undergone two construction and destruction periods.7.A systematic controlling factors study has been performed in order to unravel the formation model of high-quality lacustrine mixed sediment reservoirs with a medium-deep burial.Contemporaneous micrite envelope and penecontemporaneous dolomitization effectively enhance the resistance of reservoir rock,meanwhile reservoirs staying at a long-term shallow burial reduce the influence of compaction and cementation,which results in well-preservation of primary interparticle porosities.In addition,as the result of lake-level fluctuations,the reservoirsare strongly affected by meteoric dissolution during contemporaneous period,generating highly secondary selective dissolution pores.This process further increases preponderance of reservoirs.In summary,this paper reveals the depositional process and model of a mixed sediment system in a lacustrine basin.Meanwhile,italso illustrates the formation mechanism for the high-quality reservoirs with the background of a medium-deep burial depth.This is expected to contribute to the new theoretical support for the prediction of reservoirs distribution in the Member 1?2 of the Paleogene Shahejie Formation in the Bohai Bay,as well as to provide a new example for case studies in the similar basin background.