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Abstract: Geology and Geochemistry of the La Luna Formation Type Sections in the Maracaibo Basin, Venezuela

Noelia Baptista, Wolfgang Scherer

The Upper Cretaceous La Luna Formation is the most important source rock of hydrocarbons in Western Venezuela. Curiously enough it has two type sections, the formation was first defined in 1926 in Perija on the western flank of the Maracaibo basin; 30 years later the La Aguada, Chejende and Timbetes members were defined 260 km toward the east, on the shelf of the basin. The purpose of this study is to compare these sections and to define the vertical and horizontal variability of geological and geochemical characteristics that might have influenced the generation of hydrocarbons.

The study consisted of detailed, bed level sampling, macroscopic sedimentary descriptions, petrography of 168 thin sections with 40 variables recorded in a statistical data matrix for determination of lithomicrofacies, as well as geochemical analysis of total organic carbon (TOC), visual kerogen, Rock-Eval pyrolysis and gas chromatography.

The western type section is characterized by alternating thinly laminated and massive bedded limestones. Thermally immature, amorphous organic matter of marine origin is abundant in this section; TOC values range from 0.23% to 8.56%, generally increasing toward the top. Hydrogen index values range from 327 to 1078, indicating good to excellent oil generating potential. The eastern type sections have a higher level of thermal maturity; they show increasing amounts of clastic material, less authigenic minerals and abundant Favreina sp crab fecal pellets. The mainly terrestrially derived organic matter concentrations are considerably less, ranging from 0.07 to 3.39, again increasing toward the top of the section.

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela