Abstract
Petroleum composition is a function of the source rock kerogen type or mixtures thereof, thermal maturity, and any alteration effects. Alteration may result from any number of processes, but also include expulsion and production fractionation, where non-polar compounds are preferentially enhanced as polars are sorbed to organic matter and minerals. Thus, the composition of the produced petroleum will be different from the composition of the petroleum in the source rock. These differences will range from differences in hydrocarbon and non-hydrocarbon (polar) fractions, i.e., the amounts of saturates, aromatics, resins, and asphaltenes (SARA) but also API gravity and GOR values.
Cracking of petroleum formed from kerogen occurs in the oil and gas windows, although the latter is more commonly associated with the phrase ‘oil cracking’. The reduction of polar compounds in the oil window results in an increase in saturated hydrocarbons. This is obvious as black oil is transformed to volatile oil and condensate at thermal maturity values up to about 1.20%Roe. normal-Alkane profiles demonstrate this fact as does the reduction in polar compounds. The transformation of normal-alkanes can be assessed by exponential slope factors that are related to both API gravity and GOR values. Illustration of cracking of the tetratriacontane (n-C40) by random numbers in Excel shows an excellent correlation to volatile oils and condensates profiles and also to recombined PVT analysis.
API gravity and GOR values can be calculated and used to assess the quality and phase of petroleum in a reservoir. This can be achieved from either oil or oils extracted from reservoir or source rocks.
While thermal maturity is a good pointer for the quality of a given reservoir rock or its petroleum, an enhanced application of geochemistry to predict API and GOR greatly improves the predictive power over thermal maturity alone.
