Leakage Dynamics through Fault and Top Seals
Many of the techniques commonly used to evaluate both fault and top seal capacity are static in nature, based primarily on the definition of threshold capillary entry pressure. Measured capillary pressure data suggest that extremely large column heights are theoretically possible behind fine-grained mudstones and shale smears. However, at least two lines of evidence suggest that most or all seals leak on geological timescales. First, in many systems large volumes of petroleum have migrated across thick mudstone sequences between source and reservoir; second, routinely acquired headspace gas data show the common or perhaps ubiquitous presence of thermogenic gas hundreds of metres into cap rocks overlying both oil and gas accumulations. Furthermore, diverse evidence suggests that at least when petroleum leaks into seals, the wetting state of the pore system can change from water-wet to oil-wet. In this case, there is no capillary entry pressure for the petroleum to overcome; charge rate, fluid viscosity, and seal thickness & relative permeability and are now controlling factors on accumulation and leakage. Simple charge - leakage models allow us to understand the key risks on column presence and height in dynamic petroleum systems. The models show that substantial petroleum columns are still possible in many cases, as long as petroleum continues to be supplied from an active source, and especially where seals are (a) thick and (b) have a low (relative) permeability. In contrast, thin silts and faults which do not operate by juxtaposition will not generally sustain significant columns. The models also show (a) how gas and oil can be effectively segregated by migration through low permeability units and (b) that if charge ceases, columns will be lost on short geological timescales unless they are held behind thick seals with very low permeabilities. A complete seal analysis should include dynamic seal properties as well as the commonly applied static methods.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009