(Tech Connections Column, June 2007, American Oil and Gas Reporter)

Exploration remains a focus in the Central and Eastern Gulf of Mexico. Reporting on work performed by the University of Alabama and Louisiana State University in a project funded by the Department of Energy’s National Energy Technology Laboratory, PTTC’s Central and Eastern Gulf regions outlined their assessments of the deep gas resource in the North Louisiana and Mississippi Interior salt basins in a workshop in Shreveport, La.

The bottom line message is that the deep gas potential is significant. Such conclusions are based on a three-year study that includes a thorough assessment of the thermogenic, secondary (nonassociated) gas resource in deeply buried (greater than 15,000 feet) natural gas reservoirs of the onshore interior salt basins of the north-central and northeastern Gulf of Mexico areas through an integrated petroleum system and resource assessment methodology. Using this methodology allows the researchers to estimate the volume of the deep gas resource available for potential recovery, and to identify those areas in the onshore interior salt basins with high potential as well.

State-of-the-art geochemical techniques for analyzing source rocks, together with petroleum system characterization and modeling –including thermal maturation and hydrocarbon expulsion modeling–have shown that the Upper Jurassic Smackover formation served as the regional petroleum source rock in the onshore interior salt basins. The estimates of the total hydrocarbons generated and expelled are based on the assumption that the Smackover formation is the main petroleum source rock in these basins.

Using two calculation methods (Schmoker and a software application), researchers estimate total hydrocarbons generated for the North Louisiana Salt Basin at 2.870 trillion and 2.640 trillion barrels, respectively. Previous estimates ranged from 2.000 trillion to 2.500 trillion barrels. The estimate of gas generated for this basin has a range from 6.400 quadrillion cubic feet (Qcf) to 12.800 Qcf, depending on the calculation method applied. The estimates of the total hydrocarbons generated for the Mississippi Interior Salt Basin were 910 billion and 1.540 trillion barrels, while the estimates of gas generated for this basin were 3.130 Qcf and 4.050 Qcf.

Most of the gas is estimated to be from late cracking of oil in the source rock. Potentially, the secondary gas produced in these basins results from cracking of oil to gas in deeply buried reservoirs. The efficiency of expulsion, migration and trapping has been estimated to range from 0.5 to 10.0 percent for various basins.

The areas in the North Louisiana and Mississippi Interior salt basins with high potential for deeply buried gas reservoirs have been identified. In the North Louisiana Salt Basin, the area includes several central parishes from Upper Jurassic and Lower Cretaceous facies, especially the Smackover, Cotton Valley, Hosston, and Sligo units. The estimate of the gas generated from cracking of oil in the source rock from depths below 12,000 feet in this basin is 4.800 Qcf. Assuming an expulsion, migration and trapping efficiency of 1-5 percent, 48 Qcf-240 Qcf of gas is potentially available.

In the Mississippi Interior Salt Basin, the potential area for deeply buried gas reservoirs includes several southern counties from Upper Jurassic and Lower Cretaceous facies, especially the Norphlet, Smackover, Haynesville, Cotton Valley, Hosston, and Sligo units. The estimate of the gas generated from cracking of oil in the source rock from depths below 16,500 feet in this basin is 2.350 Qcf. Assuming an efficiency of 1-5 percent, 23.5 Qcf-117.5 Qcf of gas is potentially available.

The final recoverable gas is some percent of this estimated secondary gas resource based on the recovery factor for the specific reservoir. To date, 29 trillion cubic feet of gas have been produced from the North Louisiana Salt Basin and 13 Tcf of gas have been produced from the Mississippi Interior Salt Basin. Also, the secondary gas, whether generated from late secondary cracking of oil to gas in the source rock or from oil to gas conversion in deeply buried reservoirs, migrates updip and vertically into shallower buried reservoirs, including the Monroe and Jackson gas rocks at depths of some 2,000 feet.

Baker Hughes rotary rig statistics for the week of May 4 showed three rigs drilling in Alabama, 16 in Mississippi, and 60 in North Louisiana. Some are undoubtedly targeting deep gas.

Unconventional shale gas plays are another regional interest. Companies are learning how to more quickly adapt technologies to a given shale play. Keys to Oklahoma’s Woodford Shale and Arkansas’ Fayetteville Shale gas plays are emerging. Maybe the Floyd Shale will be next. Speaking of shale gas plays, I encourage anyone who missed the Oklahoma Geological Survey’s May 23 workshop on the Woodford Shale to contact Brian Cardott at 405-325-3031 to request a copy of the workshop notebook. The premeeting agenda looked really solid.