ROZ Potential of Tensleep Reservoirs in the Bighorn Basin, Wyoming

Residual oil zones (ROZ) are oil-bearing intervals below main pay zones (MPZ) in existing reservoirs or in oil-bearing rocks within non-commercial traps. The oil resources in ROZ have not been produced during primary and secondary production due to high water cut and unfavorable economic climate. According to core measurement and well log analysis, Tensleep reservoirs in the Bighorn Basin, Wyoming, contain thick ROZ with oil saturation as high as 80%. ROZ potential in the Bighorn basin has been evaluated using basin tectonic movement, hydrocarbon migration and accumulation, oil composition, and reservoir properties. Oil in the Tensleep reservoirs of the Bighorn Basin was originally sourced from the Phosphoria Formation in the west and migrated into the Tensleep Sandstone in stratigraphic traps before the Laramide Orogeny. After the Laramide movement, the Tensleep oil re-migrated into structural traps on the basin flanks. Expulsion of the Tensleep Sandstone on the surrounding mountain areas due to erosion caused the meteoric water to flush downward, changing oil distribution in the Tensleep reservoirs. Re-distribution of oil during the recent period left massive oil in ROZ below the MPZ and areas surrounding the existing reservoirs, as well as in the non-developed oil-bearing structures. Permeability heterogeneity of the Tensleep reservoirs and heavy oil also contribute to thick ROZ. Some of the oil in Tensleep ROZ is mobile based on drilling stem tests and its composition is similar to that of MPZ oil. After decades of water flooding, the remaining oil saturation in the MPZ of Tensleep reservoirs has been reduced to that in the ROZ or even lower, and the average water cut in currently produced oil from MPZ is over 98%. Development history of ROZ in the Permian Basin reservoirs has demonstrated that CO₂-EOR is a promising technique for recovering oil from ROZ in the mature Tensleep reservoirs and undeveloped oil-bearing structures. It is estimated that the unconventional EOR resources in Tensleep reservoirs will double the EOR reserve in the Bighorn Basin, and the recovery of oil from ROZ will revolutionize the concept of oil recovery.  

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California