Sedimentology and dolomitization of the Middle Devonian (Givetian) Slave Point Formation, Cranberry Field, northwestern Alberta, Canada

Abstract

The Middle Devonian Slave Point Formation of the Western Canada Sedimentary Basin is host to several gas fields, most notably the Cranberry Gas Field, the focus of this thesis. The Slave Point Formation can be subdivided into a Lower Slave Point (platform) that is unconformably overlain by the Upper Slave Point (reefal buildup). The Lower Slave Point is a basin-wide argillaceous limestone. Upper Slave Point reefal developments are related to slight buildups on the underlying Lower Slave Point. Colonization by 'Thamnopora' created subtle platform topography within the Lower Slave Point. These subtle 'highs' acted as the loci for Upper Slave Point reefal development. Upper Slave Point sedimentation is characterized by two growth phases: (1) early progradational growth followed by; (2) backstepping to aggradational growth. The termination of Upper Slave Point reefal development occurred during the trangressive or 'drowning' phase of the Waterways Formation. This transgression is characterised by an argillaceous crinoid-brachiopod deposit, informally known as the 'Cranberry Member' of the Waterways Formation. The Slave Point Formation has undergone at least three dolomitizing events. The major types of dolomite that occur in the Slave Point Formation in the study area are: replacive coarse crystalline dolomites (Type 2) and late-stage saddle dolomite cements (Type 3). The majority of dolormitization occurred in a burial environment. The linear orientation, restricted occurrence and stable isotopes of Type 2 dolomitized wells is suggestive of fault controlled fluid circulation during the Late Cretaceous - Early Tertiary. Dolomites formed from deep, warm, saline, basinal fluids and/or hot brines that ascended along fault conduits from deeper parts of the basin. The widespread occurrence and stable isotopes of Type 3 dolomite cements suggest the updip migration of compaction-driven dolomitizing fluids that occurred in response to basin tilting during the Late Cretaceous - Early Tertiary.