Abstract

In the first comprehensive study of the Talara basin petroleum system of onshore and offshore northwest Peru, we test oil–source rock correlation through molecular biomarker analysis of oil samples from wells scattered throughout the basin, as well as purported source rocks. The new data presented in this manuscript suggest that the oils constitute one oil family, and that the source rock was a predominant marine clay deposited in an oxic to suboxic environment. Substantial relative amounts of oleanane in each oil sample indicate a notable input of terrestrial organic matter deposited in a mixed marine and terrestrial environment (probably deltaic). The high ratio of 24-norcholestanes to 27-norcholestanes and C25 highly branched isoprenoid (HBI) alkanes suggests a significant upwelling component in the source rock depositional environment. In addition, the high oleanane indices (oleanane/hopane) of the oils are not paralleled in any alternative source rock candidate in this study. The values are as expected for Tertiary source rocks and are at levels that exceed any reported Cretaceous or older source rock or oil. This result, in concert with high nordiacholestane ratios, norcholestane ratios, and HBI concentrations, indicates a Tertiary age source rock.

Possible source rocks were selected and analyzed from different outcrops and wells and compared with the oils. A negative correlation suggests that Upper Cretaceous intervals of limestone, marl, and black shale previously believed to be important source rocks can be discounted as an important contributor to Talara basin oils. Instead, the new data suggest a Tertiary source rock (Eocene–Oligocene[?]) comparable to that of the Progreso basin. However, no such source rock strata have yet been identified within the Talara basin. Certain Upper Cretaceous samples with good source potential could support another petroleum system not yet identified in the coastal areas of Peru.

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