الملخص الإنجليزي
Core and log data from Lower Cretaceous limestones of the Upper Shu'aiba (US) Formation in northwestern Oman were examined for evidence of diagenesis and for variations in seismic attributes along the top surface where this reservoir is overlain by shale of the Nahr Umr Formation.
Study of cores from six vertical wells in three oilfields found that bulkcarbonate stable-isotope analyses, bulk-rock strontium concentrations, and porosity/permeability data do not show trends indicative of upwardincreasing meteoric diagenesis below the top-US surface. Meteoric leaching and cementation may nevertheless be pervasive throughout the US reservoirs, at least partially accounting for extensive aragonite dissolution and filling of the resulting macropores with coarse calcite cement.
Three of the cores show trends of upwards-increasing bulk-rock iron, manganese, phosphorous, and uranium, which are attributed to sea-floor authigenesis driven by reducing conditions within the upper 1.6 meters of sediment following each cycle of US sedimentation. In addition, the top 7 m of limestone in one of the cores contains several percent late saddle dolomite, possibly resulting from addition of magnesium from diagenesis of clay in the Nahr Umr Formation.
The results of this study thus indicate both early sea-floor mineralization and late dolomitization in some places below the top-US surface, but do not confirm locally more intense meteoric diagenesis underlying this surface.
Examination of seismic attributes along the top-US surface showed that RMS amplitudes can be resolved to show zones of density and porosity changes, although zone thickness is an important control of amplitude resolution. Amplitude polarity corresponds to facies variation. Depositional geometries of the clinoforms can be mapped using amplitude data if the distance between traces is adequate to resolve the lateral extend of the clinoform. Volume attributes gave trends of low frequencies in the same direction as the amplitude trends.
