DeMIS, WILLIAM D., and JEFFREY V. MILLIKEN, Marathon Oil Company, Houston, TX

ABSTRACT: Shongaloo Field: A Recent Smackover (Jurassic) Discovery in the Arkansas-Louisiana State-Line Graben

The new North Shongaloo/Red Rock/Haynesville/East Haynesville (herein called Shongaloo) field is a recent discovery (1988) with reserves of 20-30 MMBOE. The field has over 50 wells producing from the Smackover Formation at about 11,000 ft depth, and is the largest field discovered in the very mature Arkansas-Louisiana state-line Smackover play in the past 20 yr. More significantly, the field is located within the state-line graben, an area considered by industry as barren of Smackover potential because Smackover reservoir rocks were assumed absent.

Shongaloo field pay is from the Smackover "C" sand and "B" carbonate. The "C" sand is overlain by the "marine shale" (a key marker), which is overlain by shallowing-upward carbonate cycles including porous ooid grainstones of the "B" carbonate. The "C" sand has average porosity of 5-7% and average permeability of less than 1 md (average initial flow rate is 1500 MCFGD with 430 BCPD). The "B" carbonate reservoirs consist of ooid grainstones with average porosity of 14% and average permeability of 60 md, ranging up to 1800 md (average initial flow rate is 550 BOPD and 950 MCFGD). The majority of the field reserves are from the Smackover "B."

Shongaloo field is an upthrown fault trap within the overall downthrown central graben. Vertical separation on the trapping fault ranges from 500 to less than 200 ft. The trapping fault can be resolved only on modern, post-1982 seismic data.

Marathon geoscientists found Shongaloo field by overcoming two technical hurdles. First, reservoir rock distribution was shown to follow paleoshorelines through the graben. Porous ooid grainstone facies within a mappable cycle of the Smackover "B"

are parallel to, and immediately downdip (<1.5 mi) of paleoshorelines. Paleoshorelines are documented by the updip pinchout of "B" cycles into time-equivalent Buckner red beds. Second, the critical trapping fault was resolved by shooting and interpreting modern proprietary seismic data.

The discovery of Shongaloo field proves that significant reserves can be found in mature domestic plays by applying new technology and stratigraphic concepts.

AAPG Search and Discovery Article #90989©1993 GCAGS and Gulf Coast SEPM 43rd Annual Meeting, Shreveport, Louisiana, October 20-22, 1993.