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Down Dip Variability in Ichnology and Sedimentology of a Fine-Grained Deltaic System: the Medicine Hat Member, Southwestern Alberta

La Croix, Andrew 1; Yang, ByongCheon 1; Gingras, Murray 1; Pemberton, S. G.1
1 Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada.

The upper Cretaceous Medicine Hat Member of the Niobrara Formation in southwestern Alberta is a giant low permeability gas-play. Due to the distribution and occurrence of sand, the Medicine Hat was previously interpreted as a shelf deposit. This study offers a contrasting interpretation based upon ichnologic characteristics and subtle changes in sedimentary facies. The deltaic model proposed is important because it changes the current reservoir geometries and volumetrics, among other exploration and production aspects. This is one of the first documented occurrences of a fine-grained delta in the rock record, a poorly represented depositional system.

Sedimentary facies were analyzed from ten cores in terms of grain size and primary sedimentary structures, supplemented with trace fossils. Thirteen dominant facies are recognized in the study area, which are grouped into 4 facies associations (FA). The FA1, composed of multiple sets of sandy tempestites with rare bioturbation produced by diminutive Planolites and Psilonichnus, is interpreted to have been deposited dominantly under the influence of storm waves. We interpret the FA1 as representing deposition in the proximal delta front. The FA2 comprises planar-laminated fine sand and bioturbated silt and mud, in which a low diversity suite characterizes the ichnology, including recumbent Skolithos, Planolites and Siphonichnus. From this, the FA2 is interpreted as deposits of the middle to distal delta front. FA3 is composed of thin planar-laminated fine sands interstratified with relatively thicker, moderately bioturbated silt and mud. The trace fossils are diminutive, but a slightly more diverse suite is observed, which consists of recumbent Skolithos, Planolites, Phycosiphon, Teichichnus and rare horizontal spreitenated traces. We interpret FA3 to be the proximal prodelta deposits. Finally, FA4 is characterized by moderately to well bioturbated silt and mud with rare pin-stripe laminated sands. This facies association was deposited in interdistributary bays between active delta lobes. Overall, the Medicine Hat Member in the study area is interpreted to have accumulated in a prograding deltaic system where the dominant depositional process was storm waves.

The new interpretation of the depositional environment has direct implications for improved production strategies. The results of this study will help refine depositional models of fine-grained deltas elsewhere.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009