Searching for a La Luna Shale Oil Resource Play in the Magdalena Basins of Colombia
Schamel, Steven1; Ardila, Luis E.; and Mora, Cesar
The search for shale-oil resource plays in the Cretaceous-age source rock successions of the Magdalena Basins of Colombia, herein collectively referred to as “La Luna”, presently relies on identifying key characteristics of the proven plays elsewhere, principally in North America. However, a La Luna shale oil resource play cannot be expected to be fully analogous to any other due to the unique geology of the oil-rich basins that lie between the Central and Eastern Cordillera.
Successful oil resource plays have all of the elements of a viable petroleum system but one, an efficient carrier system. Consequently, the oil generated in the source rock can only seep into any relatively porous-permeable beds intercalated within or adjacent to the source rock, normally very tight carbonates and/or siltstones, or natural fracture networks. Strata with higher porosity-permeability are likely efficient carriers that permit migration of oil to distant conventional oil traps.
The La Luna petroleum system in the Magdalena basins is supported by a proven world-class oil-prone source rock. It has charged many and frequently large conventional oil fields, which requires the existence of efficient carrier systems linking source rock to traps. The “sweet spots” of the La Luna shale oil resource play will be controlled, in part, by the stratigraphic framework and distribution of clastics across the Cretaceous basin. Sands entering the La Luna basin from the east and southeast, the area corresponding to the present-day Llanos and Eastern Cordillera basins, are both effective carrier beds draining the La Luna oil and conventional reservoirs in the Upper Magdalena basin. The stratigraphic carrier system is augmented by an array of Miocene reverse faults. The Cretaceous sands are not present in the Middle Magdalena basin. There a network of sub vertical late Cenozoic faults serve as efficient carrier systems for charging Eocene-Oligocene clastic reservoirs in conventional traps.
The very complex, and laterally variable, maturation history of the La Luna source rocks across the Magdalena basins place constraints on the size of a possible shale-oil resource play. Factors constraining the viability of a play include: (1) late Cretaceous-early Tertiary first-cycle thrusting and erosional stripping facilitating drainage of first cycle oils, (2) La Luna source rock that had reached high levels of thermal maturity in the first tectonic cycle and thus are capable of little additional oil generation in the late Cenozoic, and 3) inadequate post-Paleocene burial to drive still oil-capable La Luna source rock back into the oil generative window.
Virtually all commercially successful shale-oil resource plays are in source rock successions in or near ‘peak oil’ thermal maturity. The Indicators that oil, once generated, is being retained in or in proximity to the source rock succession are (1) a high oil saturation index and (2) abnormal formation pressures. These criteria have been documented in a few wells within the Middle Magdalena Basin, but data of this type are very sparse.
Regional-scale exploration for a La Luna shale-oil resource play will require collection and synthesis of information for the La Luna source rock intervals not now widely available in the public domain, including petrophysical and mechanical properties of the tight rocks, character of natural fracture systems, levels of thermal maturity and maturation histories, presence and character of potential carrier systems, oil saturation index from programmed pyrolysis, formation pressures, and reports of oil tests or shows. Much of this information is most effectively gathered from core analysis. A shale-oil resource play likely will be viable and most successful in the areas of the Magdalena basins that are least prospective for conventional oil accumulations, such as the little-deformed keels of the large footwall synclines along the eastern flank of the Middle Magdalena basin.
AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013