Diagenesis System Of Turbidite Reservoir In The Middle Of The Third Member Of Shahejie Formation,niuzhuang Sag

Sedimentology,element and stable isotope geochemistry,hydrogeology,thermodynamics,oil and gas reservoir geology,and sedimentary basin analysis were integrated to investigate the diagenesis system of delta-turbidite deposits in the middle of the third member of Shahejie formation,Niuzhuang Sag.Information from diagenesis sequence,and burial and thermal history were systematically analysed to reconstruct diagenesis evolution history.Origin and distribution of authigenic minerals were investigated to better understand the diagenetic fluid flow pattern and mass transport.Two distinct hydrogeochemical facies were identified with the formation water composition within and outside the source rock.The Diagenesis system consists of three-stage including the initial burial period,tectonic uplift period and secondary burial period,whereas the diagenesis system was controlled by episodic overpressure fluid and high salinity fluid during the tectonic uplift and secondary burial stage in many cases.The Dongying delta-turbidite system deposited during Eocene has undergone significant diagenetic alteration.Petrographic examinations identified multigeneration of authigenic minerals from early to late including compaction,percolating clay（montmorillonite）,pyrite framboids,siderite and dolomite,early calcite cementation（C1）,1^st quartz overgrowth（Qz-ovg1）,1^st albitization（Ab1）,ankerite（Ak）,1^st kaolinite（K1）,2^nd quartz overgrowth（Qz-ovg2）,late calcite cementation（C2）,2^nd albitization（Ab1）and kaolinite（K2）.Carbonate cements,authigenic kaolinite and quartz overgrowth are the most abundant cements in the study area,whereas two dissolution events occurred before and after late calcite cementation.Three stages of diagenesis have been divided according to burial history whereas the most active fluid flow and diagenesis occurred during tectonic uplift and secondary burial period.Carbonate cementation including C1,Ak,and C2 from early to late,are the most important cements.Trace elements,rare earth elements,and carbon and oxygen isotope data show that C1 precipitation is tightly related to calcite diagenesis in shale;Thermal decarboxylation of organic matters has contributed bicarbonate to C2 precipitation,whereas Ca²⁺are mainly derived from aragonite dissolution with some contributions from other sources;Ak cements precipitated from reducing fluid with high Mg/Ca under the setting of active tectonic activity during the uplift stage,with some contribution of bicarbonate from magmatism.Si O₂ for Qz-ovg1 cementation yielded from pressure dissolution,whereas those for Qz-ovg2 from 1^stkaolinization of feldspar.Thermodynamic calculations indicate no conditions for coprecipitation of kaolinite and K-feldspar at high temperature（>100℃）.The occurrence of authigenic kaolinite indicated an open diagenesis system with acid fluid injection.Petrography,cuttings,cores,well logs,seismic data,and analysis data were integrated to better understand the distribution of carbonate and authigenic kaolinite cements in the Eocene turbidite-delta sandstones of the Niuzhuang Sag.The significant abundance of carbonate and kaolinite cements near fault zones on the regional and the local scales implies migration of fluids from deep settings through late fractures developed by compaction under deep burial conditions.The C2tight cemented zones occur near today’s top seal of the overpressure cell,whereas Ak zones lie in the lower part of C2,indicating the position of paleo top seal.The relationship of C2 tightly cemented intervals with oil-bearing sandstones suggest that the pervasive calcite cementation started as soon as the hydrocarbon and brine mixed fluid was displaced.Chemical analyses of formation waters in Shahejie Formation in Niuzhuang Oil-Field suggest that the formation waters in Shahejie formation have two main geochemical facies:chloride-magnesium（Cl-Mg）and bicarbonate-sodium（HCO₃-Na）water with low salinity（TDS=7.3±7.8 g/L）and alkalescence inside the source rock;and chloride-calcium（Cl-Ca）water with high salinity and weak acidity outside the source rocks.The mixing of alkaline fluid with high p CO₂from the inside source rock with acidic fluid with high calcium concentration from outside will produce a great volume of calcite cementation,which might be responsible for the C2 precipitation.The increasing Na⁺in formation water outside source rock yield from underlying halite dissolution induced albitization and water acidification,and then kaolinization of K-feldspar.The dissolution process induced by brine injection can be responsible for the massive secondary porosity yield from 2^nd feldspar dissolution event.The evolution of diagenesis system of delta-turbidite deposits in the middle of the Third Member of Shahejie formation are summarized as follow:Compaction flow dominates the initial burial period.Sandstone-mudstone system might be responsible for the early cementation such as pyrite framboids,siderite and dolomite,and early calcite cementation,which might derive diagenetic material from the biodegradation of organic matter.Pressure dissolution of quartz provided silica for Qz-ovg1 at the late stage of the initial burial period.Diagenesis system was active and controlled by episodic overpressure fluid and high salinity fluid during tectonic uplift and secondary burial stage.During the period of tectonic uplift,brine originated from underlying halite dissolution migrated upwelling through faults under the extrusion stress,inducing the 1^st albitization event.Simultaneously,the early overpressure cell was destroyed with releasing of volumes of high Mg/Ca and reducing fluid,leading to the precipitation of ankerite,and interrupted the albitization process as well.Organic acid with the episodic fluid injection is responsible for the 1^st feldspar dissolution event,and 1^st kaolinite and Qz-ovg2 precipitation.The range of hydrocarbon generation and its related overpressure cell was enlarged during the second burial period.Episodic fluid from inside the source rock with hydrocarbon and high p CO₂ should be responsible for the late calcite cementation,as indicated by the geochemistry data from both cements and formation waters.After that,the reinjection of brines induced albitization and water acidification,and then kaolinization of K-feldspar.