Solubility of calcite in petroleum basins—Implications for reservoir diagenesis

ABSTRACT We present calculations of calcite solubility as a function of temperature from 50°C to 200°C (122°F to 392°F) using the trends of partial pressure of CO2 and salinity with temperatures typical of many petroleum-producing basins. These results are presented within the framework of a general review of what is known about subsurface calcite solubility and how this applies to processes of both burial cementation and dissolution causing changes in the porosity of petroleum reservoirs. Unlike quartz and most other common minerals, calcite has retrograde solubility (increasing with decreasing temperature) at constant p(CO2) and salinity, but the observed trends of basinal increase of these parameters with depth result in an overall positive correlation of calcite solubility with temperatures at >50°C. Nevertheless, retrograde solubility can still result in carbonate dissolution if water moves upward to lower temperatures while maintaining its composition, as might occur by flow along a fracture having nonreactive (carbonate-free) surfaces. This provides a plausible explanation for how the porosity of both carbonate and sandstone reservoirs can be increased by mesogenetic dissolution, but such situations may be exceptional rather than widespread. Various other proposed mechanisms for reservoir porosity increase by mesogenetic dissolution of carbonates are either implausible or limited to special local conditions that are unlikely to occur generally in petroleum basins. The example of burial karst in the deeper flanks of the Tengiz field (Kazakhstan) nevertheless shows that major burial dissolution is possible, although the mechanism for this is obscure.