ABSTRACT: Detailed Mapping of Reservoir Structural Geometry in Detached, Shortened Fold Belts, Ortega (Aptian-Albian) Field Area, Girardot Basin, Colombia

George D. Allen

Discovery and development of oil fields in shortened compressional fold belts require recognition that the largest reservoirs occur in intermediate or third-order scale folds. Third-order folds generally are preserved in the footwall of larger scale fold thrusts, and may be concealed beneath smaller, detached surface folds of nonreservoir condition. A successful reservoir mapping procedure involved (1) detailed surface mapping, (2) depth conversion of seismic data, (3) construction of a network of true-scale balanced cross sections, and (4) contour mapping above and below the major zones of detachment.

Structure at the 11 million bbl Ortega field consists of convergent, third-order fold thrusts, with internal decollement. Tight flexural-slip folds imbricate and tectonically thicken upward on the west-verging Ortega anticline. The buried, east-vergent Salado anticline acts as a buttress to westward propagation at Ortega, but retreats along strike to allow the Porvenir anticline to develop a low, broad, fault-bend fold geometry.

At least four fault blocks in the Ortega field remain untested because balanced section analysis has not been employed to create additional control points for structure contour maps on the top of the reservoir. It is speculated that another 10-50 million bbl of primary recoverable reserves remain in the Ortega field. In addition, reservoirs like the Ortega field should occur elsewhere in the Girardot basin. These reservoirs likely will occur all along the footwall of fold-thrust structures on the flank of the intrabasin Pata high.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990