Stratigraphy and Sedimentation of Cretaceous Fine-Grained Clastic and Carbonate Deposits: Maracaibo Basin, Venezuela
Donald A. Goddard, Suhas C. Talukdar, 2003. "Stratigraphy and Sedimentation of Cretaceous Fine-Grained Clastic and Carbonate Deposits: Maracaibo Basin, Venezuela", Siltstones, Mudstones and Shales: Depositional Processes and Characteristics, Erik D. Scott, Arnold H. Bouma, William R. Bryant
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The Cretaceous interval in the Maracaibo consists mainly of fine-grained clastic and carbonate sediments deposited during a period of passive margin development. Stratigraphically, the early Cretaceous Apon Formation of Aptian age contains interbedded rich organic dolomitic and calcareous shale and black bituminous limestone (Mercedes, Tibu, Guaimaros and Machiques Members). Not only do these constitute important source and reservoir rocks in the southwest region of the Maracaibo Basin, but also represent maximum flooding surfaces within the overall depositional characteristics in the Basin. To the north, the Apon along with the Lisure and Maraca Formations of the Aptian-Albian Cogollo Group, consisting mostly of limestone and fine-grained sandstone, represent a shallow to middle shelf environment transgressive systems tract.
In the south, the Cretaceous Capacho, consisting of black shale and the La Luna Formation comprised of interbedded calcareous shale and black cherty limestone, were deposited during a period of relative sea level rise. The La Luna Formation is the primary source rock in the basin as well as a reservoir in places where microfractures produce the necessary permeability for hydrocarbon flow. Numerous geochemical studies performed in the Maracaibo Basin provide evidence for the existence of multiple stratigraphic intervals containing hydrocarbon generating organic-rich shale and calcareous mudstone within the Cretaceous. Integrated reservoir studies and production data indicate that some of the same intervals are excellent oil and gas producers where fracture systems exist. The above-mentioned stratigrahic and depositional characteristics are responsible for making the Maracaibo Basin one of the most prolific producers of hydrocarbons in the world.
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Siltstones, Mudstones and Shales: Depositional Processes and Characteristics
Siltstones, mudstones and shales have been studied mainly with regard to clay mineralogy and general transport-deposition. Recent studies on deepwater deposits from cores and outcrops have shown that fluid flow properties of deepwater reservoirs are greatly affected by the presence of finer-grained deposits in the reservoir. Initial analysis indicates that the majority of these finer grained deposits have a large silt component and are closer to siltstones rather than mudstones, commonly called shales To date, little attention has been given to their characteristics resulting from different depositional processes.