Opon Gas Field, Colombia: Part I - Geoscience Description, Well Design and Well Test Performance
Authors: Stone D.M. , Huffmyer W.A. , Greener J.M. , Mamerow B.S. 
Published: December 2007

Abstract
This paper describes the geoscience and engineering aspects of the Opon Gas Field located in the Middle Magdalena Basin, Colombia. The remoteness and extreme downhole conditions make the drilling, completion, testing and geoscience interpretation of the two most recent Opon wells technically very challenging. Multiple faults, steep dips, rugged topography, a sensitive jungle environment and variable surface velocities complicate field definition. A full assessment of the commercial potential of the reservoir requires additional development drilling. Now in the early development stages, the Opon Gas Field has first production scheduled for late 1996.

Introduction

Geographic and Geological Setting
The Opon Gas Field is located within the western fold-and-thrust belt of the Eastern Cordillera in the southeastern portion of the Middle Magdalena Valley, approximately 125 miles north of Bogota (Fig. 1). The Middle Magdalena Basin is Colombia's oldest and most prolific area in terms of cumulative hydrocarbon production.
A spectacular NE-SW trending surface anticline, 6 × 38 miles in size, the Opon structure eastern flank is bounded by the Las Salinas Fault which thrusts Cretaceous strata over the Tertiary section's elliptical outcrop. The radar image (Fig. 2) best displays this large scale anticline. The surface outcrops, mostly obscured by vegetation, form a series of parallel north-east south-west trending ridges.
Geologically complex and operationally challenging, Opon Gas Field's multiple thrust faults, high angle dips and rugged topography make seismic data acquisition, processing and interpretation difficult. To date, seismic data has had limited value in defining shallow subsurface geometries. The extreme overpressure and the high angle dips developed during Andean compression make drilling, completing and testing Opon wells a technical challenge. The remote location, thick vegetation, intense rainy seasons and the lush tropical environment complicate logistics. Helicopter support and/or new roads are required to provide access.

Opon Area Exploration History
Opon area oil exploration began in the late 1960's with the drilling of the Opon #1 and #2 wells (Fig. 3). These projects were targeted for the La Paz Formation located on what was then interpreted as the crest of the structure. Both wells tested overpressured gas which had no market value at the time. During the 1970's and 1980's, three subsequent exploratory wells attempted to test the La Paz Formation for oil. The Escocia #1, Guayacan #1, and Lilia #1 wells were unsuccessful due to a combination of mechanical problems related to pressure and the La Paz Formation being deeper than anticipated. During the 1980's and early 1990's, six additional exploratory wells were drilled to evaluate a shallow oil play in the Mugrosa Formation. These efforts resulted in intermittent oil production of 200 barrels per day from 1976 to 1982.
In the 30 years that have elapsed since the gas tests of Opon #1 and #2, numerous advances have occurred to make the technical challenges more manageable and the gas play economically attractive. Namely, the improvement of drilling and production testing technology in overpressured environments and the expansion of gas infrastructure into the interior region of Colombia.
In 1993, Amoco Colombia Petroleum Company (ACPC) farmed-in to the 124,000 acre Opon Association Contract. Upon assuming operatorship, ACPC successfully drilled and completed the Opon #3 and #4 wells. These wells confirmed commercially viable production rates and reserves from the La Paz Formation. Successful field operations and well post appraisals were performed by a multi-disciplinary technical team from Bogota, Houston and Tulsa. In 1995, the original Opon Association Contract area was subsequently reduced to its present size due to a required 50 percent relinquishment obligation.
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