Heavy Oil from an Ancient Reef

New data from the northern terminus of the Leduc reef trend in western Canada shows a karsted carbonate reservoir with exceptionally high porosities and permeabilities and, in places, bitumen-filled caves. This area is a prime prospect for bitumen development and a new technique, thermal assisted gravity drainage, is being used to recover the hydrocarbons.
This article appeared in Vol. 9, No. 6 - 2013


Devonian paleogeographic map of the Leduc reef trend. The initial discovery well was located just south-west of Edmonton, Alberta in the Rimbey-Meadowbrook reef trend. Source: Athabasca Oil Corporation This bitumen was recovered from an ancient, subsurface cave along the northern end of the Devonian Leduc reef. Source: Athabasca Oil Corporation A very porous reef containing bitumen-filled caves is enough to get anyone excited about exploring an area. The Devonian Cairn Formation which outcrops near Canmore, Alberta, Canada exhibits caves and vuggy porosity very similar to that found in the subsurface at the northern terminus of the Leduc reef. Source: Athabasca Oil Corporation
The 1947 Imperial Oil Leduc No. 1 discovery near Edmonton, Alberta ushered in a new and modern era for oil exploration in Alberta. The oil discovery was in a north to north-east Late Devonian reef trend that extends over 480 km across central Alberta. More discoveries of light gravity oil followed along the trend with recovery rates exceeding 80%. About one quarter of Alberta’s recoverable conventional oil reserves have been found in the Leduc Formation carbonates.

The northern terminus of the reef trend is about 100 km north-west of Ft. McMurray, Alberta, where this reef subcrops beneath the well-known Cretaceous oil sands reservoir, the McMurray Formation of the Athabasca deposit. This deposit contains the world’s largest accumulation of hydrocarbons, estimated at 1.7 trillion barrels that are primarily bitumen: a heavy, viscous oil. The porosity in the underlying reef trend in this area is also filled with this very heavy oil and new data has confirmed excellent reservoir quality. Athabasca Oil Corporation is currently testing a unique recovery method called Thermal Assisted Gravity Drainage (TAGD) for this portion of the reef.

Northern Terminus of Reef

The northern terminus of the Leduc reef trend, showing location of the Leduc subcrop and 3D seismic survey. Source: Athabasca Oil Corporation The northern terminus of the Leduc reef trend has been poorly understood because of the limited well control and the lack of core information. However, recent exploration activity is providing new subsurface information that has made it possible to characterize the Leduc’s reservoir potential in this area.

Similar to the Leduc reefs in south-central Alberta, the Leduc Formation in north-eastern Alberta has excellent reservoir properties. Typical Leduc reservoirs are characterized by original vuggy and moldic porosities in a high permeability dolostone. The big difference is that this already excellent reservoir has been enhanced by karst-related dissolution near the subcrop area in north-eastern Alberta. This ranges from large vugs, bitumen-lined sub-vertical to vertical fractures and to the extreme cases where metre-size caves are filled with bitumen. CT scans of cores reveal the spatial connectivity of porosity and fractures in three dimensions.

A Karsted Reef on Seismic

Initial 2D seismic mapping clearly defined the western reef front, and additional 2D seismic along strike confirmed a single, continuous reef complex that extends over 100 km. This is in contrast to the many separate reef buildups encountered in central Alberta. Detailed 3D seismic has greatly enhanced the reservoir understanding by imaging the real distribution of karsted collapse features, and distinguishing between the fore-reef, reef crest and back reef.

3D seismic has proved a very good porosity predictor. Core-calibrated porosity logs are used in conjunction with dipole sonic data to forward model seismic response. The seismic data is used to derive rock mechanical properties away from well control and to produce a deterministic porosity volume which can be used in the modeling of the reservoir.

  • Source: Athabasca Oil CorporationrnttttThe 3D imaging clearly shows karsted features along the reef and, in this case, several interconnected caves. Reference: Overview of Porosity Evolution in Carbonate Reservoirs in Kansas Geological Soc. Bull, V 79, no. 1 and 2, S. J. Mazzullo, 2004.rnttttSource: Mazzullo, 2004

Producing the Heavy Oil

The TAGD recovery process for carbonate reservoirs will provide reservoir-wide heating and should lower operational costs. Proof of concept is underway and construction of the first pilot is scheduled to start in 2014. Source: Athabasca Oil Corporation Athabasca Oil Corporation has developed the thermal assisted gravity drainage (TAGD) method for the in-situ recovery of bitumen from these very porous carbonate reservoirs. TAGD uses minerally insulated electric conduction heaters installed in horizontal wells to heat the reservoir and bitumen gently through thermal conduction. Athabasca is using the conduction heaters to heat the reservoir to 140–160°C. At that temperature, the viscosity of the bitumen is similar to medium crude oil. Via gravity drainage, the bitumen flows to the bottom of the reservoir and is then recovered by the horizontal production wells.

This process offers several advantages over steam assisted gravity drainage (SAGD) in the carbonates. First of all, this gentle technique heats both the matrix and the bitumen evenly, allowing for efficient gravity drainage in this fractured and complex reservoir. Secondly, internal drive and voidage replacement is provided by in-situ vaporization of water and gas expansion through the recovery process. Finally, one of the most attractive aspects of the TAGD process is that there is no steam plant or water processing infrastructure required which results in a large upfront capital savings for the project.

The 3D Leduc surface from seismic showing well control and location of the fore-reef, reef crest and back reef facies. Source: Athabasca Oil Corporation

Athabasca Oil Corporation

Athabasca Oil Corporation was founded in 2006 as an Alberta, Canada-based company focused on the sustainable development of oil sands in the north-eastern Alberta Athabasca region and light oil resources located in north-western Alberta. They are poised to be producing 220,000 barrels of oil equivalent per day by 2020, about half from their Thermal Oil Division and the other half from their Light Oil Division.

The Thermal Oil Division is currently involved in five project areas that include over 9.2 billion barrels of contingent resources. Commercial production is expected in 2014. The Light Oil Division has three initial development areas that are targeted to be producing 10,000 – 11,000 boe/d by the end of 2012 using horizontal drilling and multi-stage hydraulic fracturing technology.

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