Duvernay Middle Carbonate Distribution and Potential Implications to Subsurface Development

Abstract

The Upper Devonian (Frasnian) Duvernay Formation of the Woodbend Group is a geological formation found over most of central Alberta. Since 2009, the Alberta Energy Regulator (AER) has seen a sharp increase in the number of operators targeting this liquids-rich shale resource, particularly in the Kaybob–Fox Creek area. Informal lithostratigraphic members include the A shale, the B carbonate (also referred to as the middle carbonate), and the C shale. The B carbonate is recognized to be nonreservoir. Recognizing the extent of the B carbonate is important. It needs to be removed from estimates of gross formation thickness that may otherwise be rich in organic carbon for credible in-place resource estimates. This can be problematic in that there are other carbonate-rich intervals of the A and C members that need to be included in gross thickness because they can contribute to reserves in these unconventional plays.

To isolate the B member, all three members were mapped at high resolution using over 2500 wells in the study area, focusing primarily on regions of viable hydrocarbon thermal maturity in the Kaybob–Fox Creek area of west-central Alberta. The Duvernay has been classified into twelve lithofacies and three end--member rock types with decreasing reservoir quality: highly siliceous organic-rich mudstones, argillaceous mudstones, and nonreservoir carbonates (Dunn & Humenjuk, 2014). The B carbonate is equivalent to those nonreservoir carbonates, which are typically limestones (Hein, 2012). A challenge to mapping the B carbonate is to distinguish it from other carbonate-rich sections on logs. For example, one important difference was observed between Leduc reef talus and B carbonate. From log analysis alone, it can be difficult to differentiate between the two, especially where the B carbonate immediately offsets Leduc reefs. In core, the Leduc reef talus is generally identified by the presence of stromatoporoid limestones; the B carbonate primary macrofacies, however, are identified by the presence of argillaceous grey crystalline limestones. On log signatures, the B carbonate often appears as cleaning upwards on the gamma ray with thin shale breaks, unlike the Leduc’s blocky and overall cleaner appearance. Characteristic B carbonate parameters include porosity ranging from 0 to 3%, determined from a sonic-density cross-plot, and water saturations between 25 to 55%, calculated using Archie’s method. The B carbonate does not contain detectable amounts of total organic carbon when calculated using Passey’s method, confirming that it should be removed from gross rock volumes for the purposes of resource and reserves estimates.

Mapping in detail shows that the B carbonate is thickest in the east, while the organic-rich shales of the A and C members are thickest to the west in the Kaybob area. There is a sharp south-trending boundary where the B carbonate thins to the west and thickens to the east. The distribution of the B carbonate suggests that its distribution is controlled by paleotopography and paleobathymetry. It is thickest to the east, where waters shallowed from the depocentre of the long-recognized Duvernay west shale basin and created conditions where biological carbonate production could flourish and still be preserved in the rock record. The sharp break in the B carbonate isopach is intriguing and may represent deeper tectonostratigraphic control on paleobathymetry, perhaps even basement-related structures.

The AER is responsible for producing and publishing estimates of Alberta’s energy resources and reserves by play, pool, and deposit. Estimates of in-place resources for the Duvernay were last generated by the Alberta Geological Survey (Rokosh et al., 2012). In order to support orderly development and optimize overall resource recovery, the AER is changing the way we calculate and classify reserves, moving to a richer and more probabilistic view in line with international reporting standards. Estimates of technically recoverable resources in the Duvernay from pilot wells have been created and can be applied to geological mapping of resources in the Duvernay. Critical to converting these resource numbers to recoverable reserves estimates are factors like gross and net reservoir thickness, total organic carbon content, etc. The isolation of the B carbonate from the total Duvernay play is a critical step to populating the resource and reserve estimates and also to link its distribution in the subsurface to its place on the land surface.

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