ORGANIC GEOCHEMISTRY OF THE CARAPITA FORMATION AND TERRESTRIAL CRUDE OILS IN THE MATURIN SUBBASIN, EASTERN VENEZUELAN BASIN

Geochemical characteristics of the organic matter in the thick shaly s equence of the Carapita Formation (lower Miocene) were analyzed in fiv e exploratory wells drilled in the Maturin Subbasin, Eastern Venezuela . The study also includes crude oils from well One, analyzed to establ ish their origins and possible sources. The quantity and quality of or ganic matter in the Carapita Formation (% TOC, H/C, HI, OI, and keroge n type) indicate vertical variation. Shales of the Carapita Formation increase in source rock potential from top to bottom. This suggests tw o facies with different hydrocarbon potential. Maturity parameters (T( max) = 428-446-degrees-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 wel l 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 15,9 00 ft (4846 m). Geochemical characteristics of oils from well One indi cate generation from a source with abundant higher land plant input. T hese are the first terrestrial crude oils of this type found in the fo ld thrust belts of the Maturin Subbasin. GC-MS analyses of saturated h ydrocarbon fractions suggest a possible correlation with the terrestri al organic matter in the Carapita Formation.