_____________________________________________________________________________________ 7 th – 9 th September 2000, Caracas, Venezuela SEPM Research Conference Paleogeography and Hydrocarbon Potential of the La Luna Formation and Related Cretaceous Anoxic Systems Latinoamericana Diagenetic Evaluation of La Luna Formation at Tachira and Merida States, Western Venezuela Egleé Zapata 1 ; María A. Lorente 2 , Olga Rey 1 ; Víctor Padrón 1 1 Universidad Central de Venezuela, Faculty of Engineering, School of Geology, Mines and Geophysics. 2 PDVSA Exploration and Production 1.- Introduction La Luna Formation, from the Late Cretaceous, has been considered the most important hydrocarbon source rock in western Venezuela and possible one of the most productive of the world. Even it importance, their biostratigraphy, diagenesis and sedimentological characterization are not well known. This deposits are represented by marine sediments of fine grain: “black-shale”, composed essentially of clay and carbonatic mud, with an abundant fossil fauna of foraminifera in association with phosphate, chert, pyrite and glauconite. Some fish bones and mollusk of thin walls remains, enrich the fossil assemblage. With the purpose of evaluate diagenetically this formation, two surface sections were chosen in Merida and Tachira states. This sections are almost complete and not affected tectonically (Fig. 1). By detailed sampling (every 50 cm.), a total of two hundred and five (205) samples where collected, for the elaboration of petrography sections and 102 samples for electronic microscope studies. Petrographycally, lithotypes with muddy texture were identified, classified according to Dunham (1962) as packstones, wackestones, and mudstones and in a minor proportion grainstones. The electronic microscopy evaluation allows to precise the composition relationships of the rocks. The whole study shows early diagenetic process of micritization, phosphatization, pyritization, cementation, etc.; and compaction effects by burial diagenesis. 2.- Lithostratigraphy: La Luna Formation, in the studied sections, is characterized by the presence of black calcareous shale interbedded with black limestones and in a minor proportion, thin layers of chert. Two intervals can be recognized: the lower one is characterized by a very compacted black calcareous shale with concretions of a variable size ( from 0.03 to 1.50 m) and very thin chert levels, the limestones levels are very rare and thin. The upper interval is characterized by limestones interbedded with black shales, concretions and chert levels are scarce. The top of the formation is characterized by the presence of a phosphate and glauconite level recognized as Tres Esquinas Member. 3.- Petrography and diagenesis The samples and petrographic sections were described with the purpose of compile the greater amount of diagenetic evidence that can be easily seen such as concretions (with diversity in size, form and distribution), highly compaction grade, fracturing (the presence of microfractures filled and open) without presence of stylolites, low primary porosity and secondary porosity by dissolution and mineralization (presence of phosphates, glauconite, pyrite, etc.) By microscopic studies, a lithologic assemblage of muddy texture can be recognized, represented by a fine fraction of abundant terrigenous clay and carbonate mud, in which the grains are usually distributed with loose contacts, becoming occasionally punctual contacts . The coarsest granular component is represented by planktic and benthic foraminifera assemblage, distributed with variable abundance and diversity throughout the sections in proportions that varies between 10% and 35%. Other components of the rocks are represented by small percentages of fishbone and mollusk remains (less than 15%). Phosphates, pyrite, silicates, sulphates and organic matter, are important components in low proportion.