Page 184 - Geology and Geochemistry of Oil and Gas

HYDROCARBON EXPULSION (‘‘PRIMARY MIGRATION’’) 153
gas. It is much greater for oil and gas than it is for the rocks. In addition, the physical
and chemical properties of mobile substances change with increasing temperature.
Their viscosity declines (except for gases) and they can evaporate, completely or in
part.
Thus, these changes promote migration. Kh. Watte (in: Eremenko and Chilingar,
1996) believed that the major force behind the oil expulsion is the temperature
changes in the Earth’s crust. According to him, the primary migration occurs by way
of hydrocarbon transfer by water (as an emulsion?), and a combination of
adsorption and diffusion due to temperature gradient. The issue of the nature of a
medium is highly important when looking for the external energy sources for the
migration and accumulation of the products of transformation of organic matter.
The issue of physical and geochemical uniformity of rocks is worth discussing. It
is especially important with respect to those sequences that are believed to be oil and
gas source rocks. Most authors, whether in an obscure or unambiguous form, base
their conclusions on a concept of uniform distribution of source organic matter in
rocks. The rock sequence, at least within the oil and gas generation zone, is more or
less uniform and the geochemical environment is maintained within a rigid range
with the same heat flow. Thus, there is a vertical zonation of catagenesis by
temperature intervals, with uniform compaction of the source-rock sediments. These
assumptions, which are based on the average organic matter content and on its
average transformation, are utilized in genetic method of the evaluation of
prognosticated reserves. But, is this correct? To answer this question, one must
review the type of changes in the major physical and geochemical parameters in a
source rock.

9.2.1. Overburden Pressure

The overburden pressure is determined by the thickness of overburden and the
rock density, both of which vary in any oil and gas basin. Even if smaller areas
within a source-rock sequence are selected, the above variables will change
substantially even if the surface topography is ideally plain. This occurs due to the
fact that:
(1) A thick source-rock sequence always includes lithofacies and thickness changes
of component formations. The boundaries of these formations do not have to
coincide with the hydrocarbon-generation zone boundaries (Nazarkin, 1979).
(2) The source-rock sediments are always deformed, which causes differences in
lithostatic pressure along them. Deformed sediments will respond differently to
the ‘‘arch effect’’ (Poisson’s ratio). Thus, the overburden pressure (pressure
potential) will differ within a source-rock sequence.

9.2.2. Pore Pressure

Source rocks usually have low permeability and usually display the ‘‘elision’’
regime with AHFP (see Chapter 3). The magnitude of pore pressure depends on the