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Comparison of the La Luna Formation Unconventional Resource Shale in Middle Magdalena Basin, Colombia and Lago de Maracaibo Basin, Venezuela


The upper Cretaceous La Luna Formation, is present along the northern part of South America and is recognized as the major oil and gas source rock in Colombia and Venezuela. It has been proposed that this formation has generated over 1.5 trillion barrels of oil. This study encompasses stratigraphic and geochemical characterization and comparison in the Middle Magdalena Basin (Colombia) and the Lago de Maracaibo Basin (Venezuela). In Colombia the formation is characterized in a 1,000ft outcrop of an upwelling shelfal zone with predominantly type II-S organic matter, and is described according to the previously subdivided members: Galembo (200 ft; calcareous shales, limestone layers, calcareous nodules and phosphate concretions; TOC 2-12%; 8.5% total porosity), Pujamana (500ft; claystone, mudstone, gray shale and cherts; TOC <2%) and Salada (300 ft; black shales, black mudstones, black calcareous claystone and concretions with pyrite; TOC 2.15-11.9%; 8.11% total porosity). In Venezuela the La Luna is characterized in a 345 ft stratotype core located along the northwest of the Maracaibo basin, with TOC 3.85 to 9.13 wt. %, Type II kerogen, and with a “Good-to-Excellent” oil generation potential. The Lower and Middle La Luna Formation in Venezuela changes from a laminated mudstone to a mudstone with recrystallized calcite and micrite lenses. The laminated packstones become more calcareous moving stratigraphically upward in the section. The Upper La Luna is composed of olive-gray, hemipelagic limestone with alternations of well-laminated layers where bioturbation and phosphates are present. Biomarker analyses for both basins reveal variations in redox and predominant marine organic matter deposited under anoxic and high water salinity conditions. The observed facies association and biomarker analysis identifies the depositional environment as shallow marine, middle carbonate shelf, in a transgressing sea. Four major third order stratigraphic cycles corresponding to La Luna Formation members are proposed and correlate for both basins. These assessments indicate a good potential for an unconventional oil and gas system, where good organic matter content is present, the formation has reached maturity levels for hydrocarbon generation and has relatively high porosity for oil and/or gas storage. A predominantly shale oil/gas system is identified in Colombia and a shale oil system in Venezuela; both have similarities to the U.S. Eagle Ford Formation.