Direct measurements
S1 : the already generated oil in the rock. These are the free hydrocarbons (oil and gas) already present in the sample, and they are distilled out of the sample at initial heating of the sample to a temperature of 350 ° C. Free hydrocarbons increase with depth. These values may be anomalously high from migration and contamination by drilling fluids and mud.
- S1 = 1.0 mg HC/g dry rock -- Minimum value for good source rocks.
S2 : the amount of hydrocarbons generated through thermal cracking of nonvolatile organic matter (kerogen) when the sample temperature is increased to 550 ° C. S2 is an indication of the quantity of hydrocarbons that the rock may potentially produce should burial and maturation continue. This parameter normally decreases with burial depths >1 km.
- S2 >= 5.0 mg HC/dry rock -- Minimum value for good source rocks.
S3 : The trapped CO 2 released during pyrolysis up to a temperature of 390 ° C. This value is proportional to the oxygen present in the kerogen. Carbonate rocks may increase S3 values.
T max : The temperature at maximum release of hydrocarbons occurs during Rock-Eval pyrolysis. This occurs at the top of the S2 peak. T max is a maturation parameter that is kerogen-dependent.
Derived measurements
HI : Hydrogen index [HI = (S2/TOC) x 100]. The ratio of S2 hydrogen (in mg HC/g dry rock) to total organic carbon (TOC), in grams. The hydrogen index is a measure of the hydrogen richness of the source rock, and when the kerogen type is known it can be used to estimate the thermal maturity of the rock. When plotted against the oxygen index (OI), the HI can be used to provide a crude assesment of the petroleum generative potential in a source rock (Peters and Moldowan, 1993).
OI : Oygen index [OI = (S3/TOC) x 100]. The ratio of S3 (mg CO2/g dry rock) to TOC (in grams). This parameter measures the oxygen richness of a source rock and can be used in conjunction with the hydrogen index to estimate the quality and thermal maturity of source rocks. This index is unreliable in rocks with high carbonate content. High OI values (>50 mg/g) are characteristic of immature hydrocarbons.
PI : Production index [PI = S 1 /[S 1 + S 2 ]. The production index is the ratio of already generated hydrocarbon to potential hydrocarbons. Low ratios indicate either immaturity or extreme postmature organic matter. High rations indicate the mature stage or contamination by migrated hydrocarbons or drilling additives. The PI increases steadily with depth and associated hydrocarbon generation.
