Why dual-phase systems demand a new assessment approach

Various work-arounds at­tempt to compensate for these limitations, but each introduce their own arte­facts. A common approach is to treat gas as a failure by assigning a “gas risk” and evaluating only oil scenarios. An­other merges the risked and success case volumes from separate oil and gas cases, but eliminates the possibility of a dual-phase outcome. A third work-around subdivides the hydrocarbon column into a gas column and an oil column, often considered the most acceptable option, but it is rarely sup­ported by justifiable parameter choices, and it fails to capture the controls on phase behaviour and column height.

To overcome these limitations and to properly estimate prospect risk and volumes in systems where oil and gas phases compete for pore volume and seal capacity, the best workflow is a pe­troleum systems-based approach. This integrates charge volume and compo­sition, PVT conditions and seal prop­erties to derive the hydrocarbon phase risk associated with the expected range of oil and gas volumes.

This difference becomes clear when applied to a prospect assessment case study from the gas-dominated Vul­can Sub-basin, which illustrates how strongly resource estimates depend on the underlying fluid assumptions, even when pore volume is held constant.

An oil-only, filled-to-spill scenario yields an estimated mean STOIIP in excess of 1,000 MMbbl; however, as discussed in our previous article (GEO EXPRO Vol. 23, Issue 2, 2026), filled-to-spill oil-only accumulations cannot be physically explained in dual-phase charge systems. Constraining the oil column height based on nearby an­alogue field data reduces the mean STOIIP estimate to 477 MMbbl, but this approach is flawed because the analogue distribution is dominated by gas columns. Limiting the analysis to oil-only analogue columns lowers the mean STOIIP to 75 MMbbl, moving closer to a plausible range but still fail­ing to account for mixed columns.

In contrast, the petroleum sys­tems-based prospect assessment yields a mean STOIIP of 109 MMbbl and a mean GIIP of 589 Bcf, within a sin­gle probabilistic assessment where oil and gas are inherently associated. The results of this approach capture the nat­ural continuity from gas-only (class 1), mixed (class 2) and oil-only (class 3) accumulations with a dual-phase sce­nario representing the most probable outcome (~60 %), and also the most prolific for oil. The increase in estimated STOIIP relative to the oil-only column case reflects the potential presence of a gas cap, which effectively displaces oil into structurally higher gross rock vol­ume portions of the trap.

The case study demonstrates why petroleum system understanding and integration are essential to realistic prospect assessment. Standard meth­ods often rely on assumptions that lead to unrealistic resource estimates, especially when evaluating an oil accu­mulation in a gas-dominated petrole­um system. A petroleum systems-based approach inherently captures the in­verse relationship between column height and oil phase in such settings, embedding phase behaviour within the methodology itself and eliminat­ing the need for separate “oil versus gas” risking.