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ABSTRACT

Integration of core data with high resolution sequence stratigraphic well-log correlations from over 300 wells indicates that the elongate sandstone bodies which compose the Upper Cretaceous Shannon Sandstone at Hartzog Draw were deposited by tide-dominated deltas consisting of stacked, upward-coarsening tidal bars. Correlation of regional stratal surfaces suggests that the Shannon Sandstone comprises a sequence set which can be informally subdivided into 3 unconformity-bounded depositional sequences: Copenhagen Blue, Crimson Red, and Canary Yellow. The Copenhagen Blue sequence boundary is a regional unconformity at the base of the Shannon Sandstone which separates distal tidal-bar deposits from offshore mudstones of the Cody Shale. The overlying Crimson Red sequence is the main reservoir interval at Hartzog Draw and is dominated by high reservoir quality, proximal tidal-bar sandstones. The highly erosional Canary Yellow sequence boundary forms the trap at Hartzog Draw by juxtaposing the reservoir sandstones of the Crimson Red sequence with the overlying distal tidal bars to offshore mudstones of the Canary Yellow sequence. Thickness trends within the Shannon Sandstone are interpreted as the product of variable incision associated with southeast-trending incised valleys.

The continuity of reservoir facies within these bar complexes is quite variable depending upon the lateral and vertical association of facies. In a strike orientation, lateral amalgamation of sandstone-prone, proximal bar complexes produces a relatively high degree of lateral continuity. Lateral amalgamation of mud-prone, distal bar complexes, however, produces a relatively low degree of lateral continuity. Bar complexes in a dip direction display a distinct “off lapping,” shingled geometry in the direction of bar accretion and a lower degree of continuity. This is due to depositional thinning, and increasing proportions of interlaminated mud and non-reservoir sandstones in a basinward (southward) direction. Understanding the distribution and depositional trend of the tidal bars therefore allows recognition of areas with potentially higher reservoir quality and targets for future infill drilling.

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