Sedimentology and Petroleum Geology, Spirit River Formation (Lower Cretaceous), Deep Basin, Alberta: ABSTRACT

Abstract

The Spirit River Formation is subdivided into three members in northwest Alberta. The basal Wilrich Member consists of two 50 to 100-m thick upward-coarsening cycles of marine shales, siltstones, and sandstones. The Falher Member consists of nonmarine clastics and coals in the southern part of the area. Around the Elmworth gas fields, it is composed of five transgressive and regressive cycles in which marine and nonmarine conditions alternated. Each cycle can be traced northward into a laterally extensive upward-coarsening marine cycle. The gas reservoirs are complexly interbedded fine conglomerates and sandstones. Conglomerates interpreted as fluvial deposits have sharp bases, moderate to poor sorting, some cross-bedding, and variable amounts of sandy matrix. Those interpreted as beach deposits are moderate to well-sorted, horizontally bedded, and may lack matrix entirely. A complete gradation exists between the types, which are closely interbedded. Shoreface and beach sandstones are fine grained, well sorted, burrowed, and have near horizontal laminations and truncation surfaces. On a large scale, this shore-zone complex is best considered a wave-dominated delta. The Notikewin Member is the final seaward progradation of this system. Most sandstones in the Falher have less than 6% porosity and 1 md permeability whereas the reservoirs may have 20% porosity, much of which is secondary, and several darcys permeability. Early cementation, then formation of secondary porosity in the delta complex followed by deep gas generation have created a combined stratigraphic-diagenetic trap. End_of_Article - Last_Page 557------------