Luncheon – May 17, 2002

Title: The Influence of Basement Linears and on Valley-Fill Depositional Patterns and Hydrocarbon Entrapment

Author: John C. Horne, Orion International Ltd.

Date: May 17, 2002

Publication: The Outcrop, May 2002, p. 5-6

Structurally and stratigraphically entrapped hydrocarbons in valley-fill reservoir facies in the Greater Green River, Wind River, and Powder River basins are influenced strongly by basement linears. Many of the valley-fill depositional patterns and their associated reservoir facies of producing formations show a direct relationship to the juncture of basement structural highs with crosscutting regional or orthogonal linear features in these basins.

Many of the valley-fill depositional patterns and their associated reservoir facies of producing formations in the Greater Green River basin show a direct relationship to the juncture of basement structural highs with crosscutting regional linears, or to other orthogonal to regional linear features. Major northwest-oriented regional basement linears controlled the distribution and orientation of Dakota and Muddy lowstand valley-fill deposits in the western part of the Greater Green River basin, while orthogonal northeast linears controlled the orientation and distribution of reservoir facies in associated shoreline deposits.

Basement Linears developed a northwest-northeast orthogonal pattern in the area of the Moxa arch. During the Late Mid-Turonian sea-level drop, fluvial valleys formed a rectilinear drainage pattern. The fluvial valleys that incised the lowstand surface followed the subtle topography created by differential movements of orthogonal-shaped basement blocks. The streams incised into more easily eroded fractured margins of the blocks. Frontier valley-fill reservoir facies accumulated in the orthogonally oriented valleys that developed during the Late Mid-Turonian sea-level lowstand. Production trends from these valley-fill deposits directly reflect the orthogonal depositional patterns resulting from the influence of the basement linears.

Similar to the association between basement linears and valley-fill deposits observed in the Greater Green River basin, a direct relationship exists between basement linears and the location of Muddy formation valley-fill deposits in the Wind River and Powder River basins. The location of several stratigraphically influenced Muddy sandstone producing fields such as Hilight field in the Powder River basin and Grieve field in the Wind River basin are controlled by basement linears.

In the Greater Green River, Wind River, and Powder River basins, production in valley-fill reservoirs commonly ends abruptly or changes trend at basement linear discontinuities. Consequently, these linears are critical controls, not only on structural development, but also for the productive limits of many valley-fill stratigraphic entrapments. These limits may occur due to diagenetic alterations or to changes in facies. In either case, these relationships indicate that basement linears were active prior to, during, and after accumulation of valley-fill deposits in these basins. Additionally, they may have influenced diagenetic fluid movement through the reservoir systems at a later time.

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