Environmental Engineering Reference
In-Depth Information
Figure 6 Eagle Shale Formation stratigraphic column, illustrating shale oil reservoir.
3.1 Organic Richness
The basic prerequisites for a productive thermogenic shale-gas play are an
organic-rich shale which has attained a gas-window level of thermal maturity
prior to any subsequent uplift. In detail, an effective hydrocarbon source
rock is required with development of porosity to store generated but non-
expelled hydrocarbons allied with a 'tight', very low permeability matrix to
isolate these pores and retain the trapped hydrocarbons.
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These shales
therefore have negligible natural flow capability except where natural frac-
tures have developed. Many such organic-rich shales can be demonstrated to
have been hydrocarbon-source rocks prior to becoming shale-gas reservoirs.
For a shale to be an effective hydrocarbon source rock requires a min-
imum organic-matter content, measured as TOC, of 2-3 wt%, comprising
oil-prone, hydrogen-rich labile kerogen with a Rock-Eval pyrolysis Hydrogen
Index (HI) of greater than 250 mg hydrocarbon(HC) g
1
TOC. Organic-rich
black shales deposited under marine anoxic conditions with a high content
of algal and planktonic kerogen form the most prolific oil source rocks,
typically with TOCs of 5-10 wt% and HI of 400-600 mg HC g
1
TOC.
Hydrocarbon generation occurs due
to burial-related increases
in
