Petroleum systems and fluid migration in Devonian reservoirs, Kaybob Alberta

Speaker: Mathew Fay | Shell Canada Inc.

November 20, 2018 |  11:30 am doors open 
Marriot Hotel, Kensington Ballroom | 110 9 Ave SE, Calgary AB T2G 5A6

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ABSTRACT 
The close vertical proximity of conventional and unconventional Devonian-aged reservoirs in the Kaybob region of central Alberta makes this an interesting study area to understand seal capacity and geochemical stratification.  This presentation will discuss how produced fluid sample geochemistry can be used to understand how fluids are generated and migrated in the region, and how the local geology impacts these processes in Devonian strata.  In addition, subtle changes in formation isochore thickness related to deep structures, and formation pressure data, are also useful when trying to understand the fluid migration in the Kaybob area.  

The prolific Duvernay Formation (Upper Devonian Woodbend Group) thickness ranges from 45 to 60 m and present day total organic carbon (TOC) contents ranging from 1 to 6 wt% (Figure 1).  There may also be minor hydrocarbon source potential in the underlying Majeau Lake Fm that has TOC ranging less than 1 wt%.  In the Kaybob area, the Duvernay Formation is a Type II organic rich, highly siliciclastic mudstone, that is variably interbedded with silty and carbonate rich intervals.  The aerial extent of  the Duvernay source rock is 130,000 km2 (Preston et al., 2016) and has generated an enormous volume of oil and gas.  The 2016 Alberta Energy Regulator best estimate of the total remaining unrisked contingent resources in the Duvernay is 1676 MMboe (354 MMboe proved reserves) which equates to about 350 to 540 trillion cubic feet of natural gas, 7 to 16 billion bbl of natural gas liquids, and 44 to 81 billion bbl of oil (Preston et al., 2016).  These hydrocarbons have migrated large distances into numerous reservoirs including the Leduc Formation carbonate buildups of the Rimby-Meadowbrook trend, Simonette, Windfall, and other Devonian aged reservoirs (e.g. Switzer et al., 1994; Allan and Creaney, 1991, Fowler et al., 2001).  

One of the interesting geochemical novelties of the Kaybob area is that the Duvernay is a sweet, clastic source rock that is surrounded by sour Devonian gas and oil reservoirs with high H2S concentrations ranging from 1 to 30 mol%.  For example, the prolific ‘North’ and ‘South’ Kaybob Swan Hills Formation oil and gas reservoirs respectively (Krouse et al, 1988) have been produced for decades.  The Duvernay mudstone has generated sweet petroleum fluids with < 0.2 wt% sulfur, that is consistent with a clastic Type II kerogen with abundant free iron.  Figure 1 shows a north to south cross section based on gamma, resistivity and sonic logs of the Duvernay and Majeau Lake mudstones underlain by the Beaverhill Lake Group (BHL) that includes the upper Waterways Member argillaceous carbonate mudstones and the Swan Hills Member carbonate platform and reef buildups.  The Duvernay Formation is overlain by the Ireton Fm. calcareous mudstones shale seal, with a thickness ranging from 160 to 330 m in the Kaybob area, that was deposited as regional scale prograding clinoforms (Stoakes, 1980).  The Ireton is an excellent regional seal from the Duvernay, that separate the overlying Nisku and Wabamun carbonates that also contain sour H2S gas and HS- water ranging from 1 to 30 mol% H2S concentration (IHS, 2017).  

Figure 1 illustrates how the upper Ireton and lower Waterways seal thicknesses surrounding the Duvernay source rock change through the region.  This seal thinning in some areas the Duvernay represent a risk for Duvernay horizontal wells to connect with to sour gas fluids in the Swan Hills, Nisku or Wabamun reservoirs.  For example, Chalmers (2016) showed that some Duvernay horizontal wells are seeing breakthrough with up to 0.7 mol% H2S concentrations reported (IHS, 2017).  This was interpreted as a downward hydraulic fracture connection to sour fluids in the Beaverhill Lake Group, and in some areas a lateral connection to the Leduc Formation.  Based on Chalmers (2016) analysis, an adequate lower seal thickness of Waterways shale, necessary to reduce H2S concentrations to 0 ppm in Duvernay horizontal wells, appears to be 40 to 60 m.  


Figure 1 – Geologic cross section Ireton, Duvernay and Beaverhill Lake (Waterways & Swan Hills) formations, plus a Duvernay isochore thickness map inset showing surrounding Leduc reefs (Figure from Chalmers, 2016).


BIOGRAPHY


Mathew Fay has worked for 6 years at Shell Canada, both as a Senior Production Geologist for the Fox Creek Duvernay Asset, and currently as Senior Geochemist for Canada. Overall, he has 15 years of industry experience in petroleum exploration and development of both unconventional and conventional resources. Mathew holds a Ph.D. from the University of Calgary related to organic & aqueous geochemistry of petroleum biodegradation, a B.Tech. in Environmental Engineering from BCIT, and a B.Sc. in Geology from UBC.