Facies Architecture of a Porous Upper Ordovician Reef in the Hudson Bay Basin — A Potential Reservoir Directly on Top of a Type II Source Rock

Abstract

Abstract

The Hudson Bay Basin is the largest intracratonic basin in North America, although the understanding of its hydrocarbon systems has only recently being initiated for this oil-prone basin. The carbonate-dominated succession of the onshore northern segment of the Hudson Bay includes in ascending order, the Upper Ordovician Bad Cache Rapids and Churchill River groups and Red Head Rapids (RHR) Formation and Lower Silurian Severn River, Ekwan River and Attawapiskat formations. The RHR formation is divided into four units and capped by thin-bedded dolomitic limestone with local bioherms with up to 500 m in diameter and 10 m of relief. The resistant buildups display massive and porous boundstone and cementstone facies. These massive facies have more in common with the sponge–microbial reefs that dominated worldwide in the Early Ordovician and include the following primary components: early calcified sponge material, microbial elements, and synsedimentary calcite cement. A close spatial relationship suggests that a poorly preserved decaying sponge framework provided substrates for the attachment and development of microbes, and the microbes played essential roles as reef consolidators. Small centimetre-scale colonial metazoans were present and locally intergrown with the sponge and microbial components. Although altered now to calcite, cement fabrics suggest that metastable aragonite was ubiquitous as sea floor precipitate. Prior to their subaerial exposure at the end-Ordovician, the RHR buildups developed in a shallow-marine evaporative epicratonic sea with physical and chemical parameters distinct from the open ocean. In the more central offshore Hudson Bay Basin, the RHR Formation consists of a mixed succession of anhydrite and minor gypsum beds, limestone, dolomite, and organic-rich shale capped by halite. In spite of its location within 10° of the paleoequator during the Late Ordovician, the study area was under relatively warm and arid conditions leading to the formation of intracratonic platform evaporite deposits.

These reefs likely have excellent potential as reservoir facies judging by their porous nature (up to 15% by visual estimation) and stratigraphic position immediately above high TOC (up to 35%), Type IIs source rock. These reefs are identified in seismic profiles and even if the precise timing of pore space evolution with respect to hydrocarbon generation and expulsion is still elusive, dead oil and pore-coating bitumen have been reported.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016