Abstract: Use of Sequence-Bounding Surfaces for Correlation and Mapping in Nonmarine, Incised-Valley Reservoirs
D. A. Leckie, N. Vanbeselaere, D. P. James
One of the problems with the application of sequence stratigraphy to nonmarine sediments is the use of effective surfaces for correlations. This case study from the Mannville Group of southern Saskatchewan demonstrates how major, regional bounding surfaces can be identified and correlated to produce a suite of maps that can be used for exploration purposes.
In southern Saskatchewan, Cretaceous Mannville sediments, termed the Pense, Cantuar, and Success (S2) formations, overlie Jurassic S1 and older deposits. The interval, which is up to 100 m thick, was deposited over 40 to 50 m.y. and is riddled with unconformities and weathered horizons. Detailed stratigraphic correlations using well logs are difficult, imprecise, and highly suspect unless corroborated by core control.
Jurassic Success S1 sediment was deposited in a restricted shallow-marine environment. The top of the S1 is extremely weathered, indicating a long hiatus. The S2 was deposited as a sheet of quartzose, braided fluvial sandstone that unconformably cuts into the S1. S2 sediment extended across the southern prairies with minimal relief at the base. The top of the< sS2 is commonly a deeply weathered, thick paleosol overlain by coal and carbonaceous shale.
The overlying Cantuar Formation consists of dominantly lithic sandstone, siltstone, and shale overlying a basal quartzose unit. The base of the Cantuar Formation has a high local relief and in places has eroded long, wide valleys into the Success and older Jurassic strata. The valleys were hundreds of kilometers long and up to 74 m deep. Remnants of the Success sediment are preserved as isolated, buried cuestas on the margins of the valley walls. Cantuar sediments represent the infill of an extensive valley system that took millions of years to fill. The fill was from meandering streams with abundant paleosols, shallow lacustrine, and splay deposits.
The top of the Cantuar Formation is represented by chert and quartzose sandstones deposited in a north-south-trending estuarine system with several tributaries. The Cantuar valley fill and Success cuestas are overlain by transgressive marine sandstones of the Pense Formation. Several play types, which are dominantly stratigraphic, have been identified and are related to the valley incision, valley fill, and preserved erosional cuesta remnants.
This study also questions whether a sequence boundary must always be present below estuarine deposits. The uppermost stratigraphic unit with the Cantuar valley fill is estuarine in origin. However, rather than implying a sequence boundary at its base, the unit may only represent the final fill of the valley before deposition of the overlying Pense marine shale.
AAPG Search and Discovery Article #90953©1995-1996 AAPG Distinguished Lecturers