Abstract

The Upper Cretaceous Alderson Member (Lea Park Formation) of Western Canada produces prolific amounts of biogenic gas from what is dominantly a fine-grained, low-permeability interval. This production has been thought to depend on the presence of laminated and bedded sandstones. However, several bioturbated intervals contain abundant silt and sand-filled burrows and likely contribute to the overall reservoir. A case study from the Hatton gas field area of southwest Saskatchewan documents the depositional setting of the Alderson Member and clarifies the contribution of biogenic permeability to production in an overall tight-gas play.

In the study area, the Alderson Member encompasses eight facies, the numbering of which follows that of previous workers in the region: Facies 2A (Phycosiphon-dominated sandy mudstone); Facies 2B (sandy mudstone bearing a mixed-ethology assemblage); Facies 2C (burrow-mottled sandy mudstone); Facies 3A (interlaminated fine-grained sandstone, siltstone and mudstone); Facies 3B (bioturbated heterolithic bedding); Facies 4 (low-angle cross-stratified sandstone); Facies 5 (massive-appearing sandy shale); and, Facies 6 (conglomerate/massive medium- to coarse-grained sandstone). Facies successions are consistent with deposition in mud-rich, shallow offshore (shore-margin) and deltaic coastline settings. Shore-margin deposits comprise subtly upward coarsening and shallowing successions that, in ascending order, comprise organic-rich, bioturbated mud (F2A), heterolithic bedding (F3 and F4), and potentially root-bearing sandy mudstone (F5). Delta-margin successions record a gradual vertical increase in organic matter content, sedimentation rates, and wave influence as Phycosiphon-dominated sandy mudstone (F2A) grade vertically into interlaminated mudstone and sandstone (F3A), bioturbated heterolithic bedding (F3B), and thin intervals of low-angle, cross-stratified fine-grained sandstone (F4).

Facies 4, 5 and 6 exhibit low permeabilities and are not likely contributors to overall gas production. The highest permeability values are associated with sand-dominated Facies 3 (2 to 7 x 101 md), and slight permeability enhancement was observed in Facies 2A (2 x 10−1 to greater than 1 md). Due to the local 3-D morphology of the dominant trace fossil (Phycosiphon), the bioturbated units constitute a comparably isotropic flow media. At the reservoir scale, these beds are likely variable in thickness (decimetre- to metre-scale) and dominantly planiform. The burrowed media can be characterized as a dual-porosity unit in which the large total surface area of burrows interacts extensively with gas-bearing matrix and likely contributes to overall gas deliverability.

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