Abstract: Organic Geochomistry of the Tithonian Source Rocks and Relate Oils from the “Sonda de Campeche,” Mexico

Santamaria-Orozco, Demetrio - IMP, Mexico; Horsfield, Brian - Forschung Zentrum Juelich, Germany; Di Primio, Rolando -Oslo University

The Tithonian source rocks facies were deposited in a anoxic marine carbonate shelf environment. They are carbonate shales and argillaceous limestones. Chemical and petrographical data show high-sulfur organic content, the presence of chlorophyllinite, high amounts of amorphous preserved organic matter and the absence of bioturbation. The main molecular features of Tithonian organic extracts and their related oils are: Pr/Ph ratio < 1, presence of C₂₈ bisnorhopanes; these features are consistent with anoxic conditions. The cause of sulfur enrichment was probably the degradation of the inorganic sulfates, due to enhanced bacterial activity, creating large amounts of H₂S which limited the benthonic faunal activity. These conditions helped to preserve the planktonic organic matter deposited (e.g., algae, calcisphaerulids, radiolarians, fish bones and ammonites). Thermal maturity in the kerogens ranges from 0.35 to 1.29% of R_r. This variation can explain the broad physico-chemical differences observed in the Tithonian related oils. The oils were expulsed from 0.45-1.0 % of R_c. Kinetic modeling of the Tithonian kerogens indicate that oil generation starts at 0.45 % R_c and the oil generation window ends at 1.0 % of R_c. These results are consistent with the measured maturity parameters. The abundance of sulfur-rich organic compounds is responsible for the early oil generation in the Sonda de Campeche.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil