The Black Hollow field is located in Weld County, Colorado, on the W. flank of the Denver-Cheyenne Basin. The producing reservoir is the Lyons Sandstone of Permian age. Seismic work was begun in the area in 1944, following up gravity leads resulting from a gravity survey done in 1943. A minor lead resulted, but it was not until 1949 when The California Co. moved in a Western Geophysical crew for additional seismic work that structural closure was recognized. This led to the drilling of a well to test the Lower Cretaceous Dakota and Lakota sands, which were then the major objective. The well was abandoned in 1952. During the next year, interest increased in Permian objectives and the well was re-entered and deepened to the Lyons, thereby becoming the discovery well at Black Hollow. Subsequent development drilling indicated anomalous thickening and stratigraphy on the W. flank which led to more seismic work. Results indicated that this could be mapped. Furthermore, a velocity gradient across the area was recognized, and applying this, the possibility of extending the field to the E. was recognized. More seismic work was done in an attempt to confirm this prior to additional drilling.