Page 248 - Geology and Geochemistry of Oil and Gas

MODELS OF STATIC GEOLOGIC SYSTEMS 215
of rocks with increasing depth. This relationship is typical for both the individual
anticlinal zones and for the entire area of sedimentation during the Kala time.
Thus, the conclusions can be summarized as follows:
1. Entropy of a geologic system may serve as a criterion of its heterogeneity. This
makes it possible to use entropy as a quantitative rating of heterogeneity of rocks.
2. Relative entropy is a convenient measure of heterogeneity of rocks and can be
calculated using natural logarithmic tables.
3. Heterogeneity of different rocks with respect to their grain-size distribution, and
heterogeneity of formations with respect to the proportions of different rock
types in the section, may be expressed in entropy values.
4. In studying the heterogeneity of sediments, the concept of entropy may prove to
be useful not only in the fields of lithology and petrography, but also in the fields
of geochemistry and hydrochemistry.

11.2.1.2. Anisotropy of sedimentary rocks
The migration and accumulation of hydrocarbons in stratified heterogeneous
deposits differ from those in massive homogeneous reservoirs. The fluid motion
in various directions is subject to a considerable anisotropy, because the permeability
of the stratified rocks is dependent on the direction of flow. One needs to deter-
mine the anisotropy coefficients for such rocks in order to simulate hydrodynamic
scenarios.
For a three-component medium composed of layers of (1) good-quality reservoir
rocks, (2) low-quality reservoir rocks, and (3) non-reservoir rocks, the permeabilities
can be expressed as follows:
k 2 ¼ ak 1 and k 3 ¼ bk 1 (11.7)
where k 1 , k 2 , and k 3 are the permeabilities of the above three classes of rocks,
respectively, whereas a and b are empirical factors which are less than 1.
The initial equations for the permeability parallel (k k ) and perpendicular (k ? ) to
the bedding, using Eq. 11.7, are as follows:
k k ¼ ðk 1 h 1 þ ak 1 h 2 þ bk 1 h 3 Þ=ðh 1 þ h 2 þ h 3 Þ
¼ k 1 ðh 1 þ ah 2 þ bh 3 Þ=ðh 1 þ h 2 þ h 3 Þ ð11:8Þ
and

k ? ¼ ðh 1 þ h 2 þ h 3 Þ=ðh 1 =k 1 þ h 2 =ak 2 þ h 3 =bk 3 Þ
¼ k 1 ðh 1 þ h 2 þ h 3 Þ=ðh 1 þ h 2 =a þ h 3 =bÞ ð11:9Þ
where h 1 , h 2 and h 3 are the thicknesses of layers of good-quality reservoir rocks, low-
quality reservoir rocks, and non-reservoir rocks, respectively.
The anisotropy coefficient can be expressed as follows:
1=2 1=2
l ¼ ðk k =k ? Þ ¼ ½ðh 1 þ ah 2 þ bh 3 Þðh 1 þ h 2 =a þ h 3 =bފ =ðh 1 þ h 2 þ h 3 Þ
(11.10)