Comparative analysis and geological significance of kerogen isolated using open-system (palynological) versus chemically and volumetrically conservative closed-system methods

Organic Geochemistry 41 (2010) 800–811

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Comparative analysis and geological significance of kerogen isolated using open-system (palynological) versus chemically and volumetrically conservative closed-system methods Rovshan A. Ibrahimov *, K.K. (Adry) Bissada Department of Earth and Atmospheric Sciences, University of Houston, 312 Science and Research Building 1, Houston, TX 77204-5007, USA

a r t i c l e

i n f o

Article history: Received 6 August 2009 Received in revised form 3 May 2010 Accepted 5 May 2010 Available online 8 May 2010

a b s t r a c t Characterization of kerogen, the insoluble organic fraction of sedimentary rocks, is of key importance in investigations of the geochemistry of ancient depositional systems, paleo climates, basin thermal history, and petroleum generation systematics. Furthermore, exploitation of organic-rich oil shales as unconventional petroleum resources requires understanding of the physical chemistry of the kerogen conversion reactions and the coking behavior. These issues can be effectively addressed only if pure, representative kerogen is obtained from the rock matrix using chemically and volumetrically conservative methods that preclude losses, alteration, or fractionation of the organic matter. Isolation of representative and pure kerogen concentrates from the rock matrix is difficult. Many methods have been used over the years with various degrees of success. Most are based on traditional palynological processing approaches in open systems that commonly yield fractionated products, generally contaminated with pyrite and insoluble neo-fluorides. A few are based on complex sealed-system procedures that purport to conserve the entire kerogen with no fractionation, no alteration and no contamination. This study focuses on a comparative analysis of kerogens isolated with the conventional open-system approach (HCl/HF/heavy liquid separation) and kerogens concentrated with a fully automated, conservative, closed-system method (HCl/HF/CrCl2) under an inert N2 atmosphere. Source-rock samples of various richness, kerogen types, ages, depositional environments, lithologies and thermal histories were selected for kerogen isolation using the two approaches. Kerogen recovery efficiency, progress of the processes and quality of the isolated kerogens were monitored via mass balance calculations, organic elemental analysis and X-ray diffraction analysis. The inherent inadequacy in the open-system approach for geochemical research and the improved features of the chemically and volumetrically conservative method are documented. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Characterization of kerogen, the major form of organic matter (OM) in sedimentary rocks, is important for investigations of the geochemistry of ancient depositional systems, paleo climates and basin thermal history. Kerogen is thermally degraded in a predictable manner, with the solid organic structure becoming condensed. Tracking the physical chemistry of kerogen thermal conversion reactions and studying its coking behavior as it undergoes thermal transformation, is essential for assessing petroleum generation systematics. This is especially critical as we endeavor to exploit kerogen-rich oil shales as alternative petroleum resources using in situ conversion processes (Andrews, 2008). Investigations of kerogen are implemented by first isolating it from the rock matrix and studying the morphology with several * Corresponding author. Tel.: +1 832 213 9602. E-mail address: [email protected] (R.A. Ibrahimov). 0146-6380/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2010.05.006

methods. These include transmitted light microscopy, reflectedlight microscopy and chemistry. The morphology, used alone, is sometimes ambiguous inasmuch as highly degraded humic kerogen may appear amorphous and might be misinterpreted as sapropelic (hydrogen-rich), when, in fact, it is hydrogen-poor. Kerogen chemical composition and, specifically, its hydrogen enrichment and aromaticity, influence the quantity and type of hydrocarbon products that can be generated. Elemental analysis of the isolated kerogen to determine C, H and O composition, and nuclear magnetic resonance (NMR) spectral characteristics to determine aromaticity, should provide a more incisive view of any hydrogen enrichment and reactivity. All these can be addressed effectively by use of unaltered, unfractionated, indigenous kerogen concentrates. Kerogen can be isolated using chemical and physical means. Robinson (1969) and Durand and Nicaise (1980) reviewed the physical methods for separation. These methods include mineral–organic fractionation based on specific gravity differences

R.A. Ibrahimov, K.K. (Adry) Bissada / Organic Geochemistry 41 (2010) 800–811

(sink-float method), differential wetting of the kerogen and minerals with two immiscible liquids, etc. The physical methods of isolation deliver chemically unaltered kerogen, but frequently results in incomplete recovery and fractionation of the natural kerogen composition. Chemical isolation of kerogen is a complicated process. Traditionally, it involves the use of strong, non-oxidizing acids and bases that are deemed effective for dissolving the rock matrix without modifying the chemistry, morphology or color of the kerogen. The method entails the dissolution of carbonates, sulfates, oxides, hydroxides and some sulfides with HCl, and dissolution of silicates with HF. The reactions are normally carried out in open, plastic beakers at 70 °C, a temperature beneficial to dissolving carbonate, but not too high to affect oxidation and degradation of the OM (Dancy and Giedroys, 1950; Forsman and Hunt, 1958; Smith, 1961; Saxby, 1970; Durand and Nicaise, 1980). After reaction, the aqueous liquids are removed from the beakers by decanting. The dissolution with acids is repeated until kerogen isolation is judged to be relatively complete. The samples are washed between acid treatments. The residue of the HCl and HF treatments commonly consists of a concentrate, intimately admixed with pyrite, some heavy mineral oxides such as rutile and anatase (TiO2) and silicates such as Zircon (ZrSiO4). Frequently, there are Ca and Mg neofluorides such as Ca and Mg ralstonite [(Na1.47Ca0.52)(Mg1.49Al0.55P0.04)F6[(OH)0.43O0.36F0.21)], which, once formed, are virtually impossible to remove (Durand and Nicaise, 1980; Saxby, 1970). Further refinement of the kerogen concentrate is required, commonly using heavy-liquid floatation, to separate the pyrite and other insoluble heavy minerals. Pyrite removal without loss of kerogen is particu-

801

larly difficult because of the intimate association between kerogen and authigenic pyrite in many rocks. Kerogen obtained using these two standard methods is seldom representative of the whole organic make-up of the rock because the recovery efficiency is generally low. The presence of un-dissolved minerals, particularly pyrite and neo-fluorides in the concentrate, interferes with characterization of the kerogen, especially through elemental analysis. This paper describes and documents problems inherent in the conventional isolation process and compares the yield, chemistry, and other characteristics of kerogen separated using the conventional open-system process with those of kerogen concentrated using a fully-automated, chemically and volumetrically conservative process carried out in a flow through, sealed system under an inert N2 atmosphere (Royle and Nolte, 1992).

2. Materials and methods 2.1. Samples A suite of diverse source rock samples was selected for the isolation of kerogen using the conventional and conservative methods. The samples are mostly from different basins in the USA (Fig. 1), but include rocks from one additional country (Fig. 2). The suite was selected to represent lacustrine, marine and deltaic source rocks of various richness, kerogen type, ages, lithologies and thermal histories (Table 1; Figs. 3 and 4). Most of the sample preparation work and the geochemical analyses of the native rock

Alaska

Indonesia tra

ma

Su Fig. 1. Sample locations.

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R.A. Ibrahimov, K.K. (Adry) Bissada / Organic Geochemistry 41 (2010) 800–811

W

E

MINAS FIELD

SIHAPAS GROUP PEMATANG GROUP

SIHAPAS

PEMATANG BROWN SHALE

0

10

5

15

KM

AMAN TROUGH

Migration Direction Top Oil-Window Top Condensate-Window Carrier/Reservoir System Source Interval Basement

Fig. 2. Geological cross section of Aman trough, Central Sumatra Basin, Indonesia, showing sampling from the Eocene/Oligocene Pematang Brown Shale Formation and the Miocene Sihapas Formation.

samples and produced kerogens were carried out at the geochemistry laboratories of the Center for Petroleum Geochemistry at the University of Houston. A brief description of the geologic settings and geochemical attributes of the selected rocks is given below. 2.1.1. Sample 1: Monterey Formation, Santa Maria Basin, California (Fig. 1) This is an upper Miocene siliceous shale with a total organic carbon (TOC) content of 2.4% and a total hydrocarbon generation potential (S1 + S2) of 8.21 mg HC/g rock. Isaacs (1984) documented enrichment of up to 17% TOC, with an average value of about 5%. The kerogen is Type II marine OM with a hydrogen index (HI) of 216 mg hydrocarbons (HC)/g OC. Baskin and Peters (1992) stated that the formation was deposited in a deep water, highly reducing marine environment, with high concentration of organic sulfur accounting for the enrichment in Type II-S kerogen. Our sample was obtained from the Harvest Platform, offshore California (Fig. 1). 2.1.2. Samples 2-A, 2-B and 2-B0 : Mahogany Shale, Green River Formation, Piceance Basin, Colorado (Fig. 1) These samples are Eocene, lacustrine, extraordinarily organicrich, highly dolomitic and highly laminated, with an average TOC content of 17.1% and average S1 + S2 value of 164 mg HC/g rock. They were obtained from an excavated outcrop in the Mahogany ledge zone near Anvil Point, Colorado. The OM in this area is an algal, Type I kerogen (HI > 900 mg HC/g OC), developed in a marginal lacustrine environment (Fouch et al., 1994). Sample 2-B was divided into two portions. One was used as is. The other, sample 2-B0 , was subjected

to Fischer Assay pyrolysis to 500 °C for 4 h, i.e. past the HC generation stage and well into the coking stage. 2.1.3. Samples 3-A and 3-B: Pematang Brown Shale and Sihapas Formations, Central Sumatra Basin (Fig. 1) These were obtained from the highly prolific rift sequence in the Aman Trough of Central Sumatra (Williams et al., 1985; Williams and Eubank, 1995). Sample 3-A, from the overlying Miocene fluvio-deltaic rift facies of the Sihapas Formation (Fig. 2), was selected to represent a gas prone, Type III source rock, with TOC values averaging ca. 1%, and S1 + S2 < 1 mg HC/g rock, and HI < 100 mg HC/g OC. In contrast, sample 3-B, from an Eocene/ Oligocene syn-rift, freshwater lacustrine facies of the Pematang Brown Shale Formation (Fig. 2), was chosen particularly to represent oil prone, Type I source rocks. It had a TOC value > 4%, an S1 + S2 > 28 mg HC/g rock and HI > 500 mg HC/g OC. 2.1.4. Sample 4: Yegua Formation, South-East Texas Basin This sample is an Eocene rock from the Boyt Heirs Gas Unit, Liberty County, Texas (Fig. 1). It was chosen to represent a Type III OM source rock. It has TOC averaging ca. 1.8%, S1 + S2 < 1.5 mg HC/g rock and HI < 100 mg HC/g OC. The formation is composed of carbonaceous mudstones, interbedded with fine-grained to mediumgrained sand, silt and lignites, deposited in a marginal marine environment with notable terrestrial input (Fisher, 1990). 2.1.5. Sample 5: Shublik Formation, North Slope, Alaska This is a Triassic sample, with an average TOC content of 4.9% and total S1 + S2 of 31.6 mg HC/g. It is from the Tenneco Phoenix

2.8 0.8 0.9 0.9 0.1 0.6 1.0 2.0 3.3 2.4 1.2 2.5 1.2 0.7 3.7 N/A 2 3 0 0 10 1 N/A 10 10 0 0 2 0 14 N/A 0.4 0.4 — 0.4 1.3 0.8 0.8 1.3 1.4 0.6 0.7 0.8 N/A 0.8 441 439 439 543 432 442 455 436 458 463 434 446 457 432 457 216 958 868 3 84 504 50 637 297 122 412 222 164 261 194 0.8 1.8 1.3 0.1 1.0 1.6 0.2 0.2 0.6 0.2 2.0 0.3 0.2 0.7 0.1 2.4 17.7 16.4 10.9 0.8 8.56 1.8 4.9 2.6 2.8 10.6 3.8 3.3 0.99 5.78 1 2-A 2-B 2-B0 3-A 3-B 4 5 6-A 6-B 7-A 7-B 8-A 8-B 8-C

Miocene Monterey Fm, California (83700 ) Eocene Green River Shale – 1, Colorado Eocene Green River Shale – 2, Colorado Pyrolized Green River Shale – 2, Colorado Miocene Sihapas Fm., Central Sumatra (34020 ) Eoc.-Olig.Pematang Brown Sh., Central Sumatra (45240 ) Eocene Yegua Fm, S.E Texas Basin, TX (14,2650 ) Shublik Fm, Triassic, N. Slope Alaska (79560 ) Pennsylvanian Desert Creek, Utah (55570 ) Pennsylvanian Desert Creek, Utah (56430 ) U. Dev -L. Mississip. Woodford Sh., TX (87400 ) U. Dev -L. Mississip. Woodford Sh., TX (87440 ) Upper Devonian New Albany Sh., Illinois (37810 ) Upper Devonian New Albany Sh., Illinois (37860 ) Upper Devonian New Albany Sh., Illinois (38030 )

3.0 7.5 1.1 0.0 0.1 8.2 0.5 0.6 3.2 1.9 1.1 3.9 1.8 2.0 2.0

5.3 169.8 142.4 0.4 0.7 43.2 0.9 31.1 7.7 3.4 43.7 8.5 5.4 2.6 11.2

8.3 177.3 143.5 0.4 0.8 51.4 1.4 31.7 10.9 5.3 44.8 12.4 7.2 4.6 13.2

31 10 8 1 119 19 11 3 25 6 19 7 5 75 1

0.36 0.04 0.01 0.16 0.14 0.16 0.33 0.02 0.3 0.36 0.02 0.31 0.25 0.44 0.15

Ro or Equiv. RE HI (mg HC/g TOC) S1 + S2 (mg HC/g rock) S3 (mg CO2/g rock) S2 (mg HC/g rock) S1 (mg HC/g rock) TOC (%)

Properties of native rocks (source quality and thermal maturity)

Stratigraphic and geographic affinity No.

Sample

Table 1 Geochemical characteristics of samples (see text for abbreviations, N/A, not available).

OI (mg CO2/g TOC)

KTR

Tmax (°C)

Pyrite (%)

TS (%)

R.A. Ibrahimov, K.K. (Adry) Bissada / Organic Geochemistry 41 (2010) 800–811

803

# 1 well, north of the Colville Delta, Arctic Alaska (Fig. 1). It represents the marine, laminated, black phosphatic limestones, marls and mudstones of the area (Bird, 1987; Robison et al., 1996). These organic rich rocks are thought to have been deposited in an active upwelling margin (Peters et al., 2006; Robison et al., 1996). The selected sample is a Type I source rock, with an average HI of 637 mg HC/g OC and an optimum thermal maturity for petroleum generation (vitrinite reflectance, Ro, 0.8%). 2.1.6. Samples 6-A and 6-B: Desert Creek Formation, Paradox Basin, Utah These two Pennsylvanian samples, with an average TOC content of 2.7% and total S1 + S2 ranging between 10.9 and 5.3 mg HC/g rock, are from the Aneth oil field in south-east Utah (Fig. 1). They represent the autochthonous, evaporitic, marine, shaly dolomitic facies of the Chimney Rock Shale member (Peterson, 1992). Sample 6-A, has a S1 + S2 value of 10.9 mg HC/g rock and is from a highly mature (Ro 1.3%), highly sapropelic source rock, enriched in Type II algal kerogen (HI ca. 300 mg HC/g OC). The facies are considered the most likely source for the Aneth oil (Hite et al., 1984; Grammer et al., 1996; Nuccio and Condon, 1996). Sample 6-B has a distinctly lower hydrocarbon generation potential (S1 + S2 ca. 5.3 mg HC/g rock), a significantly lower HI (122 mg HC/g OC). The somewhat higher thermal maturity, with Ro 1.4%, falls within the condensate generation/preservation stage. 2.1.7. Samples 7-A and 7-B: Woodford Shale, Permian Basin, Texas (Fig. 1) These Upper Devonian–Lower Missisipian marine, cherty black shale samples represent rocks of deep water origin, deposited under generally anoxic conditions and enriched in oil prone Type II kerogen (Comer, 1991; Landis et al., 1992). Sample 7-A has TOC content 10.6%, S1 + S2 value of 44.8 mg HC/g rock and HI value of ca. 412 mg HC/g OC. The rocks are considered the principal source for one of the most prolific petroleum systems in Texas and New Mexico, the Woodford - San Andres Petroleum System (Ellison, 1950; Corboy and Cook, 2003). Sample 7-B displays a distinctly lower TOC content (3.8%), a lower hydrocarbon generation potential (S1 + S2 ca. 12.4 mg HC/g rock), and a lower HI (222 mg HC/g OC). 2.1.8. Samples 8-A, 8-B, and 8-C: New Albany Shale, Illinois Basin, Illinois (Fig. 1) These brownish-black to greenish-gray shale samples of Middle to Late Devonian age represent deep water source rocks with Type II and Type III kerogen (Barrows and Cluff, 1984). Samples 8-A and 8-C have TOC contents of 3.3% and 5.8%, respectively, and S1 + S2 values of ca. 7 and 13 mg HC/g rock, respectively. In contrast, sample 8-B has a TOC content of ca. 0.9 wt% and a S1 + S2 value of ca. 5 mg HC/g rock. The New Albany Shale accumulated under a variably oxygenated water column (Frost, 1996), with the paleo-oxygenation of the bottom waters affecting both the sulfur and pyrite contents and pyrite framboid size distribution. The lithofacies varies from an organic-rich, variably bioturbated, dark shale lithofacies with up to 20% TOC, to organic-poor, strongly bioturbated, lighter colored shale lithofacies with <2% TOC, depending on the paleo-redox conditions during deposition (Lazar and Schieber, 2008; Schieber and Lazar, 2004). In this study, samples 8-A and 8-C represent the organic-rich facies and sample 8-B represents the organically lean facies. 2.2. Analytical methods Before kerogen isolation, the geochemical properties of the individual rock samples in the suite were documented (Table 1 and Figs. 3 and 4). Furthermore, the geochemical properties of

804

R.A. Ibrahimov, K.K. (Adry) Bissada / Organic Geochemistry 41 (2010) 800–811

Type I (Oil Prone)

1000

Green River 2-A

900

Hydrogen Index (mg HC/g TOC)

Green River 2-B

800 Type II (Oil/Gas Prone)

700 Shublik

600 Pematang Brown Sh.

500

Woodford Sh. 7-A

400 300 New Albany Sh. 8-B Monterey Sh.

200

Type III (Gas Prone)

New Albany Sh. 8-C & A Desert Creek 6-B

100

Sihapas

Yegua Pyrolyzed Gr. Riv. 2-B'

0 0

50

100

150

Oxygen Index (mg CO2/g TOC) Fig. 3. Modified van Krevelen diagram showing organic matter types in sample suite used.

1000

3. Kerogen isolation

THGP (S1+S2 - mg HC/g rock)

Oil Prone Green River Sh. 2-A Green River Sh. 2-B

100

Pematang Brown Sh.

Gas Prone

Woodford Sh. 7-A Shublik

10

1

New Albany Sh. 8-C Monterey Sh.

New Albany Sh. 8-A

Yegua Sihapas Pyrolyzed Gr. Riv. 2-B'

0.1 0.1

(iii) RockEval analysis was carried out using a RockEval II-Plus instrument following the standard procedures of Espitalié et al. (1985, 1986). This ensured that the sample selection met the diversity requirement of the study. The classification of the various members of the sample suite is depicted in Fig. 4 (plot of TOC vs. S1 + S2), and Fig. 3, (plot of hydrogen index vs. oxygen index). (iv) Mineralogy and pyrite content determination on the rocks and the kerogen was made using standard powder X-ray diffraction procedures a Rigaku Miniflex II Desktop X-ray Diffractometer. (v) Organic elemental analysis of the kerogen was contracted to Intertek QTI Laboratory for C, H, N, O and S determination. The analysis entails combusting a 5 mg aliquot of kerogen to CO2, H2O and NO2 (N2) in a pure O2 environment with a Perkin-Elmer 2400 Elemental Analyzer to determine C, H and N contents, which are separated under steady state conditions and measured as a function of thermal conductivity. Another aliquot is introduced into the same analyzer equipped with an oxygen accessory kit that uses pyrolysis to convert the oxygen to CO. The CO is then separated from the other pyrolysis products under steady state conditions, and measured as a function of thermal conductivity. A third aliquot is analyzed for S by combustion to SO2, which is measured using coulometric titration. The elemental analysis data are reported in Tables 4a and 4b.

1

10

100

TOC (wt%) Fig. 4. TOC vs. total hydrocarbon generation potential (S1 + S2) for sample suite used.

Kerogen can be isolated from its rock matrix using chemical and physical means. The physical means include mineral–organic fractionation based on specific gravity differences. Because this process most often results in an incomplete recovery due to fractionation of the kerogen blend, it is not discussed further. Only the chemical processes were examined, i.e. those that involve the use of strong, non-oxidizing acids and bases to dissolve the rock matrix without modifying the kerogen chemistry, morphology or color. We compared and contrasted two methods: (i) the first, used most frequently by commercial laboratories (adapted from palynological practices), is normally carried out in batch mode in open beakers where the demineralizing reagents are removed from the beakers by decanting. The second method (ii) involves reactions conducted in a closed-system under an inert N2 atmosphere. This method was developed explicitly for conserving the entire kerogen concentrate in a filter-sealed, flow-through reaction vessel that precludes the use of decanting and the attendant loss of products. The following section describes our experiments using the closed-system process (Fig. 5). 3.1. Kerogen isolation with open-system method

the concentrates produced from both the conventional open-system and the conservative closed-system isolation processes (described in Section 3) are documented in Tables 3–5 and Figs. 6 and 7. A brief description of the analytical work is as follows: (i) An aliquot (50 g) of each sample were ground to pass 60 mesh. A 20 g split of the original material was used for the open-system kerogen isolation and another 20 g for the closed-system process. The remainder was used for the geochemical and mineralogic characterization of the original rock samples. (ii) TOC and total sulfur (TS) contents of the rock samples were determined using a Leco-type HORIBA EMIA-220 V Carbon/ Sulfur Combustion Analyzer.

A variety of different methods of palynological preparation for the isolation of organic constituents from the rock matrix are found in the palynological and micropaleontological literature (e.g., Gray, 1965; Barss and Williams, 1973; Doher, 1980; Herngreen, 1983). Most commercial laboratories and geochemical research groups employ their special adaptation of these methods, except that the final HNO3 bleaching step is eliminated to preclude oxidation of the OM. In this study, the conventional/traditional open-system isolation work was contracted to a commercial service laboratory. 3.2. Kerogen isolation using conservative closed-system method The isolation process using the conservative closed-system method began by extracting any bitumen from the 20 g aliquots of ground sample using MeOH–acetone–CHCl3 (15:15:70 v/v) by

R.A. Ibrahimov, K.K. (Adry) Bissada / Organic Geochemistry 41 (2010) 800–811

805

Reagents & N Select 2 Valves Reaction vessels with membrane filter seals

CPG

Hot a ir Supp ly

Fill / D rain v a

lves

Fig. 5. Automated, closed-system kerogen isolation instrument.

Table 2 Typical kerogen isolation protocol in conservative closed-system method. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

FILL HCl DRAIN FILL HCl DRAIN FILL HCl TIME/DRAIN FILL H2O DRAIN FILL HCl DRAIN FILL HCl TIME/DRAIN FILL HCl TIME/DRAIN FILL H2O DRAIN FILL NH4OH DRAIN FILL H2O DRAIN

21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

FILL H2O DRAIN FILL/HCl:HF DRAIN FILL/HCl:HF TIME/DRAIN FILL HCl DRAIN FILL H2O DRAIN FILL NH4OH DRAIN FILL H2O DRAIN FILL NH4OH DRAIN FILL H2O DRAIN FILL/HCl:HF TIME/DRAIN

41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

FILL HCl DRAIN FILL H2O DRAIN FILL HCl DRAIN FILL H2O DRAIN FILL/HCl:HF DRAIN FILL HCl DRAIN FILL H2O DRAIN FILL NH4OH DRAIN FILL H2O DRAIN FILL H2O DRAIN

61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80

FILL HCl DRAIN FILL H2O DRAIN FILL CrCl2 DRAIN FILL HCl DRAIN FILL CrCl2 DRAIN FILL H2O DRAIN FILL HCl FILL H2O DRAIN FILL H2O DRAIN FILL H2O DRAIN FILL H2O

soxhlet extraction. The bitumen-free sample was air dried and heated overnight in a vacuum oven at 60 °C. The kerogen isolation process was then performed in a pressurized reaction cell (Fig. 5). The method comprises multiple reaction steps between acidic and basic reagents and the mineral sample (Table 2). The closed cell permits reaction at pressures ca. >30 psi (two atmospheres) and provides for removal of all liquids from the cell without any loss of sample. The automated kerogen isolator used (Fig. 5) is capable of processing up to eight samples simultaneously. All steps are executed by a microprocessor system under computer-command. The samples are placed in the cells between two membrane filters (<0.45l) at the entrance and exits of the cell. Once the ‘‘start” command is initiated, a protocol similar to that in Table 2 is implemented. The process starts with a 50:50 mixture of concentrated HCl and de-ionized water introduced into the reaction cells. After the desired period of reaction, the fluid is removed automatically through the membrane filters and concentrated

HCl is added to the reaction cells. At the completion of the reaction, the HCl is removed and the reaction cells are flushed with de-ionized water to remove any remaining metal ions and to bring the samples close to neutrality. Concentrated NH4OH is added and the mixture allowed to react for the desired time and removed. The reaction cells are then flushed with de-ionized water. Concentrated HF is added for the desired reaction time and then removed. Concentrated HCl is added again and removed. The cells are flushed once again with de-ionized water before a second treatment with NH4OH, followed by another flush with de-ionized water. In our study, these steps were repeated a number of times, depending on the mineralogy of the rocks. The dolomitic rocks required more extensive treatment with HCl, whereas the siliciclastic rocks needed more treatment with HF. At the end, the kerogen in the sealed reaction cells was treated numerous times with acidic CrCl2 (Acholla and Orr, 1993) to dissolve pyrite. Samples high in S required at least three treatments. Finally, the kerogen was removed from the membrane filters of the reaction cells, dried in a vacuum oven and packaged until ready for analysis. 4. Results and discussion As indicated above, our objective was to assess the efficiency of the kerogen recovery approaches, the effectiveness of the kerogen/ mineral separation, the utility of the products for organic microscopy studies and the chemical integrity of the recovered products. Mass balance calculations, organic elemental analysis, X-ray diffraction analysis were carried out on the kerogen concentrates prepared using the two approaches. The results are summarized in Tables 3–5 and Figs. 6 and 7. 4.1. Kerogen recovery efficiency For the purpose of this study, kerogen recovery efficiency (KRE) of the isolation process is defined as the quantity of recovered kerogen relative to the amount of OM initially in the rocks prior to the isolation process. It can be expressed qualitatively by simply comparing the initial kerogen content, as wt% of the rock, to the

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Table 3 Comparative analysis of kerogen recovery efficiency: isolation with traditional open-system method vs. conservative closed-system method. No.

1 2-A 2-B 2-B0 3-B 5 6-A 7-A

Sample

TOC (wt%)

Monterey Green River – 1 Green River – 2 Pyrolyzed Green River – 2 Pematang Brown Sh. Shublik Desert Creek Woodford

2.44 17.72 16.40 10.90 8.56 4.88 2.58 10.60

C content of kerogen (wt%)

77.69 81.90 76.81 87.51 85.34 79.16 78.64 74.78

Initial kerogen (% of rock)

3.14 21.64 21.35 12.46 10.03 6.16 3.28 14.17

recovered kerogen, also expressed as wt% of the rock. The initial kerogen content is determined from TOC of the parent sample, converted to OM content via the elemental carbon content from the kerogen’s elemental analysis, where

Initial Kerogen Contentðwt%Þ ¼ TOCðwt%Þ=ðKer: Elem: Carb: FractionÞ

ð1Þ

More precisely, KRE can be expressed in terms of amount of kerogen recovered as % of initial OM in the rock. Thus

KRE ð%Þ ¼ ½Recovered kerogen ðgÞ=Initial kerogen ðgÞ  100 ð2Þ where

Initial Kerogen ðgÞ ¼ Rk: wt ðgÞ  TOC ðwt%Þ=ðKer: Elem: Carb: ðwt%Þ

ð3Þ

Table 3 compares the kerogen recovery efficiency for eight of the samples. Column 4 shows the calculated initial kerogen content of the rocks and columns 7 and 8 the kerogen recovery efficiency expressed as % of the initial kerogen for the two isolation approaches. The recovery efficiency for the conventional open-system method is as low as 2% to a maximum of about 14%, whereas the efficiency of the conservative closed-system approach can exceed 90%, with a minimum of ca. 64% in one of the eight cases examined. 4.2. Kerogen/mineral separation effectiveness The effectiveness of the acid digestion processes in the two approaches was assessed from powder X-ray diffraction analysis (XRD). Generally, bulk XRD is used for semi-quantitative determination of the minerals in a ground rock sample. Routine bulk mineral analysis includes identification of quartz, feldspar, calcite, dolomite, siderite, ankerite, pyrite, pyrrhotite, marcasite, apatite, zircon, rutile or other crystalline mineral species. Examples of XRD patterns for the kerogen isolated using the two techniques are shown in Fig. 6. Note the presence of a variety of minerals detected in the kerogen residues generated by the traditional method and the purity of the residues recovered with the unconventional kerogen isolation approach. Only major minerals have been annotated on the XRD traces of Fig. 6. The broad peak on the XRD patterns of the kerogen concentrates isolated using the conservative method (patterns on the right hand panels of Fig. 6) represents the amorphous OM background. It is interesting to note that the Monterey concentrate, isolated with the closed-system approach, shows two very small peaks riding above the broad OM background. These are attributed to a minor amount of quartz that escaped HF digestion. The Monterey is a highly siliceous mudstone

Kerogen recovery of rock (%)

of Initial kerogen (%)

Open system

Closed system

Open system

Closed system

0.33 5.70 0.46 0.53 0.64 0.85 0.01 0.98

2.00 21.15 18.20 11.05 4.55 5.61 2.85 12.96

10.35 2.29 2.15 4.21 12.46 13.79 0.30 6.88

63.68 91.74 85.24 88.71 88.23 91.00 86.87 91.40

and should have received another HF digestion step to achieve complete removal of the silica from the kerogen (Section 3.2). 4.3. Kerogen chemical integrity Organic elemental analysis was carried out on kerogen isolated by both methods. The results are tabulated in Tables 4a and 4b. The calculated H/C and O/C atomic ratio values are shown in Table 5. Values for the 15 sets of concentrates are plotted on eight van Krevelen diagrams in Fig. 7a and b, where the original plot base (Tissot et al., 1974) is modified to reflect interpretation of the H/ C and O/C atomic ratios in terms of ‘‘Sapropel Index”, a measure of oil-proneness (Bissada, 1982). Invariably, kerogen isolated with the closed-system approach shows either higher H/C values or lower O/C values than that isolated with the open-system method. In cases where the kerogen is homogeneous, such as the Green River Oil Shale, there is negligible or no fractionation of the OM. In all other cases, where the kerogen is a heterogeneous mixture, losses during processing are accompanied by serious fractionation. It is not immediately evident whether oxidation of OM plays a significant role in the open-system isolation process, though the shift in O/C ratio in the Monterey, the pyrolyzed Green River, the Sihapas, the Shublik and the Woodford kerogen is quite strong (Fig. 7a and b). Another salient observation is the difference in relative concentration of ash and S in the two sets of pyrolyzed isolates. Generally, the concentrates from the open-system approach show higher enrichment in S and ash (ash is not in the rock – it is a combustion product) than the remains from the closed-system approach (Table 4a). Evidently, the traditional method was not as effective in removal of all the mineral matter, especially pyrite. 5. Summary and conclusions Robust chemical and physiochemical characterization of kerogen is important in investigations of basin thermal history and petroleum generation systematics. It is especially so in studying the physical chemistry of kerogen conversion reactions and coking behavior of organic-rich oil shales as unconventional petroleum resources to be exploited by in situ heating technologies (Kelemen et al., 2007; DiRicco and Barrick, 1956). These chemical and physiochemical studies require concentrate of very pure, unaltered, unfractionated kerogen representative of the complete organic make-up of the rocks. We focused on examining the quality and integrity of kerogen concentrates prepared using the traditional method and compared the attributes of the concentrates with those prepared under an inert atmosphere using a closed-system approach. Our observations lead to the following conclusions on the relative effectiveness of the two methods:

807

R.A. Ibrahimov, K.K. (Adry) Bissada / Organic Geochemistry 41 (2010) 800–811 Table 4a Composition of organic concentrates from traditional method vs. conservative method. (elements and ash content as determined). No.

Identity

1 2-A 2-B 2-B0 3-A 3-B 4 5 6-A 6-B 7-A 7-B 8-A 8-B 8-C

Monterey Green River – 1 Green River – 2 Pyrolyzed Green River – 2 Sihapas Pematang Brown Shale Yegua Shublik Desert Creek Desert Creek Woodford Woodford New Albany New Albany New Albany

Traditional open-system kerogen and ash C (wt%)

H (wt%)

39.42 3.44 80.11 11.24 Not analyzed 58.22 2.53 39.85 3.35 82.84 7.99 55.27 4.56 44.02 4.51 56.37 4.18 49.00 3.68 55.64 5.19 41.16 3.10 50.27 4.64 31.95 0.96 64.83 4.55

Conservative closed-system kerogen and ash

O (wt%)

S (wt%)

N (wt%)

Ash (wt%)

C (wt%)

H (wt%)

O (wt%)

S (wt%)

N (wt%)

Ash (wt%)

4.41 5.73

11.28 0.52

1.21 0.20

40.24 2.20

7.82 6.58 3.35 3.82 3.92 4.39 1.89 7.78 2.72 2.23 1.23 2.58

5.63 1.05 1.14 3.62 10.53 8.62 18.24 4.72 30.76 8.34 26.61 18.17

3.58 0.62 1.66 0.75 1.02 1.40 0.99 1.69 0.64 0.99 0.52 1.19

22.22 48.55 3.02 31.98 36.00 25.04 26.20 24.98 21.62 33.53 38.73 8.68

72.07 81.49 73.78 83.87 68.95 84.47 52.69 72.20 72.06 61.82 72.68 59.83 63.90 64.09 63.08

6.07 11.34 9.87 3.16 5.93 8.61 5.17 7.37 4.62 4.76 6.95 5.61 6.91 6.66 6.74

7.73 5.98 7.84 3.32 5.75 3.40 3.55 5.00 4.39 2.21 10.31 2.88 2.82 2.74 2.65

4.27 0.49 1.93 0.80 1.28 1.05 1.75 4.88 8.62 4.12 5.03 0.62 1.38 2.90 5.92

2.63 0.20 2.64 4.69 0.82 1.45 0.69 1.76 1.94 0.66 2.22 3.16 1.13 0.60 0.68

7.23 0.50 3.94 4.16 17.27 1.02 36.15 8.79 8.37 26.43 2.81 27.90 23.86 23.01 20.93

Table 4b Elemental composition of kerogens concentrated with traditional method vs. conservative method (kerogen elemental composition normalized to 100%). No.

Identity

Traditional open-system kerogen C (wt%)

1 2-A 2-B 2-B0 3-A 3-B 4 5 6-A 6-B 7-A 7-B 8-A 8-B 8-C

Monterey Green River – 1 Green River – 2 Pyrolyzed Green River – 2 Sihapas Pematang Brown Shale Yegua Shublik Desert Creek Desert Creek Woodford Woodford New Albany New Albany New Albany

H (wt%)

65.96 5.76 81.91 11.49 Not analyzed 74.85 3.25 77.45 6.51 85.42 8.24 81.26 6.70 68.78 7.05 75.20 5.58 66.40 4.99 74.17 6.92 52.51 3.96 75.63 6.98 52.15 1.57 70.99 4.98

O (wt%)

Conservative closed-system kerogen S (wt%)

N (wt%)

C (wt%)

H (wt%)

O (wt%)

S (wt%)

N (wt%)

7.38 5.87

18.88 0.53

2.02 0.20

10.05 12.79 3.45 5.62 6.13 5.86 2.56 10.37 3.47 3.35 2.01 2.83

7.25 2.04 1.18 5.32 16.45 11.50 24.72 6.29 39.24 12.55 43.43 19.90

4.60 1.21 1.71 1.10 1.59 1.86 1.33 2.25 0.82 1.49 0.84 1.30

77.69 81.90 76.81 87.51 83.34 85.34 82.52 79.16 78.64 84.03 74.78 82.98 83.92 83.24 79.78

6.55 11.40 10.27 3.3 7.17 8.70 8.10 8.08 5.04 6.47 7.15 7.78 9.08 8.65 8.52

8.33 6.01 8.16 3.46 6.95 3.44 5.56 5.48 4.79 3.00 10.61 3.99 3.70 3.56 3.35

4.60 0.49 2.01 0.83 1.55 1.06 2.74 5.35 9.41 5.60 5.18 0.86 1.81 3.77 7.49

2.83 0.20 2.75 4.90 0.99 1.46 1.08 1.93 2.12 0.90 2.28 4.39 1.49 0.78 0.86

Table 5 Elemental composition and sulfur content of kerogen concentrates isolated with the traditional open-system vs. the conservative closed-system method. No.

1 2-A 2-B 2-B0 3-A 3-B 4 5 6-A 6-B 7-A 7-B 8-A 8-B 8-C

Sample Identity

Monterey Green River – 1 Green River – 2 Pyrolyzed Green River – 2 Sihapas, Sumatra Pematang Brown Sh. Yegua Shublik Desert Creek Desert Creek Woodford Woodford New Albany Shale New Albany Shale New Albany Shale

Traditional open-system kerogen concentrate

Conservative closed-system kerogen concentrate

Atomic H/C

Atomic O/C

Atomic S/C

Atomic N/C

Atomic H/C

Atomic O/C

Atomic S/C

Atomic N/C

1.04 1.68 Not analyzed 0.52 1.00 1.15 0.98 1.23 0.62 0.90 1.12 0.90 1.10 0.36 0.84

0.11 0.05

0.107 0.002

0.026 0.002

0.13 0.12 0.03 0.05 0.09 0.03 0.03 0.14 0.05 0.03 0.03 0.03

0.036 0.010 0.005 0.025 0.090 0.057 0.140 0.032 0.280 0.062 0.312 0.105

0.053 0.013 0.017 0.012 0.020 0.021 0.017 0.026 0.013 0.017 0.014 0.016

1.01 1.67 1.60 0.45 1.02 1.22 1.17 1.22 1.28 0.92 1.14 1.12 1.29 1.24 1.27

0.08 0.05 0.08 0.03 0.06 0.03 0.05 0.05 0.03 0.03 0.10 0.04 0.03 0.03 0.03

0.022 0.002 0.010 0.004 0.007 0.005 0.012 0.025 0.045 0.025 0.026 0.004 0.008 0.017 0.035

0.031 0.002 0.031 0.048 0.010 0.015 0.011 0.021 0.023 0.009 0.026 0.045 0.015 0.008 0.009

1. Quantitative mass balance calculations and qualitative reflected light microscopy of kerogen indicate that the traditional isolation process recovers a small fraction of the kerogen originally

in place (<14%). In contrast, the closed-system isolation process delivered a much higher yield, most frequently exceeding 85%.

808

R.A. Ibrahimov, K.K. (Adry) Bissada / Organic Geochemistry 41 (2010) 800–811

Traditional open-system method 5000

Quartz

3000

4000

Pyrite

4000

2000

Monterey Sh.

Counts

5000

Counts

Conservative closed-system method

1000

Organic matter Quartz?

3000 2000 1000

0

0 0

10

20

30

40

50

60

70

80

0

10

20

30

2θ (degrees)

Dolomite

Green River

Pyrite

Counts

Quartz Calcite

Counts

1000

50

60

70

80

60

70

80

Organic matter

3000

3000

2000

40

2θ (degrees)

0

2000

1000

0 0

10

20

30

40

50

60

70

80

0

10

20

30

2θ (degrees)

40

50

2θ (degrees)

Organic matter

Pyrolyzed

Insufficient recovery

Gr. Riv.

Counts

800 600 400 200 0 10

20

30

40

50

60

70

2θ (degrees) 5000

3000 2000

4000

Pematang Br. Sh.

Counts

Counts

4000

Calcite

Quartz

5000

Pyrite

1000

Organic matter

3000 2000 1000

0

0 0

10

20

30

40

50

60

70

0

80

10

20

40

50

60

70

80

60

70

80

2θ (degrees)

2θ (degrees) 8000

4000

Woodford Sh.

2000

Counts

Quartz

8000

6000

Counts

30

6000 4000

Organic matter

2000

0

0 0

10

20

30

40

50

60

70

80

2θ (degrees)

0

10

20

30

40

50

2θ (degrees)

Fig. 6. Examples of bulk XRD analysis of kerogen samples isolated using traditional open-system and conservative closed-system methods.

2. Comparing kerogen/mineral separation effectiveness for the two processes using powder X-ray diffraction showed a variety of minerals in the kerogen from the open-system method compared to the kerogen concentrates from the closed-system method.

3. Chemical integrity of the kerogen from both techniques was assessed from organic elemental analysis and stable carbon isotope analysis. The kerogen from the closed-system method showed higher H/C and lower O/C atomic ratios with no evidence of OM fractionation, unlike that from the open-system

809

R.A. Ibrahimov, K.K. (Adry) Bissada / Organic Geochemistry 41 (2010) 800–811

OIL

OIL 1.8

1.8

GreenRiver 2-A 1.6

1.6

1.4

1.4

1.2

1.2

1.0

Atomic H/C

Atomic H/C

GreenRiver 2-B

GAS

1.0

GAS

0.8

0.8

0.6

0.6

Pyrolyzed Gr. Riv. 2-B'

Monterey 0.4

0.4 0.2 0.00

0.05

0.10

0.15

0.20

0.2 0.00

0.25

0.05

Atomic O/C

0.10

1.8

1.6

1.6

1.4

1.4

Pematang

Sihapas

GAS

0.8 0.6

Atomic H/C

Atomic H/C

0.25

OIL

OIL

1.0

0.20

Atomic O/C

1.8

1.2

0.15

1.2 1.0

GAS

0.8 0.6

Yegua 0.4 0.2 0.00

0.4

0.05

0.10

0.15

0.20

0.25

0.2 0.00

0.05

0.15

0.20

0.25

Atomic O/C

Atomic O/C Traditional open-system kerogen

0.10

Conservative closed-system kerogen

Fig. 7. (a) Modified van Krevelen diagrams with atomic H/C and O/C ratios for kerogens isolated using traditional open-system vs. conservative closed-system methods. (b) Modified van Krevelen diagrams with atomic H/C and O/C ratios for kerogens isolated using traditional open-system vs. conservative closed-system methods.

technique. In almost all cases, the kerogen concentrates from the open-system approach showed greater enrichment in S, higher ash yield and higher O/C values, perhaps indicating some degree of oxidation.

vative separation procedures that ensure quantitative recovery, effective mineral removal and chemical preservation and maintenance of the OM. Acknowledgements

Overall, the traditional open-system approach afford fractionated, impure and otherwise unrepresentative kerogen not useful for chemical investigations. Research quality kerogen for chemical and physiochemical studies most certainly requires special conser-

We thank M. Darnel, E. Szymczyk, T. Szymczyk and T. HendrixDoucette of the University of Houston Center for Petroleum Geochemistry for invaluable help with the analytical work.

810

R.A. Ibrahimov, K.K. (Adry) Bissada / Organic Geochemistry 41 (2010) 800–811

OIL

OIL

1.8

1.8

1.6

1.6

1.4

1.4

1.2

1.2

1.0

Atomic H/C

Atomic H/C

6-A

GAS

0.8 0.6

1.0 0.8

6-A

0.6

Shublik

Desert Creek

0.4 0.2 0.00

GAS

6-B

0.4

0.05

0.10

0.15

0.20

0.2 0.00

0.25

0.05

Atomic O/C

0.10

0.15

0.20

0.25

Atomic O/C

OIL

OIL

1.8

1.8

1.6

1.6

1.4

1.4

8-A

7-B

7-A

1.2

7-A

1.0

GAS

7-B 0.8 0.6

Atomic H/C

Atomic H/C

8-C 1.2

8-B 8-A

1.0 0.8

GAS 8-C

0.6

Woodford

New Albany Sh.

0.4

0.4

0.2 0.00

0.2 0.00

8-B 0.05

0.10

0.15

0.20

0.25

Atomic O/C

0.05

0.10

0.15

0.20

0.25

Atomic O/C

Traditional open-system kerogen

Conservative closed-system kerogen Fig. 7 (continued)

We acknowledge with thanks the advice and constructive review of Etuan Zhang. Special thanks go to Zheng Huang for help with XRD interpretation, and M. Pawlewicz and an anonymous reviewer for constructive comments. Financial support to R.I. Ibrahimov came from BP Caspian Sea Exploration and the University of Houston’s Center for Petroleum Geochemistry. Associate Editor—Maowen Li

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