applied source rock geochemistry

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Magoon, L. B, and W. G. D ow, eds., 1994, The petroleumsystemfrom source to trap: AAPG Mem oir 60.

Chapter 5

I * A pp l i e d Sou rce Rock G eoc he m is t ry

Ke nne th E. Peters* Mary Rose CassaChevron Overseas Petroleum Inc. California E nvironmental Protection AgencySan Ramon, California, U.S.A. Department of Toxic Substances Control

Berkelet/ California, U.S.A.

AbstractApplied organic geochemistry provides the information needed to make maps of the richness,

type, and thermal ma turity of a source rock. These map s are a necessary step tow ard determiningth e stratigraphic and geographic extent of a pod of active source rock in a petroleum system, andthey are based o n geochemical analyse s of rock samples from outcrops and w ells that are display edon logs. These geochemical well logs are based on Rock-Eval pyrolysis, total organic carbon,vitrinite reflectance, and other rapid , inex pen sive screening me tho ds. The log s defin e (1)potential, effective, and sp ent petro leum source rock (2) the thermal maturation grad ient, includingimmature, mature, and postmature zones, and (3) in situ and migrated petroleum shows. Usefulgeochemical logs require proper sample selection, preparation, analysis, and interpretation.Detailed studies, including oil-source rock correlations by biomarker and supporting techniques,are undertaken on selected sam ples only after the screening meth ods are comp leted.

INTRODUCTIONThe goal of this chapter is to show how geochemical

screening technology is applied to petrole um exploration. This chapter pro vides a conceptual fram ework for

later discussions in this book by de fining key term s usedto describe source rock characterist ics and reviewingpr inc ip le s and r ecen t deve lopmen t s in source rockgeochemistry. Major emphasis is placed on (1) criteria forsamp ling, prepa ration, an d a nalysis of rocks and oils; (2)geochemical logs; and (3) geochem ical m aps .

The main contr ibut ion of organic geochemist ry tosedimentary basin analysis is to provide analytical datato identify and map source rocks. These maps includethe richness, type, and thermal maturity of a source rockand are a necessary step toward determining the strati-graphic and geographic extent of a pod of active sourcerock in a petroleum system. The volume, richness, andthermal maturity of this pod of active source rock determines the amount of o i l and gas avai lable for t raps .Because of this, maps that show the pod of active sourcerock red uce exploration risk (e.g., Dem aison, 1984).

Geochemical ivell logs are essential for ma pp ing activesource rocks. These logs plot various geochemical paramete r s ve r sus dep th and can be made f rom su r facesections and during or after drilling. Certain criteria thatare largely omitted from the l i terature must be met toensu re useful geochemical logs. These criteria include

wel l s i te sampl ing, type of samples (core , s idewal lcut t ings) , sample spacing, sample prepara t ion procedures, and methods of analysis and interpretation.

SOURCE ROCK PROPERTIES ANDTERMSSedimentary rocks commonly contain minerals and

organic matter with the pore space occupied by waterb i t u m e n , o i l, a n d / o r g a s . Kerogen i s the par t icula tefraction of organic matter remaining after extraction opulverized rock with organic solvents. Kerogen can bisolated from carbonate- and sil icate-bearing rocks bytreatment with inorganic acids, such as HC1 and HF (e.g.,D ur an d, 1980). This is only an opera tiona l definit ionb e c a u s e t h e a m o u n t a n d c o m p o s i t i o n of i n s o l u b l eorganic mat ter or kerogen remaining af ter ext ract iondepends on the types and po la r i t i e s o f t he o rgan icsolvents. Kerogen is a mixture of macerals and reconstituted degradation products of organic matter. Maceralsare the remains of various types of plant and animamatter that can be distinguished by their chemistry andby the i r morpho logy and r e f l ec t ance us ing a pe t rograp hic m icroscope (Stach et al. , 1982). This term w aoriginally applied to components in coal but has beene x t e n d e d t o s e d i m e n t a r y r o c k s . Palynomorphs a r eresistant, organic-walled microfossils such as sporespollen, dinoflagellate cysts, and chitinozoa.

'Present address: Mobil Exploration and Producing TechnicalCenter, Dallas, Texas, U.S.A. 93


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5. Applied Source Rock Geochemistry 95

Table 5.1. Geochemical Parameters Describing the Petroleum Potential (Quantity) of an Immature Source Rock

PetroleumPotential

TOC(wt. )

Organic MatterRock-Eval PyrolysisS ^ S 2 b (wt. )

Bitumen^(ppm)

Hydrocarbons(ppm)

PoorFairGoodVery GoodExcellent

0-0.50.5-1

1-22-4

>4

0-0.50.5-1

1-22-4

>4

0-2.52.5-55-1010-20>20

0-0.050.05-0.100.10-0.200.20-0.40>0.40

0-500500-1000

1000-20002000-4000

>4000

0-300300-600600-1200

1200-2400>2400

amg HC/g dry rock distilled by pyrolysis.

hmg HC/g dry rock cracked from kerogen by pyrolysis.cEvaporation of the solvent used to extract bitumen from a source rock or oil from a reservoir rock causes toss of the volatile hydrocarbons below about n-Cis. Thus, most extracts

are described as C^ . hydrocarbons."Lighter hydrocart)ons can beat least partially retained by avoiding complete evaporation of the solvent (e.g., Cio+).

Table 5.2. Geochemical Parameters Describing Kerogen Type (Quality) and the Character of Expelled Products 3

Kerogen Type

IIIll/lllIIIIV

HI(mg HC g TOC)

>600300-600200-300

50-2001510-155-101-51.51.2-1.51.0-1.20.7-1.0


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96 Peters and Cassa

Table 5.3. Geochemical Parameters Describing Level of Thermal Maturation

Stage of ThermalMaturity for OilImmature

MatureEarlyPeakLate

Postmature

Ro(%)

0.2-0.6

0.6-0.650.65-0.90.9-1.35

>1.35

Maturation' m a x

(C)470

TAIa

1.5-2.6

2.6-2.72.7-2.92.9-3.3>3.3

Bitumen/TO O


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5. Applied Source Rock Geochem istry 97

o- f % R 0^ B otryococcus

ResiniteTasmaniles

Macera lGroups

%TYPE x H / C = A H / C

0 . 2 0 x 1 . 3 = 0 . 2 6

i0 . 6 0 x 0 .8 5 = 0 . 5 1

- 0 . 2 0 x 0 . 5 0 = 0 . 1 0

E S T I M ATE D H / C = 0 . 9 0

0.1 0.15A to m ic O / C

0.20

Figure 5.2. Combined use of organic petrography, elemental analysis, and Rock-Eval pyrolysis and TOC improves confidence in assessment of the quality and maturity of kerogen in rock samples. A sample analyzed by Rock-Eval pyrolysis wascharacterized as being marginally mature Tmax = 435C) and gas prone HI = 150 mg HC/g TOC). Organic petrography showsa TAI of 2.5, an R0 of 0.5% (supporting the maturity assessment from pyrolysis), and the following maceral composition:type II20%, type III 60%, and type IV 20%. The calculated atomic H/C (0.90) corresponds with that determined by elementalanalysis, supporting a dominantJy gas-prone character. (Concept for figure courtesy of T. A. E dison.)

ca rbon . Dur ing ca t agenes i s and me tagenes i s , a l lkerogens approach graphite in composition (nearly purecarbon) near the lower left portion of both diagrams(Figure 5.1).

Maceral G roups

The three principal maceral groups in coal and sedimentary rocks are liptinite (exinite), vitrinite, and inertinite(Stach et al., 1982). Liptinite macerals, such as alginite,sporinite, cutinite, and resinite, generally mature alongthe type I or II kerogen pathways on the van Krevelendiagram (Figure 5.2). Preserved remains of the algaeBotryococcus and Tasmanites are examples of structuredalginite. Vitrinite macerals originate from land plantsand m ature along the type HI kerogen pathway. Collinite

is the structureless consti tuent of vitrinite, whereastelinite is the remains of cell walls of land plants. Figure5.2 shows two types of collinite: telocollinite contains noinclusions and is the maceral recommended for vitrinitereflectance measurements, whereas desmocollinite showssubmicroscopic inclus ions of l ip t in i te and othermaterials. Because of the inclusions, desmocolliniteshows a higher atomic H /C , has a lower reflectance(Figure 5.3), and comm only fluoresces un der ultravioletlight, unlike telocollinite. Inertinitic macerals, such assemi-fusinite and fusinite, mature along the type IVkerogen pathway. Because of the combined effects ofdiagen esis, therm al maturity , and differing org anicmatter input, a kerogen can plot anywhere on the vanKrevelen diagram and need not fa l l on any of theindicated maturation curves.


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98 Peters and Cassa

Desmocollinite

TelocoliiniteOxidized Vitrinite

Semi-Fusinite

N = 100% R0= 0.61 (Tdocollinite)

Liptinites

ft Fusinite

0.4 0.6 0.8 1.0 1.2 1.4

% REFLECTANCE1.8 2.0 2.2

Figure 5.3. A complete reflectogram showing thereflectance of all macerals in a kerogen sa mple. In caseswhere selection of the "true" vitrinite population (telocoliinite) is difficult, the trend of Ro versus depth estab lished bymany sam ples is useful for selecting the correct population. Here, telecollinite (hatchured) has a m ean % R 0 of0.61 . This sample contains significant amounts of oxidizedvitrinite and sem i-fusinite that could be mistaken forvitrinite. (Courtesy of S. C. Te erman.)

Petrography alone is too imprecise to evaluate thepetroleum potential of a source rock, primarily becausehydrogen-rich and hydrogen -poor ke rogen is difficult tod i s t ingu i sh . Am orp hou s ke rogen i s comm onlypresumed to be hydrogen rich and oil prone, but not allamorphous kerogens can generate o i l . Ul t raviole t -induced fluorescence microscopy of samples of lowthermal maturity distinguishes hydrogen-rich, oil-proneamorphous (fluorescent) from hydrogen-poor, non-gene ra t ive amorphous (nonf luo rescen t ) ke rogen ,suggesting that petrographic methods might be furtherrefined to better predict generative potential (Senftle etal., 1987).

Organic Fades

Various workers have used the term organic fades as asynonym for kerogen fades (based on chemical data) orpalynofacies or maceral assemblage facies (based onpetrographic data). Jones (1984,1987) propose a condsedefinition:

An organic facies is a mappable subdivision of a designatedstratigraphic unit, distinguished from the adjacent subdivisionson the basis of the character of its organic constituents, withou tregard to the inorganic aspects of the sedim ent.

Jones (1984, 1987) has defined organic fades using acombination of three types of kerogen analyses: atomicH/C ratios, Rock-Eval pyrolysis and TOC, and trans-mitted-reflected light microscopy. He showed that all

organic facies can exist in either carbonates or shales andthat there is little evidence that TOC requirements arelower for carbonate tha n for shale source rocks. Integration of organic facies studies with the concepts ofsequence stratigraphy is a step toward improving ourability to predict the occurrence of a source rock (e.g.,Pasleyetal.,1991).

When used together, elemental analysis, Rock-Evalpyro lys i s and TOC, and o rgan ic pe t ro g rap hy a repowerful tools for describing the richness, type, andthermal maturity of organic matter. Jones and Edison(1978) and Jones (1984) have sho wn how maceral com po

sition and thermal maturity from microscopy can beused to estimate the atomic H/C ratio of a kerogen(Figure 5.2). If the mea sured atomic H /C differs by m orethan 0.1 from the estimated value, both analyses aresuspect and are repeated. These maturity and atomicH /C results are commonly supported by Tm ax and HIdata obtained from each whole rock sample using Rock-Eval pyrolysis and TOC.

Coal

Coal is a rock contain ing m ore th an 50% org anicmatter by weight. Both coals and sedimentary rocks cancontain any combination of macerals. Various classifications of these organic-rich rocks are found in the literature (e.g., Cook and Sherwood, 1991). Not all coals arecompo sed of humic organic matter (higher plant, type IBkerogen). Humic and sapropelic coals contain less than 10%and more than 10% liprinite, respectively. Humic coalhas long been recognized as a source for gas, primarilymethane and carbon dioxide. However, boghead andcannel coals are dominated by type I and II kerogens,respectively, are oil prone, and thus show high oilpotential.

Coals can generate oil, as exemplified by major accumulations in Indonesia and Australia. Two principallimitations for coals as effective source rocks are (1)expulsion efficiency and (2) organic matter type (sufficient hydrogen). Because of the physical properties ofthick coal seams, generated liquid products are usuallyadsorbed and generally escape only when cracked to gasand condensate (Snowdon, 1991; Teerman and Hw ang,1991). Coals that can generate and release oil mustcontain at least 15-20% by volume of liptinite maceralsprior to catagenesis, corresponding to an HI of at least

200 mg H C /g TO C and an atomic H /C ratio of 0.9(Hu nt, 1991).

Kerogen and Bitumen Com position

Detailed structural information on kerogen is limitedbecause of its heterogeneous composition and difficultiesassociated with the chemical analysis of solid organicmatter. Kerogen has been described as a geopolymer,which has been polymerized from a rand om mixtureof monomers. These monomers are derived from thediagenet ic decomposi t ion of biopolymers, inc ludingproteins and polysaccharides (e.g., Tissot and Welte,1984). This view has led to many publications showing

generalized chemical structures for kerogen, none ofwhich are particularly informative.The discovery of insoluble biopolymers in l iving

organisms, sediments, and sedimentary rocks has led toa reappraisal of the structure of kerogen (Rullkotter andMichaelis, 1990). In the modified scheme, more emphasisis placed on selective preservation of biopolymers andless on reconstitution of monomers. Progress has beenachieved by the application of specific chemical de gradation (Mycke et al, 1987), pyrolysis (Larter and Senftle,1985), and spectroscopic techniques (Mann et al., 1991).Structural elucidation techniques are bey ond the scope of


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this chapter, although the reader should be aware thatthese studies are likely to impact our understanding ofkerogen.

Asphaltenes in bitumen are lower molecular weightfragments of kerogen and may be intermediates betweenkerogen and b i tumen . For example , a l thoughasphaltenes are soluble in polar solvents, they showelemental compositions similar to associated kerogens(Orr, 1986) and similar distributions of hydrocarbons(Band urski, 1982; Pelet et al., 1985), includ ing sterane sand triterpanes (Cassani and Eglinton, 1986).

Lipids can be incorporated into kerogen du ring diage-nesis, but many survive as free consti tuents in thebitumen and are known as molecular fossils, biologicalmarkers, or biomarkers. Biological markers are complexorganic compoun ds composed of carbon, hydrogen, andother e lements which show l i t t le or no change instructure from their parent organic molecules in livingorganisms (Peters and Mo ldowan, 1993).

Expelled Products

Petroleum expelled from an active source rock,primary migration) (Lewan, Chapter 11, this volume) can

migrate along a fault plane or permeable carrier bedsecondary migration) (England, Chapter 12, this volume)

to a porous reservoir rock (Morse, Chapter 6; Jordan andWilson, Chapter 7, this volume) capped or surroundedby a comparatively impermeable seal (Downey, Chapter8, this volume) that together form a trap (Biddle andWielchow sky, Chap ter 13, this volume ). Examples ofhow this happens are described in the case studies in thisvolume. Factors controlling the quantities of petroleumneeded to saturate the pore space in a source rock priorto expulsion and the efficiency of expulsion are poorly

understood and represent active research topics (e.g.,Wilhelms et al., 1990; Macke nzie an d Q uigley, 1988).Accurate estimates of these quantities will improve massbalance calculations.

Shows of petroleum are proof of a petroleum systemand when encountered during drilling are useful exploration clues, particularly when they can be quantifiedand regionally mapped. Cuttings or cores that bubble orbleed oil and gas during removal from the well are called

live sho ws , in contrast to the asphaltic staining ofdead shows. The quality of shows can be evaluated by

their fluorescence under ultraviolet light, by the color oforganic solvent extracts, or by the geochemical screeningmetho ds described later. Q uantitative bitumen or hydro

carbon yields from reservoir rocks assist in distinguishing between commercia l and noncommercia lsubsurface petroleum occurrences (Swanson, 1981).

Oils inherit biomarker distributions similar to those inthe bitumen from the source rock, thus allowing oil-oiland oil-source correlation or fingerprinting and paleo-reconstruction of source rock depositional conditions(Peters and Mo ldo wa n, 1993). An adv an tag e ofbiomarkers is their resistance to biodegradation byaerobic bacteria in the reservoir. For heavily biode gradedoils wh ere biomarke rs have been partially altered, correlation sometimes requires sealed tube pyrolysis of

asphaltenes, followed by biomarker analysis of thegenerated bitumen (e.g., Cassani and Eglinton, 1986).Biomarker and other correlation technologies, such asstable carbon isotope analysis and pyrolysis-gas chromatography, are among the most powerful tools formapping petroleum systems to reduce exploration risk,particularly w hen oils migrate large distances from theirpod of active source rock or when more than one sourcerock pod exists in the basin fill. Based on these fingerprinting techniques, the level of certainty for a petroleumsystem is determined. This level of certainty indicates theconfidence that the petroleum from a particular accum ulation came from a specific po d of active source rock.

SCREENING METHODSSedimentary bas in analys is (Magoon and Dow,

Cha pter 1, this volume) of frontier areas begins w ithgeologic and geophysical reconnaissance. Early evaluations focus on sample and data collection to assess the

presence of thick sedime ntary sequences, regional hydrocarbon seals, and appro priate reservoir lithologies. Mapsusing well control, outcrop, and geophysical data mustbe prepared or revised.

Geochemical screening analyses are practical exploration tools for rapid and inexpensive evaluation of largenumbers of rock samples from outcrops and wells.Outcrop samples from measured stratigraphic sectionsare better than random outcrop samples because theycan easily be made into a geochemical log that can becompared to nearby geochemical logs of wells. Rocksamples from wells include d rill cuttings, sidewall cores,and co nve nt iona l cores , in orde r of dec reas ingabundance. Large numbers of analyses of these rocksamples are used to make geochemical logs to evaluatethe thickness, distribution, richness, type, and thermalmaturity of source rocks in the basin fill. Evaluating thesource rock in the basin fill is an important part of sedimentary basin analysis. The next step is to identify thepod of active source rock, which is the first step in evaluating a petroleum system.

The most effect ive screening method for largenumbers of rock samples f rom wel ls and outcropscombines Rock-Eval pyrolysis and TOC measurements.These da ta a re usua l ly supp lemen ted by v i t r in i t ereflectance and spore coloration results to constructdetailed geochem ical logs (see Figures 5.4-5.11).

Chapter App endix B describes key criteria for usefulgeochemical logs. These include proper (1) samplespacing, (2) sample quality and storage, and (3) samplepreparation.

Rock-Eval Pyrolysis and Total Organ icCarbon

Total organic carbon (TOC, wt. ) describes thequantity of organic carbon in a rock sample an d includesboth kerogen and bitumen. TOC can be determined inseveral ways, and geologists should be familiar w ith theadvan tages and d i sadvan tages o f each (Chap te r


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Appendix C). TOC is no t a clear indicator of petro leumpotential. For example, graphite is essentially 100%carbon, but it will not generate petroleum . Some Tertiarydeltaic marine shales contain u p to 5 wt. % TOC b utgenerate l i t t le if any petroleum because the organicmatter is gas prone or inert. The theory and pitfalls ofRock-Eval pyrolysis interpretation are discussed byPeters (1986) and are not repeated here. Key parametersare defined in Chapter Ap pendix D.

Gas AnalysisResidual gas (Q -C5) an d heavier hydrocarbons in

drill cuttings and m ud arriving at the shaker table can beliberated with a blender and analyzed by gas chromatog-raphy (GC) at the we ll site as par t of a process calledhydrocarbon mud logging. Some systems use a simple hotwire detector to make only two measurem ents, methaneand ethane-plus hydrocarbons. Hydrocarbon mud loggas curves are commonly available from wildcat wellsand provide useful information on hydrocarbon shows(e.g., see Figure 5.7).

Alternately, gaseous hydrocarbons can be detected atthe well si te or in the laboratory using an oil showanalyzer (Espitalie et al., 1984) or by hydrogen strippingGC (Schaefer, et al., 1978). In GC, an inert carrier gas(mobile phase) passes through a column coated with anonvolatile, high molecular weight liquid (stationaryphase). The temperature of the column is graduallyr a i s ed u s i n g a t e m p e r a t u r e - p r o g r a m m e d o v e n .Petroleum com ponents are separated d epending on theirvolatility and affinity for the mobile versus stationaryphases as they pass through the column. A plot ofdetector response versus time shows separated peaksrepresenting single or multiple comp onents a nd is calleda chromatogram.

Headspacegas analysis is sometimes used as a screeningtool because it assists in quantitative show detection(Tissot and Welte, 1984). For this method, cuttings arefrozen or canned w ith water and a bactericide. Agitationand/or heating releases some of the hydrocarbons fromthe cuttings into the headspace over the water, which canbe sampled th roug h a sep tum w i th a sy r inge andanaly zed b y GC (e.g., Be rnard, 1978; W helan , 1984).Many choose not to use this technique because it is costlyand time consuming and metal cans rust or leak instorage. Furthermore, this method is not particularlyuseful for establishing maturity profiles because gasreadily migrates. Vitrinite reflectance and Rock-Evalpyrolysis are more reliable methods for establishingthermal maturity profiles tha n gas analysis.

Light hydrocarbon gas distributions combined withisotopic compositions can be used to describe the originand level of thermal maturity of the gas (e.g., Rice andClay pool, 1981; Jame s, 1983; Schoell, 1984). Reliablesampl ing me thods a re impor t an t because samplehandling can alter these gas compositions. For example,drill cuttings used for headspace gas analyses should bekept in gas-tight containers at deep freeze temperaturesto avoid evaporative loss of components. Examples ofprocedures for sampling gases in dril l ing muds and

cuttings are g iven in Schaefer et al. (1978), Reitsema e t al.(1981), and Whelan (1984). Other procedures are used forsampling gases under pressure (Gas Processors Association, 1986). The m ore adv anced aspects of gas geochemistry are beyond the scope of this chapter, which dealsprimarily with rapid screening methods for evaluatingoils and source rocks. How ever, readers sh ould be aw arethat analysis of gases is likely to become increasinglyimportant as future exploration shifts from oil to gas.

Organic PetrographyThermal Alteration Index

Thermal alteration index (TAI) is a numerical scalebased on thermally induced color changes in spores andpollen. Strew-mount slides of kerogen are examined intransmitted light, typically using a split-stage comparison microscope. The analyst matches the color of thespecimen unde r one ocular with that of a standar d u nde rthe other ocular of the microscope. Several TAI scales

have be en p ublish ed (e.g., Staplin, 1969; Jones a ndEdison, 1978). An advantage of TAI is tfiat the greatestcolor changes occur in the oil window. TAI measurements are imprecise because description of color issubjective, palynomorph thickness and type affectresults, and many samples contain few palynomorphs.Q uantitative spore color measurem ents (M arshall, 1991)offer the possibil i ty of more precise assessment ofthermal maturity. Despite limitations, TAI commonlyprovides useful data, even when other maturity parameters fail.

Vitrinite Reflectance

Vitrinite reflectance (RQ) increases during thermal matura t ion due to complex, i r revers ib le aromat iza t ionreactions. A pproximate RQ, TAI, and Tmax values havebeen assigned for the beginning and end of oil generation (Table 5.3). RQ versus de pth plots generally showlinear trends on semi-log paper. Dow (1977b) showedhow these plots can be used to support the existence offaults, intrusions, and changes in geothermal gradientand how to estimate the thickness of a section lost at anunconformity. This information provides valuable calibration for reconstructing burial histories.

For vitrinite reflectance, kerogen isolated from sedimentary rocks is embedded in epoxy on a slide or in anepoxy plug and polished to a flat, shiny surface (Bostickand Alpern, 1977; Baskin, 1979). Mea suremen ts are ma deof the percentage of incident light (usually at a wavelength of 546 nm) reflected from vitrinite particles(preferably telocollinite) under oil immersion (Stach etal., 1982). The subscript o in RQ refers to oil immersion.Some old papers refer to Ra and Rw, reflectance in airand water, respectively. Vitrinite becomes anisotropic athigh levels of maturity (above about 1% R


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Several factors based on the experience of the analystare weighed in the process of selecting vitrinite particles.Ro trends established above an d below the sam ple can beused to eliminate certain populations of macerals fromconsideration. B ecause TAI an d RQ are related (Jones andEdison, 1978), a measured TAI can be used to estimatethe R

0 of the vitrinite population. This process is not

always reliable, however, because TAI is commonlymeasured on less than a dozen palynom orphs and thesemight represent recycled organic matter or contamination from drilling mu d.

Reliability of RQ measu rements from single sam plesincreases when suppor ted by independent matur i typarameters (e.g., TAI and T max ) and R0 versus depthtrends established by multiple samples in a well. Forexample, Tm a x can be used to support R, particularly inthe thermally mature stage. In situ vitrinite in somesamples can be ove rwhe lmed by r ecyc led (h ighmaturity) or caved (low maturity) particles. Selection ofthese particles as the true vitrinite migh t result in

anomalous values compared to the R


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late petroleum from larger drainage areas compared tovertically drained systems. For this reason, lower limitsare used to define SPI categories for laterally drainedsystems (low, SPI < 2; mo dera te, 2 < SPI < 7; high, SPI >7) than for vertically drained petroleum systems (low,SPI < 5; moderate, 5 < SPI < 15; high, SPI > 15) (see figure4.4 of Lew an, Chap ter 4, this volume).

SPI is a measure of the pe troleum potential of a sourcerock and ideally is determined from thermally immaturerock. After a source rock shows a favorable SPI rating,maps of SPI and thermal maturity are used to evaluatewhich areas of a basin have the highest petroleumcharge. Areas with the highest charge are most likely tobe nearest the source rock w here it is the m ost thermallymature , or neares t the pod of ac t ive source rock.Conversely, areas most likely to have the lowest chargeare farthest from the mature source rock, or farthest fromthe pod of active source rock. Whether this charge ismostly gas or mostly oil is determined from the ke rogentype and matur i ty. Demaison and Huizinga (1991;Chapter 4 , this volume) pro vide a complete discussion ofmigration drainage and entrapment styles for differentpetroleum systems and show h ow to estimate the SPI forsource rocks, even when they have undergone thermalmaturation beyond the immature stage.

Mass Balance Calculations

Mass balance calculations, either by a ccumulation (orprospect) or petroleum system, can be used to provideano the r compar i son o f the amoun t o f pe t ro l eumgenerated with the amount that has accumulated. Thegeochemical data for screening can also be used for SPIcalculations (Demaison and Huizinga, Chapter 4, thisvolume) and for mass balance calculations as suggestedby Schmoker (Chapter 19, this volume), who se techniqueis used in many of the case studies in this volume.

EXAMPLESGeochemical Logs

Geochemical logs are among the most valuable toolsfor basin an alysis, yet few e xamples are given in the literature (e.g., Clementz et al., 1979; Espitalie et al, 1977,1984, 1987; Peters, 1986; Magoon et al., 1987, 1988).Proper use of geochemical logs allows identification ofthe following features in penetrated intervals:

Occu rrence of poten tial, effective, and spen t as wellas active and inactive source rocks

Main stages of thermal evolution: diagenesis(immature), catagenesis (mature), and metagenesis(postmature) zones

Occurrence of varying amoun ts of in situ andmigrated petroleum

When geochemical logs are unavailable, geophysicalwireline logs and interpretive techniques can be used asqualitative indicators of organic content (e.g., Passey etal., 1990; Sch mo ker a nd H este r, 1983; Stocks an d

Lawrence, 1990; Hester et al., 1990). These methods aremost reliable within small areas wher e wireline responsehas been calibrated to geochemical da ta.

Geochemical logs for eight exploratory wells areincluded to show their usefulness for detecting freehydrocarbons and identifying source rocks. The firstthree geochemical logs (Figures 5.4-5.6) are from threewells (I, II, and III) that are in the sam e area a nd dem onstrate the lateral continuity of two different source rocks.The last five geochemical logs (Figures 5.7-5.11) are fromwells that are in different areas, but a re used as examplesof different ways to identify and evaluate a source rock.

We lls I through III

The high-quality geochemical log for well I is basedon closely spaced Rock-Eval pyrolysis and TOC datasupplemented by vitrinite reflectance (Figure 5.4).Closely spaced samples allow a critical evaluation ofsource and reservoir rock intervals (note the widersample spacing in the C formation, a Lower Cretaceous

reservoir rock). The penetrated section contains twosource rocks. The Upp er Cretaceous B formation sourcerock interval at 780-1540 m is a potential source rock thathas the capacity to generate significant quantities of oil(SPI = 42 t HC/m2). The Tm a x versus depth trend iss l ight ly depressed through th is in terval , probablybecause this sulfur-rich kerogen undergoes thermaldegradation at lower temperatures than many type IIkerogens. Because the Lower Cretaceous is at maximumburial de pth, the F formation source rock at 3120-3620 mis an active source rock that is presently generating oil(SPI > 81 H C/m2). The production or productivity index(PI) gradually increases below about 3200 m, reflectingthe onset of generation, which is also indicated by Tm^

an d RQ data. Vitrinite is generally absent in the carbonatesect ion and in the s t ra ta conta ining par t icular lyhydrogen-rich kerogen. PI anomalies (e.g., at 100-600 mand 1600-3050 m) are mathematical artifacts caused byrelatively low S2 yields wh ere Si yields ma y be slightlyelevated by sm all quantities of organic drilling ad ditivesor minor shows. The F formation penetrated in well I ispresently an active source rock.

The geochemical log for well II, which is located in thesame basin about 120 km southeast of well I (Figure 5.5),shows that the U pper C retaceous potential source rock isthicker than in well I. This potential source rock is stillimmature and shows a similar source potential index(SPI = 40 t HC/m2) to that in well I. The Lower Creta

ceous source rock in well II is thicker and shows morediscrete zones of higher and lower source potential thanin well I. The total thickness of the Lower Cretaceousinterval in well II is 700 m, but the net source rockthickness is only about 550 m and shows an SPI of 25 tH C / m2 . Only the deeper portions of the Lower Cretaceous source rock are actively generating petroleum(because the onset of petroleum generation for thissource rock occurs at 0.6% R J. Stratigraphically equivalent Lower Cretaceous source rocks buried more deeplyadjacent to this trap are the probable source for hydrocarbon shows in the Lower Cretaceous sandstone in well


11/28


O X Y G E N H Y D R O G E N O R G A N IC

IN D E X IN D E X C A R B O N

W E L L H Y D R O C A R B O N S O U R C E

D ATA P O T E N T I A L

S =HCS ALREA DY IN ROCK S ? = H C S FROM KEROGEN PYROLYSIS. S3 = C 0 2 FRO M KER OGEN PYROLVSIS.

Figure 5.4. Geochemical tog for welt I, showing immature and mature source rocks in the Upper and Lower Cretaceous(see Tables 5.1-5.3). Mud gas data were unavailable for this well.

II. High product iv i ty indices (a t 100-600 m and1820-2600 m) are an artifact of low S\ and S2 values inrocks containing highly oxidized organic m atter.

Well III is located about 80 km northwest of well I(Figure 5.6). Sidewall cores are indicated by dotted bars,while ditch cuttings are indicated by solid bars. Only theUpper Cretaceous potential source rock was penetratedin this well. This unit is less oil prone than in wells I andII, but it is approaching the onset of oil generation (R,,

equivalent 0.5% for this unit). Because it is very thick, theUpp er C retaceous source rock still shows a high SPI of 15t HC/m2 . Variations in TOC and HI in the Upper Cretaceous source rock in this well are more obvious than inwells I and U and can b e explained by local sea level fluctuations. Geochemical param eters on the logs in Figures5.4-5.6 allow individual units to be correlated amongthese three wells, similar to conventional correlationsusing wireline logs and pa leontology.


12/28


S | = H C S A L R E A D Y IN ROCK S; =HCS FROM KEROGEN PYROLYSIS. S3 = C 0 2 FROM KEROGEN PYROLYSIS.

Figure 5 .5. Geochemical log for well II, located about 120 km southeast of well I (Figure 5.4). The Upper Cretaceous potentialsource rock in well II is thicker than in well I but is still immature. The Lower Cretaceous section in well II is thicker andcomprises more discrete zones of organic-rich and -lean rock compared to well I.

W ells IV through VIII

The geochemical log for well IV shows a largely poorsource section above 3500 m (Figure 5.7). Tick marks inthe samp le location column to the left of the hydroc arbonyield colum n (S2) in the figure show locations of samples.The hydrocarbon yield column shows that most samplesabove 3500 m contain kerog en w ith little or no petroleumpotential. PI data indicate the presence of hydrocarbon

shows in the sandstone and siltstone interval between2400 and 3500 m. Deeply buried shale below 3500 mrepresents an active source rock that is presently generating oil (SPI > 8 t HC/m2 ) . T max values increase withdep th in this shale, establishing a therma l maturity trendthat is consistent w ith that in the poo r source shale aboveabout 2400 m. The thermal m aturity of the sandstone an dsiltstone interval between 2400 and 3500 m can be extrapolated from the T max trend established by the overlying


13/28


OXYGEN

INDEX

HYDROGEN

INDEX

Oil GASNONE 1 NONE ,

ORGANIC

CARBON

W E L L

D A T A

H Y D R O C A R B O N S O U R C E

P O T E N T I A L

F E E T

1000

2000

3000

4000

5000

(S,/TOC)1

m m

^^S

j g 3

: _ j

:3

S, =HCS ALREADY IN ROCK SE =HCS FROM KER0GEN PYROLYSIS. S 3 = C 0 2 FROM KEROGEN PYROLYSIS.

Figure 5.6. Geochemical log for well III, located about 80 km northwest of well I (Figure 5.4). The Upper Cretaceous section inwell III is not as oil prone as in wells I or II and is more thermally mature, approaching the onset of oil generation. Geochem

ical parameters on the logs in Figures 5.4 to 5.6 allow individual units to be correlated among the wells. Sidewall coresindicated by dotted bars, drill cuttings by solid bars.

and underlying shales. Measured T ^ * values are eitheranomalously high or low in the sandstone and siltstoneinterval compared to the established trend based on Tm a xin the shales. The anomalously high Tm a x values resultfrom dominance of recycled organic matter in thesecoarse-grained, organic-poor rocks. The low T m^ valuescorrespond to zones impregnated by migrated oil orwh ere S2 peaks are too sm all (


14/28


DEPTH

O X Y G E N

I N D E X

H Y D R O G E N O R G A N IC

I N D E X ' C A R B O NW E L L

D ATA

H Y D R O C A R B O N S O U R C E V T R t N I T E. . , < T I T . . I REFLECTANCEP O T E N T I A L , A N D T MA X

7000

> ' ';:^ I H.1 M U D

je=;i HYDROCARBON I5 3 S , YIELD .m \ S j / S ,a * I l I I S io , J 1 " ' 1 mg H C /g rock.

The geochemical log for well VI shows a depressedtrend in Tm ax in the Lower Cretaceous lacustrine rocksbelow 2500 m (Figure 5.9). Organic petrography data


15/28


S, =HCS ALREADY IN ROCK S 2=HCS FROM KEROGEN PYROLY SIS. S 3 = C 02 FROM KEROGEN PYROLYSIS .

Figure 5.8. Geochemical log fo r well V showing organic-lean, marginally mature Jurassic source rock. In situ bitumen from

this source rock has been geochemically correlated with oils in the area. An SPI of 21 HC/m2 has been calculated from theJurassic source rock (see text). Sidewall cores indicated by dotted bars, drill cuttings by solid bars.

indicates a significant alginite component in the kerogenfrom these rocks. Espitalie et al. (1985) and Huizinga etal. (1988) have shown that Tm a x is seldom useful as amatura t ion parameter for o i l -prone a lgal kerogensbecause, unlike other kerogens, i t shows li t t le to noincrease throughout the oil window. Fades effects onT m a x are discussed in Peters (1986). In this case, vitrinitereflectance represents a more useful thermal maturityparameter than Tm ax .

Well VII penetrates Mesozoic and Tertiary marineshales and sandstones, reaching Precambrian basementat total depth (Figure 5.10). The geochemical log showsthe results for sidewall cores (clotted bars) and drillcut t ings (sol id bars) . Thin shale beds sampled bysidewal l cores are more organic-r ich than nearbysamples, but are too thin to generate significant quantities of petroleum . W hen m ixed w ith adjacent lithologiesin drill cuttings composited over 10-20 m intervals, these


16/28

10 8 Peters and Cassa

Si =HCS ALREA DY IN ROCK FRO M KEROG EN PYROLYSIS.

Figure 5.9. Geochem ical log for well VI showing a depressed T m a x trend compared to R 0 in Lower Cretaceous lacustrine

rock. Type I kerogen, comm only ass ociated with lacustrine source rock, show s little change in T m a x during maturationthroughout the oil window. For example, Espitalie et a l. (1985 ) show that Tm a x for type I kerogen does not exceed 440-445Cthroughout the maturity range defined by R 0 = 0 .4-1 .5%.

thin organic-rich shales are difficult to recognize. If thedrill cuttings samples had been used alone, these sourcerocks would have been overlooked in the penetratedsection. These thin beds could represent the feather-edgeof a source rock that thickens and is more deeply buriedsomewhere else in the basin fill. Based on the results inthis well, more of these types of geochemical analyses innearby wells could be obtained to determine if this

source rock does thicken and become more maturewhere it is more deeply buried. If these thin shale bedsdo not increase in thickness and maturity, there is littlechance they could charge nearby traps. In this case, othersource rocks must be identified if this area is to remainprospective for oil exploration.

Well VIII penetrates a thick Cretaceous marinesequence dominated by gas-prone organic matter


17/28


FROM KEROGEN PYROLYSIS.

Figure 5.10. Geochemical log for well VII. The thin shale beds sam pled by the sidewall cores are organic-rich, but are too thin

to generate commercial quantities of petroleum. When mixed with adjacent lithologies as in cuttings samples, these organic-rich shales can be overlooked. If drill cuttings had been used alone, no source rock intervals would have been recognized inthis well. Sidewall cores indicated by dotted bars, drill cuttings by solid bars.

(Figure 5.11). The geochemical log shows the effects of amajor thr ust fault on Tmax, PI, and vitrin ite reflectancetrends. Thrusting of more thermally mature over lessmature rocks results in an offset in the maturity trend,with higher maturity values above the fault plane thanbelow. The rocks above the thrust fault are postmature(>1.35% Ro). If a source rock had been present in thiszone, it would now b e described as a spent source rock.

Geochemical MapsGeochemical maps are made from geochemical logs.

The properly designed and implemented geochemicallog allows the geologist and geochemist to evaluate asource rock in one dimension. The power of thegeochemical log is obvious wh en several logs in the samearea are used to make source rock maps and crosssections. An example is a series of maps and a cross


18/28


S, =HCS ALREADY IN ROCK S2=HCS FROM KEROGEN PYROLY SIS. S 3 =C 0Z FROM KEROGEN PYROLY SIS.

Figure 5.11. Geochemical log for well VIII shows the e ffects of a major thrust fault at about 2000 m in this well on T ma x , R0,

and PI. The footwall shows good quantities of mature, gas-prone to inert organic matter, while the hanging wall containspostmature organic matter. Sidewall cores indicated by dotted bars, drill cuttings by solid bars.

section for the fictitious Dee r Shale source rock.The ficti t ious Deer-Boar(.) petroleum system was

int roduced by Magoon and Dow (Chapter 1 , th isvolume) through four figures an d a table (Figures 1.2-1.5and Table 1.4). The map and cross section were d raw n toshow the essential elements of the petroleum system atthe critical mom ent, or at th e end of the Paleozoic. At thattime, the Deer Shale was a n active source rock, wherea snow, because of uplift, it is inactive. In addition, a rift

graben formed on the right (east) side of the cross sectionduring the Tertiary. To elaborate on this example, foursource rock maps and one cross section (Figure 5.12) thatrepresent the present-day geology show how a sourcerock is evaluated and placed into the context of apetroleum system that was operating in late Paleozoictime. Sedimentary basin analysis techniques are used toevaluate the source rock, whereas the petroleum systemis used to evaluate the h ydrocarbon s.


19/28


INDEX MAPFOII WELL

NUMBERS ONTABU *

\ \ 8 \

\l_ i \ ^

\ / \ * \V15 4 ^ v^ l \ ^ v

130'.


20/28


Table 5.4. Geochemical Information

WellNo.

12

345

6789

10

1112131415

1617

R0(%)

0.430.42

0.380.380.35

0.450.500.520.500.65

0.780.560.580.450.60

0.600.55

h

(m)

2537

80125123

105121

864575

15080

150115146

156130

on the Fictitious Deer Shale 3

S,b

1.71.8

2.84.14.0

3.11.93.12.93.0

2.05.35.16.45.1

5.15.2

S 2b

88

142221

1617151615

1027263426

2526

S-, + S 2 b

9.79.8

16.826.125.0

19.118.918.118.918.0

12.032.331.140.431.1

30.131.2

P(g/cm3)

2.42.4

2.42.42.4

2.42.42.42.42.4

2.42.42.42.42.4

2.42.4

SPI(t HC/m2)

0.60.93.27.87.4

4.85.53.72.03.2

4.36.2

11.211.210.9

11.39.7

TOC(wt. %)

2.02.0

2.84.44.3

3.64.03.13.03.9

4.85.25.26.45.1

5.05.2

HI(mgHC/gTOC)

400400

500500488

444425484533384

208519500531510

500500

aSee equation 1 in text for definition of terms.bmg HC g dry rock.

Sedimentary Basin Analysis

The data used to construct this hypothetical examplemight include geochemical logs for the 32 exploratorywells, as well as other types of geologic and geophysicalinformation that are no t show n (Figure 5.12A). Information from geochemical logs is summarized for 17 of the sewells in Table 5.4. The symbols in Figure 5.12A indicate

that the exploratory well is a dry hole, encountered gasor oil shows, or is an oil well. A dry hole indicates thatfrom well site drill cuttings examination, mu d log, Rock-Eval pyrolysis, or other geochemical analyses, there is alack of petroleum. Oil and gas shows indicate that somepetroleum was enc ountered. The oil wells correlate withthe fields shown on the Deer-Boar petroleum systemma p an d table (Figure 1.3 and Table 1.3, Chapter 1). Thedashed line represents the geographic extent of the sam epetroleum system, and the wavy unconformity l inerepresents the erosional edg e of the source rock.

The isopach map of the Deer Shale indicates that itthickens from 25 to 150 m (Figure 5.12B). Because ofessentially uniform lithology vertically throu gh the shale,this is both a gross and net source rock thickness. Thecontours trend in a northwesterly direction and indicatethat before or during the deposition of the overlyingreservoir rock erosion stripped aw ay the source rock eastof the unconformity line.

The TOC contour map of the Deer Shale source rockranges from 2.0 to >6.0 wt. % (Figure 5.12C and Table5.4). The average TOC contoured is derived from the netsource rock thickness, that is, TOC data is first avera gedover 20 m intervals then these 20 m intervals areaveraged. This procedure is used to average TOC

because the frequency with which each 20 m interval issampled and analyzed is different. For example, a 10-mcore may be sam pled and analyzed every 1 m, whereasdrill cuttings samples may be sampled every 10 m. Thelowest TOC values are nearest the unconformity at thetop of the map, and the highest TOC values are wherethe source rock is immature at the bottom of the map.

From right to left, TOC values increase to 5.0 wt. %, butthen decrease with burial depth to about 3.0 wt. %because of thermal maturity. This TOC map suggeststhat the Deer Shale is a very good to excellent source rock(Table 5.1).

To determine the present-day thermal maturity of theDeer Shale source rock, a vitrinite reflectance value wa sdetermined for the base of the overlying Boar Sandstonereservoir rock, and this value was contoured (Figure5.12D). The isoreflectance map indicates that the base ofthe reservoir rock was buried enough to be in the oilwindow (>0.6 RQ) in two areas and has reached the gaswindow (>1.35 %Ro) on the left. Because the source rockis only present in the w est (left) an d is thermally m ature,

it is logical to conclude that the oil and ga s shows and oilaccumulations originated from this pod of ma ture sourcerock. When the thermal maturity pattern is comparedwith the TOC m ap, the TOC is interpreted to decreasewh ere the thermal maturity is highest.

Rock-Eval pyrolysis data are sum marize d for 17 of the32 wells (Table 5.4) to determine the hyd roge n index (HI)and (SPI). The HI show s the present-day a mo unt ofhydrog en in the source rock organic matter and indicatesthe kerogen type or quality (Table 5.2). HI is the S2 peak(mg HC/g rock) divided by the TOC (mg TOC/g rock)


21/28


times 100. The average HI values were determined in thesame way as the average TOC values, that is, over 20-mintervals. The average H I contours range from 400 to 583mg H C /g TOC, indicating type II kerogen.

The source po ten t i a l i ndex r equ i re s Rock-Eva lpyrolysis data as well as the thickness and density of thesource rock uni t , as d iscussed by Demaison andHuizinga (Chapter 4, this volume). If the density, netthickness, and quality of a source rock remain constant,SPI decreases as thermal maturity increases. Becausethese SPI values indicate remaining potential, they arecalled residua l SPI (Figure 5.12E). Resid ual SPI valuesfor the Deer Shale increase to the southwest (lower left),but are interpreted to be less than 4 where the sourcerock is bur ied deepest . The source rock densi ty i sconstant (Table 5.4). If prior to increased thermalmaturity, the SPI values all increased to the southwestfrom 2 to 10, then alon g strike there is a decrease of up to8 SPI uni ts caused by thermal matur i ty. These aremoderate to high SPIs (Figure 4.4).

These map s indicate that the Deer Shale is a very goodto excellent source rock that contains type II kerogen andthat there is a pod of thermally matu re source rock in thewest-central part of the map. Additional information,such as paleontology, indicates that the Deer Shale wasdeposited in a marine environment, and organic petrography indicates that the kerogen macerals are in partwaxy pollen (terrigenous), but mostly marine algaldebris.

The cross section that represen ts present-day geologyindicates that the ov erburd en rock to the east in the riftedgraben is Tertiary in age, whereas the overburden rock inthe foreland basin to the west is late Paleozoic (Figure5.12F). Because the source rock is absent from either sideof the graben, and the graben is filled with nonmarinesiliciclastics, the conclusion is that the hydrocarbons originated from the west side of the cross section from thepod of mature source rock ma ppe d earlier.

The map s and interpretations outlined here are part ofsedimentary basin analysis because organic and rockmatter are investigated. The thickness, TOC content, %Ro, and SPI are all mappable properties of the sourcerock. However, the well control shown on the base mapindicates that hydrocarbons have been detected in thearea (Figure 5.13). These hyd rocarbons are proof that inthis area a petroleum system exists. To determine theorigin and economic significance of these hydrocarbo ns,a petroleum system study is carried out.

Petroleum System StudyThe presence of hydrocarbons in such close proximity

to a pod of ma ture source rock strongly suggests that thetwo are genetically related, that is, the oil came from theDeer Shale source rock. This makes the Deer Shale aneffective source rock.

Oil has been discovered to the east of the graben,whereas the pod of mature source rock is on the westside of the graben. Unless the oil came from the east,south, or north of the rift graben, it had to m igrate beforethe rift grabe n formed in Tertiary time . This observation

ft

/

ft11

ftx

\\ v - %

A +

ftA

O

Aft

ft __

ftft

ft

ft ft

4 - ~ " \

ft

ft-

ft

N

A

" >\

ft ^\ft

\+ ++ft)

I- _ ^ft

No Shows -fr 'l Show

Q Gas SIow Oil Recov ered, DST

HYDROCARBON SHOWSOVERBURDEN ROCK INTERVAL

//A

/1\+\\

V

B

ft

ftft

ft

ft-ft

-~ ft

ft

ft

ft

ft

ft ft

ft

A

ft

X

ftX .

ft?\

ft ^Vfr\

*M/

- -_ J^ft

HYDROCARBON SHOWSSEAL ROCK INTERVAL

ft

/ft

/1

\\

v -r 'ft

c +

_ -

ft

ft

- ft

ft

ft-

.

ft ___

ft

ft

~"~ft x

ft

~_

ft

ft

\

ft

f\ft \

\ft\

ft \+ /

y- -_ J^ft

HYDROCARBON SHOWSRESERVOIR ROCK INTERVAL

Figure 5.13. Maps indicating the oil and gas shows enco untered in exploratory wells penetrating the Deer Shale andBoar sandstone. Few shows were detected in (A) the overburden rock and (B) the seal rock, whereas numerousshows w ere detected in (C) the reservoir rock horizon,indicating that this is the interval through which the oilmigrated.


22/28


indicates that the Deer Shale was an active source rocksometime in pre-Tertiary time. Burial history charts(Figure 1.2, Chapter 1) in several places in the areaconfirm that the source rock was active in late Paleozoictime.

Because the hydrocarbon sh ows in the wells are proofof the petroleum system, an unders tanding of the distribution of these shows is important to determine thegeographic and stratigraphic extent of the system. Mostof the wel ls were dr i l led to basement . Where thebasement rock, underburden rock, and source rockintervals were penetrated, shows were not detected.Hydrocarbon shows were detected in the overburdenrock and seal rock intervals.

Slight oil shows (streaming cut-fluorescence) weredetected near the base of the overburden rock in thenorth-central part of the area, and gas shows weredetected at a depth of 350 m in the central part of the area(Figure 5.13A). Well site evaluation of the gas showsindicated substantial gas on the ho t wire that w as 99.7%methane on the GC (dry gas). Laboratory analyses ofheadspace gas from canned cuttings collected throughthis interval indicate a carbon isotope value of -71.6%o,or biogenic gas. The conclusion is that th is gas is part of aseparate petroleum system probably related to Carboniferous coals in the overb urden rock.

Within the seal rock, two sh ows were detected nearthe base of the unit (Figure 5.13B). The show in the north-central part of the area und erlies the oil show in the overburd en rock. Both shows are interpreted to be verticallymigrating oil from the reservoir rock below. South of thisshow is another show in the seal rock, which is also interpreted to be oil that migrated from the reservoir rockbelow.

The reservoir rock interval, the Boar Sandstone, hasthe most abu ndan t oil shows of any interval investigated(Figure 5.13C). Of the 32 wells that p ene trate d thereservoir horizon, 7 have oil shows and 6 recovered oilwhen drill-stem tested. Based on the distribution of theseoil shows and their proximity to the pod of maturesource rock, there is little doubt that the oil originatedfrom this source rock. If this is true, the pattern of oilshows should give some indication as to the migrationpath of the oil. Two east-west bands of show s are show n,one at the top and on e in the middle of the map with fivewells in between that lack shows. These bands are goodcircumstantial evidence that the oil migrated from themature source rock on the west through the reservoirrocks along tw o corridors to the east (Figure 1.3, Chapter

1). The distribution of these oil shows and the pod ofmature source rock provide a basis for the geographicextent of the petroleum system.

Gas chromatography of the Deer Shale source rockextract shows tha t the pristane- phyta ne ratio is above 1.5and the carbon preference index (CPI) is 1.2. Organicpetrography indicates that the organic matter is mostlymarine amorphous material with up to 35% terrigenousmaterial.

The API gravity of the oil ranges from 32 to 43 andsulfur content is less than 0.3%. The pristane -phyta ne

ratio is 1.6 and the CPI is 1.1. A more definit iveoil-source rock correlation using biomarkers was notcompleted. Based on the rock extract and oil geochemistry, the oil probably originated from the Deer Shale,therefore the level of cer ta in ty for the Deer-Boarpetroleum system is hypothetical.

The maps and cross sect ions in Figures 1.2-1.5(Chapter 1) and in Figures 5.12 and 5.13 indicate thestratigraphic and geographic extent of the Deer-Boarpetroleum system. Based on the burial history chart, theDeer Shale was an active source rock in late Paleozoictime. The oil show map of the Boar Sandstone reservoirrock in terval indicates the o i l migra ted a long twocorridors into anticlinal and stratigraphic traps. InTertiary time, a rift graben formed, isolating the oil accumulations on the east from those on the west side of thegraben and the pod of inactive source rock. Oil samplesfrom the shows and accumulations were used in anoil-oil correlation. Results showed that all these oils originated from the same source rock. The confidence thatthis oil originated from the Deer Shale is based on thesimilarity of certain geochemical param eters for both theoil and rock extract even though a detailed oil-sourcerock correlation was not completed.

After the source rock is demonstrated to be effective,that is, it generated and expelled hydrocarbons, then thequestion remains as to how effective. Decreased TOCand pyrolysis yields of source rocks resulting fromthermal maturation must be taken into account to assesstheir original generative potential accurately and to ma kevolumetric estimates of petroleum generated (Dow,1977b; Schmoker, Cha pter 19, this volume). For example,TOC is little affected by maturation of rocks containingtype IV kerogen, but TOC can be reduced by 12-20 wt. %

for type III and b y as muc h as 50 and 70 wt. for types IIand I, respectively (Daly and Edman, 1987). Failure toaccount for these effects on mature or spent source rockscan cause source intervals to be overlooked on geochemical logs and can result in underestimates of originalsource rock potential or oil generated.

The volume of hydrocarbons generated and accumulated can be demonstrated in many ways. The cases tudies in th is volume use a mass balance method(Schmoker, Chapter 19), and D emaison and Huizinga(1991; Chapter 4) use the SPI. Because the case studiesadequately explain the mass balance method, only theSPI metho d is outlined here.

The residual SPI map is constructed from the net

source rock thickness ma p and the genetic potential (Sj +S2) (Table 5.4) of the source rock from Rock-Evalpyrolysis (Figure 5.12E). The SPI ranges from 0.6 to 11.3.The residual SPI where the source was buried deepestsuggests that up to 8 SPI units were lost as the sourcerock generated oil. Based on the preliminary SPI classification (Figure 4.4), the buria l of this source rock ind icatesa large drainage area with moderate to high potential.Using regional geology and SPI, this petroleum systemcan be classified as a supercharged, low impedance,laterally drained petroleum system.


23/28


SUMMARYSource rock character izat ion using geochemical logs

and maps i s an exerc i se in sed imenta ry bas in ana lys i swith the objective of identifying the pod of active sourcerock. The pod of act ive source rock contr ibutes hy drocarbons to the petroleum system.

This chap te r emp has iz es (1) t e rms use d to desc r ibesource rocks ; (2) sam pl ing , p re par a t io n , and ana lys i scr i ter ia ; (3) geochemical logs and their use to descr ibesource rocks and pe t ro le um show s in one d im ens ion ;and (4) geochem ical map s and their use for interpolat ingb e t w e e n o n e - d i m e n s i o n a l c o n t r o l p o i n t s f o r a t h r e e -d imens iona l unders tand ing o f the pe t ro leum sys tem.

Pro per u se of terms is cri t ical for c lear ly descr ibingpetroleum systems. Some examples of source rock termsi n c l u d e r i c h n e s s , k e r o g e n t y p e , t h e r m a l m a t u r i t y ,p roduc t genera ted , t ime genera ted , and p rovenance o rdeposi t ional enviro nm ent . Source rock organic r ichnesscan be poor, fa i r, good, very good, or excel lent (Table5.1) . Kerogen can be descr ibed as type I , I I , I I I , or IVbased on elemental analysis (Table 5 .2) . Organic petrography provides information on organic mat ter type andt h e r m a l m a t u r i t y, b u t is c u r r e n t l y t o o i m p r e c i s e t od e s c r i b e g e n e r a t i v e p o t e n t i a l . T h e r m a l m a t u r i t y i sd iv ided in to immature , ma tu re , and pos tmature basedon such pa ramete r s a s v i t r in i t e r e f l ec tance , Tm a x , andtherma l a l terat ion index (Table 5.3) . A s ource rock can bedes c r ib ed as po te n t i a l ( cou ld gen era te o il ) , e ffec tive( g e n e r a t e d o r c u r r e n t l y g e n e r a t i n g o i l) , o r s p e n t(generated oi l) . A s pent so urce rock can s t i l l gener ate gas .An inact ive source rock is not genera t ing oi l today, but inthe past i t wa s an act ive source rock. The term mar inesource rock implies ma rine deposi t ion, while the terms

m a r i n e o rg a n i c m a t t e r a n d m a r i n e k e r o g e n c o u l d

imply an or igin from marine organisms. A marine sourcer o c k m i g h t c o n t a i n d o m i n a n t l y l a n d p l a n t o r g a n i cmatter.

G e o c h e m i c a l l o g s o f c l o s e l y s p a c e d R o c k - E v a lpyrolysis and TOC , vi t r ini te ref lectance, li thology, m udlog gas , an d related da ta are indispe nsab le tools in thesedim entary basin evalu at ion process . Useful geoc hemi c a l l o g s r e q u i r e a d h e r e n c e t o p r o p e r p r o c e d u r e s f o rsam ple se lect ion, p repara t ion , ana lys i s , and in te rp re tat ion . The se logs iden t i fy pe t ro leu m sou rce rocks (aspo ten t i a l , e ffec tive , o r spen t ) , t he the rm al ma tu ra t iongrad ien t ( inc lud ing imm ature , ma tu re , and pos tm aturezones) , an d in situ and migra ted pe t ro leum shows .

B e c a u s e o f t h e r a p i d a n d i n e x p e n s i v e s c r e e n i n g

m e t h o d s u s e d , i t i s p r a c t i c a l t o g e n e r a t e l i b r a r i e s o fgeochemical logs that progressively reduce the r isk assoc i a t e d w i t h p e t r o l e u m e x p l o r a t i o n a s a p e t r o l e u mprov ince becomes m ore thorough ly samp led . Logs f romvarious locat ions can be used to map the pod of act ivesource rock, regional var ia t ions in organic fades , and thevolume of generated petroleum. This information can beu s e d a s i n p u t t o r e f i n e m a t h e m a t i c a l b a s i n m o d e l s .Final ly, the two-step procedure consis t ing of screeningfol lowed by detai led geochemical analyses on selectedsam ples reduce s cost an d s implifies interpreta t ion.

Acknowledgments We thank L . B. Magoon and W . G. Dowfor their invitation to prepare this paper and G. J. D emaisonforhis suggestion that it be written. The following peoplecotitributed timely reviews that improved the manuscript: L. B.Magoon, S. C. Teerman, D. K. Baskin, T. A. Edison, G.J.Demaison, J.T. Smith, and W.G. D ow. Tables 5.1,52, and 5.3were improved by input from D. K. Baskin, L. B. Magoon, and

Miles. Concepts for Figures 5.2 and 53 were provided by T.A. Edison and S. C. Teerman, respectively. S. D. Northam andB. R. Borden coordinated the production of figures and text.We thank E. L. Couch and N. Schneidermann for their supportand the management of Chevron Overseas Petroleum Inc. forpermission to publish this work.

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CHAPTER APPENDIX A:Kerogen Types

There are four principal types of kerogens found in coalsand sedimentary rocks which are defined using atomic H/Cversus O/C or Rock-Eval HI versus OI diagrams (see Figures5.1 and 5.2.)

Type I

Immature type I kerogens are oil prone, show high atomicH/C (>1.5), low O/C ( 0.04) and appe ar to generate pe troleu m at

We recommend Rock-Eval pyrolysis and TOC analysesevery 10-20 m and vitrinite reflectance data every 100-200 mthroughout each well. Closer sam ple spacing res ults in bettergeochemical logs. The strength of the pyrolysis and TOCscreening approach lies in sheer numbers of analyses. Trendsare established by statistically significant amounts of data, andoccasional anomalies become obvious (e.g., Figure 5.6). Incomplete geochemical logs based on isolated measurements are oflittle exploration va lue.

Because screening analyses are inexpensive, it is practical togenerate libraries of detailed geochemical logs. As provincesbecome better explored, libraries of logs progressively reduce

exploration risk by clarifying the three-dimensional distributions of organic fades, thermal maturity, and prospective reservoirs. Consistent scales for geochemical logs simplify comparisons of source rock intervals between wells.

Rock Sam ple Preparation

Rock sample quality generally decreases in the followingorder: conventional whole core, sidewall core, drill cuttings,and outcrops. Cuttings can be contaminated by particulate orfluid (e.g., oil-based m ud) drilling additives or can contain rockchips cave d from higher in the section du ring drilling.Cuttings polluted with diesel can be cleaned with a solvent, but

lower therm al m aturity than other type II kerogens (Orr, 1986;Baskin and Peters, 1992). Type II kerogens are also dominatedby liptinite macerals.

Type III

Imm ature type III kerogens show low atomic H/ C (


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5. Applied Source Rock Geochemistry 119

be light-colored on w eathered surfaces. Burrows and abund antbenthic macrofossils indicate poor conditions for preservationof organic matter. When in doubt, it is better to sample acandidate source rock for laboratory confirmation.

Most geochemical logs are based on cuttings, complemented by s idewal l co res and conven t iona l co res . Thefollowing discussion is directed toward cuttings, although thesame general criteria for preparation and interpretation applyto cores and outcrop samples. Each sample should weigh a bout50 g. Cuttings are best washed at the well site prior to shipmentand storage because mudcake can become more difficult toremove with time. Cuttings are washed of mudcak e using freshor salt water, and in the process, wet-sieved with a 2-mm topsieve and a 180-um bottom sieve. Cuttings must not be w ashedwith organic solvents, which remove soluble components.Many particulate additives can be floated off by p anning in anevaporating basin. Samples are air-dried at about 40C.

After arrival in the geochemical laboratory, cuttings arerewashed and described for lithology. Washed and air-driedcuttings are exam ined using a binocular microscope, and contaminants, such as walnut hulls, woodchips, metal, and obvious

caved material, are removed by negative picking. We do notrecommend positive picking, where an inferred representative lithology is selected for analysis from a mixture of litholo-gies in a sample. Positive picking generally results in nonrepre-sentative samples. We do not recommend composite samplesof cuttings from several intervals. Natural mixing of cuttings inthe mudstream during drilling is a form of compositing thatneed not be compounded in the laboratory. When severe,caving or bypass of lithologies on the shaker table can causeproblems in interpretation. We have found that natural mixingis reasonably representative of significant rock lithologies. Forexample, thin, organic-rich beds that might be sampled bysidewall cores are averag ed and do not appear as organic-rich spikes on geoc hemical logs (Figure 5.10).

A small portion of the dried cuttings is crushed to fine sandparticle size (0.125-0.25 mm). Grinding to a smaller size is notrecommended because powdered samples can resu l t inanom alous Rock-Eval results, including poor S2 peak definition, low S2 yield, and erroneous Tm a x values. The crushedsamples (100 mg) are analyzed using Rock-Eval pyrolysiswhe re every twentieth sam ple is a rock standard . If very rich inorganic matter (>10 wt. TO O , sample size is reduced an d thesample is rerun to ensu re linearity of response. Sample size alsoaffects Rock-Eval pyrolysis response (Peters, 1986). Anotherapproach that avoids this problem is to dilute an organic-richsample with pu re carbonate, followed by pyrolysis of 100 m g ofthe mix ture (Peters, 1986).

Samples showing high Sj values result from (1) potential or

effective source rocks or (2) rocks containing migrated oil orcontaminated by dr i l l ing addi t ives . Samples containingmigrated oil or drilling additives are readily distinguished fromsource rocks by anomalously high production indices for theirlevel of thermal matur i ty. Samples that do not meet thefollowing criteria (Table 5.3) are assumed to be contaminatedby drilling additives or migrated oil:

If Tm ax is in the range 390-435C, then PI mus t be


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applied source rock geochemistry

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