Sedimentology and Geochemistry of the Heavy Oil Bearing Leduc Formation of East-Central Alberta: New Insights Into a Historic Formation

Abstract

Following the early successes of platform and pinnacle reef exploration in central Alberta, the Devonian Leduc Formation of eastern Alberta has seen sporadic exploration and production in the last two decades, and limited publication. In the cold heavy oil belt south of Lloydminster, the uppermost several meters of the Leduc are oil bearing within erosional remnants formed by the sub-Cretaceous unconformity. The highs have proven producible by horizontal drilling, and the presence of reservoir quality dolomite, which is interpreted to have replaced the original mineralogy of the Upper Leduc early, in the shallow burial realm. In east-central Alberta the Leduc Formation is comprised of an impressive 200 meter platformal accumulation that typifies rapid carbonate growth of the Frasnian Stage. From bottom to top, the Leduc becomes increasingly more restricted, as evidenced by the decrease in fossil abundance and diversity. The Lower Leduc is comprised of prograding stromatoporoid reefal facies, which grade upward into back reef facies, then restricted subtidal, and peritidal facies of the Upper Leduc. Leduc subdivisions have been thoroughly correlated in the area, and the youngest two from which oil is produced were sampled across the region for petrographic and geochemical analyses. Dolomites range in size, reflecting facies variations in the original depositional fabric they replaced. With burial, the Upper Leduc dolomites recrystallized to produce non-ferroan dolomites, with an average δ18O value of -4.1‰ VPBD, and an average δ13C value of 1.0‰ VPBD. The dolomites are nearly stoichiometric, with cloudy cores, and in some samples, clear rims. After initial burial, the laterally extensive sub-Cretaceous unconformity formed, creating an approximate 260 million year gap between Upper Devonian carbonates, and upper Lower Cretaceous siliciclastics. The proximity of the sub-Cretaceous unconformity to the Upper Leduc dolomites likely led to dedolomitization in some samples, yielding low magnesium calcites with very negative, meteoric δ18O compositions. Late replacement calcite and calcite cements occlude porosity, and are thus a detriment to reservoir quality. Proximity of the sub-Cretaceous unconformity to the Leduc reservoir may increase the likelihood that dedolomitization took place, and that calcite cements formed. Knowledge of the depth of the unconformity relative to the Upper Leduc reservoir thus provides an additional tool in predicting reservoir quality.

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