Evidence for Precambrian Stratigraphy in Graben Basins Below the Eastern Llanos Foreland, Colombia

Arminio, Juan F.¹; Yoris, Franklin; Quijada, Carlos; Lugo, Jairo M.; Shaw, Dave; Keegan, James; and Marshall, John
¹[email protected]

Deep and extensive graben basins are preserved below the South American sub-andean foreland basins. These features are the subject of frontier exploration in Colombia and Venezuela. Indeed, "traditional" exploration models were based on the superposition of two Wilson Cycles that considered one or more pre-Cretaceous petroleum systems within pre-Cretaceous grabens. Recently, exploration drilling and new seismic interpretation in the Eastern Llanos Basin of Colombia has yielded evidence for thick sedimentary, non-metamorphosed and well preserved Precambrian (Neoproterozoic) successions that may have filled pre-Paleozoic grabens in at least four locations in the Meta region. These grabens were reactivated and/or continued their uppermost fill-up until Cambrian times; Late Paleozoic-Mesozoic graben/sag/impactogen reactivations are not discounted, but they are difficult to detect because the entire region is still poorly drilled with deep exploration wells. Biostratigraphic findings include well preserved examples of acritarchs of the genera Kildinosphaera and Leiosphaeridia asperata, recovered in a section of compact shales and sands as deep as 1000 feet below the Eastern Llanos foredeep uncomformity. Chronostratigraphic ages range from Early Cambrian to Ediacaran and Mid-Cryogenian, with color indexes in the range of 3+ to 4- and reflectance results showing that the Neoproterozoic samples are in the upper gas generation window. The evidence collected provides an unexpected window into the early expressions of life on the planet.

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013