High-resolution molecular stratigraphy of Oligocene–Miocene sequence of Tumaco-1-ST-P well, Tumaco forearc Basin, Colombia

摘要

In the present work, high-resolution molecular stratigraphy technique has been applied to the rock samples from Tumaco-1-ST-P well to investigate vertical variations of the deposited organic matter (O.M.) based on screening parameter and high-resolution biomarkers in both saturate and aromatic fractions 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. The O.M. is immature with a high concentration of biolipids (early diagenesis) and with poor to fair generation potential originated from a type II/III or III kerogen. Original total organic carbon (TOCo) and original hydrocarbon index (HIo) values for each formation and lithology are determined to evaluate the hydrocarbons potential in the basin. 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 O.M. was deposited on siliciclastic rocks in a marine-continental (transitional) environment, although carbonate rocks were also deposited in the bottom of the column (late Oligocene). The molecular stratigraphic interpretation allows to conclude that organic facies and lithology in Tumaco-1-ST-P are characterized in Cascajal and Tangareal del Mira Fms. (1000–5000 ft) by a marine-deltaic paleoenvironment with an important terrestrial contribution and varying between deltaic plain and prodelta during the late Miocene. At the top of Tumaco Fm (5000–9000 ft) in the late to medium Miocene, the depositional environment was like the marine delta of the Tangareal del Mira Fm, changing to a marine platform environment at 8000 to 10,000 ft depth in the middle to early Miocene. Toward the bottom of the column (10,000–12,000 ft), Tumaco Fm was deposited in the late Oligocene under a marine shelf environment. In the present paper, we propose a modeling approach, based on the evolution of the thermal maturity through the stratigraphic column, that at 17,000 feet depth could exist rocks with O.M. in the hydrocarbon generation window for Tumaco Basin, Colombia.