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Geology of the Turner Sandstone, Finn-Shurley Field, Powder River Basin Wyoming


The Finn-Shurley field produces petroleum from the Upper Cretaceous Turner Sandstone of the Powder River Basin. The Turner is a member of the Carlile and is overlain by the Sage Breaks and underlain by the Pool Creek members of the Carlile. The Turner is interpreted to be a shallow marine shelf sandstone deposited along the eastern side of the Western Interior Cretaceous Seaway. Sand-shelf-bar orientation across the field is roughly east-west. Trapping occurs where sandstone beds get shalier up-dip. The field is located along the shallow east margin of the Powder River Basin south of the Clareton lineament. Three to four coarsening upward cycles are present in the Turner in the field. Most of the production comes from the lower two cycles. Each cycle consists of burrowed to bioturbated heterolithic mudstones and sandstones coarsening upwards into fine-grained laminated to burrowed sandstones. Trace fossil present fall into the shelf Cruziana ichnofacies. The sandstones are largely litharenites. Porosities range from 11-17% and permeabilities range from 0.06 to 0.5 md. Source rock analysis of the Turner shales indicate Ro values averaging 0.63 and Tmax values of 433oC. Source beds for the oil and gas in the Turner is thought to be the Mowry Formation. The low thermal maturity suggests lateral migration of oil into the stratigraphic trap. The field extends over an area roughly circular in shape of ~65 square miles. Productive depths across the field are 4450 to 5700 ft. First production is reported as 1974 and cumulative production from ~750 vertical wells is 22.5 MMBO and 38 BCFG. Cumulative gas oil ratio is 1688 cu ft gas per barrel oil. Average production per well is approximately 30 MBO and 50 MMCFG. Horizontal drilling activity in the field area has recently commenced. Although the production is fair to marginal, the field provides an excellent example of trapping style as well as a depositional model for Turner Sandstone elsewhere in the deeper parts of the Powder River Basin. Recent drilling in the deeper overpressured parts of the Powder River Basin has encountered excellent production from the Turner (> 1,000 bbls oil equivalent per well).