A complex bimodal fluid gradient in a simple tilted sheet reservoir is explained and modeled by (biogenic) gas addition to an oil column

A deepwater Gulf of Mexico reservoir exhibits a simple geologic structure of a tilted sheet reservoir, three-way seal against salt. However, the (well-connected) oil column is complex and bimodal. The top half of the oil column exhibits an enormous gradient of gas-oil ratio (GOR) of 8000 scf/bbl to 2000 scf/bbl (standard cubic feet per barrel) while the lower half of the oil column exhibits a small GOR gradient which is in “local equilibrium”. In addition, the methane carbon isotope shows that there is a large increase in the fraction of methane that is biogenic towards the top in the upper half of the oil column, while the lower half of the column shows an invariant 50% solution methane that is biogenic. The asphaltene concentration is very low where solution gas is high, and shows an equilibrium gradient to appreciable values in the lower half of the column. The GOR gradient of the top half of the column has previously been shown to result from a diffusive gradient of biogenic gas into the top of the column. But the diffusive flux is nearly zero into the lower half of the column. Here, we show the simple fluid geodynamic processes that gave rise to this enigmatic bimodal distribution of GOR and biogenic solution gas. 2D reservoir simulations with different reservoir geometries are used to show the fluid properties over the course of oil and biogenic gas charges into this reservoir. The sequence of events is as follows: 1) oil charge into reservoir 2) biogenic gas charge at the oil-water contact (OWC) into an undersaturated oil resulting in increased GOR and excellent fluid mixing across the reservoir. 3) With sufficient biogenic gas charge, the reservoir oil reached saturation pressure. Continued gas charge caused formation of a gas cap. 4) Reservoir subsidence with its concomitant pressure increase led to higher saturation pressures; the gas in the gas cap then diffused into the oil column. Complex fluid columns can be understood with thermodynamic and geochemical analysis coupled with reservoir simulation of charge history.