SUBDIVISION OF THE BARNETT SHALE (MISSISSIPPIAN) IN THE NORTHERN DELAWARE BASIN

The Mississippian interval in the northern Delaware Basin can be subdivided into an upper clastic unit and a lower limy unit using well log markers. The limy unit can be further subdivided for detailed mapping into upper, middle and lower “limestones” separated by shale intervals. Two electrofacies—serrate and massive—are present in the lower limestone. Mississippian as used here extends from a pronounced gamma ray excursion at the top of the underlying Woodford Formation (Devono-Mississippian) to the lowermost ten foot-thick sand in the overlying Morrowan clastics (Pennsylvanian). Operators in the area refer to the massive facies of the lower limestone as the “Mississippian limestone” and the remaining section as the “Barnett Shale.” The Mississippian lime and Barnett Shale accumulated in the ancestral Tobosa Basin, which was fragmented into the Delaware and Midland Basins by uplift of the Central Basin Platform at the end of the Mississippian and beginning of the Pennsylvanian. Structure contour maps on the top of the Woodford, the top of the lower limy unit and the top of the upper clastic unit are nearly identical, suggesting no differential tectonic movement took place while the strata were being deposited. The Mississippian interval reaches a maximum thickness of almost 2000 ft in Loving County. The Misissippian lime, including both the serrate and massive facies of the lower limestone, thickens from 120 ft to over 600 ft from west to east across the study area. Most of the increase in thickness occurs in eastern Loving County and western Ward County in a NW-SE trending belt of closely spaced contours along the axis of the Delaware Basin. The isopach maps for the overlying units and subunits of the Barnett generally thicken from south to north across the study area with pronounced isopach thicks along the southern borders of Eddy and Lea Counties, New Mexico, and the northern borders of Culberson and Loving Counties, Texas.