Geochemical characterization of source rocks, crude oils and gases of Northwest Cuba

Abstract

In order to characterize the Cuban petroleum system, we analyzed source rocks as well as crude oils and gases from carbonate producing reservoirs from northwest onshore Cuban fields. The geochemical oil and gas characterizations were based on bulk analyses, chromatography (liquid and gas), mass spectrometry and mass spectrometry-coupled to stable carbon isotope ratio determination. Analyses of sampled source rocks intervals were performed with Rock Eval 6 and provided valuable data for the appraisal of the active petroleum system in Northwest Cuba. Our paper focused on the genetic semblance between carbonate reservoired crude oils and source rocks to assess the offshore extension of the onshore Cuban carbonate petroleum system. Crude oils-source rocks and crude oils-offshore DSDP source rocks were correlated to a common type of carbonate depositional environment. An understanding of the geochemical compositions of carbonate crude oils, grouped in families including heavy sulphur rich oils, light or mixed and biodegraded fluids helped to underline the different processes operating in the reservoirs today. The high degree of anoxicity during the organic matter deposition in the Upper Jurassic and Lower Cretaceous was confirmed. We observed low hydrocarbon cracking levels and non biodegraded fluids of Type II/IIS organic matter in Upper Jurassic and Lower Cretaceous reservoirs, while shallower Tertiary reservoirs, although more mature were altered by microbial biodegradation. There, reservoirs with temperatures below 80 °C provided a good habitat for aerobic/anaerobic bacteria and the possibility of hydrocarbon biodegradation.