Abstract

Geochemical and biological marker analyses of oils and rock samples from the Sureste Basin of Mexico were effective in identifying and geographically limiting four major oil families related by age and source rock depositional environment; Oxfordian, Tithonian, Early Cretaceous, and Tertiary. The source rocks giving rise to the Jurassic and Cretaceous oils are associated with marine carbonate environments. In contrast, the source rocks giving rise to the Tertiary oils are associated with a marine deltaic siliciclastic depositional setting. Biomarker and isotope differences observed in the oils derived from marine carbonate environments can be interpreted in terms of salinity, clay content, and oxygen depletion variations. These differences provide diagnostic criteria for recognizing and differentiating five distinct organic-rich depositional regimes as the sources for these oil types; an anoxic hypersaline marine-carbonate environment associated with a narrow and shallow semirestricted sea (Oxfordian age, family 1 oil); an anoxic marine-carbonate environment associated with a silled basin geometry (Tithonian age, family 2 oils, subtype 2a); an anoxic marine-carbonate environment associated with a shallow, gentle, broad marine-carbonate ramp in a distal position (Tithonian age, family 2 oils, subtype 2b); a clay-rich suboxic/anoxic marine-carbonate environment associated with a carbonate platform in a proximal position (Tithonian age, family 2 oils, subtype 2c); and an anoxic marine-evaporitic environment (Early Cretaceous age, family 3 oils). The Tertiary oils (family 4) are derived from bacterially reworked terrigenous and marine organic source materials deposited in a marine-deltaic environment. The Tithonian-related oils in the Mexican southern side of the Gulf of Mexico accumulated both offshore and onshore and throughout the stratigraphic column from Kimmeridgian to Pleistocene reservoirs, suggesting vertical pathways as the principal secondary migration mechanism. The lateral variations of these oils can be interpreted to reflect the Tithonian paleogeography in the area and could be useful in predicting differences in the oil compositions.

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