EXPLORATION OLD, NEW PLAYS MAY REVITALIZE COLOMBIA'S LLANOS BASIN

Martin Keeley
Consulting petroleum geologist
Leighton Buzzard, U.K.

Egon Castro Arevalo
Ecopetrol
Bogota

Reevaluation of Colombia's Llanos basin has identified 10 plays and 94 leads and prospects.

Some of the plays are conceptual and unproven, such as dip and fault closures within Paleozoic clastics, hydrodynamic trapping in the Paleozoic, and fractured/weathered granitic basement. But all of these higher risk plays have giant field analogs elsewhere in the world.

The state oil concern Ecopetrol during third quarter 1993 set in motion a strategy to increase exploration investment in Colombia, specifically in three areas set aside for licensing: Llanos, Putumayo, and Upper Magdalena.

The company commissioned Intera to conduct a review of the association contract terms and other fiscal conditions, a re-evaluation of petroleum potential, and a promotional campaign for April-May 1994.

This, the first licensing round by competitive tender in Colombia, opened Apr. 18 and closes after 90 days. Awards will be based on a points system in which committed work program attracts the greatest points.

The review has led to the introduction of a formula for improving production sharing terms for the associate, a scheme for reimbursement of the associate's exploratory well costs out of production, and a substantial reduction in pipeline tariffs, by as much as 46%. Together with the heightened technical prospects identified in the course of petroleum re-evaluation, Colombia has once again become one of the more attractive exploration venues in the western hemisphere.

NEW LLANOS POTENTIAL

In the first of a series of review articles on the areas set aside for licensing, new potential is identified in the Llanos basin for both deeper higher risk but large Paleozoic targets and for smaller, shallower traps involving proven lower risk plays.

Ecopetrol is releasing 14 blocks for bidding in the Llanos Oriental basin and adjoining Llanero foothills of the eastern Cordillera (Fig. 1), each averaging about 520,000 acres (2,100 sq km).

Shell drilled the first exploration well in the Llanos basin in 1944, and Chevron discovered the first field, Castilla, in 1968. To date 51 fields have been found, producing at about 310,000 b/d. This level is set to rise steeply when Cusiana field is brought fully onstream.

Giant fields include the very different Cano Limon-La Yuca (1.4 billion bbl) and Cusiana-Cupiagua (1.5 billion bbl) complexes. Proved reserves now stand at 3.6 billion bbl. Yet with an exploration well density of only 1:123,000 acres (1:500 sq km), the Llanos basin can hardly be considered mature.

PETROLEUM GEOLOGY

Very thick sequences of Cambro-Ordovician dark marine shales and moderately porous sandstones everywhere onlap strong relief in the granitic basement of the Guyana shield (Fig. 2).

Deep Paleozoic grabens are evident from seismic, with major strike-slip inversion taking place in the Permo-Carboniferous (Figs. 3, 4). Basement topography established during CambroOrdovician basin formation was subsequently reactivated repeatedly under extension, compression, and strike-slip.

The Upper Paleozoic record is poorly preserved. Triassic-Jurassic sedimentation is thought to be continuous with the La Quinta graben system of western Venezuela, with the possibility of a restricted lacustrine play.

A Late Cretaceous transgression from the northwest led to the deposition of the rich La Luna-Gacheta marine source shales respectively over and beneath the prolific Une and Guadalupe reservoir sands. Source potential within the La Luna-Gacheta is largely restricted to beneath the Llanero foothills, with thrust burial-induced maturation and expulsion taking place in several phases during the Tertiary.

Long range oil migration has taken place, reaching Rubiales field more than 200 km to the southeast. In these more distal traps, oils are biodegraded and water washed, as a result of a strong meteoric hydrodynamic flux northwestwards into the basin. Oil Gravities fall from around 40 with moderate gas-oil ratios in the Llanero foothills to 10-15 with very low GOR. However, at ambient reservoir temperatures, these heavy oils are nonviscous.

Cyclic Paleogene sedimentation continued in response to foreland basin loading. The Barco, Los Cuervos, and Mirador sand-shale sequences form important proven reservoir-seal associations. The thick Carbonera formation comprises up to four sand-shale cycles. Found stacked, these are good producers in the majority of tilted fault block fields in the Llanos basin. They are formed antithetic to the regional asymmetric dip, trapping oil migrating updip against the downdip hydrodynamic flux. The overlying Leon shale provides a thick regional blanket seal.

PLAYS, PROSPECTS

Seismic mapping has revealed a number of very large structures involving the 10 plays identified in the re-evaluation.

However in any economic assessment these should be discounted by high geologic risk factors quantified in the range 1.7-12%. They do nevertheless represent attractive opportunities for risk-takers seeking a company-making strike.

Lower risk plays are linked to proven production mechanisms, such as Carbonera antithetic traps (e.g. Trinidad, La Gloria), Guadalupe-Paleogene thrust traps (e.g. Cusiana, Apiay), and Paleogene-Mirador strike slip traps (e.g. Cano Limon). Structures are mapped in license blocks near these analogs, thereby providing both development models and pipeline facilities. Geologic risk factors are much lower, calculated at 11-26%, but recent drilling success ratios on the antithetic play by Ecopetrol would indicate risks as low as 50%.

The traps identified in this re-evaluation have a capacity for almost 16 billion bbl of initial oil in place. With the application of the above geologic risk factors and recovery factors of about 40%, this equates to an expectation of some 823 million bbl of oil reserves. If this were distributed evenly amongst the mapped traps, then the average most likely discovery size would be 8.8 million bbl. However, risked reserves for individual traps reach up to 70 million bbl.