Geological Framework of The Pacific Coast Sedimentary Basins, Western Colombia

The Pacific Coastal Province comprises the Tumaco, San Juan, Atrato and Urabá Basins. Although

these basins are laterally contiguous and related to an active margin (convergence of Nazca

and South America Plates) each one is characterized by a distinctive geological framework. The comparative

regional stratigraphic and structural setting of these active basins as well as their petroleum

potential is presented here using 2000 km of 2D seismic profile and information from fifteen exploratory

wells. Due to the lack of biostratigraphic control within these basins, only second order sequences

were recognized based mainly on seismic information.

The Tumaco Basin is a forearc basin that comprises four second order sequences, one pre-Miocene

and three post-lower Miocene, which were deposited in deep marine environments. The base of

the Sequence B (lower Miocene) and C (upper Miocene) were deposited in submarine fan systems

providing turbiditic coarse clastics. The sequence A (pre-Miocene sequence) could have a thick potential

source section of shale.

The San Juan Basin represents a pull-apart basin comprising rocks from Paleocene to Quaternary,

deposited mainly in deep marine environments as well as in neritic environments. The Sierra

(Oligocene), Conglomerado La Mojarra (lower Miocene) and Munguidó (upper Miocene) Formations,

could provide potential clastic reservoirs). In the Iró Formation also occurs competent rocks (cherts

and limestones) highly fractured that represents potential fractured reservoirs. The shaly section of the

Iró Formation is a potential source rock in the basin.

The Atrato basin is an interarc, which comprises by five second order sequences deposited

mainly in deep marine environments from Late Cretaceous to Quaternary. The base of the C and D

sequences (Oligocene and middle Miocene respectively) were possibly deposited in submarine fan

systems. These levels could provide sandstones and conglomerates as potential reservoirs. Source

potential rocks could occur in sequence A (upper Cretaceous-lower Miocene) and sequence B (lower

Eocene-Oligocene).

The Urabá Basin represents a backarc basin which contains five second order sequences, probably

deposited from Late Cretaceous to Quaternary, mainly in marine to transitional environments.

There are three levels with coarse clastic facies. These three levels are in the upper part of the

Sequence B (Oligocene), in the Sequence C (Oligocene-middle Miocene) and in the Sequence D

(Middle Miocene-Pliocene?), which have coarser facies toward the southwest. The Sequences A (upper

Cretaceous?-Eocene) and B (Eocene) contain pelitic facies in the depocenter of the basin that

could be potential source rocks.

The present study is a preliminar evaluation of the regional geology of the sedimentary basins of

the Pacific Coast Province. This evaluation provides new ideas about the different geologic features

of basins developed in a same tectonic setting (convergent margin) and the implications for petroleum

potential. The different characteristics of these basins could be related to differential local rate of

subduction, different angle of the Benniof zone, transcurrent faults, etc. factors beyond the scope of

this paper.