OIL COMPOSITION VARIATION AND RESERVOIR CONTINUITY - UNITY FIELD, SUDAN

A suite of oils from stacked reservoirs in the Unity Field in Sudan ha s been analyzed by various geochemical techniques for molecular inform ation to elucidate the geological processes which cause variations in oil composition and their resulting oil fingerprints in different rese rvoir units. Analyses of these highly paraffinic oils indicate that th e chromatographic fingerprint variations are due to differences in the abundances of saturated compounds, including branched and cyclic alka nes. Neither aromatics nor NSO compounds have any significant effect o n the observed fingerprint variations. This association of saturates, instead of aromatics and NSO compounds. with the fingerprint variation s precludes rock fluid interactions as a cause of the variations. Biom arker analyses show that variations in thermal maturity and organic fa cies of the source rock are responsible for the fingerprint variations . Thermal maturity increases with the depth of the reservoir, suggesti ng a multiple-charge process for the oils to fill these reservoirs ove r an extended period of time. Apparently the source rock generated and expelled progressively more mature oils and little mixing occurred du ring migration. Thus, knowledge of oil compositional variations from o ne reservoir to another, organic facies variation and source rock matu rity combined with tectonic history may help explain charging and timi ng of oil emplacement.