Paleogeomorphy evolution of the Ordovician weathering crust and its implication for reservoir development,eastern Ordos Basin

Gas reservoir development of the Ordovician weathering crust in the Ordos Basin is closely controlled by the pre-Carboniferous paleogeomorphy.Previous studies show that the paleogeomorphy is high in the west and low in the east,and the karst highland,karst slope and karst basin are developed from west to east.With further exploration in recent years,many karst breccia that represent strong karstification,are found in the east area which previously is considered to be the Ordovician karst basin.Thus,it is necessary to revaluate controlling factors of karst paleogeomorphy development from a viewpoint of the dynamic paleogeomorphy evolution,to investigate the paleogeomorphy evolution of the Ordovician weathering crust in geological history and guide further research and prediction of development law of reservoir spaces.In order to reconstruct the paleogeomorphy of the weathering crust in the top of Ordovician in the east Ordos Basin,paleogeography,thickness of residual strata and paleokarst characteristics are well studied.The result shows that a wide range of paleokarst highland occurred in the central to east part of Ordos Basin in the early period,and the karst reservoir spaces were formed;but in the late period,the east part of the basin gradually evolved into the paleokarst basin,the current pre-Carboniferous paleogeomorphy was thus formed,and the dissolution reservoir spaces formed in the early period were mostly filled,accordingly the reservoirs were tight.However fracture networks formed by cave collapse connect intercrystalline pores of dolomite,therefore,the reservoirs can still be well preserved locally and are worthy targets for hydrocarbon exploration.

Tightness and sweet spot formation in moldic-pore-type dolomite reservoirs: The middle Ordovician Majiagou Formation in the eastern Ordos Basin, central China

The diagenetic evolution of the moldic-pore-containing gypsum dolomites of the Middle Ordovician Majiagou Formation in the eastern Ordos Basin,central China,was studied by means of petrological,mineralogical,and geochemical analyses,to improve our understanding of heterogeneity in high-quality reservoirs.The aim of the study was to elucidate the processes that resulted in reservoir tightness and the formation of sweet spots,to guide future exploration.Results show that the moldic-pore-containing gypsum dolomites are the most favorable reservoir in the study area,with a mean porosity of 4.96%and a mean permeability of 0.748 mD.The development and preservation of gypsum moldic pores were the main factors causing reservoir heterogeneity.Specifically,moldic pore development was controlled by the sedimentary microfacies,whereas pore preservation was related to dissolution and filling during telogenetic diagenesis.There were three main dissolution-filling stages that took place in three settings:penecontemporaneous,epigenetic,and burial.These processes controlled the formation of reservoir sweet spots.Early-consolidated dolomite deposits were frequently exposed to the atmosphere in the penecontemporaneous environment under the influence of high-frequency sedimentary cycles,which led to the dissolution of evaporite minerals and consequent formation of gypsum moldic pores,accompanied by infilling by freshwater calcite.During epigenesis,the porosity initially increased due to karstification,then significantly decreased because of calcite infilling resulting from long subaerial exposure(120 Myr),which contributed to reservoir heterogeneity.Finally,during burial the high-temperature and high-pressure conditions led to chemical compaction and continuous tightening of the reservoir,although some burial dissolution also took place.In conclusion,the variable paleo-topography resulted in differences in the intensity of pore filling among the blocks in the study area,resulting in reservoir heterogeneity.