Foreland Llanos Basin of Colombia: A 3D Modelling of Geochemical Signatures Associated to the Fluid Flow History and its Implications in the Petroleum System

Gonzalez-Penagos, Felipe¹; Moretti, Isabelle; and Guichet, Xavier
¹[email protected]

The Eastern Llanos foreland basin is the largest petroleum province of Colombia. Its Petroleum system involves hydrocarbon accumulations in Cretaceous and Tertiary units in a silisiclastic shale/sandstone stratigraphic sequence. Shale units represent both, source and seal rocks within a shale-dominated column and sandstones the reservoir rocks. Hydrocarbon maturation and migration are currently active but a strong regime of water circulation play a determinant role in the global fluid flow. Production of fresh water in oil fields represents often a high cut (BSW~70%) and heavy oil accumulations due to biodegradation are present.

New geochemical analysis performed in formation waters and natural gas sample give us the clues to understand the present-day fluid origins and highlight the geochemical interaction and water source mixtures in depth. Results show a wide variation in salinity (0.1 to 40 g/L), variable anionic composition (bicarbonate to chloride dominated) and stable isotopes (52 analysis δD and δ¹⁸O) of formation waters. For the natural gas samples, carbon isotopic composition and noble gases abundance and isotopic ratios have been measured.

Distribution of geochemical information allows us to determine the genetic fingerprints preserved and to propose a mixture model of origin of fluids in the hydrocarbon fields. This mixture has 3 sources: the connate water (corresponding to the sediment environment), the shale diagenesis (mainly here dehydration of the smectite) and meteoric recharge (topography driven flow).

Shallow reservoirs seem to be controlled primarily by an active modern meteoric water flow, whereas deepest reservoirs, westward, contain mainly diagenetic. Geochemical distribution maps of water show that connate water is migrating upward and suggest that the salinity could be locally increased by dissolution of salty layers as the ones known in the Cordillera.

We carried out a 3D basin model to reconstruct present-day hydrocarbon accumulation and water geochemical distribution. Pressure, temperature, shale transformation and the source rock maturation are calibrated on subsurface data. Salinity transportation and topography-driven meteoric water infiltration are taken into account in this advanced modelling.

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