Detailed Sedimentology to Sequence Stratigraphic Interpretation of Organic-Rich Mudstones of the Duvernay Formation, Alberta, Canada

Abstract

Development of a liquid-rich gas play in the Duvernay shale has increased the need for models that explain and predict rock properties such as lithology, organic-richness, porosity, permeability, and fracturability. We present here detailed depositional and sequence stratigraphic models based on examination of drill cores across the basin, integrated with organic and inorganic geochemistry, that enable us to predict key rock properties. Shale lithologies show systematic variation related to sequence stratigraphic systems tracts. Transgressive and early highstand deposits are composed of laminated to massive, organic-rich, siliceous mudstones. Sedimentation is dominated by suspension settling of fine grained calcite, silt, organic matter, and calcareous and siliceous micro-organisms. Uncommon traction currents form millimeter-thick debris beds enriched in silt- to fine-sand sized calcite shell debris and limestone intraclasts. Bioturbation is uncommon in basinal areas but is minor to moderate on paleobathymetric highs. Highstand deposits show an increase in abundance of bioclast- and intraclast-rich debris beds. Minor increases in terrigenous material are seen as clay- to silt-sized quartz deposited predominantly from suspension. Bioturbation is more common and TOC values are typically lower. Stillstand or lowstand deposits are commonly composed of nodular carbonates with increased argillaceous content and show increased fissility, intense bioturbation, and moderately to drastically reduced TOC values. In locations proximal to reef buildups, coarse-grained intraclastic and fossiliferous wackestone-packstone debris beds and breccias become common. Subsequent transgression may result in hardground cementation at the top of lowstand deposits. Transgressive and highstand deposits are the most prospective for unconventional reservoir exploration. Organic matter content is highest in TST and HST deposits and increased biogenic silica creates brittle, non-fissile strata. Lowstand/stillstand deposits are organic-lean, may have increased fissility and are therefore less prospective. Lowstand/stillstand deposits may also be capped by heavily-cemented hardgrounds that, if thick enough can result in reservoir compartmentalization.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015