Three stacked reservoirs in the deepwater Gulf of Mexico received similar oil and gas charges, yet they were subject to different reservoir fluid geodynamics (RFG) processes that led to entirely different present-day fluid distributions: a near-critical condensate in the upper zone, black oil in the middle zone, and dry gas in the lower zone. Extensive wireline logs and fluid data analysis show that the three zones initially contained oil, then received a large influx of biogenic gas that led to gas-washing processes. Oil-filled fluid inclusions and precipitated asphaltene inclusions are found in the gas-filled lower zone suggesting that oil first arrived before gas charged the reservoir. Our objective is to use compositional reservoir simulation to model the fluid mixing processes over geologic time, constrained by wireline logs and fluid data, to explain and reproduce present-day complex and spatially variable fluid distributions across the stacked reservoirs.
Mixing of oil and gas charges involves various mass transport processes and rock-fluid interactions, leading to present-day spatial distribution of fluid properties. Wireline logs and fluid data are vital to describe complex mixing outcomes. The middle zone exhibits extreme gas washing in the crude oil with a methane-to-ethane mole ratio of 70 and a methane carbon isotope of −67 ‰. Thus, the solution gas is 90% primary biogenic gas. However, the oil in this zone only has a gas-oil ratio (GOR) of 1000 scf/bbl precluding a process of preserved massive gas addition to this moderate maturity oil (Ts/(Ts+Tm) ∼ 0.63). Additionally, there is no way this Neogene formation degassed the thermogenic gas in the geologic past. The only plausible explanation for these observed fluid properties is that the oil in the middle zone was gas-washed by added primary biogenic gas. The upper zone contains a high GOR oil with a gas cap. We model gas-oil mixing dynamics in the middle zone, driven by a point gas charge into undersaturated oil, and leading to present-day oil state. In the examined reservoirs, fluid mechanics evaluations and modeling results indicate that biogenic gas arrived after the oil charge. Moreover, the petrophysical evaluation indicates that the middle zone consists of main and stray sandstones which are connected but baffled. There is a compositional offset of oil between the two sandstones which must be accounted for by compositional simulation. We investigate gas-liquid equilibria, convective and diffusive mixing dynamics, as well as the effects of baffles and mechanical subsidence on compositions of liquid and gas phases.