The distribution of thermogenic, bacterial and inorganic fluid sources in the petroleum systems of the Llanos Basin (Colombia) – Insights from the noble gases and carbon stable isotopes

The Colombian Andean foreland is a rich petroleum province, where various source rocks and an active circulation of fresh water are present in the subsurface, resulting in a complex fluid mixture within the reservoirs. Moreover, some of the traps are shallow, and low °API gravity oils are found. Massive meteoric water infiltration and local biodegradation have been proposed to explain these observations, but require some in-depth investigation to be confirmed. In order to provide some new insights into the Llanos Basin petroleum system, we performed a natural gas geochemical survey over different areas of the basin, including stable isotopes of hydrocarbon, non-hydrocarbon and noble gases. Results show that the influence of meteoric water infiltration is dominant in the shallow area eastward, but decreases towards the deepest part of the basin westward. A general gas/oil phase separation differentiates gas-depleted biodegraded, shallow reservoirs from deep gas-rich less-altered reservoirs. Data suggest that there is a shallow biodegradation associated with meteoric water circulation in the Carbonera Fm, while some of the deeper heavy oils (Mirador, Une fms) were more likely produced by an early mature source rock. However, biodegraded oil later mixed with less altered oil and associated gas are plausible. This study also indicates a contribution of mantle fluids in the deepest parts of the basin near the contact with the Guaycaramo Fault System. There, older rift sequences may have recorded mantle fluid fluxes, or alternately the presence of diffuse mantle fluxing along the deep-rooted thrust front during the formation of the Llanos Basin. •We sampled natural gas associated to complex organic and inorganic fluid mixtures.•Geochemistry shows biodegradation and meteoric water circulation at shallow depths.•Hydrocarbon gas and oil undergo phase separation likely during oil migration up-dip.•Deep heavy oils are due to low maturity of the source rocks.•Mantle fluids reach the accumulations along the fault systems near the thrust front.