In August of 1989, the Exxon No. 1 Sweet Lake flowed at a rate of 21.9 MMCF per day with 384 bbl of condensate per day in initial production tests, confirming a significant discovery in the Miogypsinoides ("Miogyp") sands of the Oligocene Frio Formation of southwestern Louisiana. The discovery well, together with subsequent development wells, have delineated a deeper productive interval associated with the estimated 400 BCF Chalkley Field, discovered in 1938. This new deep pool may contain reserves of approximately 300 BCF and could prove to be among the largest onshore Gulf of Mexico discoveries of the 1980s.

The Chalkley Field Deep Pool is located within the Miogyp Embayment of southwestern Louisiana, a low-angle normal fault-bounded reentrant into the late Frio shelf that acted as a locus for lowstand fluvio-deltaic deposition. This new production extends the play by approximately 20 miles to the west of the trend established by the discovery of Miogyp production in the Southeast Gueydan Field in 1966 and the South Lake Arthur Field in 1979. Several deep tests in the Chalkley arm, dating from 1956, had previously encountered the Miogyp interval without success. Regional stratigraphic correlations within the Embayment, however, indicate that the newly productive interval at Chalkley is, indeed, both the temporal and facies equivalent to established Miogyp reservoirs to the east.

Interest in the deep Miogyp (14,000 to 20,000 feet) trend has particularly increased over the last several years, fueled not only by the South Lake Arthur discovery, but also by the numerous speculative seismic surveys which have sparked a fast-paced and competitive leasing effort for the large Miogyp structures. Exploration in the Miogyp Embayment is characteristic of many mature basins where new, deeper resolution seismic data is only now imaging high-potential, deeply buried structures that had previously gone unrecognized.

Modern geophysical applications were particularly critical in the early recognition of deeper potential in the Chalkley area. Improved seismic methods, most notably DMO processing, resulted in the first adequate resolution of buried structures beneath existing Miocene production. Additionally, the early recognition of a velocity gradient across the Chalkley area, below the top of abnormal pressure, allowed the development of a refined time-depth conversion which resulted in structural closure being recognized updip of previous wet Miogyp tests.

The Chalkley Deep trap consists of a buried, rollover anticline lowside to the listric, Miogyp Embayment Fault. Trap closure at the Miogyp interval has been further enhanced by syn- and post-depositional salt movement. The approximately 800 feet of hydrocarbon column height at Chalkley Deep can be attributed to a combination of both fault-independent and fault-dependent closure.

The overall pay interval at Chalkley Deep consists of up to 900 feet of Miogypsinoides fluvial and shallow marine sandstones, and associated siltstones and shales. Effective reservoir sands have porosities ranging from 14 to 22 percent, permeabilities from 80 to 150 millidarcies, and may exceed 300 feet (net) across the pay interval. Detailed physical correlation and sequence stratigraphic modeling suggests that the gross reservoir interval may be locally reduced as a result of both lateral facies changes in fluvial and marine sequences, as well as by the erosional downcutting of fluvial facies into underlying reservoir units.

The further delineation of the Chalkley Field Deep Pool is ongoing. Two successful development wells, the Exxon No. 2 and No. 3 Sweet Lake, have been drilled to date. Additional development wells are planned, at least one of which will test the deeper stratigraphic interval beneath known Miogyp pay.

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