Page 69 - Fundamentals of Gas Shale Reservoirs

KEROGEN QUANTITY AND QUALITY 49
Finally, it should be noted that additional societal benefit typically having original hydrogen indices (HI) from 350 to
comes from the royalties paid to mineral right owners, 700 mg/g TOC, these source rocks are classified as Type II
whether landowners or governments. In North Texas, kerogen based on chemical and visual analyses that reflect
independent school districts received approximately $51 the depositional settings (Jones, 1984; Tissot and Welte,
million, municipalities $86 million, and the University of 1978). These kerogens are organic‐rich, typically greater
Texas $5 million in royalty payments, bonuses, and tax than 2.00 wt% TOC with sufficient hydrogen content to form
revenues from Barnett Shale operations in 2010 alone abundant amounts of hydrocarbons. In order to explain the
(The Perryman Group, 2011). An independent petroleum high content of gas in commercial shale gas systems, there
company paid $271 million in royalties in 2010 to individ­ must be sufficient organic matter to yield petroleum upon
uals, cities, counties, school districts, and other public and cracking, but also to account for retention of secondary prod­
private entities; this same firm has paid over $1.1 billion ucts, for example, bitumen. Retention is a function of various
since beginning operations in the Barnett Shale (The factors such as permeability and pressure, but also the affinity
Perryman Group, 2011). of the polar constituents of bitumen and petroleum, that
is, resins and asphaltenes, as well as hydrocarbons to be
adsorbed and imbibed in the organic matter matrix. The
3.2 OBJECTIVE AND BACKGROUND molecular size and structural complexity of organic matter
also act as a molecular sieve retaining a range of petroleum
The key objective of this chapter is to describe shale gas compounds. This combined retention by adsorption and
resource systems in terms of their organic geochemical absorption may be best referred to as sorption (Levine, 2013).
characteristics. Shale gas resource systems are petroleum‐ The most labile portion of kerogen cracks in the oil
source rocks, that is, organic matter from which petroleum is window as do most of the asphaltenes and a substantial por­
generated, that also act as a reservoir rock. Organic matter is tion of the resins. Saturated hydrocarbons begin to crack in
typically only a minor constituent of the total rock matrix the late oil window depending on molecular size and branch­
that consists primarily of clays, silicates, and carbonates in ing. This is most obvious in condensates where the hydro­
varying proportions. It will often only be about 5% by mass carbon composition is highly concentrated in the C carbon
20‐
or about 10% of the volume of a typical marine petroleum‐ range. Refractory kerogen, C hydrocarbons, hydrocarbon
20‐
source rock. It is this organic matter that yields petroleum, gases, and alkylated aromatics crack in the gas window. This
that is, gas and oil, upon increasing temperature exposure portion of cracking occurs above about 150°C (Claypool and
due to various sources of heat such as burial and heat flux Mancini, 1989). This not only results in enhanced volumes
from the mantle, hydrothermal heating, and heating by of gas but also removes the more polar and viscous compo­
radioactive decay. Insoluble organic matter or kerogen is nents of petroleum that tend to restrict petroleum flow from
formed at temperature below 90°C, and petroleum genera­ such tight rocks with sorptive kerogen and bitumen.
tion occurs over a temperature range of about 90–150°C The extent of kerogen conversion is indicated by thermal
with peak generation occurring at about 120°C for marine maturity, although this must be in the context of under­
carbonates and about 135°C for marine shales using BP’s standing the kinetics of decomposition of organic matter. Oil
organofacies “A” and “B” kinetic data (Pepper and Corvi, window maturity indicates that the primary product is oil, but
1995) at 2°C/ma. gas is also generated albeit in substantially lesser quantities
While kerogen cracking is thought of as being the principal depending on the source rock type with Type III source rocks
source of hydrocarbons, they are in fact primarily derived generating more gas relative to oil (Tissot and Welte, 1978).
from decomposition of soluble bitumen (Behar and Jarvie, Thus, gas kicks during drilling can be found in the oil window
2013; Behar et al., 2008a), the primary product of kerogen despite lower volumes of generated gas, and as such, do not
cracking. In laboratory experiments, kerogen yields less than necessarily indicate a shale gas play. It is in the gas window
35% hydrocarbons, whereas bitumen cracking yields the where shale gas resource systems become commercially pro­
remainder of generated hydrocarbons. Hydrocarbons are ductive. This is due to the volume of gas being highest due to
strictly carbon‐ and hydrogen‐bearing molecules, whereas both kerogen and retained petroleum cracking. In addition,
nonhydrocarbons, such as resins and asphaltenes, are the this process also removes the larger, more polar molecules,
primary constituents of bitumen. To distinguish insoluble thereby enhancing flow out of such low permeability rock.
kerogen cracking from petroleum cracking, the terms primary
and secondary cracking are used, respectively.
Kerogen is derived from deposited, preserved, and insolu­ 3.3 KEROGEN QUANTITY AND QUALITY
bilized biomass, which can be deposited in lacustrine, marine,
fluvial–deltaic, or terrestrial settings. However, the predomi­ A key component in the volume of petroleum that can be gen­
nant setting for highly productive shale gas systems to date erated is the relative quantity and quality of organic matter
has been in marine depositional settings. With such systems preserved from the deposited biomass. Organic richness is