Myron P. Maslanyj
Intera Information Technologies Ltd.
Henley, U.K.
Luis E. Pena
Ecopetrol
Bogota
During third quarter 1993 Ecopetrol set in action a strategy to increase exploration investment in Colombia resulting in the first licensing round by competitive tender, which opened for 90 days on Apr. 18, 1994. The Upper Magdalena Valley basin (UMVB), one of three areas set aside for licensing, lies at the head of the Magdalena River between 2-5 N. Lat. The acreage offered consists of 4,900 sq km, some 23% of the total basin area (Fig. 1). Historical attention has focused on the Cretaceous Caballos and Monserrate sandstones, but new play potential exists for reservoirs in laterally equivalent facies and at other stratigraphic levels.
PREVIOUS ACTIVITY
Exploration in the UMVB began in 1922 with the Guataqui wells in the Girardot sub-basin. By 1951, Texaco had made the Ortega discovery.
Subsequent exploration focused on the Girardot and Neiva sub-basins, which have become the main producing regions. Up to December 1993, 210 exploration wells have been drilled resulting in discovery of 30 fields.
REGIONAL GEOLOGYThe UMVB separates the Central and Eastern Cordillera mountain ranges of the Colombian Andes.
The basin is presently a bivergent foreland basin subsiding under the over-thrust load of the Central and Eastern Cordilleras. It has evolved through various tectonic settings including a Triassic-Aptian rift phase, an Aptian-Campanian passive margin phase, and a Campanian-Recent foreland basin phase. The regional chronostratigraphy is summarized in Fig. 2.
STRUCTURE
During the Campanian, following collision of the northeast-migrating Farallon plate a subduction zone developed beneath Colombia uplifting the Central Cordillera.
The collision produced east-directed thrusting and development of a foreland basin. Most of the east-verging thrusts in the UMVB are related to this event. Subsidence resumed in the Mid-Late Eocene and the thrust front advanced eastward through the UMVB region.
The UMVB was first isolated during the ?Mid-Late Eocene-Miocene during uplift of the Eastern Cordillera, producing west-directed thrusting and reactivation of the Central Cordillera structures.
The latest phase of movement was dextral strike-slip during the Late Miocene-Pliocene, related to the northeastward migration of the entire North Andean block. This has produced northwest-striking synthetic faults, such as the Altamira and Ibague-Piedras faults. These systems consist of steep faults which cut the earlier thrust faults, producing pull-apart basins.
Antithetic sinistral faults which are parallel to the Santa Maria fault have not been documented but may account for the sinistral offsets observed between the Middle and Upper Magdalena Valley basins.
PETROLEUM SOURCE ROCKS
The principal source rock is the La Luna (Villeta) formation, which was deposited during the Late Cretaceous in a partially restricted marine basin. The distribution of La Luna formation in the UMVB is much more limited than in the Middle Magdalena Valley basin and complicated by the structural complexity of the basin.
The Lower La Luna formation is up to 400 in thick with TOC values of 1.2-3.1% and up to 65% oil prone kerogen. Gross composition and carbon isotope analyses demonstrate the La Luna has sourced most oils with variations attributable to post migration alteration and regional variations in maturity. For the acreage on offer gravities range from 23 in the south to between 8 (Chicoral-1.) and 36 (Gualanday-3) in the north. The Guaduala shales also offer some source potential.
RESERVOIRS
The principal reservoirs are the clastic Cretaceous Caballos (10-16% porosity) and Monserrate (10-18% porosity) formations.
Of secondary importance are the Cretaceous Tetuan limestones of the Simiti formation and the Tertiary Honda and Doima groups.
In the south, the Monserrate formation passes into the Tobo limestone, and in the north the Caballos formation grades into the Tablazo limestone. Little data are available on the reservoir properties of the carbonates and more work is required to evaluate fully their potential.
Two Tertiary fields produce from fluviatile sandstone reservoirs of the Honda group: Dina-T And Andalucia Sur.
SEALING MECHANISMS
The La Luna formation transgressive marine shales form an excellent regional top seal to the basal transgressive Caballos sandstone play. The ductile thick and uniform lithological character of the shales is of primary importance in maintaining trap integrity in the thrust prospects.
The Guaduala and Barzalosa sealing shales are likely to show a more irregular distribution and are, therefore, less reliable. Proven analogs in the Middle and Upper Magdalena Valley basins suggest that many thrust faults seal. The timing and mechanism of fault sealing is critical because many traps rely initially on migration along the same thrust faults.
TRAPS
The most common trap type is the fault-bend anti-cline and associated footwall structures (Fig. 3). These structures mostly formed prior to or during the main phase of oil generation, and have juxtaposed various reservoir rocks against thick source and sealing horizons, producing attractive multi-play prospects.
The structures can be traced over tens of kilometers and show structural relief in the range of 200-1,000 m. Many of the major basin margin thrusts are untested but have high risk trap Potential with considerable thicknesses of pre-Cretaceous basement in their hanging wall. Seismic data are, however, of low density coverage and with poor resolution in these areas. Thrust imbricates, duplexes, and sub-thrust traps are in general difficult to define due to velocity variations.
Tertiary onlap and pinchout traps may be developed locally adjacent to thrusted anticlines. The Honda group fluviatile channel sandstones also offer stratigraphic trap potential, dependent on hydrocarbons migrating through the Guaduala and Brazalosa sealing horizons.
PLAY TYPES
Fractured pre-Cretaceous reservoirs may offer exciting new high risk targets. General stratigraphic play concepts for the Cretaceous transgressive-regressive depositional megacycle are presented in Fig. 4.
The Caballos formation clastic play offers enhanced source rock potential and excellent sealing potential. This play, however, depends on the Caballos formation reservoir being thrust above the younger La Luna source rock. The Monserrate formation clastics represent a good play but with less reliable sealing horizons.
Little data are available to assess the Caballos and Monserrate formation carbonates, and this factor together with the absence of proven analogs represents high risking of these plays.
Villeta formation reservoirs have produced hydrocarbons but are higher risk on account of the small number of proven analogs.
The Tertiary play is based on limited production from the Honda group and Doima unit. The Tertiary sandstones are excellent reservoirs, but shallow prospects may have considerable seal and trap risk. Tertiary plays depend on migration through thrusts which cut the Guaduala and Barzalosa regional seals.
TRAP CHARGE, INTEGRITY
The onset of oil Generation for the La Luna formation was Late Oligocene-Pliocene, with a peak during the Late Miocene. Given suitable migration routes, this oil would have charmed the Central Cordillera structures.
Miocene-Pliocene deformation of the Eastern Cordillera produced additional structures for charging, and introduced the possibility of remigration into Miocene-Recent traps. Late Miocene-Pliocene reactivation of Paleocene-Oligocene thrusts probably led to breaching of traps, since there are many oil seeps in the UMVB, in particular along the basin margin thrusts.
Proven field analogs suggest that under suitable conditions this was not a problem. Cooling of the over-thrust sheets during uplift would have prevented further petroleum generation in the overthrusts, although enhanced generation would occur in the loaded subthrust continuing to the present-day. Hydrodynamic flushing and Biodegradation is a potential problem, particularly in the north in the Girardot sub-basin.
The kinematics of reservoir charge and sealing in relation to structuring represents an element of exploration risk which requires further investigation. Secondary migration is dependent on thrust faults acting as conduits rather than barriers.
Given that most of the thrust faults are sealing in the Middle Magdalena Val]eN?, lateral migration may represent a considerable risk element. Secondary migration losses are not possible to quantify but are likely to be considerable given complex structuring and facies changes.
TECHNICAL MERIT
The Upper Magdalena Valley basin acreage has considerable technical merit.
There are few difficulties with source rock distribution, maturation and migration, or with sealing mechanisms, and many volumetrically large structures remain untested.
Discoveries to date, however, are not full to spill point and breaching and remigration related to late Tertiary structuring are likely to be the most obvious threats to trap integrity.
The greatest exploration challenges are associated with trap definition, both in terms of structural modeling and geophysical definition. Discovery of future reserves will depend on the development of plays sustained by the acquisition of more detailed geological data and application of improved geophysical techniques.
BIBLIOGRAPHY
Beltran, N., and Gallo, J., The geology of the Neiva sub-basin, Upper Magdalena basin (southern portion), 9th Am. Field Conf., May-June 1968, Colomb. Soc. Pet. Geol. Geophils., Bogota, pp. 253-274.
Schamel, S., Middle and Upper Magdalena basins, Colombia, in Biddle, K.T. (ed.), Active Margin Basins, AAPG Memoir 52, pp. 283-301.
Copyright 1994 Oil & Gas Journal. All Rights Reserved.
