Abstract: Geochemical Features of Oils from Western and Southern Gulf of Mexico: Reconstruction of the Depositional Environment of Their Source Rocks

Mario Guzman, Marcio Mello, Noel Holguin

Three main source rock depositional environments can be interpreted from the western and southern Gulf of Mexico oils: marine deltaic, marine carbonate and marine carbonate evaporitic. Marine deltaic oils are geographically limited to the Burgos and Macuspana basins. Their molecular features suggest a source from reworked, bacterially degraded terrestrial higher plants deposited in siliciclastic dysaerobic depositional environments. They can be recognized by their ^dgr¹³C values (between 23.6 to -21.7 ^pmil), low sulphur content (0.06 to 0.12 wt % S), high pristane/phytane ratios (1.95 to 3.43), high concentrations of oleanane, and very low sterane/hopane mbbs (<0.05).

Marine carbonate oils have a widespread distribution throughout the western and southern Gulf of Mexico. They are present from Tampico Basin to Campeche Sound. Isotopic values of these oils range from -24 to -28.1 ^pmil. They are characterized by medium to high sulphur content, pristane/phytane <1, abundant extended hopanes C₃₅ hopanes greater than or in similar abundance to their C₃₄ counterparts, and presence of hexahydrobenzohopanes. Subtle internal differences in biomarker composition of this oil set can be interpreted in terms of salinity, clay content and oxygen depletion variations.

Marine carbonate evaporitic oils are present in the Veracruz Basin and the Chiapas-Tabasco area. The oils are recognized by their heavy ^dgr¹³C values (between -23 to -24 ^pmil), very low sterane/hopane ratios, high C₂₄-tetracyclics/C₂₆-tricyclics terpanes ratios, and C₃₃ hopanes greater than their C₃₄ counterparts. In this oil set the organic matter is dominated by a strongly pre-vailing bacterial input.

The diversity of oil families from the western and southern Gulf of Mexico explains the enormous petroleum potential of this region.

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela