Karsten Schjoedt Nielsen1,
Dale A. Leckie2
(1) Carleton University, Ottawa, ON
(2) Canadian Occidental Petroleum Ltd, Calgary, AB
Abstract: A dense grid of resistivity logs permits development of high resolution sequence stratigraphy in a shallow gas basin - Medicine Hat Sandstone, Western Canada
The Upper Cretaceous Medicine Hat sandstones, in southern Alberta and Saskatchewan, Canada contain an estimated 425 BCF of shallow biogenic gas. A dense drilling grid has resulted in a large data base of publicly-accessible digital wireline logs. The resistivity log in this shale-dominated interval has proven a superb indicator of grain-size variations and recognition of stratal patterns. Correlation of a detailed, closely-spaced grid of resistivity log cross-sections in marine settings allows reconstruction of stratal patterns, which resembles that of geophysical seismic lines, but with considerably more detail. Within the Medicine Hat sandstones, application of sequence stratigraphic principles to this stratal pattern recognition technique allowed delineation of more than ten distinct shingled sandstones; previously, traditional lithostratigraphic analyses only allowed for the recognition of three reservoir sandstones.
The Medicine Hat sandstones occur within regional strata that onlap a thick eastward progradational clinoforms. The stratal geometry of the Medicine Hat Sandstones indicates a backstepping morphology in an overall transgressive systems tract. The Medicine Hat sandstones are interpreted as detached shelf sandstones deposited during higher order lowstand events during the Niobrara transgression. The transgressive backstepping pattern associated with the Medicine Hat sandstones culminated in the highstand deposits of the First White Speckled Shale (Niobrara Formation equivalent). Clinoforms within this First White Speckled Shale prograde eastwards, downlapping onto the upper Medicine Hat surface. Sandstone bodies within the highstand deposit are related to higher-order sea-level fluctuations, which are inferred from sharp boundary units and strata geometry evident on the resistivity logs.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana