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.