Re: Assessing whether a seal can hold oil or gas?
Top seals can withhold HCs, but it is a dynamic process of leaking very little, and all gradations toward total loss of an accumulation. There exist beautiful examples of "chimneys" on seismic where gas leaks in sufficient quantities to show up. For gas the gas saturation of the overlying sediments show (in a seismic section) even at, say 5%!
Mohler and myself have made a study of "unfaulted" caprocks in about 160 well-documented cases. Statistical analysis showed that thickness of the seal was important, a bit in contradiction to the idea that only capillary pressure is the limiting factor. I think this observation can be explained by the definition of 'Unfaulted". In practice, a seismic survey will not show the minute cracks or even small faults, that make a seal leak. The thicker the seal, the more difficult it is to get a leakage path(es), so you get a correlation of seal capacity with topseal thickness. (Nederlof, M.H. & H.P. Mohler, 1981, "Quantitative investigation of trapping effect of unfaulted caprock", Abstract, AAPG Bulletin, v.64, p.964. The results of this study are used to estimate the length of possible HC column in prospect appraisal, hence before drilling.