The Norphlet Trend in the northeastern U.S. Gulf of Mexico region is one of the largest gas-producing trends in the United States. Reserve estimates are nearly 10 Tcf and current production averages 1.2 Bcf of gas per day. Several studies describe the onshore Norphlet but very little is published relative to the offshore region. By means of seismic, well logs, cuttings, and core data, this paper attempts to characterize this important gas reservoir in the OCS region of the Gulf of Mexico.

The offshore Norphlet Formation is an aeolian sandstone formed along the northern margin of the early Gulf of Mexico. Buried to depths greater than 21,000 feet, the offshore Norphlet includes wind ripple, grain flow, and rare interdunal facies. High-angle cross stratification with second- and third-order truncation surfaces characterizes the wind ripple facies, whereas grain flow facies are structureless sandstone. Interdunal facies are interpreted from low porosity zones measured by wireline logs and core data. Thickness of individual facies is highly variable, ranging from inches to tens of feet. Total formation thickness is as much as 700 feet. Seismic “pods” indicate areas of greatest thickness and best reservoir-quality rock. The pods are resultant from downbuilding into the underlying Louann Salt. In some areas thickness patterns appear to mimic original dune morphology, whereas in other areas depositional patterns are not clearly evident. Tightly cemented with silica and calcite, the uppermost offshore Norphlet has a ubiquitous tight-cap. This section of nearly 0% porosity ranges in thickness from a few feet to nearly 200 feet. Minimum porosity and permeability thresholds for productive Norphlet are about 6% porosity and 1 md permeability. Porosity (mostly secondary) and permeability generally increases with depth. Bimodal distribution of water saturation generates a funnel-shaped resistivity curve which exhibits decreasing resistivity with depth.

Structures are salt-cored anticlines bounded and cut by listric down-to-the-south faults. Many are growth faults with a greatly expanded Smackover Limestone section which overlies the Norphlet. Traps are both fault and dip closure, or a combination of both. The nonpermeable lower Smackover and the Norphlet tight cap provide seals.

The source rock is the overlying Jurassic Smackover Limestone. Gas composition is primarily methane, ethane, propane, and carbon dioxide. Solid and liquid diamonoids also occur in most reservoirs. The H2S content ranges from .001% to nearly 10%, reservoir temperatures exceed 400°F, and pressures are as high as 18,000 psi. Pyrobitumen coating of quartz grains indicates Norphlet reservoirs originally contained oil that cracked to gas as burial proceeded.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah