Middle Carbonate Member in Black Shale – a view from core, thin sections, and logs – the Duvernay versus black shale analogues

The important distinction between carbonate cement and depositional limestone is addressed via core observations, thin sections petrography and, XRD and XRF characteristics. We reviewed the carbonate mid-member by using log facies maps (Figure 1) and continuous XRF to complement core descriptions (e.g. Figure 2). Geometrical analysis of relative bed thicknesses has been done using Bischke plots, also called Delta D/D plots (Tearpock and Bischke, 2003); such plots help understand some of the reasons for bed thickness variations (Figure 3). In order to achieve a reliable interpretation, a Bischke Plot is prepared using a dataset consisting of many well tops (of upper and lower shale and carbonate units see Figure 3). As a complement, high resolution vertical profiling of 256 shades of grey-colour has been used as a first pass to discriminate between carbonate types as well as to define accommodation space and carbonate factory supply. Shale and carbonate/siliciclastic base-lines, created using gamma ray logs (Figure 1) can help determine the 3rd and 4th order sedimentary cycles.