Petroleum Geology of Llanos Basin of Colombia

J. A. Sutherland, L. E. Pena, F. Munoz

The Llanos basins located in eastern Colombia between the Guyana shield and the Cordillera Oriental, the most easterly branch of the Andes. Currently, the basin produces 224,000 bbl of oil/day from a mixed marine and continental section that ranges in age from Cenomanian to Holocene and reaches over 7,500 m in thickness.

It is important to realize that until the Pliocene the basin lacked an independent existence; rather, it formed the nearshore sector of a much larger depocenter that opened westward to the Pacific and northward to the Caribbean. The bulk of the oil accumulated in the Llanos was probably generated outside the present basin from a much more extensive marine Cretaceous section that crops out in the Cordillera. Subsurface pressure data indicate that these rocks are now isolated from the Llanos, implying generation and primary migration before the elevation of the Cordillera in the early Pliocene.

Oil accumulations are mostly fault controlled and fall into three classes. The giant Cano Limon complex in the north is closed against a lateral fault that appears to be a rejuvenation of a regional pre-Cretaceous normal fault. The central Llanos fields of Casanare are controlled by north-south-trending up-to-the-basin normal faults. The Apiay-Castilla fields to the south, a short distance from the mountain front, are located on an anticlinal trend bounded to the east by a high-angle reverse fault that also appears to have had horizontal movement.

Oil gravity varies widely, from below 10° to 40° API, with most of the reserves in the 30° API range. However, with very few exceptions and regardless of the gravity, the oil has an abnormally low dissolved-gas content. To date, with relatively low density of drilling, over 1.3 billion bbl of oil have been discovered and prospects for further major discoveries are excellent.

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