Nolley Wolfcamp Unit, Andrews and Gaines Counties, Texas: Classic Shallowing-Upward Sequence
The Nolley Wolfcamp unit is located in west Texas within Andrews and Gaines Counties and is approximately 13 mi northeast of the city of Andrews. In this area, deposition of Lower Permian sediments took place on the Central Basin platform. The unit produces from skeletal and oolitic carbonate shoals and carbonate sheet sand bodies, which cap a series of shallowing-upward cycles. Regional tectonic and/or global eustatic events controlled sedimentation. Sporadic uplift along the eastern edge of the Central Basin platform was initiated by regional tectonic events while global eustatic events influenced relative sea level fluctuations. Shallowing-upward cycles in the Nolley Wolfcamp unit result from and are contemporaneous with these events.
The reservoir averages 400 ft thick and has been divided into upper, middle, and lower zones based on log response and core description. The upper and lower zones have average porosities of 11%, and the middle zone is essentially devoid of reservoir-quality rock. Porosity types include biomoldic, vuggy, interparticle, intraparticle, and solution enhancement of any of the above. Important diagenetic factors that influence reservoir quality involve dissolution of allochems, cements, and/or matrix by phreatic fresh water. Facies distribution, proximity to the strand, and configuration of the paleoaquifer system appear to be major factors affecting the distribution of reservoir-quality rock.
AAPG Search and Discovery Article #91026©1989 AAPG Southwest Section, March 19-21, 1989, San Angelo, Texas.