Page 185 - Geology and Geochemistry of Oil and Gas

154 FORMATION OF HYDROCARBON ACCUMULATIONS

difference between the increase in the rate of fluid pressure increase and the rate of
relaxation. Both rates can vary significantly within the source-rock sequence. For
instance, the former one is determined by the (1) overburden pressure, (2) formation
temperature, (3) volume of the released interlayer water, mainly as a result of
montmorillonite-to-illite transformation, (4) volume of newly-formed substances
(due to organic matter transformations), and (5) osmotic pressure, etc. The rate of
relaxation, on the other hand, depends mainly on the permeability (because it is
determined by the diffusion and effusion phenomena). Thus, the variation in pore
pressure potential arises within a source-rock sequence. This variation is especially
pronounced in a heterogeneous source rock.

9.2.3. Rock Compaction

Rock compaction occurs mainly as a result of increasing effective pressure, which
is equal to the difference between the total overburden pressure and the pore (fluid)
pressure. Variable nature of these parameters has been discussed earlier (see Section
3.1). The non-uniform nature of source-rock compaction, with all ensuing
consequences, is obvious.

9.2.4. Temperature

The endogenous heat flow undoubtedly affects the rock–water–organic matter
system. Smirnov (1980) mentioned three types of the heat-flow anomalies: mantle,
crustal, and near-surface ones. The latter two types may be associated with the
occurrence of non-stationary heat sources in the Earth’s crust and the sedimentary
cover (Sergiyenko, 1995; Nazarkin, 1994). These authors described in detail the
arising non-uniformity and unsteadiness of the heating of sedimentary rocks. It is
sufficient to emphasize here the emerging variation in the potentials of temperature
within the sedimentary rocks (including source rocks).

9.2.5. Geochemical Non-Uniformity
Geochemical heterogeneity is caused not only by the original deposition of
chemically diverse sediments, but also by the combined effect of diagenetic and
catagenetic alterations. The recrystallization processes, the emergence of new
minerals, and the disappearance of the other ones are accompanied by appreciable
changes in temperature and pressure. These changes occur because endothermic and
exothermic processes may be happening at the same time within different parts of the
system. Just the geochemical heterogeneity alone can create a very strained
geochemical environment (see Section 3.1) within a sedimentary sequence (especially
within the source rocks). These phenomena increase in sedimentary sequences
enriched in the organic matter. Field data indicate (see Chapter 7) that the quality
and quantity of organic matter may significantly fluctuate even within a single
sample. The largest quantitative fluctuations of organic matter (both organic carbon
and bitumen) are observed within the oil and gas windows.