SAXENA, RAM S., Geo Consultants Intl., Inc., Kenner, LA

ABSTRACT: South Lake Arthur Field: A Classic Unstable Shelf Delta

South Lake Arthur Field is a giant gas field discovery with potential reserves in excess of a trillion cubic feet of 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 "Miogyp" (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 (excellent source and seal rocks). Production is between the depths of 16,000 ft 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 all around it.

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 are elongated along depositional strike.

Unstable shelf deltas are deeply buried structures of very large magnitude in which thick reservoir sandstones have been 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.

In the unstable shelf delta sequences, rapid deposition of sands on large prodelta shale platforms triggered a series of chain reactions that are self-sustaining and supportive. These include: initiation of growth fault, development of rollover anticline, geopressuring of sediments, shale movement, and diapiric activity around the sand depocenter and finally the abandonment and bypassing of the area by major distributary channels initiating marine transgression, reworking of older deltaic sands, and deposition of thick marine shales on top of the sequence.

Lack of knowledge of the unstable shelf delta model combined with structurally emphasized interpretation hindered the recognition, proper development, and accurate mapping of South Lake Arthur 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. In excess of 200 faults were mapped in the field with a map looking like a bowl of fettucini. Utilizing 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 structure with a bounding arcuate fault on the north and no other internal faulting. Other similar features exist, not only on the South Louisiana shelf, but in deltaic b sins all around the world.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.