Estimation of volume of strata that serves as reservoirs for the oil:
If a reservoir strata has relatively no variation in lithology and thickness over large areas, then, it will remain much easier to estimate the volume of the sand that serves as a reservoir – based on the measurements made on the outcrops of the oil/gas-bearing sand or the data collected from a few scattered wells.
On the other hand, how to go about estimating the volume of the reservoir strata, if the formations are very lenticular; and if the reservoir sands remain so inconsistent in both thickness as well as in lithology?
Whether the measurements made from closed intervals will do justice?
Assuming that you have seismic data for the area of interest (unless you are dealing with an unusual case for which no seismic is available) it is straightforward: map the top and the base of the interval of interest and subtract one from the other.
in the case that you have no seismic your problem is basically the same as faced in minerals exploration: how to estimate the ore volume from a couple of drill hole penetrations that show widely varying ore concentrations. This problem was addressed by Mr Krige who developed the geostatistical kriging technique. If that’s your situation I recommend you get up to speed with Geostatistics in order to tackle your problem
If you're looking to estimate volume for a particular pool, the well test and production data should be applicable to the reservoir volume connected to that particular well, especially in a gas reservoir. However, if you're looking for estimates of undiscovered resources, you will want to use a geostatistical model, looking at all wells in the area penetrating the interval of interest, not just the productive wells. Are all sands in the formation charged with hydrocarbons, or are some wet? If the latter, you'll need to look at the probability of charge as well as presence. Assuming no outcrop is available, you may want to look at analogous formations with the same sedimentary facies that have similar statistical properties, such as the total percentage of sand in the interval and mean sand thickness where encountered.
If you have any geophysical mapping tool, with a good subsurface information; then you can map easily the layer you want, regardless of its geometrical shape.
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