Wolfcampian Development of the Nose of the Eastern Shelf of the Midland Basin, Glasscock, Sterling, and Reagan Counties, Texas Wolfcampian Development of the Nose of the Eastern Shelf of the Midland Basin, Glasscock, Sterling, and Reagan Counties, Texas

by Flamm, Douglas S

The nose of the Eastern shelf of the Midland Basin is a prominent structural and depositional feature present in Glasscock, Sterling, and Howard counties, Texas. This feature has been expressed in many regional maps and mentioned in some literature, but has not otherwise been studied significantly. This study looks at the viability of using an acoustic impedance seismic inversion to interpret the 2nd and 3rd order sequence stratigraphy of the southern portion of the nose of the Eastern shelf along with its shelf to basin transition in Glasscock, Sterling, and Reagan counties during the Wolfcampian (Asselian-Sakmarian) time (Early Permian).

The Wolfcamp Formation (Wolfcampian-Leonardian) was subdivided into six units based on regionally mapped shale markers that correlate with 3rd order sequence boundaries. These horizons were mapped throughout the study area utilizing 3D seismic data and well logs.

Analysis of seismic amplitude and inversion (acoustic impedance) volumes, along with well logs were then used to create a 2nd and 3rd order sequence stratigraphic framework in the study area. Six 3rd order sequences and two 2nd order sequences were identified in the study area during the Wolfcampian. From this framework a 2nd order sea-level curve was developed.

The oldest Wolfcampian 3rd order sequence is marked by sediment bypass of the shelf and slope into the basin during a 3rd order sea level fall. Shelfal deposition resumed during subsequent sequences as sea-level rose and carbonate production resumed. Carbonate production increased during sequences four through six as part of a 2nd order sea-level highstand. During this highstand the nose of the Eastern shelf grew vertically increasing the gradient of the slope from less than 1° to 3.5°. The end of Wolfcampian deposition is marked by a large number of gravity flows into the basin resulting from subaerial exposure and erosion after a second order sea-level fall.