Andrew J. Matthews
Intera Information Technologies Ltd.
Henley, U.K.
Omar Portilla
Ecopetrol
Bogota
The Putumayo basin lies in southern Colombia between the Eastern Cordillera of the Andes an the Guyana Brazilian shield (Fig. 1). It covers about 50,000 sq km between 0 3 N. Lat. and 74 77 W. Long. and extends southward into Ecuador and Peru as the productive Oriente basin.
About 3,500 sq km of acreage in the basin is being offered for licensing in the first licensing round by competitive tender. A recent review of the available data from this area by Intera and Ecopetrol suggests that low risk prospects and leads remain to be tested.
Exploration in the Putumayo basin began in 1941 with the first commercial discovery being made at Orito in 1963 by Texas Petroleum in association with Gulf. Orito has proved to be the largest field found to date, with total recoverable reserves of more than 212 million bbl in Cretaceous Cabalos/Villeta T and the Eocene Pepino horizons. An historic success ratio of 1:5 is recorded (24 discoveries from 122 exploration wells).
TECTONIC SETTING
The Putumayo basin evolved in three successive stages throughout the Mesozoic Cenozoic, namely rift, passive margin, and foreland basin. The rift phase lasted from Triassic to early Cretaceous times during which red beds, volcanics, and marine sediments were deposited. By Aptian times the rift belt had evolved into a northwest facing passive margin. During this phase, which lasted until the late Cretaceous (Campanian), marine sandstones and shales, including important reservoir and source intervals, were deposited across the present day Putumayo, Llanos, and Magdalena basins.
During the Campanian the passive margin was converted to an east facing foreland basin due to collision of the allochthonous Western Cordillera terrane against the Central Cordillera. This event caused the western part of the Central Cordillera to be uplifted and thrust eastward. A later collision of the Panama Baudo arc in the Eocene forced the thrust front to advance to the eastern limit of the Central Cordillera. A final eastward shift of the thrust front in the Miocene caused uplift of the remainder of the Eastern Cordillera, thus separating the Upper Magdalena and Putumayo basins.
STRATIGRAPHY
The Putumayo basin is underlain by the Precambrian crystalline igneous and metamorphic assemblage of the Guyana shield. The overlying sedimentary section thins eastwards on to this shield area while in the deeper parts of the basin up to 11,500 ft of Cretaceous and Tertiary are present (Fig. 2).
The Mesozoic rift sequence is floored by the continental facies with volcanics of the Triassic Jurassic Motema formation. However, the potential for encountering lacustrine shale facies in the deeper rift sequence remains an interesting possibility.
The basal Aptian Albian sandstone of the Caballos formation forms an important reservoir in western parts of the basin (e.g. Orito), where it consists of more than 300 ft of transgressive shallow marine sandstones. The thickness of this unit diminishes eastwards eventually pinching out onto the Guyana shield.
The comformable Albian to Coniacian Villeta formation is mainly composed of marine shales and limestones and forms the main source rock for the Putumayo basin. In the lower part of the formation shallow marine and deltaic sandstones, derived from the exposed Guyana shield, become gradually better developed eastwards and form important reservoirs.
These transgressive regressive sequences have been named from the base upwards as the C, T, B, U, A, and M intervals. West of the Nancy Burdine Maxine area these horizons shale out. The Villeta formation is succeeded unconformably by the Paleocene continental red bed shales of the Rumiyaco formation which form an excellent regional seal.
STRUCTURE
The Putumayo basin falls into two major structural provinces, with the Sub Andean fold belt to the west closely associated with the uplift of the Eastern Cordillera. This is characterized by northeast southwest trending asymmetrical hanging wall anticlines related to major westerly dipping thrust faults. The Orito fault has a throw of several thousand feet. The intensity of the structural deformation can be clearly seen to decrease eastwards away from the Eastern Cordillera Thrust Front.
East of the Orito fault the Putumayo basin takes the form of a platform area with gentle northwesterly dip. Moderately steep, north-south trending reverse faults are the major structural features offering the best trapping possibilities (Fig. 3). The easterly dipping Puerto Asis fault runs for 70 km across the study area and extends southwards into Ecuador. Two sizable closures are associated with this fault in the Alea area (Fig. 4). The Alea structure is particularly attractive in that the down-flank Alea 1 well tested 394 b/d of 30.9 gravity oil from the Lower U sandstone.
HYDROCARBON GEOLOGY
The Villetta formation is the primary source horizon for the Putumayo basin. Average TOC values have been measured at 1.03% by weight with a maximum value of 1.69%.
Organic matter in the Villeta is predominantly marine derived amorphous Type II kerogen, although an influence of continentally derived material is seen to the northeast of the basin. Hydrocarbons generated from the Villeta formation have a sulfur content in the range of 1.08-1.16%.
Oil gravities vary across the basin, being in the range of 33 38 in the west and 25-30 in the east, showing the effects of an active groundwater flow and biodegradation.
The timing of the onset of oil generation in relation to trap formation is still a matter of debate, although two phases, late Eocene to early Oligocene and Miocene Pliocene, have been proposed.
Expulsion has almost certainly continued into the present day.
RESERVOIRS
The Cretaceous Caballos and Villeta sandstones and the Eocene Pepino conglomerates have proven reservoir potential. Porosities in the Caballos formation increase eastwards, but are generally in the range 10 16% with an average permeability of 76 md. Porosities in the Villeta formation sandstones are slightly more variable, but average between 12 18%, with an average permeability of 67 md.
The Pepino formation becomes prospective where faulting has created migration pathways through the thick regional seal of the Rumiyaco formation (e.g. Orito).
TRAP TYPES
Four way dip closures created by gentle compression have been proved to be productive at Burdine and Nancy 1. Some subtle time structure closures of this type remain untested but are susceptible to velocity effects.
Dip closures on the foot-wall have been tested successfully (e.g. Cohembi 1). Similarly, dip closures against the hanging wall have proved productive as at Alea 1. Due to the stacked nature of the Cretaceous reservoirs, lateral sealing across faults is an important risk consideration. An element of independent anticlinal closure (ramp anticline) may also be present in zones of greatest deformation (e.g. Orito).
Stratigraphic trapping potential is possible where sandstones lap out onto paleo highs. The Caballos and Villeta sandstones all pinch out eastwards, although the quality of the overlapping seal is obviously a critical factor. The Villeta sandstones also shale out westwards into the basin. Where structuring has reversed the regional northwest dip, up-dip pinchout plays become prospective. High risk targets are provided by the on-lap of unmetamorphosed Motema formation clastics onto impermeable basement or by subcrop beneath a shaIy facies of the Caballos formation.
The hydrodynamics of strong groundwater flow can also provide possibilities for trapping hydrocarbons, even in nonstructural closures. However, at present this remains a conceptual play only for the Putumayo basin. The presence of a strong hydrodynamic regime in Orito field is well known, being the cause for the tilted oil water contact.
CONCLUSIONS
Throughout the open acreage Ecopetrol is offering for competitive tender there are low risks associated with oil sourcing, maturation, timing, and sealing. The acreage has considerable technical merit, in terms of opportunities for both low risk, low return plays that have proven oil potential, and new, higher risk, higher return plays. Leads and prospects for both types of plays are distributed throughout the license round acreage.
Although most obvious time structures have been tested, uncertainties still remain regarding accurate depth conversion, and the potential for oil being held in nonclosing structures by a hydrodynamic flux.
REFERENCES
- Robertson Research, Putumayo Basin, Colombia Petroleum Geology and Hydrocarbon Potential, Ecopetrol, Informe Geologico No. 1482, 1985.
- Caceres, H., and Teatin, P., Cuenca de] Putumayo, Provincia Petrolera Meridional de Colombia, II Simposio Bolivariano Exploration Petrolera en las Cuencas Subandinas, 1985.
- Ecopetrol and Becip, Evaluacion Regional Geologica y Geofisica de la Cuenca del Putumayo, Contrato LEG (P) 192 87, Bogota, 1988.
- Portilla, O., The Putumayo Fold-belt, Colombia, South America: Structural Interpretation and Hydrocarbon Potential, unpublished MSc thesis, University of South Carolina, 1991.
Copyright 1994 Oil & Gas Journal. All Rights Reserved.
