Origin Of Dolomite And Dolostone Reservoir Development In Lower Ordovician Penglaiba Formation Carbonate Rocks Of Tarim Basin,NW China

The deeply buried carbonates is one of the most important regions of hydrocarbon exploration.The particular geological background of different areas determines the complomxity of the deeply buried carbonates.The deeply buried dolostones(>6000m)of the Lower Ordovician Penglaiba Formation are currently the key targets of deep hydrocarbon exploration in Tarim Basin,China.The core samples of 13 wells from the dolostone reservoirs of the Penglaiba Formation were detected and sampled in this study for the research of the formation mechanism of deeply buried dolomite reservoirs.In order to meet the need of the actual requirement of carbonate oil/gas reservoirs in Tarim Basin,the origin of dolomite and the dolomite reservoir has been interpreted and documented.The prediction of the dolomite reservoir,based on petrographic,the physical properties of log and geochemical method.Based on the petrography examination,three matrix dolomites and two dolomite cements were identified:very finely to finely crystalline,nonplanar-a to planar-s(D1)dolomite;finely to medium crystalline,planar-s to planar-e(D2)dolomite;medium to coarse crystalline,nonplanar-a(D3)dolomite;medium to coarse crystalline,planar-e to planar-s cement dolomite(Cd);and coarse crystalline,nonplanar-a saddle dolomite(Sd).D1 dolomite,which commonly occurs widely distributed throughout the study area,is characterized with?¹³C_VPDB values of-2.4 to-0.1‰,?¹⁸O_VPDB values of-8.1 to-4.1‰,⁸⁷Sr/⁸⁶Sr ratios of 0.70899 to 0.70979,and its similar REE pattern to ambient limestone,inferring the seawater origin.D2 dolomite,merely confined to one side of the stylolite,has?¹³C_VPDB values of-1.8 to-0.4‰,?¹⁸O_VPDB values of-8.3to-3.7‰,and ⁸⁷Sr/⁸⁶Sr ratios of 0.70871 to 0.70955,suggesting that D2 is precipitated in shallow-middle burial conditions and the dolomitizing fluids is derived from the residual primitive seawater in the formation.D3 dolomite is considered to be formed at higher temperatures under deep burial conditions considering its crystal textures and the?¹⁸O_VPDB values of-9.0 to-6.8‰.Although Cd and Sd dolomite cements have different petrographic textures,based on both of geochemical characteristics and their occurrence,it is considered that Cd dolomite and Sd dolomite were both related to hydrothermal dolomitization but formed at different stages.Based on the distribution,petrography and geochemistry data of D1,the mainly hydrological system for dolomitization in the Penglaiba Formation is interpreted to be the seepage-reflux model.This work unravels the whole paragenetic diagenetic processes and make a conclusion:the favorable diagenesis for the dolomite reservoir development include dissolution and fracturing,and the damage effect to the development of reservoir development including cementation,compaction,pressure solution,over-dolomitization and the filling of authigenic mineral.Based on petrographic observation and SEM,four pores types were contained in the carbonates reservoirs,including vugs,intercrystalline pores,intercrystalline dissolved pores and fracture.Based on petrography,the diagenetic sequence and the diagenetic pore evolution model are established.Based on the study of dolostone visual porosity,dolomite texture,pore types and formation environments of differernt kinds of dolomites,this paper summarized the relationship between dolomitization and pore development in matrix dolomites.Studies have found that the reflux and burial dolomitizaition did not effectively enhance pores.The destructive diagenesis has led to the densification of rocks in early-intermediate diagenetic stage before the hydrothermal fluids enterd the strata.Therefore,the secondary porosity did not inherited from earlier times.The magnesium ions in dolomite cements were mainly from the host rocks of the Cambrian to Middle-Lower Ordovician strata in the study area.Although,the hydrothermal fluids may be unsaturated with respect to dolomite in the initial stage of the hydrothermal activity,the fluids enriched magnesium ions through dissolution of the host rocks followed by the precipitation of Cd and Sd dolomite.This process resulted in a low abundance of dolomite cement and a large number of preserved residual pores.Therefore,the hydrothermal fluids effectively enhance pores and result in a net gain in porosity.The origin of porosity of the dolomite reservoir was studied based on the analysis of dolomite genesis and diagenesis.Using lateral migration model of hydrothermal fluids to interpret the origin of dolostone reservoirs.The result shows that hydrothermal fluids flowed upward along basement-rooted and strike-slip faults,and were sealed by overlying dense limestones/D1 dolostones.Cooling hydrothermal fluids flowed out laterally into the more porous and permeable strata,moved away from the fault zone,and formed a number of nearly horizontal vugs.Finally,the favorable exploration direction was pointed out based on the conditions of petroleum geology in Tarim Basin.