Cretaceous black shales in the Upper Magdalena Valley, Colombia. New organic geochemical results (part II)

This paper presents primary organic geochemical data from the Late Cretaceous succession of the Upper Magdalena Valley, Colombia. During the middle Albian through Maastrichtian, black sediments (shales, cherts) were deposited over the basin floor. High amounts of predominantly marine organic matter, especially in the Hondita and Lomagorda Formations were accumulated. The geochemical data, particularly high averages of TOC and HI, especially in the Hondita Formation (middle AlbianupperTuronian), together with the estimated sedimentation rates point toward plankton proliferation (high primary productivity) as the main driving mechanism for the origin of the organic matter. High productivity is attributable to nutrient enrichment caused by upwelling currents from the open ocean. These values diminish in the Lomagorda Formation (upper Turonian-lower Santonian) and Olini Group (lower Santonian-upper Campanian). Besides the Hondita Formation, which is by far the most valuable source rock unit, other rock intervals sampled exhibit good oil source properties, but did not achieve the necessary thermal maturity for hydrocarbon generation at the sections studied.