Page 189 - Hydrocarbon Exploration and Production Second Edition

176 Deterministic Methods

reservoir thickness is constant across the whole field. If this is not a reasonable
approximation, then the method is not applicable, and an alternative such as the
area–thickness method must be used (see Section 7.1.2).
This procedure can be easily carried out for a set of reservoirs or separate
reservoir blocks. It is especially practical if stacked reservoirs with common contacts
are to be evaluated. In cases where parameters vary across the field, we could
divide the area into sub-blocks of equal values, which we measure and calculate
separately.

7.1.2. The area–thickness method
In some depositional environments, for example fluviatile channels, marked
differences in reservoir thickness will be encountered. Hence the assumption of a
constant thickness, or a linear trend in thickness across the field will no longer apply.
In those cases, a set of additional maps will be required. Usually, a NOS map will be
prepared by the production geologist and then used to evaluate the hydrocarbon
volume in place.
In the following example (Figure 7.3), well 1 had found an oil-bearing interval
in a structure (1). An OWC was established from logs and had been extrapolated
across the structure assuming continuous sand development. However, the core
(in reality cores from a number of wells) and 3D seismic have identified a channel
depositional environment. The channel had been mapped using specific field data
and possibly analogue data from similar fields resulting in a net sand map (2). In this
case, the hydrocarbon volume is constrained by the structural feature of the field and
the distribution of reservoir rock, that is the channel geometry.
Hence we need to combine the two maps to arrive at a NOS map (3). The ‘odd
shape’ is a result of that combination and actually it is easy to visualise: at the fault
the thickness of oil-bearing sand will rapidly decrease to zero. The same is the case
at the OWC. Where the net sand map indicates 0 m there will be 0 m of NOS.
Where the channel is best developed showing maximum thickness we will
encounter the maximum NOS thickness, but only until the channel cuts through
the fault or the OWC.
We can now planimeter the thickness of the different NOS contours, plot
thickness vs. area and then integrate both with the planimeter. The resulting value is
the volume of NOS (4) and not the GRV!
It is clear that if the area–depth method had been applied to the above example,
it would have led to a gross over-estimation of STOIIP. It would also have been
impossible to target the best developed reservoir area with the next development
well.
It should be noted that our example used a very simple reservoir model to show
the principle. NOS mapping is usually a fairly complex undertaking.
As will be shown in the next section, the methods discussed so far do
not take account of the uncertainties and lateral variations in reservoir
parameters. Hence the accuracy of the results is not adequate for decision making.
The next section introduces a more comprehensive approach to volumetric
estimation.