Geochemical stratigraphy and porosity controls in Cretaceous carbonates near the Oman Mountains

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Abstract
Geochemical stratigraphy is an investigative tool useful (a) for discerning the mode and spatial extent of diagenetic processes in carbonates, (b) for making diagenetic correlations between localities, and (c) for inferring palaeotopography. δ 13 C and δ 18 O stratigraphies of a five-well transect across central Oman indicate that most of the Cretaceous U. Kahmah-Natih interval stabilized under semiclosed, shallow burial, marine-water-like conditions. A ‘depositional’ δ 13 C isotopic signal is preserved. Local anoxic-suboxic events are also recognized, as well as depth-limited effects of early meteoric diagenesis. Isotopic stratigraphic relationships clearly indicate the presence of a local palaeotopographic high in central Oman during deposition of the Natih E member. Good diagenetic-stratigraphic correlations are shown between locations in central Oman and Sharjah, a distance of about 300 miles. Porosity type and distribution within Cretaceous carbonates is related both to early diagenesis and to later structural events. Early cementation associated with marine hardgrounds and subaerial exposure surfaces occluded primary porosity in a limited depth-thickness sense. Later, all existing porosity types were regionally destroyed below the Hawasina thrust sheet in central Oman by thrusting-induced burial stylolitization and associated cementation. Foreland basin carbonates not involved in nappe emplacement retained excellent reservoir properties. Porosity in foreland basin wells consists of primary macro- and micro-porosity, and secondary selective-leach microporosity; selective leaching fluids moved along the subsurface ‘plumbing system’ of the primary porosity-permeability network. Small volumes of pore space were formed in low porosity subthrust carbonates by late aggressive (meteoric?) leaching along fractures and stylolites. Rock texture (facies) did not play a major role in defining the distribution of porosity in the immediate study area.