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Geochemical Analysis for Oil and Gas Exploration

Carbon Alteration Index

The Carbon Alteration Index (CAI) is a function of the carbonate mineralization in the soil being analyzed. CAI is a measure of the buffering capacity of bases to neutralize acids. Measuring CAI is important in determining a soils ability to precipitate carbon as carbonate byproducts of the hydrocarbon seepage and soil alteration process.  CAI is not pH, but instead refers to the ability of a soils interstitial water to resist change in pH. The presence of buffering materials help neutralize acids as they are added to soil pore water. These buffering materials are primarily the bases bicarbonate (HCO3-), and carbonate (CO32-) and occasionally hydroxide (OH-), borates, silicates, phosphates, ammonium, sulfides, and organic ligands.

Carbonate not only helps regulate the pH of a soil, but also the metal content. Bicarbonate and carbonate ions in water can remove toxic metals (such as lead, arsenic, and cadmium) by precipitating the metals out of solution. This allows many of the transition elements to be used as hydrocarbon indicators. Siderite (FeCO3) calcite (CaCO3) are early diagenetic indicators of oil with siderite detection being one of the early geochemical pathfinder minerals often referred to as the delta C carbonate.

The use of CAI as a hydrocarbon exploration tool relies on the accumulation of bacterial end-products in pore fluids which result in the precipitation of carbonate minerals. These end products affect pore water pH, Eh, and ionic strength.