Page 72 - Fundamentals of Gas Shale Reservoirs

52 GEOCHEMICAL ASSESSMENT OF UNCONVENTIONAL SHALE GAS RESOURCE SYSTEMS

3.5 KEROGEN TYPE AND COMPOSITIONAL Primary organic macerals found in kerogen include
YIELDS gas‐prone vitrinite, oil‐prone exinite and alginite, as well as
hydrogen‐poor inertinite. These organoclasts as identified by
Typical shale gas resource systems have HI values of Alpern (1980) can be used to evaluate their combined oil and
o
about 350–700 mg/g TOC typical of Type II kerogen gas potential (Fig. 3.4). Also using the approach of Baskin
(Espitalie et al., 1977; Jones, 1984). At such values, only (1997), the original H/C ratio can be estimated. Marine‐
30–60% of their original TOC can be converted to petro­ source rocks will often have a mixture of these organoclasts,
leum with the difference being the percentage of NGOC and this can alter the distribution of products expected at a
o
(Jarvie, 2012a). given thermal maturity.
A detailed visual and chemical classification scheme While kerogen type using hydrogen indices is generally
modified from Jones (1984) and Hunt (1995) provides var­ acceptable particularly with larger data sets on extracted
ious characteristics of different kerogen types and the pri­ rock samples, in new plays both elemental and visual ker­
mary products through the oil window (Table 3.1). All ogen analyses should be completed in addition to TOC and
kerogen types except Type IV can yield commercial amounts pyrolysis analyses on whole and extracted rock samples. As
of petroleum that will ultimately crack to gas. Type IV the kerogen isolation step has been utilized for elemental
organic matter will only yield minor amounts of dry gas but analysis of CHONS, it should then also be used for visual
could act as a carbon catalyst. kerogen analysis and vitrinite reflectance as visual inspection
Prior to the development of pyrolysis instruments for of the maceral colors will aid the determination of the
source rock analysis in the 1970s (Barker, 1974; Espitalie autochthonous vitrinite population.
et al., 1977), atomic hydrogen and oxygen to carbon ratios While thermal maturity is often used to provide an indica­
(H/C and O/C) were commonly used for chemical classi­ tion of the expected products based on the level of conversion
fication of kerogen type following the conventions used in of kerogen or these individual organoclasts, Waples and
the coal industry. Coal petrologists describe coals as Type Marzi (1998) demonstrated vitrinite reflectance is not a
I, II, or III depending on organoclasts identified in kerogen. universal indication of the level of conversion of a given
These organoclasts had specific ranges of H/C ratios as kerogen. If feasible, it would be preferable to complete com­
determined by elemental analysis. Atomic H/C ratio is positional analysis via laboratory maturation techniques to
more precise than HI in assessing hydrogen content due to determine what products are present at a given level of
working with isolated kerogen, thereby eliminating any transformation (conversion) of kerogen. Such maturation
clay adsorption effects as well as any oil and bitumen in the techniques in closed systems yield products akin to those
pyrolysis peak as is common with pyrolysis of organic‐ generated by geological processes. Closed system tech­
rich–source rock samples. However, isolation of kerogen is niques include gold tubes, hydrous pyrolysis cells, or high‐
not a simple process and even utilizing the most exacting purity Pyrex glass or quartz tubes (e.g., microscale‐sealed
isolation procedures will often leave small amounts of inor­ vessel (MSSV) pyrolysis) (Horsfield et al., 1989, 2015).
ganic matter including clays but particularly pyrite that is MSSV analysis has the advantage of working with small
intimately associated with the organic matter. While data amounts of sample (1–5 mg), whereas the other techniques
has been published showing the sometimes poor agreement require a few to tens of grams of sample and can be used to
between H/C and HI (e.g., Baskin, 1997), at least two measure yields of hydrocarbons and nonhydrocarbons by
things can affect measurement of pyrolysis yields: (1) the liquid chromatography.
presence of extractable organic matter or organic additives One reason for performing these maturation experiments
that carryover into the pyrolysis peak and (2) inorganic particularly early in the evaluation of new plays is because
adsorption effects particularly in leaner source rocks (<2% bulk pyrolysis instruments use a FID. While a FID is a very
TOC) (Espitalie et al., 1984). If there is double as to the useful and handy analytical tool, it responds to carbon ioni­
relative hydrogen abundance, analyze a solvent extract zation not hydrogen content. Thus, the hydrogen potential
rock sample or extracted kerogen by pyrolysis or run ele­ could be either under or overestimated in some systems,
mental analysis for carbon, hydrogen, oxygen, nitrogen and where either aromatic compounds or saturated hydrocarbons
sulfur (CHONS). are in abundance relative to one another. For example, the
Visual analysis of kerogen is another tool for assessing difference between two 6‐carbon atom compounds, benzene
the petroleum potential of a resource system. By this versus hexane, is eight hydrogen atoms (C H vs. C H ,
14
6
6
6
measurement, the population of hydrogen‐rich macerals pro­ respectively), whereas the FID will respond to each with
vides an indication of the petroleum generation potential and approximately the same response. This was demonstrated in
can be used with elemental analysis to determine original select source rocks where a high HI source rock (487 mg
H/C values (Baskin, 1997). These can be converted to HI petroleum potential/g TOC) yielded primarily gas (Alum
values using the formula of Orr (1981), that is, HI = (694 Shale), and a moderate HI (310 mg petroleum potential/g
(H/C – 0.29) – (800 O/C)). TOC) yielded primarily oil (Nigerian Type II/III source