An Investigation of Diagenesis Related to the Top Surface of the Upper Shu'aiba Formation in Cores from Northwestern Oman

摘要

Core and log data from Lower Cretaceous limestones of the Upper Shu'aiba (US) Member in northwestern Oman were examined for evidence of diagenesis related to the top surface where this reservoir is overlain by shale of the Nahr Umr Formation. Cores from six vertical wells in three oilfields were studied by profiles of conventional core analyses, petrographic observations, and bulk-rock geochemical analyses. Neither bulk-carbonate stable-isotope analyses nor bulk-rock strontium concentrations show trends indicative of upward-increasing meteoric diagenesis below the top-US surface. Porosity and permeability data also show neither increasing nor decreasing vertical trends suggestive of surface-related diagenesis. 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. These effects are attributed to sea-floor authigenesis driven by reducing conditions within the upper several 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, and minor saddle dolomite also occurs near the tops of several other cores. Mg for dolomitization can have been provided 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.