Vertical variability in crude oil composition: ES-SOC well, Socororo field, eastern venezuelan basin

Abstract

The determination of producing intervals and oil quality in a reservoir requires the study of different geochemical parameters. In particular, oil quality reflects the compositional characteristics of hydrocarbons that impact the economic viability of an exploration, development, or production opportunity. In general, oil quality may affect the direct economic value of the crude oil. Typical oil-quality properties include API gravity, viscosity, sulfur, asphaltene, and metals (e.g., vanadium, nickel) contents, and acidity. Biodegradation can significantly impact essentially all oil-quality properties of crude oils in producing zones. More specifically, oil biodegradation typically (a) decreases API gravity, (b) reduces the content of saturated and aromatic hydrocarbons relative to polar compounds (c) increases oil viscosity, (d) increases oil acidity and (e) increases the sulfur content and the concentration of certain metals (e.g., V and Ni). As a consequence, the residual oil resulting from biodegradation becomes enriched in NSO compounds (resin and asphaltene fractions), sulfur and metals (Connan, 1984; Wenger et al., 2002; Larter et al., 2006). Knowledge of the lateral and vertical variations in the oil-quality properties as a consequence of biodegradation is an important key to the development and implementation of enhanced oil recovery methods. Venezuela has one of the world’s largest reserves of heavy and extra heavy biodegraded crude oils in the Eastern Venezuelan Basin, which includes the widely studied Orinoco Oil Belt and the Socororo Major Area in the Maturin sub-basin. The Socororo field, and in particular the ES-SOC oil well, in the Socororo Major Area is the subject of this study. Here, we describe the vertical variability in the extractable organic matter (EOM) of sidewall core samples and oil composition. The latter was correlated to the characteristics of the source rock (e.g., lithology, organic-matter input, redox depositional conditions, and thermal maturity) in order to establish oil quality as a consequence of biodegradation.