Wednesday, September 11, 2013

The Complex Anadarko Basin

A classic structural geologic basin, such as the Michigan Basin, is essentially a bowl within the deep, Precambrian basement rock into which younger layers of sediment have accumulated.  In that case, the deepest portion of the basin is near its center and, as layers of deposits fill the bowl over the eons, the youngest surface sediments end up above the center as well.

The Anadarko Basin, which covers 50,000 square miles of western Oklahoma, the northern Texas Panhandle and portions of southwestern Kansas and southeastern Colorado, is not your typical structural basin.  This basin initially formed above an aborted Precambrian rift and began to fill with Paleozoic sediments from the Cambrian into the Mississippian Periods (from 550 to 350 million years ago).  Compression of that region during the Pennsylvanian Period (about 300 MYA), in concert with the uplift of the Ouachita Mountains (a westward extension of the Southern Appalachians), deformed the basin by producing a shallower segment on its northern and western flanks.  Throughout the rest of the Pennsylvanian Period and into the Permian (250 MYA), the Anadarko Basin filled to its brim.  Today, the basin has an irregular topography, with a relatively shallow platform (2000 feet deep) on its western and northern rim and a very deep section along its southern edge; the latter, the deepest structural basin in North America, dips 40,000 feet below the rim of the basin.

The major source of natural gas in the United States and a significant source of oil as well, the Anadarko Basin poses a challenge for energy geologists.  Its irregular floor and steeply dipping layers of sediment make locations of oil and gas relatively hard to predict.  The Woodford Shale, for example, late Devonian to early Mississippian in age, is one of the primary sources of fossil fuel within the basin but has an average thickness of only 125 feet.  As one might expect, there is a great deal of controversy regarding future drilling opportunities within the Anadarko Basin but new technologies, including fracking, have significantly increased the prospects of success.  Of course, for those of us who support a shift toward clean, renewable sources of energy, this is not necessarily good news.