ABSTRACT: Model for Unstable Shelf Data Sequence, Example from a Giant South Louisiana Discovery--South Lake Arthur Field
Ram S. Saxena
In onshore southern Louisiana, exploration emphasis has now shifted to deeper plays in interdomal areas as the majority of the domes have been discovered. Unstable shelf deltas develop in such areas. Several discoveries have been made during the past few years. One giant discovery is South Lake Arthur field, which ideally fits the unstable shelf delta model.
Unstable shelf deltas are deep buried structures of very large magnitude in which thick reservoir sandstones are deposited on extensive organic-rich prodelta shales (excellent source rocks). Deposition of reservoir sandstones generates the trapping structures--growth fault and rollover anticline. The sequence is sealed on top by large thicknesses of shales rich in marine organisms.
Shelf delta sequences build in localized basins on the part of the shelf that gives actively through combined effects of subsidence, shale compaction, and shale flowage. Sequences develop in stratigraphic units where a strong regressive episode of deposition is immediately followed by a large marine transgression.
South Lake Arthur field is a giant gas field discovery with potential reserves in excess of 1 tcf gas. More than 25 wells have been drilled in the field and both the log and the seismic data display the typical unstable shelf delta sequence--400- to 800-ft-thick reservoir sandstones of Oligocene age--sandwiched between greater than 2000 ft of prodelta shales at the bottom and 4000 ft or more of transgressive marine shales at the top. Production is between 17,000 to 18,000 ft. The field is bounded by an arcuate down to the basin fault on the north and is surrounded by shale and salt diapirs.
Productive Miogyp sands were deposited in three separate depositional episodes. Lower and middle Miogyp members are thick in the middle part of the field and are dominantly of distributary mouth bar origin--rapidly deposited, poorly sorted, and very shaly. The upper Miogyp sands were developed by the reworking of older Miogyp delta sands. They are clean, better sorted, and elongated along depositional strike.
Lack of knowledge of the unstable shelf delta model combined with structurally emphasized interpretation hindered the recognition, proper development, and accurate mapping of this field. Maps made without the depositional model erroneously used faults to explain variations of interval thicknesses, shale out of sands, and anomalies of gas/water levels. More than 200 faults were mapped in the field creating a map that looked like a bowl of fettucini. By using the unstable shelf delta model and the understanding of the depositional sequences, all variations of data could be mapped as a simple east-west-trending anticlinal structure with a bounding arcuate fault on the north and no other internal faulting. Similar other features exist, not only on the South Louisiana shelf but in deltaic asins throughout the world.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990