Abstract

A number of wells drilled in Southeastern New Mexico were completed and produce from Silurian and Devonian age rocks. During the Siluro-Devonian this area, known more commonly as the late Paleozoic Permian Basin, was part of the much larger early Paleozoic Tobosa Basin. The Tobosa Basin covers all of Lea County and parts of Roosevelt, Chaves, and Eddy Counties in New Mexico and extends to the south and southeast into Texas. Within the Siluro-Devonian carbonates, most of the oil reservoirs are found to lie in rocks beneath the pre-Woodford angular unconformity of Middle Devonian age. Internal stratigraphic correlations of the Siluro-Devonian carbonates has resulted in eleven sub-units. The most productive of these units correspond with the upper Fusselman Dolomite, the lower Wristen Formation, and the middle Wristen Formation.

Data obtained from 466 wells in this area shows that the eleven units are thickest to the west of the Central Basin Platform. Beds thin to the north, northwest, and over the Central Basin Platform. Depths to the tops of the Siluro-Devonian units range from approximately 5000 feet to the north and west to over 18000 feet in the central and southern parts of the basin. These large-scale changes are structural in origin, related to the uplift of the Central Basin Platform during late Mississippian-early Pennsylvanian. Small-scale, local changes in bed thickness are the result of facies changes where small-scale pinchouts and bioherms occur. Production has a correlation between proximity to the pre-Woodford unconformity, lithologic unit, and bed thickness.