Page 70 - Geology and Geochemistry of Oil and Gas

46 TEMPERATURE AND PRESSURE IN THE SUBSURFACE

Changes in temperature strongly affect such rock property as sorption, which
decreases with increasing temperature.
Certain zoning in porosity and permeability distribution is observed in the Earth’s
crust. According to Eremenko et al. (1986), the following vertical zones may be
identified for the platforms:
I. Karst zone
II. Calcite cementation zone
III. Decompaction zone
IV. Gravitational compaction and silica/silicate generation/regeneration zone.
Of greater interest is Zone III, because it contains major oil reserves of the world
and is present in all sedimentary basins.
Fig. 3.6 shows the distribution of the optimum reservoir–rock properties and oil
accumulations in the geologic section. This distribution was presented by Maksimov
and Minskiy in 1972, but is still valid today. Despite some oil and gas discoveries at
great depths, the bulk of the reserves is concentrated between the depths of 1 and
3.0–3.5 km.
The top of Zone III is located at a depth of 1.8 km (average of 32 regions of the
world). Reservoir rocks in that zone are formed mostly due to calcite dissolution,
and the newly-formed fractures are not cemented with calcite. Another favorable
factor is the montmorillonite-to-illite transformation, which is quite intense in Zone
III. Calcite dissolution is due to the decrease in pH of water and the simultaneous
aggressive action of CO 2 , concentration of which in formation water increases.

Fig. 3.6. Depth distribution of areas of (1) reservoir rocks (OR) and (2) oil accumulations (OA) with
optimum properties (after Maksimov and Minskiy, 1972).