Regional Frontier Exploration in Sinu Basin, Northwestern Colombia

F. Alan Lindberg, James M. Ellis, Larry L. Dekker

The Sinu basin has long held the interest of many explorationists, including the late Hollis D. Hedberg, because of large-scale structures and numerous oil and gas seeps. However, because of poor exposure and remote conditions, mapping of structures has always been difficult in this basin. The area is almost continually cloud covered, preventing the acquisition of useful regional aerial photographs and Landsat images. Seismic coverage was fair but generally of poor quality because of steep dips and overpressured shales.

In 1983, Gulf and Ecopetrol undertook a regional hydrocarbon evaluation of northwestern Colombia, during the course of which much of the Sinu basin was mapped by field geologists aided by low-altitude aerial photographs. Additional seismic and airborne radar data were acquired to assist in developing a regional structural model.

The dominant structures of the Sinu basin were produced by westward-vergent thrust faults, which are offset on the order of 10 to 20 km by northwest-southeast-trending compartmental faults. Numerous mud volcanos are surface expressions of overpressured shales, which migrate upward along both thrust and strike-slip faults. Thrust faults are expressed, on the surface, by steep-sided, asymmetrical anticlines, which are separated by broad synclines filled with clastics shed during Tertiary thrusting.

The extremely thick section of Tertiary sediments is dominated by shale but contains some potential reservoir sandstones. These resistive sandstones could be accurately mapped on the radar imagery and projected into the subsurface allowing traps to be better defined.

Combining field geology with geologic interpretation of aerial photographs and radar images was very effective in developing a regional structural framework of the Sinu basin.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.