--> ABSTRACT: A Sedimentological and Stratigraphic Study of the Cogollo Group Limestones (Lower Cretaceous), Maracaibo Basin, Western Venezuela

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A Sedimentological and Stratigraphic Study of the Cogollo Group Limestones (Lower Cretaceous), Maracaibo Basin, Western Venezuela

Espin, Maria E.1; Jarvis, Ian 2
(1) Exploration, Pdvsa E&P, Pto La Cruz, Venezuela. (2) School of Geography, Geology and the Environment, Kingston University, London, United Kingdom.

The main objective of this study was to characterize the Cretaceous limestones of the Cogollo Group on the basis of macroscopic and microscopic facies analysis, in order to test previous depositional models developed in the area, and to develop and improve correlation of potential petroleum source rocks and reservoirs.

The methodology consisted mainly in the macroscopic description of cores from two wells A and B to define sedimentary facies, sedimentary facies associations and the depositional environment, the petrographic analysis of available thin sections from core samples, petrographic analysis through scanning electron microscopy and cathodoluminescence.

Sedimentological and stratigraphic results based on the analysis of 539 m of core from the two representative cored petroleum wells, including 209 thin sections, through the Lower Cretaceous Cogollo Group of western Venezuela (Apón, Lisure and Maraca formations) are presented. Detailed core and petrographic studies enable the recognition of fourteen sedimentary rock types, and 41 microfacies. The composition of the associated fauna (echinoderms, 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 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 and SEM/EDS/CL analysis has enabled the geochemical characterization of dolostones beds.

The Apón and Maraca formations contain intervals with good reservoir characteristic, including secondary porosity that may reach up to 10%. In addition, intercrystal pores observed in the Apón Formation show a strong potential for acting as a flow unit. The Lisure Formation does not show reservoir properties. In general, porosity values in both carbonate and siliciclastic facies are low, up to 3%. Because of the low porosity values, fractures, which are observed throughout the cores, seems to be critical to hydrocarbon production by enhancing permeability and so are an important characteristic of the Cogollo Group in the study area.

 

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.