The Formation And Preservation Of Thin Porous Reservoirs In The Evaporation Environment

Formation and preservation mechanism of carbonate reservoir and its heterogeneity have been a prolonged and key issue in the study of reservoir diagenesis.Based on analyses of core,thin section,physical properties,and geochemical characteristics,this paper aims to investigate the formation and preservation mechanism of pore-cavity reservoir related toevaporitic environment,taking the sixth to tenth submembers of Ordovician Ma5 member in the Central Ordos Basin as an example.The pore-cavity dolomite reservoir,closely related to intraplatform shoals and microbial mounds,have typically developed in the study area.The shoals-mounds reservoir generally shows excellent reservoir properties due to its frequent stack in the vertical direction,despite the small scale and thin thickness of each single reservoir layer.The seventh and ninth submembers correspond to a relatively humid climate and secondary transgression under the background of dominant evaporitic environment during the sedimentary period of Ma5 member,mainly forming intraplatform shoals and microbial mounds.Meanwhile,penecontemporaneous exposed karstification are well developed and have important contribution to the expansion of the primary pores and the generation of new reservoir space.Therefore it is the most fundamental development mechanism of the pore-cavity reservoir in the study area,causing the porosity cyclically varies with the rhythm high frequency cycles,and achieves the maximum on the upper part of cycles and it has laid more favourable conditions for the optimization and superimposed transformation of reservoir layers by the later burial diagenetic fluids.On the whole,dolomite reservoirs in this region have experienced multi-stage dolomitization with various dolomitizing fluid.Pervasively seepage reflux dolomitization during penecontemporaneous stage leaded to mineral composition transformation from grain linestone to grain dolomite,enhanceing rock structure and resistance,and it is favorable for the preservation of primary intergranular pores.Suitable recrystallization during shallow to medium burial condition were to the benefit of the preservation and optimized adjustment of early pores,featured by grain dolomite with intergranular pores gradually transform into crystalline dolomite and intercrystalline pores,respectively.The hydrothermal-originated dolomite precipitation,however,caused partly damage to the reservoir layers,owing to its cemented fillings of hydrothermal associated minerals and partial overdolomitization.Reservoir heterogeneity,controled by diagenetic differentiation,is obvious identified during porosity evolution.The edge of shoals generally suffer strong cementation filling and porosity decrease,caused by hypersaline evaporated brine injection from surroundings during penecontemporaneous to shallow burial condition.And for this reason,it is hardly affected by later diagenetic fluid and is able be keep the granular fabric.The core of shoals,however,is less affected by hypersaline evaporated brine in early diagenetic stage and preserves abundant pores.It becomes relatively high permeability layer and preferential object of diagenetic fluid in medium to deep burial condition,causing strong recrystallization and fabric transforming,thus the crystalline dolomite reservoir came into being.In addition,the hypersaline diagenetic fluid shows a clear tendency for the larger pores to be cement filled by gypsum and dolomite while the smaller pores are left empty.This could be the result of pore-size-dependent precipitation.