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The lower Miocene (Oligocene?) Planulina interval of South Louisiana is a sequence of interbedded sandstones and deep water shales beneath the Siphonina
davisi zone and above the Abbeville facies of the Anahuac Formation. The Planulina interval is present in the subsurface along a narrow band extending from Lake Verret in Assumption Parish westward through Cameron, Louisiana, into coastal Texas.
The first discovery of gas in Planulina sandstones was made in 1945 by Magnolia Petroleum Company in Mud Lake field, Cameron Parish, Louisiana. By 1963, after 18 years of exploratory effort, there were only 4 significant Planulina fields. Operations were hampered by elusive structures, erratic sands, extreme correlation problems, high pressures, high drilling costs, inadequate seismic resolution, and a general lack of understanding of the geologic setting. During the past 8 years a sharp increase in success has changed a "bad" trend into one with promise of substantial new gas reserves. Modern drilling technology and CDP seismic techniques were responsible for this success, and the additional control has resulted in a better understanding of the geology.
Planulina sands are believed to have been deposited along the outer edge of a narrow continental shelf. Marine transgression in "late Planulina time" shifted the axis of deposition northward. Consequently, the next younger cycle of deposition and associated growth faulting lies north of, and updip from the Planulina trend. Because of this shift in the axis of sedimentary loading, growth of many Planulina structures ceased in "late Planulina time" and were buried by transgressive shale.
Typical Planulina structures are anticlines and northward-plunging, faulted noses buried beneath south dipping sediments. Where younger beds are also productive their structural crest generally is well removed from the apex of the Planulina structure. Planulina structural crests are commonly, though not exclusively, north of the shallow structure or in the upthrown fault block. Some salt domes and high relief uplifts have Planulina sandstone pinchouts on the north flanks.
Within the Planulina trend stratigraphy plays a vital role in hydrocarbon entrapment and modified stratigraphic traps are common.
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