Evaluation On Composite Paleokarst Reservoir Of Dolomite And Sulphate Rock

In the sedimentary process,marine dolomite and sulphate rock usually constituted paragenetic sequences which were widely distributed in Asia,North America,Middle East,Australia,Western Europe,Siberia and other regions.Dolomite and sulphate rock would generate composite paleokarst to form effective reservoir and permeability system of oil and gas while the solution rich in CO₂ migrated along the permeable network of the above-mentioned dissoluble rock,and according to the statistics,10-20%of world's oil and gas production was directly related with it.But in the geological history,as sulphate rocks were soluble and difficult to preserve,they were often neglected in actual research that detailed analytical instances and experiences based on composite palaeokarst of dolomite and sulphate rocks were lacked at present.Ma5⁴-5¹(the 4^th to 1 ^st submembers of the 5^th member of the Middle Ordovician Majiagou Formation)in the gasfields of the central Ordos Basin developed dolomite-anhydrite rock–halilith basin in depositional stage and then uplift with the North China Platform after deposition.After experiencing more than 140 Ma epidiagenesis weathering and denudation stage,the thick Ma6 limestone was almost completely denuded and then entered into the burial diagenetic stage.In karst imprinting of the top Ma5 member which mainly developed in composite construction of Ma5⁴-5¹dolomite and sulphate rocks,various dissolution pores and holes developed and then superimposed with crack of Indo-Chinese epoch and Yanshan period,forming main gas reservoir and producing formation of lower Paleozoic Ordos Basin.Research in view of composite construction,genesis and evolution of dolomite and sulphate rocks indicated that:the target stratum mainly developed two types of composite construction,one was group of micritic dolomite and fine crystal powder dolomite,another was group of non-isopachous interbeds of anhydritic crystal powder dolomite and crystal powder dolomite,anhydrite column crystal existed dispersively,three typical distribution patterns such as middle-large nucleation amount+middle concretion diameter,middle-large nucleation amount+small diameter and small nucleation amount+large diameter were often seen in anhydrite microconcretion.The above mentioned anhydritic minerals were all products of restricted?inner?shelf environment penecontemporaneous or eogenetic shallow burial stage replace initial dolomite,the amount depended on salinity of the sea water when the anhydrite started to precipitate,the size depended on salinity duration which was suitable for the growth of anhydrite in sea water.Moreover,??dolomitic anhydrite rock and gyprock were developed in lower Ma5² and middle part of Ma5³ and Ma5⁴.Research in view of composite paleokarst feature of dolomite and sulphate rocks had found that:1)dissolution of sulphate rocks was more conducive to formation of dolomite reservoir:on the one hand,after karst water infiltrated to the water table along longitudinal fissure,anhydritic concretion hydrated to gypsum which volume increased 30%that extruded the surrounding bedrock,and gypsum further dissolved into moldic pores which released pressure to the bedrock.A great quantity of shatter seams were generated in the moldic pores during the repetitive process.Favorable pore networks were composed by anhydritic moldic pores and shatter seams.On the other hand,after dissolving out in the process of anhydrite dissolution,SO₄^2-entered into karst water to make dolomite around the pore networks dissolve sharply,causing extension and connection of pore networks which were favour of groundwater circulation and alternation,further enhancing composite paleo-karstification of the dolomite and sulfate rocks.Moreover,it was important to note that dissolution of sulphate rock had destructive effects on formation of karst reservoir,such as layers which chick-shaped anhydrite rock concentrated,where the gypsum corroded extensively to form caves after reacting with water,making the balnstone abrupte and collapse after releasing pressure,accumulating colluvial gypsum breccia which had no pores.2)according to karst strength and type difference,various types of karst transformation rock and karst formation rock were developed after the above mentioned composite paleo-karstifications superimposed on each other:karst transformation rock could be divided into corrosion rock,deformation rock and metasomatic rock;karst formation rock included residual rock,colluvial rock,alluvial rock,aggradation rock and precipitated sedimentary rock.3)based on recognition of karst feature and analysis of palaeostructure and palaeogeologic map,karst palaeogeomorphology of Pre Carboniferous was further recovered and from the west to the east divided into karst highland,karst platform,karst basin and block between karst platform and delve.Each palaeogeomorphology unit could be identified into differently developed surface karst zone,vertical vadose zone,horizontal under flow zone and deep slow flow zone.Research in view of composite paleokarst reservoir characteristics of dolomite and sulphate rocks indicated that material base of reservoir development was composite construction of interbedded gypseous dolomite and crystal powder dolomite which deposited in a specific environment.The key to form high quality reservoir was development and preservation degree of composite paleokarst pore,hole and crack in supergene environment,and secondly was supraposition and improvement of buried karst and tectoclase.The reservoir were divided into three main types such as fractured-vuggy reservoir,fractured-porous-vuggy reservoir and porous reservoir based on the analysis of reservoir and permeable network evolution and recognition of pore configuration,and fractured-vuggy reservoir contributed the most to the reservoir.Reservoir physical property of M5¹,M5² and M5⁴ submembers were preferable.Research in view of evaluation on composite paleokarst reservoir of dolomite and sulphate rocks indicated had found that,palaeogeomorphology of the reservoir was flat as it usually distributed in the karst platform on plane,where the slow diffused composite palaeo-karstification developed and karst strength was moderate,karst corrosion rock and fractured-vuggy reservoir developed and preserved extensively.Fast channel composite paleokarst were developed in karst highland and karst basin with high karst strength that karst formation rock was extensively developed,damaging the preexisting reservoir.In the karst platform,the reservoir usually located in horizontal under flow zone-moderate corrosion subzone in the vertical direction,a great quantity of anhydritic concretion moldic pores were generated by stratified composite paleokarst with weak precipitation and packing action.Though a large number of half-filled moldic pores were developed in vertical vadose zone,they were generally repacked by aluminous mudstone from the top of weathering crust.Karst strength of horizontal under flow zone-strong corrosion subzone was too high that the country rock was drastically damaged,forming karst caves cavern water of deep slow flow zone tended to be supersaturated that precipitation and packing action were very strong.The comprehensive evaluation had demonstrated that high quality reservoirs were mainly distributed in karst tableland in central part of the study area in the plane,and in the vertical direction mainly distributed in Ma5₁³ and Ma5₁²,secondly distributed in Ma5₂²,Ma5₄¹,Ma5₁⁴ and Ma5₁¹.