2019 EJRNL PP TONG NIU 1.pdf
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Terbatas Suharsiyah
» ITB
Terbatas Suharsiyah
» ITB
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.