Page 66 - Geology and Geochemistry of Oil and Gas

42 TEMPERATURE AND PRESSURE IN THE SUBSURFACE

Fig. 3.4. Stress vs. strain in: (1) salt and (2) anhydrite (after Savchenko, 1977). Circled figures are
confining (hydrostatic) pressures (MPa). 1 ¼ upper curve, 2 ¼ lower curve.

Fig. 3.5. Salt deformation diagram under non-uniform triaxial stress (after Pavlova, 1970). 1–3 —
Effective stress: 1–25 MPa at 201 C; 2–50 MPa at 501 C; 3–100 MPa at 1001 C.

texture. This on the one hand, improves their sealing capacity due to increased
plasticity. On the other, non-uniform mineral composition and lamination result in
the increased anisotropy and deterioration of their sealing capacity.
Experiments conducted by Savchenko and Bereto (1977) showed increased
plasticity of evaporites under hydrostatic and, even, uniaxial pressure (Fig. 3.4).
Much higher plasticity of salt compared to anhydrite was recorded. Evaporite
plasticity increases as temperature increases. An example of the effect of temperature
on salt plasticity is presented in Fig. 3.5.

3.1.2. Temperature

Temperature affects the physical (see Fig. 3.5) and physicochemical properties of
rocks. It distorts the molecular bonds up to a total disruption, accelerates the