MIGRATION OF HYDROCARBONS WHEN CLOSURE EXCEEDS SEAL CAPACITY

Two main end member classes of hydrocarbon traps are those in which seal capacity exceeds closure and traps in which closure exceeds seal capacity. Migration processes in these two classes of traps are fundamentally different and require separate techniques to delineate hydrocarbon pathways and risk traps. The Niger Delta is an example of a petroleum system with closure that exceeds seal capacity. Because of the relative uniformity of the source rock, the physical processes associated with hydrocarbon migration can be measured.

In the Niger Delta, hydrocarbon liquids are at bubble point pressure to depths of 4 km. Equilibrium is maintained through changes in chemical composition. The mole percent methane in the liquid increases from 40% at a depth of 1.5 km to greater than 60% at a depth of 4 km. Fluid composition is constant at depths greater than 4 km and exhibit near critical state properties.

Bubble point hydrocarbon liquids over a large depth range plus the prevalence of fault bound structural traps provides insight into migration processes in the Niger Delta. Hydrocarbons charge the traps but they also leak oil and gas. Gas preferentially leaks due to its lower viscosity and greater buoyancy. Fault plane maps indicate that the faults create a lower permeability relative to the juxtaposed sands; consequently, they impede, but do not stop, the flow of the hydrocarbons.

In summary, when closure exceeds seal capacity, fluids are filling and leaking from traps. Seal analysis methods or algorithms incorporating capillary-buoyancy balances to estimate column height are not applicable because traps have already leaked. Phase behavior algorithms must model compositional changes in hydrocarbons over a large range in pressure and temperature. These results indicate that shortcut methods employed to evaluate traps in which closure exceeds seal capacity are inadequate and require more rigorous and complete basin modeling methods.