Organic geochemistry of Cuban oils .1. The northern geological province

The isoprenoid, hopanoid and steroid compositions of 15 oils from the most productive oil fields in Cuba were studied to determine source-rock depositional environments and organic matter sources. The oils, which are from the northern geological province of Cuba and can be defined by the position with respect to the overthrust belt, can be grouped into two families: those from the Remedies (1) and those from the Placetas (2) tectonostratigraphic units. Remedies oils contain 17 alpha(H)-diahopane, high relative amounts of 18 alpha(H)-22,29,30-trisnorneohopane and diasteranes, which is indicative of generation from clay-rich source rocks. The crude oils of the Placetas Unit exhibit a sterane/hopane composition consistent with a carbonate origin. Nevertheless, these polycyclic hydrocarbons exhibit significant changes in composition, indicating that several organic matter sources, e.g. a carbonate/evaporitic origin of Varadero and Varadero Sur oils, have contributed to the oils from this Unit. The Remedies oils are more mature [evaluated from the C-29-alpha alpha 20S/(S + R) and C-29-20R beta beta/(beta beta + alpha alpha) sterane indices] than the Placetas oils. A wide range of biodegradation levels are encountered in these oils (from 0 to 7-8 using the scale derived by Volkman et al. (1983). The high relative abundance of 25-norhopanes is a distinctive feature of Remedies oils. The presence of these compounds in lightly biodegraded or nondegraded oils corresponds to a mixing of paleobiodegraded oil with more recently sourced nondegraded oil in the reservoir. The most biodegraded oil, Cantel, exhibits C-27 rr/(rr + sd) and C-29-alpha alpha 20S/(S + R) sterane ratios, C-30 alpha beta/(alpha beta + beta alpha) and T-s/(T-s + T-m) hopane ratios that have been altered by microbial attack. (C) 1997 Elsevier Science Ltd.