Abstract
It is important to estimate sealing capacity for traps before drilling, because of the direct influence on the estimation of hydrocarbon reserves. In general, sealing capacity can be measured by a maximum hydrocarbon column height that retains a hydrostatic equilibrium condition between the buoyancy exerted upward and the capillary pressure exerted downward. Gas column height is smaller than that of an oil column because the buoyancy of gas caused by the density difference relative to water is much larger than that of oil, although the gas interfacial tension is greater than that of oil. For a carbonate sequence with salt as a cap rock, we can apply this concept to estimate the trapped reserves.
In the Karachaganak Field, North Caspian Basin, the gas/oil contact (GOC) and oil/water contact (OWC) are observed at −4950m and −5150m respectively, while the top of the structure is at −3526m. Oil was initially accumulated and gas migrated into the structure afterwards. There are two possible scenarios for the way oil and gas accumulated.
The application of the concept of estimating sealing capacity for the case of the “Independent Buoyancy Model” reveals that an equivalent pore throat for salt is about 10–5mm, equivalent to that formed by a shale of equivalent grain-size of 9.8 in phi-scale. If we take the case of “Total Buoyancy Model”, shale of the same grain-size could have held 1000 m oil column height at the depth of 2900 m. As the formation was being buried, migrating gas pushed down the oil downward, resulting in the present situation.
In either case, a maximum gas column of 1424 m would be expected over this area. If we try to find oil trapped as an economical reserve in this area, it is necessary to find a structure that has a relief of more than 1400m. Thus the sealing capacity estimation plays an important role for hydrocarbon exploration.