Abstract

The Yates oil field is one of the largest oil reserves in the United States. Yates was discovered as a result of surface structure mapping, but published geologic studies are primarily concerned with aspects of the reservoir and contain little information on surface geology.

Detailed surface mapping described here of Cretaceous strata and crosscutting joints and faults has delineated surface draping directly related to subsurface reservoir geometry and fluid migration paths, as well as joint sets that define preferred directions of fluid movement. Documented structures include folds, draping, joints, faults, and rotated and slumped blocks. Mappable draping of Cretaceous strata at the surface closely reflects the structure of Guadalupian strata on the Central Basin platform, and likely originated from depositional and diagenetic processes including differential compaction and dewatering of Late Permian evaporites.

Unidirectional near-vertical joints exposed on three sides of the Yates Field parallel the pinch-out of the halite in the Salado Formation and spatially coincide with anomalous thinning of Cretaceous Trinity Group strata and anomalous thickening of the salt beds. The joint sets are in part thought to have originated as a result of the formation of salt anticlines during Early Cretaceous time. Mapped joints elsewhere are locally filled with petroliferous calcite and other minerals, and pre-date hydrocarbon migration.

Investigations of this type will doubtless prove valuable to reservoir studies and secondary recovery efforts. Detailed surface mapping is an inexpensive and overlooked source of subsurface structural information.