Pacific potential exists offshore Colombia

Colombia's National Hydrocarbons Agency (ANH) has defined two Pacific basin blocks, Tumaco Offshore 6 and 7, offered under a technical evaluation agreement modality in the country's 2012 licensing round.

Both blocks lie in 100 m to 3,000 m (328 ft to 9,843 ft) water depth. PGS has acquired and processed 2,730 km (1,700 miles) of high-resolution multiclient 2-D seismic data offshore the Pacific Coast of Colombia between Buenaventura and Tumaco and has reprocessed 2,404 km (1,500 miles) of legacy 2-D seismic to complement the modern survey coverage. Processed ship-borne gravity and magnetics complete the dataset.

Structural framework, stratigraphy

The Choc?-Pacific coastal and offshore region makes up a little-explored geologic province with underestimated petroleum potential. Surface onshore seeps and oil shows in the few wells drilled to date confirm the existence of mature source rocks, and new biostratigraphic data provide a greatly improved understanding of the basin's history and deepwater depositional environment.

Colombia’s ANH is offering two frontier Pacific Ocean permits in its 2012 licensing round, Tumaco Offshore Blocks 6 and 7. (Images courtesy of PGS)

The North-Andean margin is being deformed by the subduction of the Nazca plate (5 cm/y to 7 cm/y) along a N80° direction. The Nazca plate carries the Carnegie ridge, a 200-km (124-mile) wide buoyant ridge that subducts under the Ecuadorian central margin involving major crustal deformation. The northern flank of the Carnegie ridge divides the Ecuador-Colombian margin in two tectonically and seismically contrasted segments.

In Western Colombia, the Pacific Coastal basin divides into the Uraba, Atrato-San Juan, and Tumaco basins, whereas the Borb?n, Manab?, and Progreso basins have formed along Coastal Ecuador. In North Ecuador-South Colombia, the Borb?n and Tumaco basins may form a single basin that extends dominantly onshore from the Jama-Quininde fault in Ecuador to the Garrapatas fault system to the north.

The structural style of this area corresponds to forearc basins in convergent margins developed over folded oceanic sediments adjacent to the subduction zone. Such a framework could be conducive to an attractive array of hydrocarbon-bearing traps. Along the southern Pacific margin of Colombia, thick Cenozoic sedimentary sequences accumulated over blocks of oceanic or transitional crust that accreted between the Late Cretaceous and Early Cenozoic and formed the onshore Borb?n (Ecuador) and Tumaco (Colombia) basins and the offshore Pacific Manglares Frontal basin (Tumaco offshore).

Sedimentary fill in the Colombia Pacifico basin is generally of Tertiary age, with the oldest sediments dated at Late Cretaceous age. The Cenozoic sedimentary section of the basin is subdivided into important unconformities recognized by changes in faunal composition, which in turn coincide with Paleo-environmental and lithological changes.

Hydrocarbon potential

No commercial hydrocarbon fields have been discovered to date in the Colombia Pacific basin, but a number of wells drilled both offshore (four) and onshore (12) have yielded good gas shows. Source rocks are of Cretaceous and Lower Tertiary age and are oil- and gas-prone. Onshore gas shows are mainly from the Oligo-Miocene section.

PGS acquired and processed 2,730 km (1,663 miles) of high-resolution multiclient 2-D seismic data offshore Colombia’s Pacific coast. The data showed the presence of high-amplitude BSR, interpreted to be gas hydrates, in sediments at depths of 800 m below the seafloor. Gross gas volumes of these resources on Tumoca Offshore Blocks 6 and 7 are estimated to be 158 Tcf.

Surface indications of oil and gas have been reported from numerous locations, especially in the more open country to the north towards the Gulf of Urab? (North Pacific coast), where they are principally associated with mud volcanoes.

PGS MC2D seismic data acquired over the Pacific margin show the presence of high-amplitude bottom simulating reflectors, interpreted to be gas hydrates, in sediments at depths of ~800 m (~2,625 ft) below the seafloor. Gas hydrates contain immense amounts of energy – almost twice that of fossil fuels. The company has estimated gross gas volume of these resources on Tumaco Offshore 6 and 7 to be 158 Tcf.

Subsurface shows of oil and gas were encountered in the Buchado-1, Chagui-1, and Majagual-1 wells. Two wells, Sandi-1 and Tambora-1, have been drilled in the offshore Tumaco basin. The Tambora-1 well had gas shows with small impregnations of asphalt in Lower Tertiary clastics.

Although oil and gas seeps, shows, and offshore hydrocarbon anomalies attest to the presence of mature source rocks in the Pacific coastal basins, hard geochemical data are sparse and confined to the younger part of the section. Analysis of Sandi-1, Tambora-1, Remolino Grande-1, and Majagua-1 suggest good total organic content values but strong propensity toward gas-prone kerogen. This is at odds with the numerous oil seeps associated with mud volcanoes immediately northeast and suggests that none of the wells analyzed, which apart from Remolino Grande-1 all bottomed in Miocene or younger, reached the section that is generating the oil.

Exploration play types

Exploration play type is defined here as structural or stratigraphic closures fed by various Oligo-Miocene sources. Eight play types are recognized in the Tumaco Offshore 6 and 7 blocks:

• Type-1: Folds, anticlines, narrow mud cored anticlines, and anticlines induced by basement;
• Type-2: Inversion/transtensional structures, complex folding, and faulting. Thrust-related anticlines rollovers;
• Type-3: Change in seismic character, indicating strong facies changes and usually changing to sands;
• Type-4: Extensional system. Blocks highly faulted;
• Type-5: Eocene to Miocene units against Paleo-highs;
• Type-6: Eocene to Miocene units against normal faults;
• Type-7: Wedging on the edge of the basin. Intra-formational wedging; and
• Type-8: Updip erosional truncation of strata at the base of recent units .