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calcarenite is still potentially an excellent hydrocarbon producing reservoir.

A wedge provides good structural perspective for hydrocarbons to be trapped. Although not an entire
wedge is observed in the field, it might have been deposited and eroded. Other studies — which will
be discussed in the next chapter — produce wedge shaped conceptual models as well. Figure 6.3 is
a schematic cross section of how such a trap works. Of course it is assumed that a seal — like an
impermeable clay or salt layer — is present to prevent the hydrocarbons from escaping the reservoir rock.
An illustrative oil water contact (OWC) and gas oil contact (GOC) is shown as well.

The volume of oil in place is an important factor that influences the development of a field. A rather
elementary expression to calculate the STOIIP (Stock Tank Oil Initially In Place) from average properties
is:

ST OIIP = Vbφ (1 − Sw) (6.1)
Bo

In some cases the expression is subsequently multiplied with N/G, which represents the net-to-gross ratio.
This is used to separate the net producing reservoir units from the non-producing parts. Other variables
include (Schlumberger, 2010):

• Vb, which is the bulk volume of the reservoir [m3]. Usually this volume is bounded by the reservoir
seal, faults, stratigraphic traps, and the oil water contact (OWC)

• φ is the porosity [-], which is obtained from the volume property described in the property modelling
section

• Sw, which is the water saturation [-] of each grid cell. For simplicity an initial water saturation of
0.2 is assumed, which is not unusual for such high quality reservoir rock.

• Bo is the oil formation volume factor, expressed in reservoir barrels per standard barrel [rb/st]. It is
used to compensate for oil shrinkage due to gas that comes out of solution because of the pressure
drop during production, so it is in the majority of the cases larger than one.

The Petrel software calculates all these variables for each grid cell that will be part of the bulk volume,
and then sums everything to obtain the STOIIP [m3]. For simplicity it is assumed that only oil — gas
will be ignored — is present, with a water saturation given before. Table 6.2 presents the results of the
calculations. Apart from values for the entire pseudo-reservoir, the contributions of each separate facies
are given as well. The total potential volume of oil in place is approximately 35 million cubic metres,
which corresponds to 220 million barrels. These volumes, however, need some careful consideration before
they can be adopted:

1. As seen before, a calcarenite wedge system can potentially be much larger than the outcrops
observed on Favignana, increasing the oil in place and associated reserves.

2. The fact that the Favignana calcarenite has never been buried significantly affects the reservoir

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