A sedimentological and stratigraphic study of the Cogollo Group limestones (Lower Cretaceous), Maracaibo Basin, Western Venezuela

The sedimentology and stratigraphy of two representative cored petroleum wells through the Lower Cretaceaous Cogollo Group of western Venezuela (Apón, Lisure and Maraca formations) has been studied. Results based on the analysis of 539 m of core from wells A and B, including 209 thin sections, and 274 geochemical analyses are presented. Detailed core and petrographic studies enable the recognition of fourteen sedimentary rock types, and 41 microfacies. The composition of the associated fauna (echonoderms, molluscs [rudists, oysters, other bivalves], calcareous green algae, bryozoans), and non-skeletal grains [intraclasts, peloids and ooids], the sedimentological parameters, and the analysis of facies associations have been used to interpret the depositional environment, and to define nine sedimentary units that can be correlated throughout the Maracaibo Basin. The Cogollo Group is interpreted as having been deposited on an open carbonate platform characterized by shallow-marine depositional environments. Petrographic analyses indicate that the succession has undergone a complex diagenetic history that included compaction, cementation, neomorphism, micritization, dissolution, silification, dolomitization, fracturing, controlled by marine, meteoric and burial events. Elemental geochemistry (major and trace elements) and carbon-isotope analyses were undertaken for the Cogollo Group succession. The chemostratigraphic study has provided important information about the lithogeochemical characteristics of the units, quantitative information on the amount of carbonate and siliciclastics through the succession, and the location and extent of dolomitization. Carbon isotope ([delta][sup]13C) values in both inorganic ([delta][sup]13C[sub]carb) and organic ([delta][sup]13C[sub]org) carbonate show consistent stratigraphic trends that can be correlated with coeval sections elsewhere. The [delta][sup]13C curves show similarity with global trends, permitting the identification of OAE1a. The identification of OAE1a in the succession is support by the presence of high TOC calues in a clay-rich facies of the Apón Formation.