Chemostratigraphy of the Lower Cretaceous Basal Quartz in Southern Alberta

Abstract

Abstract The Lower Cretaceous Basal Quartz (BQ) of the Western Canadian Sedimentary Basin (WCSB) is the basal part of the Lower Mannville Formation, which, together with the overlying Upper Mannville, is considered one of the most prolific hydrocarbon- bearing successions in the WCSB. However, due to the reservoir complexity often associated with low accommodation fluvial settings, detailed stratigraphic studies are required to fully exploit the potential. Preliminary findings of this chemostratigraphic study on the Horsefly, BAT, and Ellerslie units show that considerable and systematic elemental variation exists between the claystones of the three units. These variations allow the stratigraphic affinity of a claystone core sample to be determined using chemostratigraphy, thereby demonstrating that the technique is a viable additional stratigraphic tool in the WCSB, and more generally, in low accommodation fluvial settings. Introduction The Lower Cretaceous Basal Quartz (BQ) is an informal term for the Lower Mannville succession, which together with the "Ostracode," forms the Lower Mannville Formation (see Figure 1). In the study area of southern Alberta, the BQ is thin (<100 m), and is characterized by a complex stratigraphy punctuated by multiple unconformities(1, 2) (Figure 2). Based upon detailed stratigraphic and petrographic differentiation, the BQ is divisible into seven mappable units (Regional A, Mesa IV, Valley and Terrace, Horsefly, BAT, and Ellerslie). This chemostratigraphic study has concentrated on the fine grained facies of the Horsefly, BAT and Ellerslie units, which form the upper of two cycles within the Lower Mannville Formation (see Figure 1). The technique of chemostratigraphy is used to differentiate between the finer grained sediments of the Horsefly, BAT, and Ellerslie units based on variations in their major and trace element compositions. By the selection of cored intervals in wells whose stratigraphy had already been determined by detailed petrographicstudies(1), it is shown that the associated finer grained facies of the Horsefly, BAT, and Ellerslie units exhibit significantly different elemental compositions. Chemostratigraphy, or chemical stratigraphy, involves the characterization and correlation of strata using major and trace element geochemistry. For this study, data for a total of 47 elements (ten major elements, 23 trace elements, and 14 rare earth elements) have been determined using inductively coupled plasma optical emissions spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS). The sample preparation and analytical procedures are detailed in Pearce et al.(3) and Jarvis and Jarvis(4). Conventional cores from six wells (Table 1) were sampled for the study. The intervals sampled have already been stratigraphically assigned [Zaitlin et al.,(1) Table 1]; when sampling the core, apparently homogenous silty claystone chips with volumes of approximately 2 - 3 cm3 were selected for analysis. Results and Discussion Geochemical Differentiation The principal aim of this paper is to demonstrate that significant geochemical variations exist between the silty claystones of the Horsefly, BAT, and Ellerslie units. The most effective way to demonstrate any such variation is in the form of geochemical profiles, binary and ternary diagrams (see Figures 3, 4, and 5). Geochemical Profiles Geochemical profiles for a single section are constructed by plotting element concentrations, or element ratios, against depth.