

Could free gas production from overmature cretaceous rocks underneath the Sabana de Bogotá be sustainable?
Exploring solutions for Colombia's energy crisis
Approximately time lecture:
5 minutes
Exploring solutions for Colombia energy crisis.
Technical Aspects
Free gas is not chemically tied to the rock and is allocated in natural shale formations' natural pores, fractures, and voids. Free gas can flow through these spaces and can be extracted relatively easily. Free gas can indeed increase in highly overmature rocks. In these rocks, the organic matter that has once generated oil can further break down into natural gas. The process known as cracking increases the amount of free gas present in the formation. In overmature roks, the natural gas generated is mainly in a free state rather than being absorbed into organic matter. The cracking of any residual oil and kerogen into methane increases the volume of free gas in the pores and fractures. Overmature rocks may become more brittle, creating natural fractures that enhance permeability.
Indications of gas in global anoxic organic late aptian rocks
Paja formation: (Vitrinite reflectance Ro 3.5 %; thickness 900 feet; Total Organic Carbon up to 10%; porosity up to 36%; permeability up to 0,254 mD). A couple of stratigraphic wells, Tinjaca and Jesus Maria, drilled at shallow depths of 3,000 feet at both sides of the Chiquinquira syncline by Nexen in 2014, show high gas readings at this global anoxic organic interval (sweet spot) producing gas flares at the surface. It is proposed to drill a conventional well at 9,000 feet at the core of the Chiquinquira syncline at the Saboya location to reach the best compressibility spot for free gas production.
Estimated prospective resources
For the entire Eastern Cordillera of Colombia, a precise estimate of 117.6 TCFs of recoverable prospective resources has been made by the ANH 2020. This precision extends to the high overmature stratigraphic units Fomeque-Paja or equivalents, where the presence of high free gas on sweet spots leads to an estimated recoverable prospective resources of 96 TCFs (80% free gas; 20% absorbed gas).
Suggested production strategy
Overmature rocks could provide high initial gas production rates due to the large free gas volume in the global anoxic organic intervals. However, the production may decline as the easily accessible free gas is depleted. To keep output continuous, it is recommended that the production be navigated along the global anoxic organic intervals with horizontal wells.
Conclusions
Significantly free gas contained in Global Anoxic Organic intervals (sweet spots) that have suffered high thermal maturity are present underneath the Sabana de Bogotá area. Porosity and permeability are enhanced due to the thermal maturity conditions that help to improve free gas accumulation. The overmature Fomeque and Paja Formations Late Aptian in age have shown important free gas indications. Recoverable prospective resources on these stratigraphic units are estimated in 96 TCFs (80 % free gas; 20 % absorbed gas). It is recommended that these formations be reached at the core of regional synclines to get the best compressibility spot for free gas production. It is also recommended to navigate along these Global Anoxic Organic intervals (sweet spots) through horizontal wells.