Page 304 - Geology and Geochemistry of Oil and Gas

MODELS OF DYNAMIC GEOLOGIC SYSTEMS 265
11.3.3.3. Prediction of rock properties
To predict the reservoir rock and caprock properties at various depth of burial,
the models of porosity f, permeability k, and formation bulk density g were used as
mathematical descriptions of the rock properties (see Eqs. 11.97–11.100). Initial oil
or gas saturation was evaluated from the following equation:

S o=g ¼ 1 ak b (11.101)

where a and b are the empirical coefficients.
Input data (used by the authors) for the prediction of properties of Neogene rocks
of the South Caspian Basin were:
Geologic age: 10–12 My
Dynamic deformation: 1.1–1.2 tectonic-stratigraphic units 20
Depth of occurrence: up to 6.5 km
Formation temperature: up to 1101C
Rate of sedimentation: 500–800 m/My
Quartz content: 40–80%
Carbonate cement content: 8–12%
Trask sorting coefficient: 2–4
Initial porosity before compaction: 0.35–0.45
Initial permeability before compaction: 2000–3000 mD
Density of rock matrix: 2.6–2.7 g/cm 3
Empirical coefficients in Eq. 11.101: a ¼ 0:80 and b ¼ 0:225.
The parameters under study were predicted up to a depth of 9,000 m (Figs. 11.32
and 11.33). These figures show that as stratigraphic depth (to the bottom of Middle
Pliocene interval) and hypsometric depth (along the certain bed) increase up to
6,000 m, the absolute and effective porosity decrease from 19–20% to 14–15% and
from 14–15% to 9–10%, respectively. The corresponding permeability of sandstones
and siltstones decreases from 130–150 to 40–60 mD. At a depth of 9,000 m, porosity
varies from 7% to 10%, whereas permeability changes from 2 to 11 mD. Table 11.13
shows the predicted values of reservoir-rock properties of the Productive Series of
some offshore fields in the South Caspian Basin.

11.3.3.4. Prediction of hydrocarbon reserves
The model of determining hydrocarbon resources (a model of Eqs. 11.77 type, at
f ðx 1 Þ ¼ V 1 and f ðx 2 Þ ¼ V 2 , where V 1 and V 2 are hydrocarbon volumes in growing
2
1
(V 1 ) and dwindling (V 2 ) accumulations) was proposed. At one-stage oil accumu-
lation, the model is as follows:
V 1 ¼ ½BV o expð 1 Btފ=½B V o ð1 expð 1 Btފ (11.102)

where B is the volume of trap containing a hydrocarbon accumulation.

20
The intensity of subsidence of the sedimentary basin floor during the Middle Pliocene time decreased
with depth, and increased in the direction from the Absheron Peninsula to the Absheron and Baku
archipelagos and from the Cheleken Peninsula to the Turkmenian shelf, i.e., southward.