Abstract: Organic Carbon and Phosphate Distributions in the La Luna Formation, Western Venezuela

P. Stella Stoufer, Wolfgang Scherer

The Upper Cretaceous La Luna Formation in Western Venezuela is believed to be the major source rock in the Maracaibo basin. In order to obtain an approximation of the total amount of hydrocarbons that could have possibly been generated in this basin, the vertical and areal distribution of total organic carbon (TOC) was studied. Since important phosphate deposits were found in the uppermost portion of the La Luna Formation, their distribution was also determined on a regional scale.

A total of 263 samples corresponding to 19 surface sections and 9 wells were collected and analyzed with geochemical methods including LECO carbon analyzer, Rockeval and X-Ray fluorescence. The vertical distribution was represented for all sections and wells. For mapping purposes of TOC and P₂O₅ only the upper part of the formation was considered.

It was determined, in general, that TOC increases towards the top of the formation; geographically the highest concentrations were found in the central and western part of the Maracaibo basin, close to the Alturitas-Catatumbo area. Higher TOC concentrations correspond to finely laminated limestones and calcareous shales, where bioclasts are more frequent.

Phosphates are found only in the uppermost 8-20 m of the stratigraphic section; geographically it is concentrated in northern Tachira and in the central Maracaibo basin. Phosphate is mostly of primary origin, concentrated in massive limestones, glauconitic sandstones and argillaceous rocks. Phosphate mineralogy includes collophane, wavellite and apatite oolites, diagenetic fossil replacements and authigenic precipitates.

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