Abstract

The ability to estimate the maximum possible column height of potential oil and gas accumulations is extremely valuable in hydrocarbon exploration. The techniques used generally focus on the sealing capacity of the trap, in particular the relationship of the capillary entry pressure of the sealing strata to the buoyancy force of the hydrocarbons, and the resistance to seal failure by mechanical means, such as fracturing. The study of faults as seals or conduits also incorporates these analytical techniques, although their use is far from straightforward.

In this paper I focus on the issue of the appropriate subsurface value of hydrocarbon-water interfacial tension, as used in the capillary pressure equations that define maximum hydrocarbon column heights. It appears from the literature on the subject that there are major discrepancies, depending on the source, that have an extremely important bearing on oil and gas column height calculations and the subsequent conclusions so derived. Published models that use a widely accepted technique for calculating interfacial tension of oil-water in the subsurface estimate, for example, that gas column length will exceed oil column length at depths of greater than around 10,000 ft (3048 m). Other published approaches suggest the opposite. In addition, the former technique virtually precludes the existence of oil at reservoir temperatures in excess of 270°F (132°C), yet documented fields exist at this temperature that contain significant oil columns. The resolution of these issues is of paramount importance for exploration geologists, particularly as the search for hydrocarbons is being carried out at increasingly deeper levels.

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