Organic geochemistry of the Carapita Formation and terrestrial crude oils in the Maturin Subbasin, Eastern Venezuelan Basin

Abstract Geochemical characteristics of the organic matter in the thick shaly sequence of the Carapita Formation (lower Miocene) were analyzed in five exploratory wells drilled in the Maturin Subbasin, Eastern Venezuela. The study also includes crude oils from well One, analyzed to establish their origins and possible sources. The quantity and quality of organic matter in the Carapita Formation (% TOC, H/C, HI, OI, and kerogen type) indicate vertical variation. Shales of the Carapita Formation increase in source rock potential from top to bottom. This suggests two facies with different hydrocarbon potential. Maturity parameters ( T max = 428–446°C, %R o = 0.35–0.73%) indicate that the upper section is immature and the lower section is mature. Thus, the lower section is sufficiently mature to generate oil. MATOIL modeling in well One shows that the lower section of the Carapita Formation began to generate oil in Late Miocene time (6.4 m.a.), at a depth of about 15,900 ft (4846 m). Geochemical characteristics of oils from well One indicate generation from a source with abundant higher land plant input. These are the first terrestrial crude oils of this type found in the fold thrust belts of the Maturin Subbassin. GC-MS analyses of saturated hydrocarbon fractions suggest a possible correlation with the terrestrial organic matter in the Carapita Formation.