Page 70 - Fundamentals of Gas Shale Reservoirs

50 GEOCHEMICAL ASSESSMENT OF UNCONVENTIONAL SHALE GAS RESOURCE SYSTEMS

TOC (wt.%) TOC (wt.%)
o
o
Generative organic carbon Nongenerative organic carbon Nongenerative organic carbon
(GOC in wt.%) (NGOC o in wt.%) GOC pd
o
Carbon in expelled (NGOC pd in wt.%)
or otherwise
Carbon in pyrolysis yield (S2 )
co
in wt.%) lost petroleum S2
(GOC o cpd
FIGURE 3.1 A diagrammatic model of TOC showing a
hydrogen‐rich portion as generative organic carbon (GOC) and a
hydrogen‐poor portion as nongenerative organic carbon (NGOC). TOC (wt.%)
pd
The percentage of GOC is determined from the original hydrogen
content. Subscript “o” indicates original and “pd” indicates FIGURE 3.2 In this diagrammatic model of TOC, there is a net
present‐day. loss of organic carbon as a result of expulsion by the amount of
carbon in expelled petroleum. There is additional organic carbon
loss in the sample retrieval, handling, storage, and processing steps.
quantified by the amount of organic carbon, which excludes Subscript “o” indicates original and “pd” indicates present‐day, and
any inorganic carbon in carbonates. The quantity of organic “cpd” means corrected present‐day.
carbon is measured and reported as total organic carbon
(TOC) and is reported in weight percent of organic carbon in
the rock matrix. The volume of the rock occupied by TOC is cracking of the GOC portion of the TOC, providing an
approximately double the weight percent due to its low indirect indication of its hydrogen content by calibration
density relative the inorganic rock matrix. with hydrocarbon or other rock standards. The pyrolysis
A simple diagrammatic model of immature (and oil‐free) yield, referred to in instrument terminology as S2 (origi­
TOC referred as original TOC (TOC ) shows a somewhat nally, signal 2 from the instrument), is the remaining or
o
arbitrary differentiation between hydrogen‐rich organic present‐day petroleum generation potential. Except for
carbon and hydrogen‐poor organic carbon (Fig. 3.1). The immature source rocks, this value is the present‐day or
hydrogen‐rich organic carbon is referred to as the gene­ remaining petroleum generation potential (S2 ). It is
pd
rative organic carbon (GOC) as it is this portion of TOC that reported in mg petroleum potential per gram of rock (mg/g
generates petroleum with increasing thermal exposure (mat­ rock). Prior to generation of petroleum, this pyrolysis yield
uration). The remaining portion of the TOC is referred to as reflects the original GOC (GOC ) portion of TOC and is an
o
o
nongenerative organic carbon (NGOC), which has no indication of the original petroleum generation potential
commercial petroleum generation potential. Cooles et al. (S2 ) (see Fig. 3.1). The S2 can be converted to organic
o
o
(1986) referred to these constituents as labile and inert, carbon by multiplying by 0.085 (assuming 85% carbon in
which is less descriptive and somewhat inaccurate as the petroleum), which is then the carbon in GOC . The original
o
so‐called inert organic carbon functions as an adsorbent and HI (HI ) value is S2 /TOC × 100 (mg/g TOC).
o
o
o
appears to play a catalytic role in directing hydrocarbon This original petroleum potential (S2 ) can also be con­
o
product distributions (Alexander et al., 2009; Fuhrmann verted to barrels of oil equivalent per acre‐foot by simple
et al., 2003). conversion. A pyrolysis generation potential of 1 mg
While the quantity of organic carbon is important to petroleum/g rock would be equivalent to a generation poten­
measure a source rock’s capacity to generate petroleum, tial of about 22 boe/acre‐foot assuming a rock density of
hydrogen content must be assessed to determine the amount 2.7 g/cc and a petroleum density of 0.85 g/cc. If the thickness
of petroleum that can be generated from a given amount of of the source rock is known, this can be converted to boe/
TOC. Hydrogen is always the limiting factor in gas genera­ section remembering that it is an indication of the total
tion, for example, while alkanes take at least two hydrogens petroleum generation potential and does not account for
per carbon, methane requires four. A qualitative measure of incomplete maturation of kerogen. If S2 is known, the
o
the hydrogen content of a source rock is determined by actual amount of petroleum generated at a given thermal
pyrolysis of the source rock. This is part of the data mea­ maturity can be determined by subtracting S2 from S2 .
pd
o
sured from the Rock‐Eval or HAWK SM2 pyrolysis instru­ The extent of kerogen conversion or transformation ratio
®1
ments as the hydrogen index (HI). Such pyrolysis results in (TR) can be calculated as, e.g., 1176 (HI – HI )/(HI
pd
o
o
(1176 – HI )) or a simple version as (HI – HI )/HI .
o
pd
o
pd
When petroleum is expelled, there is a net loss of organic
®
1 Rock‐Eval is a registered trademark of Institut Francais du Petrole. All
rights reserved. carbon due to the carbon in the expelled petroleum (Fig. 3.2).
2 HAWK is a sales mark of Wildcat Technologies. All rights reserved. There is also carbon loss from cuttings, SWC, or core due
SM