A Sedimentological Assessment of Dolomitized Bioclastic Accumulations Within the Lower Triassic Montney Fm., Western Canadian Sedimentary Basin

Abstract

With the evolution of the Montney Formation as western Canada's premier unconventional exploration play, renewed attention has also focussed on bioclastic accumulations within the Coquinal Dolomite Middle Member, a unit that was previously the target of conventional drilling during the 1990s. Previous exploration focussed on thick intervals of porous coquina adjacent to the subcrop limit of the facies. Despite a long exploration history, bioclastic units in this interval still hold significant potential as horizontal drilling targets. This includes both thick, conventional units as well as thin bioclastic units that occur interbedded with siliciclastic siltstone and sandstone. The depositional and diagenetic history of these accumulations is of considerable importance to the exploration community; however, minimal research has been published on this topic. Previous work has not discussed the internally heterolithic, event-bed characteristics displayed by the bioclastic accumulations, nor were geochemical and SEM analyses incorporated into their interpretation. This study provides a detailed core and thin section-based interpretation of bioclastic accumulations within the Montney Fm. Interpretations are supported by δ18O/δ13C isotopic series, as well as SEM images. Additionally, modern analogues are used in this study to model the three-dimensional geometry of these accumulations. Bioclastic accumulations in the Montney Fm. are laterally continuous, and contain a large quantity of moldic/vuggy porosity as well as intergranular, intragranular, stylolitic, shelter, and fracture porosity. Porosity and permeability within these accumulations are variable, but estimations from core and thin section range from 12-21% and 80-200md. Cement stratigraphy within the bioclastic accumulations reveal early diagenetic calcite and dolomite rims forming prior to the dissolution of shell material, as well as late stage chert cementation that postdates karstification. Dolomitization appears to occur in multiple stages, and is interpreted as a combination of seepage reflux and evaporative drawdown. This research provides implications for the depositional processes that control the accumulation of shell material, as well as the diagenetic processes that affect preservation. Understanding these processes is essential to characterizing reservoir quality within the Montney Fm., and providing a refined model for exploration targets.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016