La Luna Formation is of crucial importance in the petroleum geology of northern South America, especially in Venezuela and Colombia. La Luna Formation has been considered for a long time the main focus of study for conventional oil production, since it is the most important source rock in the Maracaibo Basin and in Venezuela. However, recent works on this Upper Cretaceous prospective exploration target are very few; La Luna is now being considered an unconventional shale prospect (PDVSA-2012).
This study encompasses stratigraphic and geochemical characterization of La Luna Formation from one outcrop and a 345 foot long core along the northwest of Lago de Maracaibo Basin. TOC content from the core varies from 3.85 to 9.13 wt % (average 5.10 wt. %). Rock-Eval pyrolysis results highlight Type I and Type II kerogen, a "Good-to-Excellent" oil potential generation and a maturity indicator suggesting a greater likelihood of oil than gas in La Luna 1X core; similar values are represented in La Luna stratotype (Lago de Maracaibo Basin) where TOC content varies from 0.14 to 12.80 wt. % (average 3.60 wt. %).
The Delta Log R method (Passey, 1990) of TOC estimation showed a good agreement with geochemical Rock-Eval results in the core. Eight lithofacies were defined in the La Luna core. From bottom to top they are: Dark gray, laminated mudstone; Fossiliferous wackstone; volcanic ash; laminated mudstone with limestone concretions and packstone; calcareous – siliceous laminated mudstone interbedded with black chert filled with calcite veins; calcareous-siliceous mudstone interbedded with wackstone; calcareous slightly siliceous laminated black mudstone interbedded with calcareous fossiliferous wackstone; siliceous slightly calcareous green mudstone with authigenic glauconite and pyrite. Planktonic foraminifera are present in the upper part of the interval.
Based upon petrography, geochemical analysis and facies characterization, the deposits are Highstand and Transgressive system tracts. A Maximum Flooding Surface was correlated with the worldwide Cretaceous oceanic anoxic event 2, and the observed volcanic ash can be correlated with the volcanic ash deposits found in La Luna Colombia, (Villamil, 2003). These results serve as a baseline for current and future study of the La Luna Formation in Venezuela.
