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Sedimentological analysis of cores taken in the Cerro Negro area of the Orinoco Tar Sands have provided lithological, textural, mineralogical, and paleontological data which have enabled facies identification. These facies have been interpreted as belonging from an upper delta plain to a semi-restricted shallow marine environment of deposition. Coal beds occur in all the wells analyzed. These occurrences are closely associated with specific facies, such as the back-barrier lagoonal facies of the semi-restricted environment of deposition and the backswamp facies of the delta plain. More importantly these coal beds present very distinguishable characteristics, such as variation in thickness from few inches in the back-barrier facies to 5 ft (1.5 m) in the delta plain facies Their organic composition also ranges from micrite/atrinite to vitrinite and exinite in different proportions. Therefore, it is believed that variations in thickness and perhaps in organic composition are related to facies changes.
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