Origin of oil in the Sureste basin, Mexico

Geochemical and biological marker analyses of oils and rock samples from th e Sureste basin of Mexico were effective in identifying and geographically limiting four major oil families related by age and source rock depositiona l environment: Oxfordian, Tithonian, Early Cretaceous, and Tertiary. The source rocks giving rise to the Jurassic and Cretaceous oils are associ ated with marine carbonate environments. In contrast, the source rocks givi ng rise to the Tertiary oils are associated with a marine deltaic silicicla stic depositional setting. Biomarker and isotope differences observed in th e oils derived from marine carbonate environments can be interpreted in ter ms of salinity, clay content, and oxygen depletion variations. These differ ences provide diagnostic criteria for recognizing and differentiating five distinct organic-rich depositional regimes as the sources for these oil typ es. an anoxic hypersaline marine-carbonate environment associated with a na rrow and shallow semirestricted sea (Oxfordian age, family 1 oil); an anoxi c marine-carbonate environment associated with a silled basin geometry (Tit honian age, family 2 oils, subtype 2a); an anoxic marine-carbonate environm ent associated with a shallow gentle, broad marine-carbonate ramp in a dist al position (Tithonian age, family 2 oils, subtype 2b); a clay-rich suboxic /anoxic marine-carbonate environment associated with a carbonate platform i n a proximal position (Tithonian age, family 2 oils, subtype 2c); and an an oxic marine-evaporitic environment (Early Cretaceous age, family 3 oils). T he Tertiary oils (family 4) are derived from bacterially reworked terrigeno us and marine organic source materials deposited in a marine-deltaic enviro nment. The Tithonian-related oils in the Mexican southern side of the Gulf of Mexi co accumulated both offshore and onshore and throughout the stratigraphic c olumn from Kimmeridgian to Pleistocene reservoirs, suggesting vertical path ways as the principal secondary migration mechanism. The lateral variations of these oils can be interpreted to reflect the Tithonian paleogeography i n the area and could be useful in predicting differences in the oil composi tions.