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240 PETROLEUM GEOLOGY/Overview Figure 14 El Furrial Trend, eastern Venezuela The petroleum traps are large rollover anticlines on the hanging walls of thrusts Most are four way, dip closed structures, whilst some have a dependence on a fault component along their south eastern thrust margin Reproduced from Gluyas JG and Swarbrick RE (2003) Petroleum Geoscience Oxford: Blackwell Science low-angle, basinward-dipping planes The drive mechanism is provided by the weight of sediment deposited by the delta at the shelf–slope break or on the slope itself In the Niger Delta, the detachment planes are highly mobile muds, whereas, in the Gulf Coast (Mississippi Delta), detachment occurs on both muds and the Louanne Salt (Jurassic) The key detachment surfaces are commonly listric, concaveup, and concave-basinward in plan view The main faults are commonly large, being tens of kilometres tip to tip Stratigraphical Trap From top to bottom of a systems tract, each depositional environment is capable of producing a juxtaposition of permeable and impermeable sediments which might one day form a stratigraphical trap for petroleum In practice, the reservoir geometry becomes the trap geometry Examples include aeolian dunes encased in lacustrine mudstone, sand-filled fluvial channels cut into mud-rich overbank deposits, shallow marine bar sandstones surrounded by marine shales, carbonate reefs isolated by enclosing marls, and submarine fan sands trapped within the domain of pelagic mud The Paradox Basin (Colorado and Utah, USA) contains a large array of small oil and gas fields in stratigraphical pinchout traps Devonian reservoirs occur within shallow marine bar sandstones and Carboniferous reservoirs within carbonate mounds The Paradox Basin traps are difficult to find, but have relatively simple shapes Their geometries are either prolate bar forms or more equidimensional carbonate mounds Pinchout traps formed in deltaic settings are often much more complex in outline and, because
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