Page 78 - Fundamentals of Gas Shale Reservoirs

58 GEOCHEMICAL ASSESSMENT OF UNCONVENTIONAL SHALE GAS RESOURCE SYSTEMS

values using a 2D basin model for the Barnett Shale (Jarvie to gas cracking and demethylation of aromatics leading to
et al., 2007). However, this is variable depending on the com­ a graphitic‐like structure of metamorphosed kerogen and
position of petroleum and values are hypothesized to be carbonaceous char.
lower due to evaporative losses of light hydrocarbons prior
to laboratory analysis and also due to petroleum being
retained in the kerogen. This is a function of sorption and low 3.11 UPPER MATURITY LIMIT FOR SHALE GAS
viscosity of the polar compounds, but also the original gener­
ation potential of source rocks in both organic carbon and While thermal maturity above the latest oil window is indic­
hydrogen content as shown by Burnham and Braun (1990) ative of increasing amounts of gas, there does appear to be
and Pepper (1991). One criterion for expulsion to occur is an upper limit for increasing gas content. From drilling
for the oil content to exceed the adsorption index (AI) as results in areas above about 3.5%R , no commercial produc­
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proposed by Jarvie et al. (2013). tion has been achieved, although an independent gas com­
In the Barnett Shale, it was found that gas flows would be pany’s well, the Hallwood Petroleum Right Angle Minerals
lower if petroleum remained in the system, that is, in the oil #1, flowed about 600 mcf of gas per day at about 3.7%R
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window, which was hypothesized to be due to the occlusion from a Fayetteville Shale well in the eastern portion of the
of nanodarcy permeability by more viscous and polar petro­ Arkoma Basin, Arkansas. While drilling tests in high matu­
leum constituents (Jarvie et al., 2007). Thus, it is important rity areas have not been extensive, no commercial produc­
to have these constituents cracked as completely as possible tion has occurred in the Fayetteville and Marcellus shales at
to limit their occluding capacity and to obtain increased such high maturities. The reasons for this are unknown, but
pressure for optimum gas flow. This is applicable to a lesser it is certainly not the stability of methane alone under related
extent in shale oil resource system plays. temperature regimes at such levels of thermal maturity.
However, alteration of methane may be transpiring in the
presence of hot mineral matrix, associated gases, and water.
3.10 SECONDARY (PETROLEUM) CRACKING For example, it was postulated by Barker and Takach (1992)
that methane oxidation by water could be a possible mecha­
The decomposition of organic matter into petroleum is nism for loss of methane and increased yield of carbon
dependent primarily on organic matter composition and dioxide as well as hydrogen. In their discussion, this was
structure as well as the temperature regime in which it has preceded by the formation of methane as a result of reaction
resided. It is well known that kerogen decomposes into of graphitic carbon with water. An additional alternative
petroleum under increasing temperature by cracking of the explanation could be alteration of the rock fabric and destruc­
weakest bonds first; as the temperature continues to increase tion of pores at such levels of maturity. However, in the
under deeper burial, the more refractory (difficult to break) experimental work of Barker and Takach (1992), loss of
bonds begin to crack until the final products are methane and methane occurred under laboratory conditions where the
carbonaceous char. initial concentration of methane was not dependent on in situ
What is commonly misunderstood is the difference bet­ storage, that is, gas and sandstone were combined in a pres­
ween petroleum versus oil cracking. Oil cracking is often surized cylinder and heated. In their experiment at ca.
defined as alkane (paraffin) cracking (e.g., Behar and 3.5%R , methane volume was reduced 50% over the starting
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Vandenbroucke, 1996; Burnham et al., 1997; Fabuss et al., volume (Fig. 3.9). In this particular experiment, only
1964; Ford, 1986; Tsuzuki et al., 1999), which is quite dif­ methane was present so no additional gas was generated
ferent from petroleum cracking. As defined earlier, petro­ from refractory kerogen or other organic matter as would
leum is any of the secondary products resulting from occur in a source rock. These results suggest that higher
kerogen decomposition, which include bitumen, oil, and yields of methane occur at equivalent vitrinite reflectance
gas. Bitumen cracking begins nearly contemporaneously values under 3.5%R . While the empirical and experimental
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with its generation from kerogen (Behar et al., 2008a, b). evidence of Barker and Takach (1992) are not proof of
Similarly, it has been shown experimentally that asphaltenes methane destruction at high thermal maturity and tempera­
decompose over the temperature range of kerogen by tures, when combined with the poor drilling results, cer­
asphaltene pyrolysis (di Primio et al., 2000). This is sub­ tainly suggests greater risk for commercial amounts of gas in
stantiated by the empirical observation of decreasing such high maturity areas.
asphaltene content in more mature source rock extracts. On the other hand, it has been shown experimentally that
The same is true of resins shown experimentally (Behar methane gas generation at elevated thermal maturities does
et al., 2008a, 2010) and again empirically in fractionation occur (e.g., Behar and Jarvie, 2013; Behar et al., 2008a;
yields from oils of various maturities. At the end of the oil Erdmann and Horsfield, 2006; Mahlstedt and Horsfield,
window, cracking has largely shifted from cracking of these 2012). Their separate experimental results show that ker­
polar compounds to the cracking of alkanes and ultimately ogen can generate about 30% of its total potential for gas at