Page 67 - Geology and Geochemistry of Oil and Gas

DEFORMATION OF ROCKS IN DEPTH 43
molecular motion, overcomes the temperature threshold of reactions, and increases
the rate of chemical reactions. Thus, temperature affects the results of postdeposi-
tional processes in rocks during diagenesis and catagenesis.
With increasing temperature, different minerals (aggregates) comprising the rock
expand differently according to their particular heat expansion coefficients. This
phenomenon causes diverse consequences. The pore diameter and volume of pores
should decrease causing a decrease in porosity and permeability. This, however, does
not usually happen. T.T. Klubova (personal communication) believes that this is due
to compaction of pore cement. Cement in the pores is supported by the rock particles
and only slightly reacts to the net overburden pressure (effective stress). If cement
does not completely fill the pores, it may have experienced only the reservoir
(hydrostatic) pressure. When the temperature increases, cement compacts releasing
the water. As a result, the pore volume increases exceeding its slight decrease caused
by the temperature expansion of component minerals (heat expansion coefficients of
rocks are very low).
On the other hand, heat expansion coefficients for liquids and gases are several
times greater than those for rocks. The expanding fluids must either move into a
different space (if available) or compress according to their compressibility, i.e., in
fact to accumulate the elastic potential energy. This is how abnormally-high
formation pressure (AHFP) occurs. Comparing AHFP with the excess pressure, one
must distinguish the difference in their emergence and existence. AHFP emerges due
to the accumulation of elastic potential energy and disappears when the stress is
relieved (the energy dissipates). The emergence of the excess pressure is associated
with the gravity division of fluids (water, oil, and gas) with different densities, and
disappears when the density difference disappears (e.g., removal of one of the fluids
and mutual dissolution at high temperature). This process occurs in the formation in
a non-uniform way because of lithological variability. As a result, energy non-
uniformity (difference in energy potentials) arises. One should recall that porosity
and permeability change upon formation of fractures (Fig. 2.4). Porosity and
permeability drastically decrease within the plastic deformation zone, and the rock
becomes non-uniform energy-wise. The difference of energy potential, which is the
major cause of fluid flow, causes additional stresses in the rocks. Thus, some changes
in the texture of subsurface rocks is possible. Energy-wise, the rock particles will
attempt to occupy a more favorable position in the new energy environment.
Changes in the orientation of mineral aggregates in the rock will affect its magnetic
properties.
Klubova (1984) conducted experiments on bentonite samples under different
confining pressures. Natural moisture content of the samples was 12.5%. The
original sample was subjected to a confining stress of 50 MPa at 201C. The particles
in the compacted sample became flatter and lost their original texture. The changes
were even more pronounced in the sample compacted at 200 MPa. There was a
further microblock separation and development of a more orderly texture. Klubova
found that changes in the texture and an increase in zones of weakness in clays
studied were affected more by the changes in temperature rather than changes in
pressure.