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Abstract

Once deeply buried rocks are elevated in thrust belts, the resulting effects on reservoir evolution, source-rock maturity, hydrocarbon phase and charge history pose major problems for thrust belt exploration. To understand the geometrical evolution and burial history of thrust belts, successive structural restorations and dynamic basin modelling are needed. The forward modelling program ‘Thrustpack’ provides a semi-quantitative way forward, and this paper presents a ‘Thrustpack’ case study. The area considered is Rio Horta in the western foothills of the Colombian Eastern Cordillera, where westward-directed frontal structures break out onto the foreland basin of the Middle Magdalena Valley. A large sub-thrust anticline underlies the frontal thrusts and provides a substantial exploration lead. Following conventional models of back-thrusting and ‘fish-tailing’, the structure can be interpreted as entirely late, post-dating the overlying thrusts and once buried by the entire sedimentary megasequence of the Magdalena foreland basin. This would imply that the prospective section had been buried to a depth of about 12 km before uplift, and suggest a hydrocarbon graveyard, or, at best, dry gas in fractured, tight rock and potential overpressure. If the structure formed early, there is a chance of preserving both original porosity and liquid hydrocarbon in the structure, and charge risk is lessened because hydrocarbons were able to migrate into a structure that already existed. Hints from geological maps and the (generally poor quality) seismic data suggest that this is the more likely situation. It is consistent with the idea of an evolving palaeo-landscape and a mountain front with a very long history, where the structure remained relatively elevated during later sedimentation and thrusting. The modelling of these two alternative possible structural histories in ‘Thrustpack’ tests their viability and quantifies the hydrocarbon maturation, migration history and porosity evolution. The model in which the structure develops early presents real exploration opportunity whereas the alternative presents unacceptable exploration risk.

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