Changes

Although kerogen diagenesis and carbonate minerals are sources of CO2 and organic acids, Gaupp and Schoener<ref>Gaupp, R., and R. Schoener, 2008, Intrareservoir generation of organic acids and late stage enhanced porosity in sandstones (abs.): AAPG Bulletin Search and Discovery article 90078, AAPG National Convention, San Antonio, Texas.</ref> noted the potential of alkanes to be converted to acids.

Although kerogen diagenesis and carbonate minerals are sources of CO2 and organic acids, Gaupp and Schoener<ref>Gaupp, R., and R. Schoener, 2008, Intrareservoir generation of organic acids and late stage enhanced porosity in sandstones (abs.): AAPG Bulletin Search and Discovery article 90078, AAPG National Convention, San Antonio, Texas.</ref> noted the potential of alkanes to be converted to acids.

A moderate to high quartz content has played a significant role in allowing shale-gas resource systems to be stimulated because of their contribution to rock brittleness. Derivation of this quartz has largely been from biogenic sources instead of detrital, meaning it is closely associated with organic matter. As such, this close association with organic matter inhibits oil flow not only because of lower permeability in an organic-rich mudstone, but also because of adsorption to organic matter. However, in organic-lean rock, adsorption is minimized, thereby enhancing the possibility of free oil flow, with the remaining obstacle of overcoming low permeability in the typical tight-oil resource system by stimulation or hydraulic fracturing.

A moderate to high quartz content has played a significant role in allowing shale-gas resource systems to be stimulated because of their contribution to rock [[brittleness]]. Derivation of this quartz has largely been from biogenic sources instead of detrital, meaning it is closely associated with organic matter. As such, this close association with organic matter inhibits oil flow not only because of lower permeability in an organic-rich mudstone, but also because of adsorption to organic matter. However, in organic-lean rock, adsorption is minimized, thereby enhancing the possibility of free oil flow, with the remaining obstacle of overcoming low permeability in the typical tight-oil resource system by stimulation or hydraulic fracturing.

Adsorption plays a very significant role in unconventional resource plays. It accounts, in part, for the retention of oil that is ultimately cracked to gas in shale-gas systems and provides varying amounts of adsorptive storage in shales (as well as in coalbed methane). Oil expelled into organic-lean lithofacies does not exhibit the high adsorption affinities found in organic-rich mudstones, thereby allowing better production characteristics. The molecular size of crude oil is important, but its adsorptive affinities may be equally or even more important in flow rates. Based on experimental data from Sandvik et al.,<ref name=Sndvk1992>Sandvik, E. I., W. A. Young, and D. J. Curry, 1992, Expulsion from hydrocarbon sources: The role of organic absorption, Advances in Organic Geochemistry 1991: Organic Geochemistry, v. 19, no. 1–3, p. 77–87, doi:10.1016/0146-6380(92)90028-V.</ref> only 14% of resins (polar compounds of low viscosity) is expelled, whereas 86% of this oil fraction is retained in the source rock. A much higher percentage of nonpolar saturated and aromatic hydrocarbons are expelled (sim60%), with the balance being retained under the closed-system experimental conditions that Sandvik et al.<ref name=Sndvk1992 /> used.

Adsorption plays a very significant role in unconventional resource plays. It accounts, in part, for the retention of oil that is ultimately cracked to gas in shale-gas systems and provides varying amounts of adsorptive storage in shales (as well as in coalbed methane). Oil expelled into organic-lean lithofacies does not exhibit the high adsorption affinities found in organic-rich mudstones, thereby allowing better production characteristics. The molecular size of crude oil is important, but its adsorptive affinities may be equally or even more important in flow rates. Based on experimental data from Sandvik et al.,<ref name=Sndvk1992>Sandvik, E. I., W. A. Young, and D. J. Curry, 1992, Expulsion from hydrocarbon sources: The role of organic absorption, Advances in Organic Geochemistry 1991: Organic Geochemistry, v. 19, no. 1–3, p. 77–87, doi:10.1016/0146-6380(92)90028-V.</ref> only 14% of resins (polar compounds of low viscosity) is expelled, whereas 86% of this oil fraction is retained in the source rock. A much higher percentage of nonpolar saturated and aromatic hydrocarbons are expelled (sim60%), with the balance being retained under the closed-system experimental conditions that Sandvik et al.<ref name=Sndvk1992 /> used.