ABSTRACT

The lower Miocene (Oligocene?) "Planulina Formation" (FOOTNOTE 2) of South Louisiana is a sequence of interbedded sands and deep water shales beneath the Siphonina davisi zone and above the Abbeville facies of the Anahuac Formation. The "Planulina Formation", as presently known, occurs 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 sands 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 four significant Planulina fields. Exploration was 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 last eight years a sharp increase in success ratio 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 probably were deposited along the outer edge of a narrow continental shelf. During deposition of upper Planulina beds, a marine transgression shifted the axis of deposition northward. Consequently, the next younger cycle of deposition and associated

FOOTNOTE 2. "Planulina Formation" as defined in this report is inconsistent with standard stratigraphic procedure because the name lacks a geographic component. Goheen (1959) attempted to solve the problem in the conventional manner. He recognized the Planulina Zone ("Planulina Formation" of this report) and the Abbeville Zone (Abbeville facies of this report) as downdip equivalents of the Anahuac Formation, and named this sequence the "Erath member". Goheen (1959, p. 91) states "The term Erath member is proposed for the so-called deep water facies of the downdip Anahuac Formation, which is regarded as upper Oligocene or lower Miocene. The middle neritic Planulina and outer neritic to bathyal Abbeville are considered here as biostratigraphic zones within the Erath member."

The present writer considers only the Abbeville and basal Planulina to be Anahuac equivalents. Most of the "Planulina Formation" is equivalent to the basal Siphonina davisi sand, the Liebusella shale, and the Mulvey sand, all of which overlie the Anahuac. The problem of using the term Planulina zone is more serious. Planulina palmarae van Bellen, after which the Planulina zone is named, is only locally useful for correlations. Downdip, specimens of P. palmarae have been observed above the first observed Siphonia davisi (J. M. Crosbie, personal communication). Hence the use of the name Planulina zone as defined by the American Commission on Stratigraphic Nomenclature would in some areas include all of the Siphonina davisi zone. (Most biostratigraphic zones in South Louisiana are beset with similar problems. In practice, the term "zone" means something quite different to most Louisiana stratigraphers than that spelled out in the Code of Stratigraphic Nomenclature.)

Gulf Coast geologists generally use the name Planulina in a rock stratigraphic sense. Thus, the Planulina is considered to be a sequence of sediments that has a characteristic lithology, is prevailingly tabular, and is (somewhat) traceable in the subsurface. For this reason, the name "Planulina Formation" is introduced. Planulina will be used unitalicized by the writer to designate a rock stratigraphic unit. When the term Planulina is italicized it will refer to either the Foraminifera or the zone terminology of other authors.

Goheen (1959, p. 94-95) had additional difficulty that this writer hopes to avoid. He designated a well in Tigre Lagoon Field as the type section for the Planulina zone and a reference well in Erath Field as a supplementary section. The contact of the Planulina zone with the Abbeville zone was indicated in both wells. In both areas, subsequent drilling discovered numerous deeper sands. Today all these lower sands are considered to be part of the Planulina by some paleontologists (W. J. Rudick, 1971, personal communication) and the Abbeville is picked much lower than it is on Goheen's type section. Goheen's Abbeville is now considered to be Planulina II (W. J. Rudick, 1971, personal communication). Goheen's type section by definition cannot be wrong, so it has been ignored.

The present writer is proposing no type section, hence no type locality, and no futile attempt will be made to popularize a geographic name. "Planulina Formation" is no more unorthdox than "Old Red Sandstone" and just as firmly entrenched in a somewhat more provincial literature. The unorthodox approach taken here is the best, if not the only solution to the communication problem between geologists and paleontologists.

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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 age structures ceased and they were buried by a transgressive shale.

Typical Planulina structures are anticlines and northward plunging, faulted noses buried beneath south dipping sediments. Where the younger beds are also productive their structural crests usually do not directly overlie the apex of the Planulina structure. Planulina structural crests are commonly, though not exclusively, north of the shallow structure or in an upthrown fault block. Some salt domes and high relief uplifts have Planulina sand pinchouts on the north flanks.

Within the Planulina trend stratigraphy plays a vital role in hydrocarbon entrapment and modified stratigraphic traps are common.