Aspects of the sedimentology, stratigraphy, and diagenesis of the Upper Devonian-Lower Mississippian Bakken Formation of west-central Saskatchewan

Abstract

The Upper Devonian-Lower Mississippian Bakken Formation of west-central Saskatchewan comprises three members: the lower black shale member (LASH), the middle sandstone member and the upper black shale member (UBSH). Seven different facies have been defined in the middle sandstone member: 1) oolitic bioclastic grainstone facies (OBG); 2) lower bioturbated siltstone facies (LBS); 3) thinly-bedded bioturbated sandstone facies (TEBS); 4) thinly-bedded sandstone facies (TBS); 5) interbedded sandstone-siltstone-shale facies; 6) upper bioturbated siltstone facies (UBS); 7) transitional facies (CSST). The upper black shale member (UBSH) shows facies relationships with facies 6 and 7 in the middle member. During the early stages of deposition of the Bakken Formation, the basin waters were relatively deep and the lower Bakken shale member (LASH) was deposited below storm wave-base under anoxic conditions. Shallowing before, or at, the beginning of middle Bakken deposition resulted in the development of oolitic shoals (OBG) in a shallow, open marine setting and fine-grained siliciclastic sediments (LBS) accumulated basin ward of the shoals. Subsequent deepening of the basin waters was accompanied by progradation of terrigenous sediments. These were distributed and reworked by storms and tidal currents (TBS, TBBS and 1SSS), ultimately burying the oolitic shoals. Late middle Bakken deposition was marked by continued basin deepening with possible localized brackish or lagoonal conditions (CSST) and open marine offshore shelf conditions (UBS). Subsequent with basin deepening, sediment supply was reduced (possibly from a distal source, or areas of reduced sediment shedding) and contained within flooded river-valley systems. The upper Bakken shale member represents basin deepening coincidental with transgression, sediment starvation and a return to anoxic marine conditions (UBSH) that were similar to, but less widespread than those of the lower Bakken shale.

Penecontemporaneous and burial processes resulted in a diagenetic signature, both facies­ specific and general, that was significant for oil migration and entrapment within the Bakken Formation. Early phases include pyrite, calcite and silica cements and micritization. Silica dissolution and reprecipitation continued for some time during early compaction stages. Later events include microdolomitization and clay mineralization. Secondary porosity development associated with the subcrop edge provides reservoir space for migrating Bakken/Exshaw sourced oils that were subsequently biodegraded by oxidizing meteoric waters.