Origin and Mixture of Fluids in the Llanos Foreland Basin of Colombia: Composition, Stable Isotopes and Noble Gases Analysis of Natural Gas

Gonzalez-Penagos, Felipe¹; Rouchon, Virgile; Battani, Anne; and Guichet, Xavier
¹[email protected]

The foreland Llanos basin is a prolific petroleum province of Colombia. Although the essential elements of its petroleum system are generally well understood, dynamic processes seem to be controlled primarily by a complex fluid flow history including an active topography-driven meteoric water recharge. Geochemical characterization of carbon and noble gases (gas phase produced in oil fields) can be used to elucidate the origin of the various fluid contributions (organic and inorganic) accumulated in reservoirs.

In order to investigate the origin, mixture and reconstruct the fluid flow history at basin scale, we have analyzed the molecular major composition, carbon isotopic composition and the abundance and isotopic ratios of noble gases (He, Ne, Ar, Kr and Xe) from gas-phase samples. Natural gas samples were recovered from 15 producing wells (water-oil-gas mixture), taken from the Une, Mirador and Carbonera C7 formations between 1-5 km depth.

The results show a wide compositional variation in natural gas accumulations main compounds: CH₄ (32 to 92%), CO₂ (1.5 to 14%) and N₂ (0.6 to 17%). Hydrocarbons clearly show a classical maturity trend for the Une and Mirador Fms (C₁/C₂: 4 to 12 and δ¹³C₁: -32 to -42‰) with biodegradation processes in shallow samples from Carbonera C7 Fm to the south (C₂/C₃: 7.6 to 11.6 and δ¹³C₁: -51 to -59 ‰). Thermogenic hydrocarbons are associated with radiogenic He (⁴He/³⁶Ar: 260 to 1469, ³He/⁴He: 8.7e^-8), while some mantle He input seems to increase towards the Guaycaramo Fault System mainly in the Une Fm (³He/⁴He: -2.8e^-7 to -2.9e^-7).

Distribution of observed geochemical patterns allows determining the multiple sources mixing of thermogenic hydrocarbon fluids, atmosphere-derived (meteoric water flush) and biodegradation processes coupled with an understanding of the fluid flow history of the Llanos basin.

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013