INTEGRATED TRAP ANALYSIS OF A GULF OF MEXICO PROSPECT; A METHOD TO REDUCE UNCERTAINTY

Understanding the potential number of hydrocarbon columns, their height and if a trap contains oil or gas or a mix of both is key to successful prospecting. ExxonMobil uses an integrated trap analysis process which examines three characteristics of a trap; 1) the presence of cross fault juxtaposition leak points, 2) the mechanical seal capacity of the top seal, and 3) the capillary entry pressure of the top seal.

Cross fault juxtaposition leak analysis focuses on the geometric relationship of faults, changes in throw across the faults and the stratigraphic stacking architecture of the seal andreservoir intervals.The analysis of mechanical seal capacity focuses on the interaction of the natural hydrofracture stress of the top seal, the depth of the trap crest below mudline, the fracture gradient of the seal and aquifer pressure in the reservoir. Capillary entry pressure, on the other hand, is a physio-chemical process which is a function of the fluid densities, pore throat radius in the top seal, fluid interfacial tension and trap closure height.

The prospect in is a normal fault bounded trap with a geophysics derived direct hydrocarbon indicator that is concordant to structure but does not reach the synclinal spill point. Cross fault juxtaposition analysis using fault plane profiles suggested that the amplitude extents are controlled by cross fault leak. Althoughseismic amplitude analysis suggested that the fluid was likely to be gas, the result was equivocal enough that the amplitude might be oil rather than gas. Therefore, a mechanical seal capacity and capillary entry pressure assessment of the top seal was undertaken to reduce this uncertainty.

Mechanical seal capacity was found not to be a control on fluid type because of low aquifer pressure and trap depth below mudline. While cross fault juxtaposition leak was thought to control column height, capillary entry pressure was shown to be the dominant control on hydrocarbon fluid type and that the trap would most likely be gas not oil.