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MODELS OF STATIC GEOLOGIC SYSTEMS 221
11.2.2. Statistical approach

In terms of model dimension, the three types of probabilistic-statistical models are
defined as follows: (a) one-dimensional, (b) two-dimensional, and (c) multidimen-
sional.

11.2.2.1. One-dimensional models
11.2.2.1.1. Rock properties. The main petrophysical parameters characterizing the
properties of rocks are as follows: f — porosity (in %), k — permeability (in mD),
C cl — clay content (in wt%), C carb — carbonate cement content (in wt%), relative
clay content, Z ¼ ð12fÞC cl =½ð12fÞC cl þ fŠ, Q 100 — cation-exchange capacity (in
mg-eq. of exchangeable ions per 100 g of rock), and A da — diffusion–adsorption
factor in mV.
Almost 4,000 analyses of sands, silts, and shales were made from more than 860
cores obtained from the Productive Series of Azerbaijan and adjacent offshore areas.
The cores from Baku Archipelago, South Absheron Offshore Zone, and Lower Kura
Depression were tested (Buryakovsky et al., 1986b, 2001). Fig. 11.7 shows the sta-
tistical distributions of various rock properties.
The surface activity parameters were determined for different types of rocks.
Table 11.3 gives the results of data processing of A da parameter taking into
consideration the grain size. This table allows: (1) approximate determination of
lithology in studied sections, (2) estimation of an average A da based on lithology, and
(3) analysis of A da distribution in all types of rocks. The same types of rocks in the
offshore areas have lower A da than in the onshore areas. This difference is largely
due to the grain size: sands — approximately 2.1–4.2 times lower; silts — 1.2–1.7
times lower; and argillaceous rocks — 1.1 times lower.
The grain-size characteristics alone, however, do not determine the surface activity
parameters. The mineralogy is also of great importance. The rocks of the North Baku
Archipelago and South Absheron Offshore Zone are composed of quartz and quartz
plus feldspar grains (these rocks belong to the oligomictic 18 and mesomictic classes).
The rocks of the Lower Kura Depression belong mainly to polymictic and graywacke
classes having a low quartz content, high feldspar content, and different rock frag-
ments (shale, flint, effusive). Fig. 11.8 shows the tendency of Q 100 and A da parameters
to decrease with increasing quartz content. Close to zero A da is predicted in oligomic-
tic rocks with quartz content higher than 75%. By decreasing quartz content to
30–40% and increasing feldspar content to 50–60%, A da increases to 50 mV.
In addition to the grain size and mineralogical composition, the surface activity is
greatly influenced by the content and type of clays, which are present mainly as a
cementing material in reservoir rocks.
The X-ray analyses of core samples from the Lower Kura Depression in Azerbaijan
are correlatable with the surface activity of these rocks. The main clay minerals are

18
Oligomictic rock is a clastic sedimentary rock composed almost of quartz grains (more than 75%);
mesomictic is a sedimentary rock with quartz content between 50 and 75%; polymictic is a sedimentary
rock composed of many rock types with quartz content not more than 50%.