Geochemical, petrographical, and petrophysical evaluations of a heterogeneous, stratiform dolomite from a Barremian oil field, offshore Abu Dhabi (United Arab Emirates)

摘要

A stratiform dolomite is developed in the Barremian carbonates of offshore Abu Dhabi, United Arab Emirates. Although the average thickness is only 1.1 m (3.6 ft), it is an important drilling target characterized by heterogeneous porosity-permeability values in the studied oil field. We discuss the origin and development of the dolomite based on geochemical, petrographical, and petrophysical evaluations using 81 cores recovered from the field. The δ~(13)C values of the dolomites are relatively high (>5per thousand), and ~(87)Sr/~(86)Sr mostly fall in the range of Barremian seawater. The thin stratiform geometry and the geochemical signatures of the dolomite suggest early dolomitization just below the seafloor that was driven by the diffusion of Mg~(2+) from the seawater on top of a shallow platform. Lateral changes in the degree of dolomitization seem to be controlled by the permeability of the precursors. Dolomitization progressed further in packstone precursors (higher permeability) than in wackestone precursors (lower permeability), which led to greater permeability improvement in the packstone by replacement of fine matrix with larger dolomite crystals. Subsequently, late burial dolomite cementation occurred during oil migration, which preferentially affected the higher-permeability early dolomite, and areally, it progressed more in the flank of the field because of the delayed oil charge there. Dolomite-to-dolomite recrystallization became dominant after the oil emplacement. The recrystallization has been continuing under current burial temperatures (>100°C [>212℉]), fully resetting δ~(18)O values and modifying the Fe and Mn concentrations of the dolomites, but has not significantly affected their petrography and petrophysics in the oil leg.