Page 172 - Geology and Geochemistry of Oil and Gas

ROLE OF ENERGY IN THE OIL GENERATION PROCESS 141
found such stress to be between 0.5 and 1.5 MPa. At the same time, only because of
organic matter transformation in the adjacent zones, pressure gradient of several to
several tens of MPa may form in the source rocks. Thus, migration paths form non-
uniformly within the formation. Energy redistribution in the rock-water-organic matter
system occurs as a result of internal and external energy sources, within each limited bed
volume containing organic matter. High-energy saturated compounds, which formed
within the system, will most likely move, according to the laws of film migration,
toward the areas with a lower energy or into the adjacent reservoirs.

8.3. ROLE OF ENERGY IN THE OIL GENERATION PROCESS
The major stimulus for the evolution of process of oil generation is the internal
energy of the organic matter. The other factors serve as peculiar catalysts that
initiate the process, affect its speed, or influence the nature of the final products.
There is a major disagreement among petroleum geologists regarding the role of
internal energy in the course of organic matter-to-oil transformation processes.
Nesterov et al. (1992) studied the thermodynamic aspects of organic matter
transformation. Some petroleum geologists believe that a vertical or lateral move-
ment of the heated fluids (heat-mass transfer) may explain the appearance of heat
anomalies. They do not rule out the possibility of heating some sequences (or zones)
due to exothermal effect resulting from: (1) gas generation within the high-temper-
ature thermal-catalytic zone; (2) exothermal reaction of hydrocarbon generation
from the dispersed organic matter; and (3) specific hydration conditions by the
donor hydrogen in the course of radical thermal decomposition reactions. A problem
arises of how to determine the level of heat effect of oil generation and the role of
catagenetic transformation of organic matter in the presence of heat anomalies.
A thermodynamic solution of individual equations is insufficient due to the com-
plexity of geological system. Studies of the thermal dissociation of organic matter
showed that the endothermal reactions are usually dominant. Various factors de-
termine the expenditure of heat energy needed for kerogen decomposition. They
include the (1) number and type of broken chemical bonds, (2) formation of different
(based on mass) stable paramagnetic centers (PMC) and free radicals (FR), and (3)
desorption of volatile reaction products.
With increasing temperature, the weakest bridge bonds in kerogen are broken.
This results in loosening of the fragmental structure, redistribution of the electron
density, and an increase in the PMC concentration. The energy necessary for break-
ing the bonds ranges from 20–40 kcal/mol. As the temperature increases above
3501C, the number of stronger bonds broken (C–C, C–H and C–OH bonds) in-
creases. A breaking thermal dissociation above 450 to 5501C (at E a ¼ 60–80 kcal/
mol) results in breaking of most alkide bonds (C–H, H–H). When the expended
energy is above 150 kcal/mol, the structure of the source organic matter is totally
destroyed. Heat release (exothermal reactions) occurs only on the condition of hy-
dration by hydrogen, formation of molecules through the recombination of FR,
polycondensation of unsaturated bonds, etc.