RESERVOIR CHARACTERIZATION OF THE LOWER GREEN RIVER FORMATION, SOUTHWEST UINTA BASIN, UTAH

The +2000-foot-thick (600-m), Tertiary-aged lacustrine deposits of the Middle and Lower Members of the Green River Formation contain the primary oil-producing reservoirs in the southwest Uinta Basin. The authors developed a log-based correlation scheme by identifying what they interpret as depositional cycles on the gamma-ray and resistivity logs of several wells. Regional cross sections were constructed and cycle boundaries revised as needed. The cycles typically range from 50 to 100 feet (15-30 m) thick. The regional correlation scheme will be used to improve their knowledge of the depositional patterns and distribution of productive intervals in the southwest Uinta Basin. Currently, each operator uses a different terminology for many of the same intervals. A regional log-based correlation scheme based on depositional cycles should make it easier to relate subsurface data to the outcrop where depositional environments and lateral continuity of the reservoir rocks can be studied in greater detail. The correlation scheme uses an alpha-numeric nomenclature avoiding local field or facies names that are difficult to use regionally. The nomenclature has three primary levels: (1) MGR or LGR for Middle or Lower Green River, respectively, (2) MGR1 through MGR18 and LGR1 through LGR3 for the different cycles in each member, and (3) MGR1a, MGR1b, and so on, for beds within each cycle. Beds are defined for local use and are not intended to be regional. The cycles can be divided into smaller subcycles, if necessary, for detailed work within a field (MGR14A and MGR14B, for example). cycles can be combined where depositional thinning or poor log quality does not allow correlation of all the individual cycles (MGR5 through MGR9, for example)