The distribution of thermogenic, bacterial and inorganic fluid sourcesin the petroleum systems of the Llanos Basin (Colombia) e Insightsfrom the noble gases and carbon stable isotopes

The Colombian Andean foreland is a rich petroleum province, where various source rocks and an activecirculation of fresh water are present in the subsurface, resulting in a complex fluid mixture within thereservoirs. Moreover, some of the traps are shallow, and low API gravity oils are found. Massive meteoricwater infiltration and local biodegradation have been proposed to explain these observations, but requiresome in-depth investigation to be confirmed. In order to provide some new insights into the Llanos Basinpetroleum 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 influenceof meteoric water infiltration is dominant in the shallow area eastward, but decreases towardsthe deepest part of the basin westward. A general gas/oil phase separation differentiates gas-depletedbiodegraded, shallow reservoirs from deep gas-rich less-altered reservoirs. Data suggest that there is ashallow biodegradation associated with meteoric water circulation in the Carbonera Fm, while some ofthe 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 studyalso indicates a contribution of mantle fluids in the deepest parts of the basin near the contact with theGuaycaramo Fault System. There, older rift sequences may have recorded mantle fluid fluxes, or alternatelythe presence of diffuse mantle fluxing along the deep-rooted thrust front during the formation ofthe Llanos Basin.