Page 74 - Fundamentals of Gas Shale Reservoirs

54 GEOCHEMICAL ASSESSMENT OF UNCONVENTIONAL SHALE GAS RESOURCE SYSTEMS

Alginite + Exinite reflected recorded by a photomultiplier. If vitrinite is present,
a histogram of the readings is recorded with the mean value
10 90 and standard deviation of the indigenous population
20 Oil prone 80 recorded. Thus, between 1 and 50 readings are completed
30 70 depending on the presence of vitrinite particles with the
microscopist determining the indigenous population of read­
40 60 ings. The mean value is then often mapped by geologists in
50 50 construction of thermal maturity maps.
Mixed As expected, such particles are very common in Type III
60 40
organic matter deposited in fluvial‐deltaic or terrestrial set­
70 30 tings. For example, coals, coal lenses, coaly shales, or near­
80 20 shore organic lean samples are generally the best samples for
Gas prone determination of vitrinite reflectivity. In Type II organic
90 10
matter that has been derived from organic matter deposition
in deep marine settings, vitrinite particles are far less
Inertinite 10 20 30 40 50 60 70 80 90 Vitrinite
common. If present, it must be ascertained whether these
FIGURE 3.4 A ternary plot of recorded percentages of primary particles are autochthonous or allochthonous. Type II kero­
organoclasts from a visual kerogen assessment are plotted, and the gens, from which all the highly commercial North American
relative abundance of oil prone versus gas prone organic matter is shale gas has been derived, have minimal amounts of indig­
visualized. enous vitrinite resulting in difficulties in measurement and
consequently, interpretation of thermal maturity.
rocks) (Jarvie, 2000, Compositional Kinetics of Select The optimum methodology for performing vitrinite
Petroleum Source Rocks, unpublished data). reflectance is to complete a well profile including samples
from above the zone of interest at 500 ft intervals. Type III
shales or coaly intervals are very helpful for constructing
3.6 THERMAL MATURITY such a profile. While samples from coal lenses are ideal for
a high concentration of indigenous vitrinite, it is also pos­
Knowing the TOC and relative hydrogen contents from H/C, sible that these may cave into lower sections, thereby yield­
HI, or organoclasts is key to evaluating the overall petroleum ing lower reflectivity values indicative of the overlying coal
generation potential. With this information, an indication of lens(es). Samples of the shale of interest should also be mea­
the maximum temperature exposure of a sediment is also sured and in particular the color and any fluorescence of
needed, that is, measurement of thermal maturity. Thermal organic particles noted. Lighter yellow and browns indicate
maturity is used to describe the organic matter decomposi­ a less mature indigenous population whereas darker colors
tion as being in the oil or gas window. While the oil window help the microscopist focus on the more mature population.
is often thought to be exclusive of gas, some gas is generated Often the organic‐rich shale of interest will have values dif­
in all source rocks. However, the oil window has predomi­ ferent from the profile either due to the presence of bitumen
nantly liquid petroleum products with varying amounts of or pyrobitumen, which have different reflectivities from
gas relative to oil depending on kerogen type, whereas the the morphologically similar vitrinite particles (Landis and
gas window has only remnants of liquid petroleum and much Castaño, 1995).
more gas depending on how mature the system is. An example maturation profile for a Barnett Shale well is
Vitrinite reflectance is an indication of the maximum illustrated in Figure 3.5. Fitting all the measurements
paleo‐temperature exposure to which a sediment has been including those in the Barnett Shale suggests a good correla­
exposed. This can be quite different from bottom‐hole tem­ tion (dotted line) but with the Barnett Shale in the late oil
perature (BHT) values as sediments are often buried deep window. When the fit is projected through the Barnett Shale
and subsequently uplifted to lower temperatures. While tem­ using only the mean vitrinite reflectance measurements from
perature is the key factor in petroleum generation and related the overlying sediments, a different interpretation is reached,
increase in reflectivity of vitrinite particles, heating rate also that is, the Barnett Shale is in the early gas window at 1.10–
plays a role and can result in variations of upward of 10°C in 1.25%Roe. To resolve this discrepancy, the chemical data
peak generation rates. such as HI values help determine the correct reading. In
pd
Vitrinite is an organoclast derived from catagenetically this well because the HI values are less than 100 mg/g TOC
altered woody plant tissues, that is, fossilized wood parti­ with over 4% TOC, it is interpreted that the Barnett Shale is
cles. When such particles are identified as autochthonous in in the gas window matching the uphole profile (dashed line).
an isolated kerogen or whole rock mount, a light beam is The Barnett Shale at this early gas window thermal maturity
shined onto the polished surface and the amount of light still contains small amounts of bitumen and pyrobitumen.