The Magnolia Field in the deepwater northern Gulf of Mexico is a Plio-Pleistocene age mixed phase reservoir whose fluids are not in compositional equilibrium. Fluid heterogeneities have arisen principally due to (1) variations in maturity of the source rock from which the hydrocarbons were derived, (2) the extent to which biogenic methane has been incorporated into the fluids and (3) phase fractionation effects. These influences express themselves both in terms of bulk fluid properties such as gas/liquid ratio, API gravity and saturation pressure and minor compositional attributes such as hydrocarbon gas isotopic composition and gasoline range molecular ratios. Significant compositional variations that cannot be ascribed to gravitational fluid segregation occur within reservoirs that are demonstrably in pressure communication. These variations challenge the notion that hydrocarbon fluid mixing is geologically instantaneous and underscore the importance of testing assumptions regarding compositional equilibria in conjunction with reservoir studies. Although the state of disequilibrium impedes compartmentalization assessments at Magnolia, it provides both opportunities for fluid property and phase predictions and potentially a development setting in which geochemical surveillance techniques may be profitably employed.

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In heavy oil accumulations, vertical variations in oil producibility are controlled by (1) vertical variations in reservoir porosity/permeability, and (2) vertical variations in oil viscosity. Unfortunately, due to perceived difficulties in quantifying oil viscosity, development strategies often focus solely on the reservoir rock properties. Such development strategies are not acceptable. Identifying zones of lower viscosity oil is critical to efficient field development, and geochemical analysis of Side Wall Cores (SWC) can yield the needed viscosity information. Currently, four non-logging methods are available for acquiring oil gravity and viscosity data for an oil accumulation. The first 3 of those 4 methods are of limited value: