--> Abstract: Erosional Tidal Ridges in the Bakken Formation (Late Devonian-Early Mississippian), Southwestern Saskatchewan, Canada, by James M. Wood, Shaun C. O'Connell, and Stephen P. Robinson; #90039 (2005)

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Erosional Tidal Ridges in the Bakken Formation (Late Devonian-Early Mississippian), Southwestern Saskatchewan, Canada

James M. Wood1, Shaun C. O'Connell2, and Stephen P. Robinson3
1 EnCana Corporation, Calgary, AB
2 Belfield Resources, Calgary, AB
3 Conoco Phillips, Calgary, AB

The Bakken Formation provides a rare opportunity to study an ancient tidal sand ridge system in the subsurface using a data set consisting of thousands of wells. Subsurface mapping and core examination show that the tidal ridge morphology is defined by a scour surface near the top of the Bakken Formation. This scour surface is part of a regional transgressive surface of erosion (TSE) that separates lowstand (LST) and transgressive (TST) systems tracts. In the study area, the LST consists of a progradational tide-dominated delta system. The overlying TST consists of a deepening-upward marine shale succession with shallow offshore deposits passing upwards into deeper anoxic shelf deposits.

The TSE is a sharp contact, or appears gradational due to modification by bioturbation. Cross-sections show the TSE has a ridge and swale morphology that truncates coarsening-upward successions in the LST. An isopach map of the LST shows a series of regularly spaced, linear, northeast-southwest oriented thick and thin sand trends. Ridges defined by the isopach map have vertical relief of 10 to 15 m (maximum 24 m) and crest-to-crest spacing of 3 to 7 km (maximum 10 km). These dimensions are comparable to those of modern shelf ridges. Bakken ridges are generally symmetrical which is typical of erosional Class I tidal ridges (Snedden and Dalrymple, 1999). Internally, Bakken ridges show no evidence of accretion deposits indicative of sand ridge migration. The ridge and swale morphology of the TSE played a major role in the migration and trapping of hydrocarbons.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005