Chert and carbonate reservoirs of the Early Devonian Thirtyone Formation comprise a major regional reservoir in the Permian basin, with more than 700 Mbbl production to date and roughly an equal amount of potentially producible reserves remaining. Most major pools in the Thirtyone Formation were discovered several decades ago and are now in relatively advanced stages of enhanced recovery, with significant variation apparent in response to waterflooding among different fields. As shown by comparison between Dollarhide and Three Bar fields, reservoirs display significant heterogeneity due to structure and to depositional/diagenetic factors affecting porosity development. Most studies of these fields to date have tended to emphasize depositional and diagenetic factors as the reason for observed variation in enhanced recovery performance; however, structural factors may be of nearly equal importance. Enhanced recovery at Dollarhide field, for example, has been extremely successful, with waterflood recoveries more than double those under primary production, and a tertiary CO 2 flood expected to do nearly as well. This extraordinary level of success is understood as a result of a combination of reservoir character and compartmentalization, with the latter especially related to division of the field into several fault block portions. Mapping of these fault blocks and their related structures has been greatly advanced by a recent 3-D (three-dimensional) seismic survey. Ultimate recovery at Dollarhide is projected at 62% of original oil in place. In comparison, production history at Three Bar field shows a lower level of overall recovery, yet a very similar increase between primary and secondary production. Reservoir heterogeneity related to depositional/diagenetic factors appears more pronounced at Three Bar field than at Dollarhide field, but faulting is also significant with respect to division of the field into separate blocks with distinct productive histories. A 3-D seismic survey may provide important discriminating data useful for a future CO 2 flood or other tertiary recovery project at Three Bar field.