--> Inorganic Geochemistry as Technique to Preliminary Assessment of Shale Plays in Cretaceous Source Rocks in the Eastern Cordillera Basin, Colombia

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Inorganic Geochemistry as Technique to Preliminary Assessment of Shale Plays in Cretaceous Source Rocks in the Eastern Cordillera Basin, Colombia

Abstract

The Colombian Cretaceous source rocks in the Eastern Cordillera Basin, have been of particular relevance to geoscientists and exploration companies in the oil industry during the last decades, because of its high conventional hydrocarbons potential. Moreover, the urgency of developing unconventional hydrocarbons in Colombia today, has increased the need for new techniques and models that not only define unconventional shale plays, but also evaluate and predict rock properties. We present here, detailed elemental geochemistry analysis using the XRF technique, on outcrop samples of the Upper Valanginian to Turonian-Coniacian succession of source rocks, including limestone and marlstones of the Rosablanca Formation, black shales of the Paja, Simití and Simijaca Formations, and finally siliceous siltstone and dark chert of the La Frontera Formation and shales with some siltstone intercalations of Conejo Formation, all deposited in a mixed clastic-carbonate ramp. Within the succession studied trace elements concentrations and paleoredox proxies were identified and then grouped in chemofacies and these, in turn, provided chemostratigraphic criteria to correlate stratigraphic sections distant from each other geographically, coupled with sedimentological descriptions. The potentially most prolific chemofacies are characterized by high enrichment factor (EF) for U, V, Mn, Zn, and Mo that outlines paleoredox conditions, and EF's for Ni, Cu which in turn, indicate areas of high amounts of organic matter preservation and are interpreted as anoxic, related closely to OAE I and II and transgressions. On the other hand, chemofacies with lower amounts redox sensitive trace elemental EF's, suggests dysoxic to oxic conditions that allowed the development of benthic fauna, which precluded conservation of organic matter. In order to establish the potentially targets within the succession, semi-quantitatively TOC values were predicted, using well-known strong correlation between Mo and TOC. Higher TOC values and EF for Ca, related to brittleness, are indicative of potential zones to produce a huge amount of unconventional hydrocarbons. Thus, the black shales of Paja, Simití and Simijaca Formations matches the main characteristics of a shale play such as deposition in anoxic environments, good preservation of organic matter, higher TOC associated with good relative fracability. Finally, this study open the way to researchers interesting in similar topics in Colombian basins.