Summary

The results of integrated interpretation of geophysical data from the Pacific San Juan Basin and Tumaco Basin areas, of offshore Colombia are presented. Approximately 8100 kilometers of gravity data, 8000 km of magnetic data and 8100 km of seismic data are utilized in this interpretation. One of the primary objectives of this interpretation was to establish the depth to magnetic basement and basement related structural elements, thereby delineating prospective areas for potential hydrocarbon accumulations. The seismic reflectivity from the basement and Cretaceous horizons is not good, making seismic interpretation ambiguous in this area. As the seismic interpretation could not resolve the geological features, such as subduction zone and associated volcanism, gravity and magnetic data interpretation has been carried out. The integrated interpretation of these data worked out well to decipher the subduction zone and thrust belt above the basement as well as other geological features like sediment thickness, volcanism and intrusions.

The qualitative and quantitative interpretations were performed by using several enhancements as well as automated interpretation techniques such as Werner and Euler techniques. These results were further refined with the studies of 2.5-D / 3-D modeling. A number of positive structural features were identified which may provide lead for future hydrocarbon exploration. The sediment isopach map indicates the sediment thickness exceeds 7,000 meters at few places and few significant sediment depocenters were identified having more than 5000 meters thickness. The sediment thickness interpretation is justified in terms of plate tectonics as thinning of the sediments towards the subduction zone and thickening of the sediments towards the mountainous onshore region. This thickening of the basin may be analogous to foreland basin-type morphology, formed due to late compression although this is only an observation and has not yet been substantiated.

Introduction

The Republic of Colombia is in the north-western corner of South America. This study area is located on the offshore of Pacific coast and was shown in Figure 1. The Choco Offshore Basin is located along the arcuate corner of northwestern Colombia, south of the Panama border. The block is bounded to the east by the Romeral fault zone, approximately 50 to 100 km east of study area (Figure 2). This fault zone is part of the north- northeast trending Dolores-Guayaquil mega-shear, which is a major crustal boundary in the region separating an oceanic crustal domain (Choco block) from the South American Craton (Plate) to the east.

The gravity and magnetic data were acquired simultaneously with seismic data in this area. The survey production period was from late 2005 to the early part of 2006. The survey area covers approximately 300 km in an east-west direction and 600 km from south to north.

Geologic Setting

On the western margin of the study area, the Nazca plate is being subducted eastwards under the South American Plate creating a classic active continental margin tectonic setting. Within this tectonic setting, the study area is defined as a fore-arc basin situated between the subduction zone and the magmatic arc represented by the Cordillera Occidental mountain ranges.

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