Organic geochemistry of the oils from the southern geological Province of Cuba

The aliphatic hydrocarbon composition (acyclic isoprenoids, hopanoids and steroids) of oils from the most productive fields in the southern geological Province of Cuba have been studied. This province is defined by its position with respect to the Cretaceous overthrust belt generated during the formation of oceanic crust along the axis of the proto-Caribbean Basin. The relative abundances of 18α(H)-22,29,30-trisnorneohopane, gammacerane and diasteranes suggest that Pina oils are related to the carbonate oils from the Placetas Unit in the northern province (low Ts/(Ts+Tm) and C27,29 rr/(rr+sd) ratios). The Cristales and Jatibonico oils exhibit some differentiating features such as higher Ts/(Ts+Tm) and absence of gammacerane. The oils from this province do not exhibit significant differences in either hopane, C32 22S/(S+R) and C30 αβ/(αβ+βα), or sterane, C29 αα 20S/(S+R), maturity ratios. However, the relative content of 5α(H),14β(H),17β(H)-cholestanes (C29 ββ/(ββ+αα) ratio) indicates that Pina oils are more mature than Cristales and Jatibonico oils. Several of these oils (Cristales, Jatibonico and Pina 26) are heavily biodegraded, lacking n-alkanes, norpristane, pristane and phytane (the two former oils do not contain acyclic isoprenoid hydrocarbons). Other biodegradation products, the 25-norhopanes, are found in all the oils. Their occurrence is probably due to mixing of severely biodegraded oil residues with undegraded crude oils during accumulation in the reservoir.