Exploration Potential Seen In Little-Known Cauca Basin

James A. Noel
Trinity Gas
Brownwood, Tex.

The geology of the Cauca basin is relatively unknown except for regional studies related to the origin of the Cordilleras Oriental and Central, the plate tectonics of northern South America, and a regional gravity study. There is little public information about the Upper Cauca basin except what has been included in passing as part of regional studies.

Interpretation of the sparse geophysical and well data available for the region nevertheless suggests that exploration potential exists.

In 1964, United Geophysical conducted a gravity and a partial ground magnetic survey for Intercol, a subsidiary of Esso, part of what is now Exxon Corp. During the next year, Intercol drilled what seems to have been a stratigraphic test, Candelaria 1, to 5,028 ft. A suite of electric logs, a dipmeter log, and a mud log were run in this well.

The state oil company of Colombia, Ecopetrol, was active in several exploration projects in the Upper-Cauca basin in the late 1970s and early 1980s, commissioning aerial magnetic, geomicrobial, gravity, oil seep, and seismic studies of the Upper Cauca basin. The aerial magnetic survey covered most of the Cauca basin.

Ecopetrol permitted us to spend 2 days reviewing some of these records. In general, the gravity and magnetic surveys show positive anomalies along both sides of the basin separated by a negative anomaly in the center. The surveys indicate a relatively shallow "basement" and thin sedimentary section, especially over the positive anomalies. En-echelon positioning of the positive anomalies indicates that they may be separated by northeast-southwest trending faults.

As a result of these preliminary studies, Trinity Gas entered into negotiations with Ecopetrol for an association contract and signed an agreement to the contract terms on Mar. 29, 1995. The contract area, known as Fallarones, covers 370,989 acres in the Upper Cauca basin.

Regional setting

The Cauca basin of southwestern Colombia is bounded on the west by the Cordillera Occidental and the east by the Cordillera Central. Named for the Cauca Valley and River, the basin is several hundred miles long, extending from a point about 90 miles north of the Ecuador-Colombia border northward to its merger with the Magdalena basin (Fig. 1 [1396216 bytes]).

The Cauca basin is about 22 miles (35 km) wide throughout the study area but narrows just north of the city of Buga to about 9.3 miles for about the next 22 miles. The basin then gradually widens to its confluence with the Magdelana basin.

The Cordilleras are high, rugged mountains reaching elevations of over 10,000 ft (3,048 m). Mt. Huila in the Cordillera Central is over 18,000 ft high.

The southern end of the Cauca basin is limited by low highlands in the vicinity of Santander. On the western fringe of the valley, the Cauca River flows in a relatively straight course out of the Santander highlands, then meanders tightly northward to its confluence with the Magdalena River.

The Cauca Valley floor is nearly flat with a gentle slope to the north and to the west. The elevation in the Cali area is about 3,300 ft, gradually decreasing to the north.

The Cordilleras are complexly folded and intruded masses uplifted along steeply dipping fault zones. They contain all three classes of rocks, and volcanic activity has been common. The mountains were formed during the westward movement of the South American Plate as it collided with the Pacific Plate.

The Cauca basin is separated from the Cordillera Central in the east by a series of faults and in the west from the Cordillera Occidental by the Cauca fault. It seems to be a rift basin formed by the eastward movement of the Caribbean Plate. Rivers flowing from the Cordilleras covered the top of the graben block with sediment. Lava flows may be part of the filling of the basin as well.

Very little is known about the geology of the Upper Cauca basin because there are no exposures in the valley or along its interior flanks. It has been theorized that the Cauca basin was tilted to the north and that the fluvial sediments may be interfingered with marine sediments to the north.

What is known about the geology of the Cauca basin comes from the studies mentioned above and Candelaria 1. With the exception of the aerial magnetic survey, the studies are in the Upper Cauca basin; therefore, anything north of the Buga area is conjecture.

Upper Cauca basin

The Upper Cauca basin lies in the southern portion of the Cauca basin between 3° and 3° 35' N. Lat. and roughly along 76° W. Long. Within this area the southern boundary is the highlands near Santander, and the northern limit is the Rio Sonso, south of the city of Buga. The east and west boundaries are valley walls marked by major fault zones and the Cordilleras Occidental and Central, respectively.

Our interpretation is based on data from logs run in Candelaria 1, the United Geophysical gravity survey, and the magnetic survey conducted by Geoterrex of Ottawa, Canada, in 1979. One short seismic line is included to indicate support for the gravity-magnetic interpretation.

Candelaria 1

Intercol drilled Candelaria 1 about 9.5 miles southeast of Cali and 6 miles southwest of the village of Candelaria. The well's location on the western edge of the negative anomaly leads to the belief that it was a stratigraphic test to determine thickness of the sedimentary section. Two large gravity anomalies to the east and west would be logical choices if the well had been a structural test.

Casing was set at the base of the alluvium. Discovery of igneous rock at 4,566 ft must have been a disappointment because this shallow depth together with the narrowness of the basin should preclude the discovery of a giant field. However, the mud log reports four thin sedimentary sections in the interval from the top of the igneous rock to total depth (5,028 ft). One of these was described as calcareous with fossils. All had gas shows.

The significance of this is that the igneous rock, although labeled diabase, is probably a lava flow instead of an intrusion. The interval between 2,300 ft and 4,566 ft consists of alternating sandstone, shale, and coal with numerous strong gas shows (Table 1).

The only other source of direct geological information is in the several hundred water wells drilled by the Environmental Section of CVC, the organization that oversees the hydrologic systems of the Cali area. Few of these wells have penetrated bedrock, and we have not been able to study the data from these wells to date.

Gravity survey

Fig. 2 [2519616 bytes] is a generalized version of the Bouguer gravity map that United Geophysical completed the year after it conducted its gravity survey for Intercol. Faults shown on the map are the interpretation of the author.

There were no near-surface rocks and no hills over which to make a density determination. The subsequently run density log is of such poor quality that using density determinations from it would result in errors. The topography of the Cauca Valley is flat and featureless so that no topographic corrections were needed. The theoretical gravity was tied to a pendulum station in Cali, and the elevations to a "benchmark" located at the west end of a bridge over the Rio Cauca.

Six strong positive anomalies are visible on the Bouguer map. The southernmost, the Santander High, is just north of the city of Santander. The oval feature roughly coincides with the Patia highland area. About 5 miles wide, the anomaly is south of the contract area.

Along the western side of the Cauca Valley are two elongate en-echelon positive anomalies. The southern anomaly, the Yumbo High, begins just north of Cali and extends 8 miles north with nearly a north-south trend. The Yumbo High is about 2 miles wide at its widest point.

North of and offset to the Yumbo High is the San Marcos-Vijes High, largest of the anomalies with length of about 9 miles and maximum width of 3 miles. The faults shown on the south end of both of these anomalies are believed to be the cause of both the shift of the anomalies' trends from paralleling the Cauca fault and the en-echelon position between them.

Along the east side of the Cacua Valley are the remaining three Bouguer anomalies, all smaller than those on the west side. They are more oval in shape than the others but are also in en-echelon position because of cross faults with apparent movement in the northeasterly direction. These anomalies are bounded on the west by the Palmira-Buga fault and on the east by the Romeral fault zone.

The trend of the Romeral fault may be more northeasterly than shown on the map. One interpretation of Landsat indicates it may branch out of the Cauca Valley north of Palmira and trend northeastward into the Cordillera Central.

Another unnamed fault forms the east boundary of the basin from that point northward to the Buga area, where it may merge with the Palmira-Buga fault.

The Candelaria High, just east of the village of the same name, is oval with a diameter of about 5 miles. North of this anomaly and more elongate is the Palmira High. It is about 7.5 miles long and 2.8 miles wide. The Manulita High is north and shifted to the west of the Palmira High. Its shape is more oval and slightly larger than the Caldelaria High. The diameter of the Manulita High is about 5 miles.

Airborne magnetic survey

Fig. 3 [2521224 bytes] shows generalized results of the part of the Geoterrex airborne magnetic survey that covered the Upper Cauca basin. The survey was flown at an altitude of 7,500 ft along lines parallel to the trend of the Cauca Valley. The flight lines were oriented N65°W about 9.3 miles apart. Profile lines have been approximately located on the magnetic map to help with comparisons to the Bouguer anomaly map.

The magnetic anomalies somewhat coincide with those on the gravity map. The notable differences are in shapes and sizes of the positive anomalies, as well as the lack of the en-echelon nature shown clearly on the gravity map. The center of the basin in both cases is marked by a strong negative anomaly, which parallels the strike of the basin. This negativity rises to positive anomalies on both sides of the basin. The "magnetic basement," therefore, runs deep along the axis of the basin and rises to shallower depths along the margins.

Fig. 4 [545616 bytes], Fig. 5 [545616 bytes], and Fig. 6 [19538 bytes] are east-west gravity and magnetic profiles across the basin that show the correlation between the gravity and magnetic anomalies. They begin on the western edge of the three prominent anomalies on the basin's western side, cover the central portion of the basin, and end near the eastern margin. Although the gravity and magnetic curves are not completely coincident, they do show agreement as to the position of the anomalies and indicate that the anomalies on the east side of the basin are weaker than those of the west side.

Interpretation

Positions of both gravity and magnetic anomalies along the margins of the Upper Cauca basin have led to different ideas as to their origin. These positions of themselves would be an anomaly because unless there is considerable drag along the basin margins caused by downward movement along the bounding faults, the positive magnetic and gravity anomalies are misplaced. If there was considerable drag on the basin rocks, it is believed that the anomalies would have a different shape.

Another interpretation would be that the positive anomalies are igneous intrusions. However, there is no indication in the sample descriptions of Candelaria 1 of contact metamorphism in the sediments encountered in the well.

The Cauca and Romeral faults may really be fault zones with the major displacemet between them being downward but with at least one other minor fault within the zone with an upward displacement that moved basement rocks and their topping sediments with them. Steep gradients of both the gravity and magnetics on the basin side of the anomalies would indicate normal faults that dropped the interior of the basin downward. Both the gravity and magnetic positive anomalies are so strong that the overlying sediments have made little contribution to them. The sediments seem to be nearly transparent to the gravity and magnetic readings.

If the sediments on top of the positive anomalies are as thick as those found in Candelaria 1 then it would seem that there may be more sedimentary rocks below the total depth of the well. A more likely conclusion is that the sedimentary section on top of the positive anomalies is much thinner than in the interior of the basin and that the blocks represented by the positive anomalies were uplifted along growth faults.

The short seismic section through the Palmira High in Fig. 7 is north of the Yumbo-Candelaria gravity-magnetic profile. On the west the seismic section is just north of the location of the gravity profile and trends eastward and crosses the Palmira High on the east side.

The west side of the seismic section shows the eastern flank of the Yumbo High. The central portion is the deeper central part of the basin. The dome on the east side represents the Palmira High, with the edge of the basin to the east of the high. Note the apparent pinch-out of events along the western side of the Palmira High and the drape of the events over the top of this anomaly.

The seismic section tends to confirm the gravity-magnetic interpretation, especially the presence of the Palmira High with its thin sedimentary section and shallow basement. It shows multiple stratigraphic traps and structural traps as well.

Bibliography

Alfonso, C.A., Sacks, P.E., Secor, D.T. Jr., "Late Tertiary northwestward vergent thrusting in the Valle de Cauca, Colombian Andes," AAPG Roundtable, 1989, p. 327.

Aspden, J.A., Litherland, M., 1992, "The geology and Mesozoic collisional history of the Cordillera Real," Tectonophysics, Vol. 205, 1992, pp. 19-16.

Case, J.E., MacDonald, W.D., "Regional gravity anomalies and crustal structure in northern Colombia," Geol. Soc. America Bull., Vol. 84, 1973, pp. 2903-2916.

Stabler, C.L., "Andean hydrocarbon resources-an overview," in Ericksen, G.E., Canas Pinochet, M.T., Reinemund, J.A., eds., "Geology of the Andes and its Relation to Hydrocarbon and Mineral Resources," Circum-Pacific Council for Energy and Mineral Resources, Earth Science Series, Vol. 11, 1989, pp. 431-438.

The Author

James A. Noel of Ashland, Ohio, is a consultant for Trinity Gas. He retired recently from Ashland University, where he was a professor of geology.

Noel formerly worked with Esso in Venezuela and the Pure Oil Co. research center. He founded the geology departments at Northwestern Louisiana University and Wright State University. He holds a bachelor's degree from Lehigh University, master's degree from Dartmouth College, and PhD from Indiana University, all in geology.

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