Page 120 - Hydrocarbon Exploration and Production Second Edition

Reservoir Description 107

Diagenetic
cement

Sand grain

Figure 6.10 Destruction of porosity by cementation.

Increasing Stability

Dolomite

Calcite

Aragonite

Mg Calcite

Figure 6.11 Relative chemical stability of carbonate minerals.

2+
Surface water is usually undersaturated in calcium ions (Ca ). Where (even
saturated) surface water mixes with seawater, mixing zone corrosion will dissolve
calcium carbonate. Evidence of this occurring may be seen on islands.
The dissolution of carbonates can create spectacular features like those found in
many caves. The process is termed karstification. Some reservoirs are related to Karst.
Examples are the Bohai Bay Field in China or the Nang Nuan oil field in the Gulf
of Thailand. These reservoirs are characterised by high initial production from the