25th International Meeting on Organic Geochemistry IMOG 2011

O-83
Evolution of ―shale gases‖ in fractured reservoirs in the foothills
of the Western Canada sedimentary basin
Barbara Tilley, Karlis Muehlenbachs
University of Alberta, Edmonton, Canada (corresponding author:btilley@ualberta.ca)
We have reported that isotopically reversed gases
predominate in Permian and Triassic fractured, mixed
clastic-carbonate reservoirs in the Western Canada
Foothills. We suggest that this resource is actually
mature shale gas in disguise (Tilley et al., AAPG
Bulletin, 2011, in press). Simultaneous cooking of
kerogen, oil and gas in closed system shales yielded
gas with light � 13 C ethane and heavy � 13 C methane.
This very mature gas was released from the shales
and trapped in fractured folds of brittle clasticcarbonate
rocks during deformation and thrust faulting
of the Laramide Orogeny, creating prolific gas pools.
Whereas conventional gas reservoirs worldwide, that
show no isotope reversal, have captured gas released
from shales during burial, shale gas is a resource
retained in place as the gas evolves from the
simultaneous destruction of kerogen, oil and gas.
The presence of reversed gases in the fractured
reservoirs in the WCSB Foothills is due to the unique
combination of (a) high maturity of the shale source
rocks, (b) tight, non-permeable rocks surrounding the
source shale (c) intense tectonic disturbance after the
source rocks reached advanced stages of maturation,
ie. after oil and gas cracking had been initiated, and
(d) deformation of the shale source rocks to both
squeeze out the gas into surrounding newly fractured,
brittle rock as well as to provide a seal.
Foothills gas reservoirs are highly compartmentalized
as reflected in the large variation of isotope ratios
from closely spaced wells (Fig. 1). Gases can be
grouped according to putative differences in the
timing of gas release from the shale and the degree of
openness or closeness of the system and whether or
not the entire gas package was preserved.
Gp 1: - Mature gas released from shale during the
tectonic disturbance.
Gp 2: - Shale gas with a greater contribution from oil
cracking than Gp 1. (The most prolific gas producing
wells).
Gp 3: - Conventional gas reservoirs. Early gas
released during burial after only kerogen maturation.
Gp 4: - Gas from oil cracking. Over mature methane
from kerogen has been mostly lost. (Poor producers)
Gp 5: - Loss of some of the Gp 1 shale gas package.
(Wells may have a water leg.)
Questions to be asked are: Can one generalize from
the Foothills‘ reservoirs to mature shale gas
worldwide? Are the isotope reversals or ―rollovers‖
that are frequently seen in productive shale gas plays
the result of closed system, simultaneous kerogen
and oil cracking? The shale gases in the Foothills
fractured reservoirs and in the Horn River Basin are
very dry compared to shale gases reported from the
U.S.A. How and why does the magnitude of the
isotope effect in gases varying with an order of
magnitude difference in gas wetness? We suggest
that prolific reserves of gas in high maturity shales
around the world have formed analogously to Groups
1 and 2 gases in the Foothills. It would seem that in
the very dry gas systems, the magnitude of the
isotopic reversal is much larger than in the wet
systems, perhaps reflecting a greater degree of oil
cracking.
Fig.1. � 13 C methane versus � 13 C ethane cross plot of WCSB Foothills gases.
Gas groups 1 to 5 are described in the text. Symbol shapes indicate location:
crosses:-Sukunka, BC; triangles –Narraway, Alberta; squares - Minnow, Alberta.
Green symbols - Permian, blue and purple - Triassic; grey -Devonian.
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