Outcrop-subsurface geological characterization of La Luna Formation as an unconventional resource in the Northwest Lago de Maracaibo Basin and North Andean Flank, Venezuela /

Abstract

In recent years, unconventional shale reservoirs have become very important around the world for their high source of energy and for their high economic value. 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 shale as a 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 five outcrops and a 345 foot long core along the North Andean flank and the northwest of Lago de Maracaibo Basin, Venezuela. TOC content from the core varies from 3.85 to 9.13 wt% (average 5.10 wt. %). Rock-Eval pyrolysis results indicate 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 lX 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. %) . In the North Andean Flank, TOC content is lower, varying from 0.46 to 5.64 wt% (average 2.37 wt. %); kerogen is Type III, indicating a potential generation of dry gas instead of oil in the subsurface near the outcrops. 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: A) Dark gray, laminated mudstone. B) Fossiliferous wackstone. C) Volcanic ash; laminated mudstone with limestone concretions and packstone. D) Calcareous - siliceous laminated mudstone interbedded with black chert filled with calcite veins. E) Calcareous-siliceous mudstone interbedded with wackestone. F) Calcareous slightly siliceous laminated black mudstone interbedded with calcareous fossiliferous wackestone. G) 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 facies comprise a third order sequence. Highstand and Transgressive Systems Tracts. A Maximum Flooding Surface was correlated with the worldwide Cretaceous oceamcnoxic event 2 and also with the volcanic ash found in La Luna Colombia,Three target intervals were identified in La Luna IX well. The first interval from bottom to top has a thickness of 14.32 m. and a BI of 0.85. The second interval has a thickness of 7.60m and has a BI of 0.99. The third interval has a thickness of 18 m and has an average BI of 0.93. These results serve as a baseline for current and future study of the La Luna Formation in Venezuela.