2- Ingles (1)

Basic Petroleum Geochemistry for
Source Rock Evaluation

As the search for oil and gas prospects grows increasingly complex, more E&P
companies are turning to geochemistry to evaluate a component that is central to the
success of each well: the source rock.

Kevin McCarthy Every oil or gas play originates from source rock. richness and thermal maturity.1 Although its
Katherine Rojas The viability of each play—conventional or volume—a function of thickness and areal
Houston, Texas, USA unconventional, oil or gas—depends on its extent—must not be discounted, this article
source rock. Without this source of petroleum, all focuses on the other two characteristics. Organic
Martin Niemann other components and processes needed to richness refers to the amount and type of organic
Roissy-en-France, France exploit a play become irrelevant. matter contained within the rock. Thermal matu-
rity refers to a source rock’s exposure to heat over
Daniel Palmowski A source rock can be broadly defined as any time. Heat increases as the rock is buried deeper
Aachen, Germany fine-grained, organic-rich rock that is capable of beneath successive layers of sediment. It is the
generating petroleum, given sufficient exposure thermal transformation of organic matter that
Kenneth Peters to heat and pressure. Its petroleum-generating causes a source rock to generate petroleum.
Mill Valley, California, USA potential is directly related to its volume, organic

Artur Stankiewicz
Clamart, France

Oilfield Review Summer 2011: 23, no. 2. Vertical drilling Vertical drilling
Copyright © 2011 Schlumberger.
For help in preparation of this article, thanks to Extended-reach drilling
Nicholas Drenzek, Cambridge, Massachusetts, USA.
FLAIR is a mark of Schlumberger. Coalbed methane
Rock-Eval is a mark of the Institut Français du Pétrole.
Rock-Eval 6 is a mark of Vinci Technologies SA. Nonassociated gas Associated gas
Conventional Seal Oil
1. In this article, the term “petroleum” refers not only to structural trap
crude oil, but also to the fluid in either its liquid or Sandstone reservoir Gas in tight sand
gaseous state. Also, the term “hydrocarbons,” which
tends to be used casually in the oil industry, will be Shale source rock Conventional
treated as a loose equivalent to “petroleum.” stratigraphic trap

2. Kerogen is the particulate organic matter disseminated > Petroleum systems. A conventional petroleum system requires four components: source rock, reservoir
within sedimentary rocks that is insoluble in common rock, trap and seal—and two processes: petroleum generation and migration. Timing between petroleum
organic solvents. Bitumen is a thermally degraded migration and creation of the reservoir, trap and seal is also critical. Unconventional systems require, at a
derivative of kerogen, but is soluble in organic solvents. minimum, the deposition of source rock and sufficient overburden to achieve thermal maturity.
The exact relationship between kerogen, bitumen and the
hydrocarbons that evolve during heating of organic
matter is still a subject of investigation. For more on these
components: Peters KE, Walters CC and Moldowan JM:
The Biomarker Guide, 2nd edition. Cambridge, England:
Cambridge University Press, 2005.

3. Magoon LB and Dow WG (eds): The Petroleum System—
From Source to Trap. Tulsa: The American Association of
Petroleum Geologists, AAPG Memoir 60, 1994.

32 Oilfield Review

The mechanisms by which oil and gas are gen- alteration as heat increases; these changes, in This basic model is integral to one of the
erated vary from basin to basin depending on turn, result in an evolution of the petroleum com- most fundamental concepts of oil and gas explo-
sedimentary facies, burial history, tectonics and pounds that are subsequently generated. Further ration: the petroleum system.3 This concept
other geologic processes; however, the general heating converts the kerogen, yielding bitumen accounts for the generation, expulsion, migra-
model is fairly straightforward. Following deposi- and petroleum.2 The kerogen becomes more tion and accumulation of oil and gas—and
tion of organic-rich sediments, microbial pro- depleted of hydrogen as it gives off petroleum. source rock lies at its foundation (previous
cesses convert some of the organic matter into Increasing maturity also causes initially complex page). In conventional petroleum systems, over-
biogenic methane gas. Greater depths of burial petroleum compounds to undergo structural sim- burden rock buries the source rock to depths
are accompanied by increases in heat in accor- plification—typically starting with oil, then wet where petroleum is generated. Petroleum gener-
dance with the basin’s geothermal gradient. This gas and ending at dry gas. ated by thermally mature source rock is expelled
heat causes the organic matter to gradually into a porous and permeable carrier bed and
transform into an insoluble organic matter
known as kerogen. The kerogen continues its

Summer 2011 33

then migrates into a reservoir rock, where it The world’s remaining oil and gas resources Source Rock Fundamentals
becomes structurally or stratigraphically trapped are becoming more challenging to find and
beneath an impermeable seal.4 However, the requi- develop. As the industry targets these resources, Source rocks result from a convergence of
site processes of petroleum generation, expulsion, the need to thoroughly understand and character- physical, biochemical and geologic processes
migration and trapping are not always carried to ize all components of prospective petroleum that culminate in the formation of fine-
completion, as evidenced by the myriad of dry systems becomes more acute. Thus, in addition to grained sedimentary rocks containing carbon-
holes drilled into clearly delineated traps. assessing the reservoir, trap and seal of their and hydrogen-rich organic matter. The amount
prospects, E&P companies must evaluate the and type of organic material incorporated into a
Ongoing advances in production technology petroleum-generating capacity of the source rock. source rock are controlled, in part, by environ-
are opening new plays to exploration and devel- Petroleum geochemistry is improving the effi- mental and depositional conditions. Source rocks
opment. Resource plays, exemplified by the boom ciency of exploration and production through form where environmental conditions support
in shale gas production, are founded on uncon- characterization of the elements and processes biologic activities that produce large quantities
ventional petroleum systems, where source rock that control the richness and distribution of of organic matter, where depositional conditions
also serves as reservoir and seal. Petroleum gen- petroleum source rocks, thus providing valuable concentrate this matter and where postdeposi-
erated in this self-contained petroleum system input for basin and petroleum system modeling. tional conditions permit its preservation.7
does not migrate but remains trapped within the This article describes basic geochemical princi-
micropores and fractures of the low-permeability ples and techniques used by geoscientists to eval- Organic content is controlled largely by bio-
source rock.5 Oil shales are yet another variation uate source rock quality, quantity and maturity. logic productivity, sediment mineralogy and oxy-
on the resource play; operators who exploit these genation of the water column and sediment.
source rocks must apply heat to produce the oil.6 Biologic contributions to organic content range
from hydrogen-poor woody fragments to hydrogen-
Sunlight and oxidation Clays rich algal or bacterial components. From these, a
variety of organic compounds may be created.
Sea level Shells,
High concentration of organic matter skeletons Within the water column, organic matter may
exist in solution, in colloidal form or in particulate
Dissolved matter form, the highest concentrations of which are
Particulate matter found near the water’s surface.8 The route from the
Colloidal matter water surface to its eventual incorporation into
the sediment varies for each form (left). The
Flocculates to organic matter is subjected to various chemical
particulates and biologic processes as it descends through the
water column to the water/sediment interface.
Sediment Bottom feeders
Oxygen in the water column supports bio-
> Organic matter in the water column. Organic matter in solution may be logic productivity of organic matter but also pro-
adsorbed onto clay particles that sink slowly through the water. Colloidal motes biodegradation and oxidation. The matter
organic matter flocculates before it settles. Particulate matter may simply can also be altered by physical abrasion or chem-
drop to the bottom. (Modified from Barker, reference 8.) ical changes in water Eh and pH.9 Once this mat-
ter settles to the bottom, bacteria, worms and
other bottom feeders take in what they can
metabolize, converting some of it to simple mol-
ecules. The net result of biodegradation and oxi-
dation is a reduction in organic richness, leaving
only relatively resistant organic materials to be
incorporated into the sediment.

4. For more on petroleum systems: Al-Hajeri MM,   8. Barker C: Organic Geochemistry in Petroleum 14. Macerals are microscopically recognizable constituents
Al Saeed M, Derks J, Fuchs T, Hantschel T, Kauerauf A, Exploration. Tulsa: American Association of Petroleum of organic matter found in coals and kerogen, analogous
Neumaier M, Schenk O, Swientek O, Tessen N, Welte D, Geologists, AAPG Course Note Series no. 10, 1997. to mineral constituents in a rock.
Wygrala B, Kornpihl D and Peters K: “Basin and
Petroleum System Modeling,” Oilfield Review 21, no. 2   9. Jacobson, reference 7. 15. Tissot B, Durand B, Espitalié J and Combaz A:
(Summer 2009): 14–29. Eh is a measure of the oxidation-reduction state of a “Influence of Nature and Diagenesis of Organic Matter
solution; pH is a measure of its acidity or alkalinity. in Formation of Petroleum,” AAPG Bulletin 58, no. 3
5. It is not unusual for source rocks that are exploited for (March 1974): 499–506.
their shale gas to also be effective sources of 10. Demaison GJ and Moore GT: “Anoxic Environments and
conventional hydrocarbons in other parts of the basin. Oil Source Bed Genesis,” AAPG Bulletin 64, no. 8 16. Klemme HD and Ulmishek GF: “Effective Petroleum
(August 1980): 1179–1209. Source Rocks of the World: Stratigraphic Distribution
6. For more on oil shales: Allix P, Burnham A, Fowler T, and Controlling Depositional Factors,” AAPG Bulletin 75,
Herron M, Kleinberg R and Symington B: “Coaxing Oil from 11. Jacobson, reference 7. no. 12 (December 1991): 1809–1851.
Shale,” Oilfield Review 22, no. 4 (Winter 2010/2011): 4–15. 12. Palacas JG: “Characteristics of Carbonate Source
17. Vandenbroucke M: “Kerogen: From Types to Models
7. Jacobson, SR: “Petroleum Source Rocks and Organic Rocks of Petroleum,” in Magoon LB (ed): Petroleum of Chemical Structure,” Oil & Gas Science and
Facies,” in Merrill RK (ed): Source and Migration Systems of the United States. US Geological Survey Technology—Revue de l’Institut Français du Pétrole 58,
Processes and Evaluation Techniques. Tulsa: AAPG Bulletin 1870. Washington, DC: US Government Printing no. 2 (2003): 243–269.
(1991): 3–11. Office (1988): 20–25.
13. Jones RW: “Comparison of Carbonate and Shale Source 18. Tissot et al, reference 15.
Rocks,” in Palacas JG (ed): Petroleum Geochemistry
and Source Rock Potential of Carbonate Rocks. Tulsa:
The American Association of Petroleum Geologists,
AAPG Studies in Geology 18 (1984): 163–180.

34 Oilfield Review

Kerogen Source material General environment
type of deposition
I Mainly algae
II Mainly plankton, some contribution from algae Lacustrine setting
III Mainly higher plants Marine setting
IV Reworked, oxidized material Terrestrial setting
Varied settings

> Kerogen types. Kerogen can be classified by its source material.

7 cm deposited.11 Wave action oxygenates the deposit Type I kerogen is generated predominantly
and flushes clay-sized materials and small parti- from lacustrine environments and, in some cases,
> Undisturbed sediments. Fine laminations, or cles of organic material away from the sands then marine environments. It is derived from algae,
varves, in this core sample denote successive deposits them together in quieter waters. Sands, plankton or other matter that has been strongly
deposits in quiet waters with no disturbance from therefore, generally contain only small amounts reworked by bacteria and microorganisms living
bottom dwellers. of organic matter relative to shales. in the sediment. Rich in hydrogen and low in oxy-
gen, it is oil prone but, depending on its stage of
Within this depositional setting, oxygen and Mineralogy also plays an integral role in thermal evolution, can also produce gas.
energy levels are perhaps the most critical aspects source rock development. Minerals, transported Responsible for less than 3% of the world’s oil and
controlling the concentration and preservation of and precipitated in the sediment, can react with gas reserves, Type I kerogens are not common.16
organic matter in the sediment. Oxygen-depleted, organic compounds and ultimately dilute the rel- An example of Type I kerogen is found in the
or anoxic, sediments provide the best media for ative concentration of organic matter within the Green River Formation of the western USA.
preserving organic matter. Low oxygen levels sediment. This dilution may affect a source rock’s
create a reducing environment that shelters capacity to generate and expel petroleum. Type II kerogen is typically generated in
organic material from oxidation while also Although many organic-rich source rocks are reducing environments found in moderately deep
restricting the activity of bottom feeders. Anoxic argillaceous, carbonates (typically marls) can marine settings. It is derived primarily from the
conditions are evidenced by source rocks that also make excellent source rocks and reservoirs. remains of plankton that have been reworked by
have minute laminations, or varves (above). Some carbonates may contain as much as 10% to bacteria. Rich in hydrogen and low in carbon,
These features are indicative of persistent, stag- 30% total organic carbon (TOC), unlike shales, this kerogen can generate oil or gas with progres-
nant anoxic water above the sediment, as well as which may contain less than 5%.12 sive heating and maturation.17 Renowned exam-
an absence of bioturbation, thus implying a hos- ples of Type II kerogen include the Kimmeridge
tile environment for sediment-churning bottom In general, quality source rocks—whether Clay of the North Sea and the Bazhenov
feeders that graze on organic matter.10 These con- carbonate or shale—share a number of common Formation of Siberia. Certain depositional envi-
ditions are also closely linked with low-energy characteristics. They form in anoxic, or highly ronments promote increased incorporation of
depositional environments. reducing, environments, are generally laminated, sulfur compounds, resulting in a variation known
have moderate to high TOC and contain organic as Type II-S kerogen. This variation is seen in the
Because quiet waters reduce the exchange of matter exemplified by atomic hydrogen/carbon Monterey Formation of California, or the La Luna
oxygen and organic matter, they create an envi- ratios exceeding 1.2.13 Formation of Venezuela. The significance of this
ronment in which anoxic conditions can exist. type is that generation of oil starts much earlier,
These low-energy environments permit the depo- Although deposited under anoxic conditions, thought to be caused by kinetic reactions involv-
sition of finer-grained sediments as well. Thus, these fine-grained, organic-rich sediments are ing sulfur-bearing compounds.
there is a relationship between grain size and still missing a critical component: thermally
organic content in source rocks. Source rocks do mature kerogen. The formation of mature kero- Type III kerogen is derived primarily from ter-
not form in high-energy environments—such as gen requires heat. rigenous plant debris, which has been deposited
beaches or sand bars—where sands are typically in shallow to deep marine or nonmarine environ-
Thermal Transformations ments. Type III kerogen has lower hydrogen and
Sediment slowly cooks as pressure and tempera- higher oxygen content than Types I or II; conse-
ture increase with burial depth. Given sufficient quently, it tends to generate dry gas. Most coals
heat, pressure and time, the sediment lithifies contain Type III kerogens.
and the organic matter contained within trans-
forms into kerogen. Kerogen can be classified Type IV kerogen is derived from residual
into four types, based on provenance, as indi- organic matter found in older sediments that have
cated by specific macerals (above right).14 It can been reworked after erosion. Prior to final deposi-
also be classified on the basis of hydrogen, car- tion, this kerogen may have been altered by sub-
bon and oxygen content. Each type has a dis- aerial weathering, combustion or biologic
tinct bearing on what kind of petroleum, if any, oxidation in swamps or soils. This type of kerogen
will be produced.15 has high carbon content and is hydrogen poor.
Considered a form of dead carbon, Type IV kerogen
has almost no potential for generating oil or gas.18

Summer 2011 35

Biogenic methaneDiagenesisBiomarkersfrom the moment of deposition until just beforebodies and natural radioactive decay within the
Immature zone the onset of significant thermal alteration pro- crust. The geologic processes that control subsid-
Oil cesses.21 For source rocks, however, this stage is ence and uplift also affect maturation within a
Increasing depth and temperature Catagenesis characterized by alteration of organic matter, basin. Maturation can be interrupted if the basin
Wet gasOil window typically at temperatures below about 50°C is subjected to uplift, only to continue when sub-
[122°F].22 During diagenesis, oxidation and sidence resumes.
Dry gasMetagenesis other chemical processes begin to break down
Gas window the material. If deposited under anoxic condi- At the depths drilled by E&P companies, the
Hydrocarbons generated tions, this material may be converted by methan- petroleum-generation process is often incom-
> Thermal transformation of kerogen. The ogenic bacteria into dry gas.23 With increasing plete, leaving the source rock with a degree of
generation of hydrocarbons in source rocks is temperatures and changes in pH, the organic generating potential.28 Thus, rock samples
controlled primarily by temperature as the kerogen matter is gradually converted to kerogen and, obtained from the subsurface may contain gener-
content evolves from reactive carbon to dead in lesser amounts, bitumen. ated hydrocarbons along with organic matter
carbon. Gas is given off during early diagenesis, • The source rock matures with increasing heat, that is reacted incompletely. Some of these reac-
primarily through biologic activity. Catagenesis and it undergoes catagenesis. During this tions can be observed and measured in the labo-
takes place with further burial, during which oil stage, petroleum is generated as temperature ratory, where pyrolysis of organic matter parallels
and gas are given off. With increasing depth and increases to between 50°C and 150°C [122°F changes in the subsurface and provides a useful
temperature, any remaining oil is cracked during and 302°F], causing chemical bonds to break technique to characterize organic matter.
metagenesis, initially giving off gas, followed by down within the kerogen.24 Within this oil win-
simpler forms of dry gas. The process varies dow, Type I and II kerogens produce both oil Basic Source Rock Analysis
somewhat from one kerogen type to another. and gas, while Type III kerogens produce Some petroleum compounds within source rock
(Modified from Tissot et al, reference 15.) mainly hydrocarbon gas. Further increases in are released at temperatures lower than those
burial depth, temperature and pressure force needed to break down the kerogen. By monitoring
In general, hydrogen-rich kerogens are respon- the source rock into the upper part of the gas the compounds released during a steady increase
sible for generating both oil and hydrocarbon gas; window, where secondary cracking of the oil in temperature, geochemists can determine the
those with lesser amounts of hydrogen will gener- molecules produces wet gas containing meth- amount of generated petroleum relative to a
ate mainly hydrocarbon gas. After hydrogen is ane, ethane, propane and heavier hydrocarbons.25 rock’s total potential. In addition, the tempera-
depleted from the kerogen, generation of hydro- • Metagenesis marks the final stage, in which ture corresponding to the maximum evolution of
carbons will cease, regardless of the amount of additional heat and chemical changes convert gas gives an indication of source rock maturity.
available carbon.19 much of the kerogen into methane and a car-
bon residue. As the source rock moves farther Geoscientists employ a variety of techniques
As temperature and pressure increase during into the gas window, late methane, or dry gas, is to evaluate the hydrocarbon-generating capacity
burial, organic materials emit oil and gas. Overall, evolved, along with nonhydrocarbon gases such of source rocks. Geochemical testing of outcrop
this thermal maturation process produces a as carbon dioxide [CO2], nitrogen [N2] and samples, formation cuttings, sidewall cores and
series of progressively smaller hydrocarbon mol- hydrogen sulfide [H2S]. These changes take conventional cores can help determine the
ecules of increasing volatility and hydrogen con- place at temperatures ranging from about amount, type and thermal maturity of organic
tent, culminating with methane gas. And, as the 150°C to 200°C [302°F to 392°F].26 matter present in the rock.29 The results help
kerogen evolves through thermal maturity, its geoscientists ascertain whether, how much, when
chemical composition progressively changes, These stages have a direct bearing on source and what kind of petroleum might have been
transforming it into a carbonaceous residue of rock maturity. Thermally immature, or potential, generated as well as determine what secondary
decreasing hydrogen content.20 source rocks have been altered by diagenesis but processes may have occurred following the expul-
have yet to be exposed to sufficient heat for ther- sion of hydrocarbons from the source rock.
The thermal maturation process can be mally generated petroleum. Thermally mature, or
divided into three stages (above). effective, source rocks that are (or were) in the Carbon is an essential element of any organic
• Initially, the sediment is subjected to diagene- oil window, have been subjected to thermal pro- compound, and one way to assess the organic rich-
cesses necessary to produce petroleum and are ness of a rock is to measure its carbon content.30
sis. In its broadest sense, this term encom- actively generating petroleum. Thermally post- Because the oil or gas potential of a formation is
passes all natural changes in sediments occurring mature, or spent, source rocks have entered the related to its carbon content, the TOC measure-
gas window and have already generated petro- ment is a priority in source rock assessment. This
leum; in so doing they have exhausted all hydro- initial carbon assessment is followed by other
gen necessary for further oil or gas generation.27 screening procedures such as pyrolysis and vitrin-
ite reflectance analysis. These tests allow rapid
Although maturation is largely related to evaluations for large numbers of rock samples and
increasing burial depths caused by continual may be supplemented by more extensive test
sedimentation within a basin, it can also be methods.
locally or regionally influenced by heat flow aris-
ing from crustal tectonics, proximity to igneous TOC values can be obtained using a direct-
combustion technique that requires only 1-g
[0.0022-lbm] samples of rock.31 The samples are

36 Oilfield Review

pulverized and treated to remove any carbon > Pyrolyzer. This Rock-Eval 6 pyrolysis analyzer records gases evolved from the source rock during a
found in carbonate samples, or other contami- programmed heating process. (Photograph courtesy of Vinci Technologies SA.)
nants. They are then combusted at 1,200°C
[2,192°F] using a high-frequency induction 19. Baskin DK: “Atomic H/C Ratio of Kerogen as an Estimate 30. By definition, organic compounds are those that contain
furnace. Carbon contained in the kerogen is of Thermal Maturity and Organic Matter Conversion,” carbon (with the exception of carbides, carbonic acid,
converted to CO and CO2. The evolved carbon AAPG Bulletin 81, no. 9 (September 1997): 1437–1450. carbonates, carbon oxides and cyanides). Within the
fractions are measured in an infrared cell, realm of geochemistry, carbon is divided into a number
converted to TOC and recorded as mass weight 20. Hood A, Gutjahr CCM and Heacock RL: “Organic of classes. Geochemists define organic carbon as that
percent of rock.32 Metamorphism and the Generation of Petroleum,” which is derived from biogenic matter, whereas
AAPG Bulletin 59, no. 6 (June 1975): 986–996. inorganic carbon is derived from mineral matter. For
The TOC measurement assesses three compo- more on the organic carbon model: Jarvie DM: “Total
nents. Carbon from extractable organic matter 21. For more on diagenesis: Ali SA, Clark WJ, Moore WR Organic Carbon (TOC) Analysis,” in Merrill RK (ed):
(EOM) is derived mostly from the thermal crack- and Dribus JR: “Diagenesis and Reservoir Quality,” Source and Migration Processes and Evaluation
ing of kerogen. This carbon is found within the oil Oilfield Review 22, no. 2 (Summer 2010): 14–27. Techniques. Tulsa: AAPG (1991): 113–118.
and gas that the rock has generated but has not
yet expelled. At the other extreme is residual car- 22. Peters et al, reference 2. 31. For a description of other techniques used for measuring
bon, which makes up that portion of kerogen hav- 23. Rice DD and Claypool GE: “Generation, Accumulation, TOC, see Appendix C of Peters and Cassa, reference 27.
ing no oil- or gas-generating potential because its
highly condensed chemical structure contains so and Resource Potential of Biogenic Gas,” AAPG 32. A TOC of 1% means there is 1 gram of organic carbon in
little hydrogen. Convertible carbon contained Bulletin 65, no. 1 (January 1981): 5–25. 100 grams of rock.
within the kerogen represents the remaining 24. Peters et al, reference 2.
petroleum-generating potential of a rock. The 25. Cracking is a process in which high temperature and 33. Jarvie, reference 30.
capacity to generate petroleum depends largely pressure act upon large, heavy hydrocarbon molecules, 34. Pyrolysis involves heating of organic matter in the
on the chemical composition of this convertible causing them to split into smaller, lighter components.
carbon fraction.33 Under such conditions, oil can be transformed into a gas. absence of oxygen. In this application, pyrolysis causes
26. Peters et al, reference 2. hydrocarbons to crack into simpler, lighter compounds.
The TOC measurement is the first screen for 27. Peters KE and Cassa MR: “Applied Source Rock For more on this technique: Espitalié J, Madec M,
quantifying organic richness. TOC values provide Geochemistry,” in Magoon LB and Dow WG (eds): Tissot B, Mennig JJ and Leplat P: “Source Rock
only a semiquantitative scale of petroleum-gener- The Petroleum System—From Source to Trap. Tulsa: Characterization Method for Petroleum Exploration,”
ating potential. TOC indicates the quantity, but The American Association of Petroleum Geologists, paper OTC 2935, presented at the Ninth Annual Offshore
not the quality, of the organic matter. If this ini- AAPG Memoir 60 (1994): 93–120. Technology Conference, Houston, May 2–5, 1977.
tial screening test demonstrates sufficient 28. Barker C: “Pyrolysis Techniques for Source-Rock 35. Other variations on this method use different
organic content, the rock should undergo addi- Evaluation,” AAPG Bulletin 58, no. 11 (November 1974): temperature ranges and heating times. For more on the
tional tests to ascertain organic matter quality 2349–2361. Rock-Eval 6 technique: Lafargue E, Espitalié J, Marquis F
and maturity. 29. Fresh cores work best; outcrop samples tend to be and Pillot D: “Rock-Eval 6 Applications in Hydrocarbon
degraded through weathering and are less desirable. Exploration, Production and in Soil Contamination
One of these tests was developed by the Studies,” Oil & Gas Science and Technology—Revue de
Institut Français du Pétrole, whose Rock-Eval l’Institut Français du Pétrole 53, no. 4 (July–August 1998):
pyrolysis analyzer has become an industry stan- 421–437.
dard in source rock assessment.34 A major break-
through in petroleum geochemistry, this
programmed pyrolysis technique subjects rock
samples to high temperatures, enabling research-
ers to obtain results that would have taken mil-
lions of years in a sedimentary basin. This
technique requires only 100 mg [0.00022 lbm] of
pulverized rock and can analyze a sample in a
matter of minutes (above right). The latest ver-
sion of the Rock-Eval apparatus uses both pyroly-
sis and oxidation ovens to heat samples in a
programmed series of stages ranging from 100°C
to 850°C [212°F to 1,562°F].35 Sample analyses
are automated, and results are computed before
they are tabulated and output to a log.

During Rock-Eval pyrolysis, samples are
heated under an inert atmosphere of helium or
nitrogen. A flame ionization detector (FID)
senses organic compounds emitted during each
stage of heating. Sensitive infrared (IR) detectors

Summer 2011 37

Pyrolyzable carbon Residual carbon Mineral carbon are generated through thermal cracking of the
Oxidation Carbonate decomposition insoluble kerogen. As temperatures rise, the ker-
Vaporization Pyrolysis ogen releases CO2 in addition to hydrocarbons.37
decomposition This controlled heating program is illustrated by
a series of peaks on the pyrogram.38
FID and IR signals Tmax S5
S2 The first peak, S1, corresponds to free oil and
S3MINC gas that evolve from the rock sample without
S3CO2 cracking the kerogen during the first stage of
S4CO2 heating at 300°C. These hydrocarbons were gen-
erated in the subsurface but came to be expelled
S1 S3CO S4CO from the rock only during pyrolysis. S1 represents
300 how many milligrams of free hydrocarbons can be
300 389 472 556 639 405 472 538 605 672 738 805 851 850 thermally distilled out of one gram of
the sample.39
Temperature, °C
The second peak, S2, corresponds to the
Hydrocarbons (FID) CO2 (IR) CO (IR) Temperature programming hydrocarbons that evolve from the sample during
the second programmed heating stage of pyroly-
> Programmed pyrolysis results. Free hydrocarbons are measured by the S1 peak, and residual sis. These hydrocarbons result from the cracking
hydrocarbons are measured by the S2 peak. Tmax of 472°C [882°F] corresponds to the temperature of heavy hydrocarbons and from the thermal
recorded when the S2 peak was achieved. CO, CO2 and mineral carbon components of the S3 breakdown of kerogen. S2 represents milligrams
measurement are also displayed. CO2 is proportional to the amount of oxygen present in organic matter of residual hydrocarbons in one gram of rock,
and provides input for calculating an important index used in determining maturity and kerogen type. thus indicating the potential amount of hydrocar-
Pyrolysis results are computed to determine amounts of pyrolyzable carbon, residual carbon, mineral bons that the source rock might still produce if
carbon and TOC. (Illustration courtesy of Vinci Technologies SA.) thermal maturation continues. This reading can
have important implications for the evaluation of
measure CO and CO2 during pyrolysis and oxida- Rock samples are heated in stages, initially oil shales.
tion. A thermocouple monitors temperatures. held at a constant 300°C [572°F] for several min-
These measurements are recorded on a chart utes, followed by programmed heating at 25°C The Rock-Eval technique yields a variety of
known as a pyrogram (above). The results help [45°F] per minute to a peak temperature of CO2 measurements. The S3 peak corresponds to
geochemists characterize the type of organic mat- about 850°C [1,562°F]. During the first stage, any CO2 that is evolved from thermal cracking of the
ter in a source rock and determine the thermal free oil and gas previously generated by the bitu- kerogen during pyrolysis, expressed in milligrams
evolution of a sample and its residual hydrocar- men are distilled and released from the rock. per gram of rock. Following pyrolysis, residual
bon-generating potential.36 During the next stage, hydrocarbon compounds organic carbon is oxidized in a separate oven to
produce the S4 peak. The S4 measurement can
Source rock quality TOC, % Pyrolysis S2, mg hydrocarbons/g rock EOM weight, % Hydrocarbons, ppm be broken down into carbon dioxide and carbon
None < 0.5 <2 < 0.05 < 200 monoxide components to yield the S4CO2 and
Poor 0.5 to 1 2 to 3 0.05 to 0.1 200 to 500 S4CO peaks. A separate CO2 peak, designated S5,
Fair 1 to 2 3 to 5 0.1 to 0.2 500 to 800 reflects carbon dioxide derived from decomposi-
Good 2 to 5 5 to 10 > 0.2 > 1,200 tion of carbonate minerals in the sample.
Very good >5 > 10
Pyrolysis temperatures are also recorded and
Product type Hydrogen index Stage Tmax produce a Tmax peak that corresponds to the
Gas 50 to 200 Onset of oil pyrolysis oven temperature during maximum
Gas and oil 200 to 300 Type I kerogen ~445°C generation of hydrocarbons. Tmax is reached dur-
Oil > 300 Type II kerogen ~435°C ing the second stage of pyrolysis, when cracking
Type III kerogen ~440°C of the kerogen and heavy hydrocarbons produces
Onset of gas ~460°C the S2 peak.40 Knowing the amount of heat nec-
essary to create various chemical compounds in
the rock can help geochemists understand the
history of the rock and the extent of thermal
maturation it has already undergone. Tmax should
not be confused with geologic temperatures, but
it can be useful in characterizing thermal evolu-
tion of the organic matter.

> Source rock evaluation criteria. Evaluation of source rock potential must be based on the appraisal
of multiple factors.

38 Oilfield Review

Important Indices gas generation by determining the quantity, type [33 ft] of depth, regardless of lithology.43 However,
and maturation of organic matter. pyrolysis is not intended for use without support-
Taken together, these pyrolysis measurements ing geochemical analysis. For critical samples,
provide insight into the chemical makeup and The power of Rock-Eval and TOC analyses in interpretations from pyrolysis results should be
maturity of the organic matter contained within wellbores is a direct function of close sample verified by other methods.
the source rock (previous page, bottom left). The spacing, thus requiring samples at every 10 m
relationship between these components forms
the basis for various indices used in the interpre- Type I Products given off
tation of rock characteristics.41 from kerogen maturation
• The hydrogen index, HI, is derived from the
CO2, H2O
ratio of hydrogen to TOC; it is defined as Oil
100 2 S2/TOC. The HI is proportional to the Wet gas
amount of hydrogen contained within the kero- Dry gas
gen, and high HI indicates a greater potential
to generate oil. Kerogen type can be inferred No hydrocarbon
from this index as well. potential
• The oxygen index, OI, is derived from the Increasing
ratio of CO2 to TOC; it is defined as maturation
100 2 S3/TOC. The OI is related to the amount
of oxygen contained in the kerogen and can be Hydrogen index 1.5
useful in tracking kerogen maturation or type. Type II
• The production index, PI, is derived from the
relationship between hydrocarbons generated 1.0 Type III
during the first and second stages of pyrolysis;
it is defined as S1/(S1 + S2). This relationship Type IV
is used to characterize the evolution of the 0.5
organic matter because PI tends to gradually
increase with depth for fine-grained rock. It 0 0.1 0.2 0.3
also tends to increase with source rock matura- Oxygen index
tion prior to hydrocarbon expulsion, as ther-
mally degradable components in kerogen are > Kerogen maturation. A modified Van Krevelen diagram shows
converted to free hydrocarbons. Anomalously changes to kerogen brought on by increased heat during burial. The
high values of S1 and PI can also be used to general trend in the thermal transformation of kerogen to hydrocarbon
identify petroleum accumulations or stained is characterized by generation of nonhydrocarbon gases; it then
carrier beds. progresses to oil, wet gas and dry gas. During this progression, the
• The petroleum potential represents the maxi- kerogen loses oxygen primarily as it gives off CO2 and H2O; later, it
mum quantity of hydrocarbons that a suffi- begins to lose more hydrogen as it evolves hydrocarbons.
ciently matured source rock might generate;
it is defined as the sum of S1 + S2. It therefore 36. Espitalié J and Bordenave ML: “Rock-Eval Pyrolysis,” respectively. However, modern pyrolysis analyzers
accounts for the quantity of hydrocarbons that in Bordenave ML (ed): Applied Geochemistry. Paris: automatically calculate these areas and annotate them
the rock has already generated (S1) and those Éditions Technip (1993): 237–261. on the pyrogram, and the peaks are now commonly
that the rock could still produce if maturation referred to as S1, S2 and S3.
continues (S2). It is expressed as kilograms of 37. Peters KE: “Guidelines for Evaluating Petroleum Source 39. Espitalié et al, reference 34.
hydrocarbons per metric ton of rock. Rock Using Programmed Pyrolysis,” AAPG Bulletin 70, 40. Peters and Cassa, reference 27.
no. 3 (March 1986): 318–329. 41. Espitalié et al, reference 34.
These indices are particularly useful in track- 42. Espitalié et al, reference 34.
ing kerogen type and maturation. When plotted 38. Early papers sometimes referred to these peaks as P1, 43. Peters and Cassa, reference 27.
on a Van Krevelen diagram, Type I kerogens have P2 and P3. For example, Espitalié et al, reference 34,
a high HI and low OI (right). Type III kerogens distinguished between the P1, P2 and P3 peaks and the
are characterized by low HI and high OI. Between area beneath each peak, designated as S1, S2 and S3,
these two extremes lie the Type  II kerogens.
During maturation, the OI tends to decrease
while HI initially remains nearly constant. As the
kerogen enters the oil window, HI decreases. PI
tends to increase with burial depth.42 This type of
information provides geochemists with valuable
clues for evaluating a rock’s potential for oil and

Summer 2011 39

Vitrinite value of vitrinite reflectivity is determined from
multiple samples, it is commonly designated
20 µm as Rm.
> Vitrinite in bituminous coal. The amount of light reflected
by vitrinite macerals is a key test for determining the As indicators of thermal maturity, Ro values
thermal maturity of a rock. The intensity of light reflected vary with the type of organic matter. And because
from a sample is measured at hundreds of points along a the temperature range of the gas window extends
microscopic sampling area, then a statistical analysis beyond that of oil, Ro values for gas will show a
determines the amount of vitrinite in the sample and its corresponding increase over those of oil. Thus,
thermal maturity. This photograph was taken in incident high maturation values (Ro > 1.5%) generally indi-
white light, with the sample in an oil immersion. cate the presence of predominantly dry gas; inter-
[Photograph courtesy of the US Geological Survey Energy mediate maturation values (1.1% < Ro < 1.5%)
Resources Program: “2011 Photomicrograph Atlas,” indicate gas with a tendency toward oil genera-
http://energy.usgs.gov/Coal/OrganicPetrology/ tion at the lower end of the range. Wet gas can be
PhotomicrographAtlas.aspx (accessed July 7, 2011).] found still lower in the range (0.8% < Ro < 1.1%).
Lower reflectivity values (0.6% < Ro < 0.8%) indi-
Other Evaluation Methods maturity over temperatures corresponding to cate predominantly oil, while Ro < 0.6% points to
early diagenesis through metamorphism—a immature kerogen.
Vitrinite reflectance is a key diagnostic tool for range spanning the sequence of petroleum gen-
assessing maturation. Vitrinite, a maceral formed eration, preservation and destruction in rocks.45 By themselves, Ro values can sometimes be
through thermal alteration of lignin and cellulose misleading and should be weighed with other
in plant cell walls, is found in many kerogens Reflectivity (R) is measured by a microscope measurements. Some of these measures of matu-
(above). As temperature increases, vitrinite equipped with an oil-immersion objective lens rity are based on the color of ancient pollens and
undergoes complex, irreversible aromatization and photometer.46 Vitrinite reflectance measure- plant spores, or on the color of tiny fossilized
reactions that increase reflectance.44 Vitrinite ments are carefully calibrated against glass- or teeth. Due to the prevalence of vitrinite reflec-
reflectance was first used to determine the rank, mineral-reflectance standards, and reflectance tance, these indicators are often correlated to Ro
or thermal maturity, of coals. This technique is measurements represent the percentage of light values. Because vitrinite is present only in sedi-
now used to help geochemists evaluate kerogen reflected in oil, designated as Ro. When a mean ments younger than the Carboniferous age,
which experienced the onset of woody plants, the
Thermal alteration index Color of organic matter Associated hydrocarbons maturity of older kerogens relies on the presence
1: none Yellow Liquid hydrocarbons to dry gas of bitumen to derive a vitrinite reflectance equiv-
2: slight Orange to brownish yellow Liquid hydrocarbons to dry gas alent (VRE).
3: moderate Brown Liquid hydrocarbons to dry gas
4: strong Black Dry gas The thermal alteration index (TAI) is deter-
5: severe Black, with additional evidence of metamorphism Dry gas to none mined through microscopic examination of spore
color.47 Carbonization causes a change of color in
> Thermal alteration ratings. Maturity of source rocks can be ascertained through changes in the color palynomorphs such as spores and pollen.
of spores and pollen contained in the rock. (Modified from Staplin, reference 47.) Thermally immature palynomorphs are yellow
but change to orange or yellow-brown during dia-
genesis. With increased heat, they change to
brown during catagenesis and then to black
during metagenesis.48 The TAI has five alteration
ratings (below left).

The conodont alteration index (CAI) is based
on color changes seen in fossil teeth.49 Conodonts
are tiny fossilized dental remains of eel-shaped
chordates. These fossils contain trace amounts of
organic matter. Their small size and carbonate-
apatite composition make them highly resistant
to weathering and metamorphic temperature
regimes; they have even been recovered from
marbles and garnet-mica schists.

Their color alteration follows a sequence that
can be reproduced by laboratory heating experi-
ments. Conodont color alteration is time and
temperature dependent, and is progressive,
cumulative and irreversible, thus making it a
good correlation tool for maximum tempera-
tures.50 Unweathered conodonts are pale yellow;

40 Oilfield Review

with heat they change progressively to light Conodont alteration index Conodont temperature, °C Vitrinite reflectance Percent fixed carbon
brown, then dark brown, black, opaque white and 1 < 50 to 80 < 0.80 < 60
crystal clear. Alteration from pale yellow to black 1.5 50 to 90
is thought to result from carbon fixing within the 2 60 to 140 0.70 to 0.85 60 to 75
organic matter contained in the fossil structure. 3 110 to 200 0.85 to 1.30 65 to 73
The color changes with carbon loss and release of 4 190 to 300 1.40 to 1.95 74 to 84
water from the crystal structure. Experimental 5 300 to 400 1.95 to 3.60 84 to 95
data show that color alteration begins at about
50°C [122°F] and continues to about 550°C > 3.60 > 95
[1,022°F]. The CAI can be determined by com-
paring samples against a set of laboratory-pro- > Conodont alterations. Conodonts change color with heat; their color can be linked to vitrinite
duced conodont color standards to estimate a reflectance. (Data from Harris, reference 50.)
temperature range. Conodont color alteration
has been correlated with other optical indices Laboratory
and with percent fixed carbon (above right). Sampling manifold

Another screening method measures the Mud logging unit
composition and concentration of light hydrocar-
bons released from drill cuttings.51 Fine-grained Shale shaker Sample Chromatograph
formation cuttings, typical of those produced by chamber
source rocks, may retain hydrocarbons even after
they arrive at the surface, making good samples FID gas unit Chromatogram
for this type of analysis. This technique, known as
gas chromatography, can be carried out at the Total gas
wellsite or under strictly controlled conditions at Methane
a laboratory (right).
Ethane
44. Peters and Cassa, reference 27. Propane
45. Senftle JT and Landis CR: “Vitrinite Reflectance as a
> Formation cuttings analysis. At the well, cuttings are collected from the drilling mud after circulating
Tool to Assess Thermal Maturity,” in Merrill RK (ed): to the surface (red arrows) and passing over the shale shaker. In the mud logging unit, the cuttings are
Source and Migration Processes and Evaluation evaluated under a microscope, and mud gas is analyzed by an FID and gas chromatograph. Gas from
Techniques. Tulsa: AAPG (1991): 119–125. the cuttings may also be analyzed in a laboratory. For transport to the laboratory, the cuttings are
46. The terms reflectance and reflectivity tend to be used sealed in sample jars containing water. Gas expelled from the cuttings will accumulate above the liquid
interchangeably, with the former being more common. surface. This gas is commonly referred to as headspace gas. At the laboratory, the gas is drawn from
However, reflectance is a ratio of the light reflected from the container then injected into the sampling port of a gas chromatograph. As the gas passes through
a surface to the light directed onto that surface—the the chromatograph’s capillary column, it splits into separate compounds, with each compound taking a
ratio of reflected radiation to incident radiation. This little longer to move through the system than its predecessor. Each compound exits the column
value can change, up to a point, depending on the individually, to be analyzed by an FID or other detector. The results are recorded on a chart known as a
thickness, and hence, opacity, of a surface. If that chromatogram (bottom right).
surface is thick enough to prevent light from being
transmitted through the surface, then reflectance
reaches a maximum. This is the reflectivity of a surface.
47. The TAI is set forth in Staplin FL: “Sedimentary Organic
Matter, Organic Metamorphism, and Oil and Gas
Occurrence,” Bulletin of Canadian Petroleum Geology 17,
no. 1 (March 1969): 47–66.
48. Anders D: “Geochemical Exploration Methods,” in
Merrill RK (ed): Source and Migration Processes and
Evaluation Techniques. Tulsa: AAPG (1991): 89–95.
49. Found in Late Cambrian and Triassic formations,
conodonts were long suspected of being fossilized
teeth. In the early 1990s, this supposition was verified
through the aid of electron microscopy. For more on
conodont research: Zimmer C: “In the Beginning Was
the Tooth,” Discover 14, no. 1 (January 1993): 67–68.
50. Harris AG: “Conodont Color Alteration, An Organo-
Mineral Metamorphic Index, and Its Application to
Appalachian Basin Geology,” in Scholle PA and
Schluger PR (eds): Aspects of Diagenesis. Tulsa: Society
of Economic Paleontologists and Mineralogists, SEPM
Special Publication 26 (1979): 3–16.
For the seminal paper on CAI, see: Epstein AG,
Epstein JB and Harris LD: “Conodont Color Alteration—
An Index to Organic Metamorphism,” Washington, DC:
US Government Printing Office, US Geological Survey
Professional Paper 995, 1977.
51. Noble RA: “Geochemical Techniques in Relation to
Organic Matter,” in Merrill RK (ed): Source and
Migration Processes and Evaluation Techniques.
Tulsa: AAPG (1991): 97–102.

Summer 2011 41

Compound separation Ionization Ion separation Ion detection
Sample injector

Ion source Quadrupole analyzer Electron
multiplier
Capillary column

Gas chromatograph Mass spectrometer

> Biomarker analysis. Biomarker carbon structures are directly related to precursor molecules of specific
biological substances. The smallest markers are found at a molecular level and can only be determined
through GCMS. With this method, separate compounds are drawn through the gas chromatograph capillary
column then pass to an ionizer. There, a metallic filament ionizes each compound. The quadrupole analyzer
filters the ions based on their mass/charge ratio. The electron multiplier detects every ion of the selected
mass filtered through the quadrupole analyzer.

A gas chromatograph evaluates gas liberated biomarker distribution that can be unique to a The past decade, in particular, has seen a
during the drilling process and records individual particular location. By comparing oil to samples renaissance in petroleum system modeling tools.
peaks for methane (C1), ethane (C2), propane of potential source rock, this chemical finger- Source rock kinetics, a key input for these models,
(C3), isobutane (iC4) and normal butane (nC4); a print can help link oil to its source.53 Because characterizes the chemical reactions and petro-
single peak is typically recorded for pentanes biomarker patterns tend to change systemati- leum compounds generated during thermal matu-
(iC5 and nC5) and heavier hydrocarbons (C5+). cally with respect to time and temperature, they ration of a rock. Two pyrolysis techniques that have
Determining the composition and concentration can help geochemists infer maturation trends. proved useful in simulating maturation processes
of these gases helps geoscientists evaluate the are microscale sealed vessel and gold tube con-
types of hydrocarbons that may be produced When potential source rocks have not been fined pyrolysis.56 Using these techniques, scientists
within a prospective reservoir. encountered in a basin, indirect correlations are able to scrutinize processes such as oil-to-gas
between oil and source rock may be obtained cracking (OTGC), in which application of temper-
Gas chromatography (GC), when supple- through source-related biomarker ratios.54 These atures greater than 150°C causes existing oil to
mented by mass spectrometry (MS), can provide biomarker ratios help geochemists infer source break down into gas. OTGC investigations have, in
a detailed analysis of organic compounds found rock thermal maturity, lithology, depositional turn, led to the tracking of thermally resistant car-
in trace amounts (above). Geochemists com- environment, organic matter input and age. For bon molecules, known as diamondoids, for deter-
monly use this technique, referred to as GCMS, example, the biomarkers contained in a specific mining how gas is generated from oil under
for identifying the masses and relative concen- crude oil might indicate that its source rock was high-temperature conditions.57
trations of organic compounds known as biologi- a marginally mature, clay-poor marine carbonate
cal markers. Biomarkers constitute molecular of Devonian age that contained algal and bacte- 52. Noble, reference 51.
fossils and are synthesized only through biogenic rial organic matter deposited under anoxic 53. This type of geochemical fingerprint can also be used to
processes.52 Their organic structures can be clas- conditions. Biomarkers can also supplement
sified into basic groups, which, in turn, contain maturity indicators such as vitrinite reflectance compare one oil to another; it has proved useful in
members having variations of the same basic and spore coloration.55 tracking “mystery” oil slicks at sea to the vessels
structure. These groups can be related to certain responsible for discharging oil wastes.
types of organisms and can help geochemists Beyond the Basic Toolkit 54. Peters KE and Fowler MG: “Applications of Petroleum
ascertain the environment in which such an Though source rocks have been studied exten- Geochemistry to Exploration and Reservoir
assemblage might have been deposited. sively for the past 50 years, the recent move to Management,” Organic Geochemistry 33, no. 1
exploit plays centered on gas shales and oil shales (2002): 5–36.
Biomarker compositions reflect the type of has spurred a resurgence of geochemical research 55. Noble, reference 51.
organic matter incorporated into the sediment as and development. This resurgence has prompted 56. Horsfield B, Disko U and Leistner F: “The Micro-Scale
well as chemical changes that occurred subse- expanded utilization of established techniques as Simulation of Maturation: Outline of a New Technique and
quent to deposition. The combination of precur- well as the development of new tools. Its Potential Applications,” Geologische Rundshau 78,
sor molecules and their chemical reactions varies no. 1 (1989): 361–374.
from one basin or field to another, producing a Hill RJ, Tang Y, Kaplan IR and Jenden PD: “The Influence
of Pressure on the Thermal Cracking of Oil,”
Energy & Fuels 10, no. 4 (1996): 873–882.
57. Dahl JE, Moldowan JM, Peters KE, Claypool GE,
Rooney MA, Michael GE, Mello MR and Kohnen ML:
“Diamondoid Hydrocarbons as Indicators of Natural Oil
Cracking,” Nature 399, no. 6731 (May 6, 1999): 54–57.

42 Oilfield Review

Mineral grains

Kerogen Pores
Kerogen Mineral
grains

Pyrite Pyrite
1 µm

> Scanning electron microscope with a backscatter image of a Barnett Shale sample at 1,000× magnification. In the shale sample (right), amorphous

organic matter (dark gray), consisting primarily of kerogen is seen as large continuous lenses or small, finely dispersed packets interwoven in a complex
mineral matrix (lighter gray components). Pyrite, a product of shale thermochemical maturation, is also present (small white crystals). Pores of varying size
and shape (small black spots) can be seen in both the kerogen and mineral components, but are more prevalent in the former. The sample was milled using
an argon ion polisher then imaged using a scanning electron microscope (left) to reveal the complex organic-mineral-pore architecture characteristic of
such mudrock deposits. (Graphic courtesy of Nicholas Drenzek, Schlumberger-Doll Research Center and Natasha Erdman, JEOL USA, Inc.)

The analysis of stable isotopes of carbon and Beyond the laboratory, new techniques for While operators devote much of their explora-
other elements such as hydrogen, oxygen or sul- monitoring gases encountered during drilling tion efforts to studying the depositional and
fur is also seeing increased application in organic are helping geoscientists determine the compo- structural characteristics of a prospect, many are
geochemistry. Recent observations of ethane car- sition of hydrocarbons, locate fluid contacts and also concerned with the processes that control
bon isotopes have led to interesting correlations aid in the identification of compositional gradi- the formation of oil and gas. Geochemical reac-
between dry gas and overpressured zones within ents in reservoirs. The FLAIR fluid logging and tions and the conversion of organic matter are
gas shales. Isotope plots show a rollover, or rever- analysis service extracts gas from drilling mud integral to petroleum generation; the character-
sal, in maturity that runs contrary to trends nor- under constant pressure, flow, volume and tem- ization of organic matter, in turn, is becoming
mally tied with depth. The origin of these perature conditions. Part of the Schlumberger increasingly critical for the development of new
reversals is unknown, but a trend toward high Geoservices suite of mud logging services, the plays. These tools are proving instrumental in
production rates has been observed in wells that FLAIR system first samples gas that has been cir- opening new frontiers of exploration. —MV
exhibit rollover. culated to surface then separates it into individ-
ual components to provide a quantitative analysis
Scientists are also focusing on the physical of gases from C1 to C5 and semiquantitative infor-
structure of kerogen within the mineral matrix of mation on the C6 to C8 components. Isotope
source rocks. The formation of secondary perme- analysis can provide information regarding the
ability and porosity in organic-rich shales during origins and characteristics of the hydrocarbons
in situ maturation is believed to be a key enabler encountered during drilling. Using δ13C/CH4
for production of shale gas in many source rocks ratios, which are originally related to petroleum
(above). Organic petrography, utilizing basic and generation,mud logging analysts can provide
advanced microscopy techniques to focus on E&P companies with preliminary information
organic matter, is therefore seeing a revival in concerning the source rock, including its kero-
kerogen evaluation. gen type and thermal maturity.

Summer 2011 43