High-resolution molecular stratigraphy technique has been applied to the rock samples from Tumaco-1-ST-P and Buenaventura-1-ST-P wells to investigate vertical variations, in concentration and distribution, of the deposited organic matter (O.M.) based on screening parameter (TOC, RE-Py, and Ro) and high resolution biomarkers by GC/FID and GC/MS in both, saturate and aromatic fractions (terpanes, steranes, aromatic hydrocarbons, MAS, and TAS) in order to establish its origin, organic facies, depositional paleoenvironment, thermal maturity, biodegradation processes and to evaluate the exploratory potential in this frontier forearc basin.
TOC and Rock-Eval pyrolysis data classified the O.M. as immature, with poor to fair generation potential originated from a type II/III or III kerogen. n-alkanes, isoprenoids, and tricyclic terpanes suggest an algal/bacterial and terrestrial input deposited on a marine-deltaic shale under oxic to suboxic conditions. The molecular stratigraphic interpretation allows suggesting that organic facies and lithology in Tumaco basin are characterized by a marine deltaic paleoenvironment with an important terrestrial contribution and varying between deltaic plain to prodelta during Late Miocene as is supported by C15+ n-alkanes profiles, CPI, Pr/Ph, TAR, Oleanane index, and Tricyclic/Pentacyclic Terpanes. In the Late to Medium Miocene, the depositional environment was likely a marine deltaic changing to a marine platform environment in the Middle to Early Miocene, which is confirmed by high ∑tricyclic terpanes/∑pentacyclic terpanes, low oleanane index, and high C35/C34 homohopanes ratios. Toward the bottom of the sequence, the O.M. was deposited in the Late Oligocene under a marine shelf environment, which is supported by Pr/Ph<1.0, DBT/P<1.0, low C26/C25 tricyclic terpanes ratios, high C24/C23 tricyclic terpanes ratios. 27DIA/27ααR ratios higher than 2.0, indicate that the O.M. was deposited on siliciclastic rocks in a marine-continental (transitional) environment, although carbonate rocks were also deposited in the bottom of the sequence (Late Oligocene).
Based on 20S/20S+20R C29-sterane, ββ/αα+ββ C29-sterane, Ts/(Ts+Tm), MPI-1, TMN, Ro, TOC, and RE-Py, the thermal maturity is very low (immature zone) and with a high concentration of biolipids (early diagenesis). Monoaromatic steroids (MAS) decrease in concentration from Pliocene to Oligocene suggesting a slight increment in maturity, while the triaromatic steroids (TAS) increase with depth because of the result of aromatization of MAS into its counterpart TAS. Finally, we propose a modeling approach, based on the evolution of the thermal maturity through the stratigraphic sequence that at 17,000 feet depth could exist rocks with O.M. in the hydrocarbon generation window for Tumaco Basin, Colombia
- Jorge Cortes, Carlos Vargas
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