Resource Assessment in the Northern Midland Basin: Detailed Mapping of Late Pennsylvanian, Wolfcampian, and Early Leonardian Margins and Flooding Surfaces Using Well Logs and Seismic Data

Sinclair, Steven W. (Pioneer Natural Resources) | Crespo, Luis (Pioneer Natural Resources) | Waite, Lowell (Pioneer Natural Resources) | Smith, Kevin (Pioneer Natural Resources) | Leslie, Caitlin (Baylor University)



Any complete resource assessment of unconventional resources in a basin must include accurate delineation of marginal areas. In the northern Midland Basin, organic-rich Late Pennsylvanian/Early Leonardian mudrocks are bounded to the east, north, and west by predominantly shallow-water carbonate platform and reef deposits comprising the Eastern shelf and Glasscock Nose, Horseshoe Atoll, and Central Basin Platform. Allochthonous deep-water carbonate and siliciclastic gravity flow deposits derived from platform areas also limit hydrocarbon reserves as well as act as potential drilling hazards in certain areas. These bounding platform regions and associated deep-water flow deposits contain a complex structural and stratigraphic history that complicates resource assessment in marginal areas. A detailed mapping project of marginal regions of the northern Midland Basin utilizing available digital well logs closely tied to available 2D and 3D seismic data was therefore initiated in order to more accurately assess the resource potential of Late Pennsylvanian – Early Leonardian mudrocks of the basin.

Development of a sequence stratigraphic framework for the basin margins offers a framework that simplifies some complex basin margin relationships. Mapping and correlation of flooding surfaces, some of which correspond to existing Wolfcamp lithostratigraphic tops, and closely tying these stratigraphic surfaces to seismic response provides a more complete picture of the nature, timing, and extent of the “mid-Wolfcamp” unconformity in the Midland Basin. Seismic analysis combined with correlation of closely-spaced well logs indicates a complex history of the Glasscock Nose including periods of rapid progradation, mass wasting, erosion, and delivery of large quantities of clastic and carbonate material to slope and basin. The Central Basin Platform margin displays variable geometry in time and space but was generally aggradational during Penn-Wolfcamp time, sourcing extensive debris flows within the upper Wolfcamp interval. Seismic data augmented by well control shows clear evidence of both structurally- and stratigraphically- controlled thinning and truncation of upper Wolfcamp units along the western and eastern margins of the basin. Detailed isopaching of these units, together with mapping of carbonate percentage maps utilizing normalized gamma-ray log curves, greatly helps refine the assessment of total hydrocarbon resource in areas proximal to the shelves.