Page 22 - Fundamentals of Gas Shale Reservoirs

2 GAS SHALE: GLOBAL SIGNIFICANCE, DISTRIBUTION, AND CHALLENGES

Shale, as a function of its traditionally low permeability, divided as marine shale, marine–terrigenous coal bed car­
also often serves as a sealing lithology within the trapping bonaceous shale, and lacustrine shale. The depositional
mechanism of a conventional gas play, which prevents oil setting directly controls key factors in shales, such as
and gas accumulations from escaping vertically (Gluyas organic geochemistry, organic richness, and rock compo­
and Swarbrick, 2009). sition. According to Potter et al. (1980), the organic matter
Generic global hydrocarbon estimates have always some­ preserved in shales depends on the dissolved oxygen
what reflected resources in‐place within tight formations level in the water.
and shale. However, it is the relatively recent technological Shale gas organic geochemistry is a function of the depo­
developments and higher gas prices that have now resulted sitional environment and is similar to conventional source
in a vast resource being considered potentially economic, rock geochemistry. Marine shale is typically associated
which had previously been considered uneconomic to with Type II kerogen (i.e., organic matter associated with a
develop (Ridley, 2011). mixture of membraneous plant debris, phytoplankton, and
Sources indicate that shale is present in a very wide bacterial microorganisms in marine sediments). Lacustrine
range of regions across the globe, with an estimated 688 shale is generally associated with Type I kerogen, due to the
shale deposits occurring in approximately 142 basins organic matter being associated with an algal source rich
(Ridley, 2011). in lipids (typically only in lacustrine and lagoonal environ­
ments). Finally, terrestrial/coal bed shale is typically associ­
ated with Type III kerogen, due to the organic matter being
1.2.1 Shale Gas Geology
associated with higher plant debris, as commonly found in
Shale gas is a natural gas produced from organic‐rich fine‐ coal‐bed‐forming environments such as delta tops (Gluyas
grained low‐permeability sedimentary rocks, such as shale, and Swarbrick, 2009).
where the rock typically functions as both the “source Target TOC (wt% kerogen) values are somewhat interre­
rock” and the “reservoir rock,” to use terms associated with lated to the thickness and other factors that influence gas
conventional plays (US DOE, 2009). The relationship bet­ yield. However, for commercial shale gas production, Staff
ween conventional and unconventional gas is illustrated in (2010) notes a target TOC of at least 3%, whilst Lu et al.
Figure 1.1. (2012) states that a TOC of 2% is generally regarded as the
Gas shale is similar to traditional shale in terms of the lower limit of commercial production in the United States.
range of environments of deposition. For example, That said, TOC varies considerably throughout any one
Caineng et al. (2010) note that organic‐rich shale can be shale gas play.

Land surface

Conventional
nonassociated Coal bed methane
gas
Conventional
associated
gas
Seal Oil

Sandstone Tight sand
gas

Gas-rich shale

FIGURE 1.1 Schematic geological section illustrating the fundamental geological principles associated with conventional and unconven­
tional hydrocarbons. Shale gas is designated as “gas‐rich shale” (from EIA, 2010).