SAXENA, RAM S., DAVID C. MILLER, and TINKA SAXENA, Geo Consultants Intl., Inc., Kenner, LA

ABSTRACT: DMB Depositional Model Leads to a Significant South Louisiana Discovery

Distributary mouth bars (DMBs) are the most prolific deltaic sandstone reservoirs. DMBs build at the mouth of distributary channels, in areas of extremely dynamic sedimentation. Rapid deposition of large quantities of sand at this site over a platform of highly unstable, water-logged, plastic prodelta shales triggered a sequence of geologic events which juxtaposed good quality DMB sands between good source and seal rocks (marine and prodelta shales) on contemporaneously developed trapping structures (rollover anticlines downthrown to the growth fault). The order of events (which are for the most part penecontemporaneous) in the DMB sequence development were as follows: (1) Deposition of DMB sands over prodelta shales. (2) Initiation and growth of a CD fault on the updip side of the DM depopod. (3) Tilting of beds on the downthrown side of the fault and development of a rollover anticline producing the critical north dip. (The crest of the rollover anticline generally coincides with the middle part of the DMB.) (4) Extensive diapirism of prodelta shales around the sand depopod (an adjustment to the imbalance caused by sediment loading), generating upthrown shale ridges behind the fault, north of the DMB depopod, and frontal shale ridges basinward of the DMB sands. Often the shales upheave in the core of the rollover anticline; as such, the sands on the crest of the rollover became thin. (5) Geopressuring of sediments as the fault seals the migration path of waters. And, (6) finally, in its abandonment stage, capping of the whole system with thick transgressive marine hales, creating a very attractive package for oil and gas generation and trapping.

Two aspects of the DMB model are critical for successful exploration in these prolific depositional sequences: (1) recognition of the north dip, especially important is the differentiation of this dip from the overlying flat fill of marine shale beds and from the multiples of south-dipping shallow events and (2) the relationship of the upthrown shale ridge to the DMB sand depopod (which is also the crest of the rollover anticline). Without understanding this, the pure structural "datum chasing" approach results in a map with the highest point being mapped on the upthrown shale ridge -- "a definite dry hole."

In an area of closely gridded, good quality seismic data, an earlier interpretation without the depositional concept resulted in several deep dry holes. Remapping the same data with the help of the depositional model resulted in a trend play of nine DMB prospects with an acreage position of more than 25,000 acres. The first of these DMB prospects (DMB 7) was drilled in August of 1992. The well logged three gas sands, one oil sand, and three other sands which could potentially be productive. The lowermost of these sands, which is in the process of being hooked up now, tested 1.6 MMCF/day with 60 bbls. of condensate at 6/64th choke from perforations at 13705-13710.5 with a flowing tubing pressure of 9500 psig and a shut-in tubing pressure of 9541 psig.

This significant discovery is the first of a trend of nine DMB prospects and is a direct result of the application of the DMB depositional concept in the mapping of the log and seismic data.

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