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 settings, detailed stratigraphic studies are required to fully exploit the potential. This paper presents the preliminary findings of a chemostratigraphic study on the finer grained sediments of the upper Cycle of the BQ, i.e., the Horsefly, BAT and Ellerslie units. The work was undertaken to determine whether or not chemostratigraphy has the potential to deliniate between stratigraphic units in low accommodation settings. By selection of cored intervals in wells whose stratigraphy had previously been determined, it is shown that the silty claystones of the Horsefly, BAT and Ellerslie exhibit significantly different elemental compositions.

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 (Fig. 1). In southern Alberta, the BQ is thin (<100m), and is characterized by a complex stratigraphy punctuated by multiple unconformities (Zaitlin et al, 2002; Lukie et al., 2002, Ardies et al., 2002). Based upon detailed stratigraphic and petrographic differentiation (Fig. 2), the BQ is divisible into seven mappable units (Fig. 3). Regional A, Mesa IV, Valley and Terrace, Horsefly, BAT and Ellerslie). This chemostratigraphic study has concentrated on the upper cycle composed of the Horsefly, BAT and Ellerslie units.

The sandstones of these three units, which are unconformably overlain by the brackish marine "Ostracode", display upward increasing mineralogical and textural maturity (Fig. 2). Extensive thin section work on the sandstones of the Horsefly, BAT and Ellerslie units has allowed recognition of distinctive petrographic features for each unit. Claystones form a significant proportion of each unit. Therefore, a method for determining the stratigraphic affinities of a claystone sample will allow greater stratigraphic understanding. The primary objective of this paper, therefore, is to demonstrate that the technique of chemostratigraphy is capable of clearly differentiating the finer grained lithologies from the Horsefly, BAT and Ellerslie units.

Chemostratigraphy, or chemical stratigraphy, involves the characterisation and correlation of strata using major and trace element geochemistry. For this study, data for a total of 47 elements (10 major elements, 23 trace elements and 14 rare earth elements) have been determined using inductively coupled plasma atomic emissions spectrometry (ICPAES) and inductively coupled plasma mass spectrometry (ICP-MS). The sample preparation and analytical procedures are detailed in Pearce et al (1999) and Jarvis and Jarvis (1995).

Study Material and Methodology

Conventional cores from 6 wells (1-35-1-20 W4, 2-4- 1-17 W4, 7-9-11-16 W4, 6-11-26-28 W4, 10-29-36-27 W4 and 3-30-37-27 W4) were sampled for this study. The intervals sampled have already been stratigraphically assigned (Zaitlin et al2002, Table 1);

A total of 90 samples have been collected from these wells (Table 1). When sampling the core, apparently homogenous silty claystone chips approximately 2-3cm3 were selected for analysis.

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