Organic petrology and geochemistry of Eocene Suzak bituminous marl, north-central Afghanistan: Depositional environment and source rock potential

Marine and Petroleum Geology 73 (2016) 572e589

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Research paper

Organic petrology and geochemistry of Eocene Suzak bituminous marl, north-central Afghanistan: Depositional environment and source rock potential Paul C. Hackley*, John R. SanFilipo U.S. Geological Survey, MS956 National Center, 12201 Sunrise Valley Drive, Reston, VA 20192, United States

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 September 2015 Received in revised form 20 February 2016 Accepted 24 February 2016 Available online 25 March 2016

Organic geochemistry and petrology of Eocene Suzak bituminous marl outcrop samples from Madr village in north-central Afghanistan were characterized via an integrated analytical approach to evaluate depositional environment and source rock potential. Multiple proxies suggest the organic-rich (TOC ~6 wt.%) bituminous marls are ‘immature’ for oil generation (e.g., vitrinite Ro < 0.4%, Tmax < 425
C, PI  0.05, C29 aaa S/S þ R  0.12, C29 bbS/bbSþaaR  0.10, others), yet oil seeps are present at outcrop and live oil and abundant solid bitumen were observed via optical microscopy. Whole rock sulfur content is ~2.3 wt.% whereas sulfur content is ~5.0e5.6 wt.% in whole rock extracts with high polar components, consistent with extraction from S-rich Type IIs organic matter which could generate hydrocarbons at low thermal maturity. Low Fe-sulfide mineral abundance and comparison of Pr/Ph ratios between saturate and whole extracts suggest limited Fe concentration resulted in sulfurization of organic matter during early diagenesis. From these observations, we infer that a Type IIs kerogen in ‘immature’ bituminous marl at Madr could be generating high sulfur viscous oil which is seeping from outcrop. However, oil-seep samples were not collected for correlation studies. Aluminum-normalized trace element concentrations indicate enrichment of redox sensitive trace elements Mo, U and V and suggest anoxic-euxinic conditions during sediment deposition. The bulk of organic matter observed via optical microscopy is strongly fluorescent amorphous bituminite grading to lamalginite, possibly representing microbial mat facies. Short chain n-alkanes peak at C14eC16 (n-C17/n-C29 > 1) indicating organic input from marine algae and/or bacterial biomass, and sterane/hopane ratios are low (0.12e0.14). Monoaromatic steroids are dominated by C28 clearly indicating a marine setting. High gammacerane index values (~0.9) are consistent with anoxia stratification and may indicate intermittent saline-hypersaline conditions. Stable C isotope ratios also suggest a marine depositional scenario for the Suzak samples, consistent with the presence of marine foraminifera including abundant planktic globigerinida(?) and rare benthic discocyclina(?) and nummulites(?). Biomarker 2a-methylhopane for photosynthetic cyanobacteria implies shallow photic zone deposition of Madr marls and 3b-methylhopane indicates presence of methanotrophic archaea in the microbial consortium. The data presented herein are consistent with deposition of Suzak bituminous marls in shallow stratified waters of a restricted marine basin associated with the southeastern incipient or proto-Paratethys. Geochemical proxies from Suzak rock extracts (S content, high polar content, C isotopes, normal (aaaR) C27e29 steranes, and C29/C30 and C26/C25 hopane ratios) are similar to extant data from Paleogene oils produced to the north in the Afghan-Tajik Basin. This observation may indicate laterally equivalent strata are effective source rocks as suggested by previous workers; however, further work is needed to strengthen oil-source correlations. Published by Elsevier Ltd.

Keywords: Afghanistan oil shale Source rock Biomarkers Thermal maturity Organic petrology Organic geochemistry

1. Introduction

* Corresponding author. E-mail address: [email protected] (P.C. Hackley). http://dx.doi.org/10.1016/j.marpetgeo.2016.02.029 0264-8172/Published by Elsevier Ltd.

This study is part of a more comprehensive investigation of the geology and natural resources of Afghanistan. Field investigation was confined to brief observations made while en route to other Afghan areas and stratigraphic intervals being investigated for coal

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resources (SanFilipo, 2005; Hare et al., 2008; Hackley et al., 2010). For this study, the organic geochemistry and petrology of bituminous marl samples from the Eocene (Suzakian) interval near Madr village in north-central Afghanistan (Fig. 1) were characterized via an integrated analytical approach. Suzak oil shales are effective source rocks in the Afghan-Tajik basin and are thought to have sourced Paleogene heavy oil accumulations in southwest Tajikistan (e.g., Klett et al., 2006). The principal goal of this paper was to evaluate the depositional environment of the Suzak marls and consider implications for source rock potential. Older work focusing on Suzak oil shales in Afghanistan and Uzbekistan included Schmitz and Weippert (1966) and Bondar et al. (1990), which are in German and Russian, respectively. Therefore, a second objective of this work was to provide organic geochemical and other data for Suzak bituminous marls to a wider audience using modern techniques. Although we used only a limited number of samples, they provide critical information in a remote area with ongoing security problems where little data is available. 2. Geologic setting and previous studies The Suzak Formation (Gavrilov et al., 2003) was deposited in the southeastern incipient or proto-Paratethys, an epicontinental sea which covered most of the European platform and western Asia € gl, 1999; Schulz et al., 2005; Bosboom, 2013; Bosboom et al., (e.g., Ro 2011, 2014, and references therein). According to Bosboom et al. (2014) the incipient Paratethys (Fig. 1) receded westward stepwise from the Tarim Basin of west China, ultimately resulting in isolation of the Paratethys at about 34 Ma (Eocene-Oligocene boundary) and deposition of anoxic organic-rich mudstones in the Caspian and Black Sea Basins. Comparison to the regional paleogeography interpretations of Bosboom (2013) shows the Madr location to be ~80 km southward of the maximum palinspastic extent of the proto-Paratethys; we suggest the presence of bituminous marls at Madr indicates slightly greater southward extent of proto-Paratethys than the previous work. The Madr bituminous beds were first described by workers from the German Geological Mission to Afghanistan (DGMA: Deutsche Geologische Mission Afghanischen, 1959e1967), including Weippert (1964) and Schmitz and Weippert (1966). Weippert (1964) assigned them to the Lower Eocene Gazak Formation, named after a prominent plateau (Dash-te Gazak, Fig. 1). Schmitz and Weippert (1966) described the 8e15 m thick bituminous interval (about 2 m of which is shown graphically as highly bituminous) at the top of the Gazak Formation in more detail and reassigned it to the Middle Eocene (Lutetian), about 100e120 m above the disconformable Paleocene-Eocene boundary based on foraminifera. These workers were able to extract 0.7% bitumen from one sample, suggesting oil potential at the Madr locality and they considered the interval a petroleum source rock, based on the extraction data, calorific value, macroscopic sample observations (petroleum odor, staining) and inorganic and organic petrography. Other bituminous beds from the same general stratigraphic interval were subsequently described by Soviet workers, notably Abdullah and Chmyriov (2008). These rocks were assigned to the Lower Eocene Suzakian (Ypresian; Fig. 2) stage with about 4 m of bituminous clay noted near the lower middle part. The Suzakian type area is in Kyrgyzstan (Fig. 1) and there is considerable difference between the Soviet and DGMA mapping of the units, particularly in the Gazak type area (Fig. 1); because the name Suzak is entrenched we will generally refer to the Madr beds as part of the “Suzak” regardless of their actual age. Suzak beds have long been considered petroleum source rocks. Bondar et al. (1990) suggested that the n-alkane signature of Suzak

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oil shale from Uzbekistan was related to oils reservoired in Paleocene strata, noting deposition under highly reducing facies. Klett et al. (2006) summarized the petroleum geology of the Suzak oil shale beds from the literature, including generalized descriptions from areas widely dispersed (100s of km) across the Afghan-Tajik and Amy-Daryu basins. Klett et al. (2006) also summarized new geochemical analyses of Suzak outcrops from Tashkurghan (Kholm, Fig. 1) and Kala-i-Kurchi, correlative in age to the Madr Suzak beds, and from Paleogene-reservoired oils in Tajikistan about 240 km north-northeast of Madr. In addition to vitrinite reflectance and screening data from pyrolysis they analyzed S content and API gravity values, SARA fractionation values, C isotopes, and sterane and terpane biomarker information. From correlation of geochemical analyses of produced oils to source rock extracts and 1-D petroleum generation modeling, Klett et al. (2006) suggested Suzak beds could source oils produced in the Afghan-Tajik Basin but stated “[geochemical correlation of the proposed Paleogene source rock with crude oil in the Afghan-Tajik Basin is inconclusive, at present]”. Herein, we compare our geochemical data to the data in Klett et al. (2006) since their study contains the only other modern geochemical data available from equivalent strata. Gavrilov et al. (1997, 2003) provided the most up-to-date and detailed descriptions of Suzak beds on a regional basis, focusing on the Paleocene-Eocene boundary in the northeastern Peri-Tethys area. Gavrilov et al. (2003) interpreted the Paleocene-Eocene boundary and its associated thermal maxima as occurring at the base of the Suzak bituminous beds based on nannofossil and C isotope proxies and considered deposition of Suzak beds to have occurred in warm, arid and hypersaline conditions. The sections described by Abdullah and Chmyriov (2008) and Gavrilov et al. (2003) are generally 100s of km north and west of Madr, and the section studied herein could be time transgressive or one of several bituminous beds within the Paleogene interval. Field mapping of the extent and volumetric significance of Suzak bituminous marl beds within most of Afghanistan currently is prohibitive due to security constraints. Until additional evidence is available, the basal Lutetian age of the Suzak beds at Madr as defined by the biostratigraphic evidence of Schmitz and Weippert (1966) should be considered the best available, but the possibility that these or similar beds may mark the Paleocene-Eocene Thermal Maximum (PETM) should be further investigated. In any case, age constraints of approximately 56e47 Ma are consistent with our interpretation that Suzak beds at Madr were deposited in an incipient protoParatethys prior to westward isolation of the Paratethys at the Eocene-Oligocene boundary (34 Ma). 3. Methods 3.1. Samples Eight samples were collected from two closely spaced outcrops approximately 3.4 km south of Madr village (Fig. 1) in MayeJune, 2006. Samples included two finely laminated marly bituminous claystones (oil shales), three massive to laminated aphanitic limestones, and three similar silty limestones. Oil seepage from bedding planes or fractures was noted at outcrop (Fig. 3) and the carbonates in particular had a fetid petroliferous odor upon breakage. All samples are from the uppermost part of the Gazak Formation of Weippert (1964) and an indeterminate part of the Suzakian equivalents of Soviet workers (Abdullah and Chmyriov, 1977; Vlasov et al., 1984; Mirzakhanov, 1989). An interval of 12.25 m with visible oil residue was measured and grab samples were taken from the most productive part, but detailed descriptions of the entire interval were not possible due to time and security constraints. Our section compares well with

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approximately 12 m of bituminous beds shown by Schmitz and Weippert (1966, Fig. 3); it is possible, however, that their section is in part below ours. Based on available information, we believe that our marl samples BI-1As and 1B are between 1.5 and 6 m below Proben 1 of DGMA. 3.2. Organic petrology and geochemical/mineralogy analyses Crushed samples were mounted into polished circular briquettes of 1 inch diameter following ASTM D2797 (ASTM, 2014a). Reflectance measurements were performed following ASTM D7708 (ASTM, 2014b) using a Leica DM4000 microscope equipped with LED illumination and a digital camera detection system. Reflectance measurements were obtained on solid bitumens, vitrinite, and inert macerals for the two bituminous marl samples and for solid bitumens in one carbonate sample. All samples also were prepared as ultra-thin (~20 mm) thin sections. Rock-Eval II pyrolysis and Leco direct combustion total organic carbon (TOC) determinations were made according to typical procedures  et al., 1977) at Weatherford Laboratories. Samples were (Espitalie not extracted or acidified prior to pyrolysis analysis. Semiquantitative X-ray diffraction (XRD) mineralogy was determined on low temperature ash (LTA) residues according to methods described in Hosterman and Dulong (1989). Results are accurate to about ±5 wt.%. Major and trace element analysis was determined by lithium metaborate/tetraborate fusion ICP-OES and MS with acid digestion at Activation Laboratories following standard procedures. Percent error in measurement is dependent on element concentration; we calculated percent error of about ±1% for V concentrations for marl samples. Extraction of marls, GC analysis of saturate fractions, SARA fractionation and GCeMS analysis of the saturate and aromatic fractions at Weatherford Laboratories was according to typical procedures, e.g., as described by Hackley et al. (2013). Extractions were in the presence of activated Cu to remove S; however, further desulfurization using Raney Ni  et al., 1988) was not performed. Stable carbon (Sinninghe Damste isotope ratios of SARA fractions were measured at Weatherford Laboratories by continuous flow isotope ratio mass spectrometer analysis, reported against NBS22 with an accepted value of 29.73 per mil (Silverman, 1964). To compare C isotope ratios with extant data (Klett et al., 2006; GeoMark, 2015) we converted to PDB using dC(PDB) ¼ {[(0.9702)*dC(NBS22)/1000)þ1]1}*1000 (Peters et al., 2005a, p. 140). 4. Results and discussion 4.1. Bulk geochemistry and mineralogy The bituminous marl samples contained 5e6 wt.% total organic carbon (TOC), indicative of excellent source rock potential (e.g., Peters and Cassa, 1994; McCarthy et al., 2011), whereas the limestones contained <1 wt.% TOC (Table 1). Loss on ignition (LOI) values from LTA were higher at 10 wt.%, possibly related to volatile loss (e.g., Veres, 2002) or suggesting some component of the TOC is lost as hydrolyzate during acidification for direction combustion. Distillate values (S1) for oil shales were identical at 1.30 mg HC/g rock and pyrolysis yields (S2) were >25 mg HC/g rock, again indicative of excellent generative potential (e.g., Peters, 1986). S1

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and S2 were <1.0 mg HC/g rock for the limestones. S3 values were 0.9e0.81 mg CO2/g rock for carbonates and 4.67e5.21 for bituminous marls, computing to moderate OI values (74e90 mg CO2/g TOC). The moderate OI values may suggest oxidation at outcrop, despite efforts to collect un-weathered sample material. Alternatively, moderate OI values may be due to CO2 generated from carbonates (Katz, 1983). OI values from Suzak samples reported by Klett et al. (2006) were much lower at 13e27 mg CO2/g rock, although their samples may have been lower in carbonate or subjected to acid treatment prior to analysis. Oxidation of Madr samples may contribute to the mixed Type II/III kerogen signature on the pseudo-Van Krevelen plot (Fig. 4) although some terrestrial kerogen is present (see petrography section below). High HI values (>450) for Madr marl samples are consistent with a Type II marine kerogen, and overall oil-proneness. High S1/TOC (235) in carbonate sample MDR-GS-06-BI-2n suggests the presence of significant live oil or solid bitumen from local migration, also confirmed by petrography. Production index (PI) values for marls were 0.03e0.05, consistent with Tmax values of 421e423
C, confirming the immature nature of the kerogen. Tmax values were inconsistent for limestones, due to low S2; PI values therefore are also suspect for limestones. X-ray diffraction (XRD) bulk mineralogy illustrated clear distinctions between bituminous marl and limestone samples. Clays (illite and kaolinite) accounted for significant portions of the marl samples (38e46 wt.%), whereas limestones contained insignificant clay and 82e100% total carbonate (Table 2). Quartz makes up the balance of the bulk mineralogy (Fig. 5) with trace amounts of feldspar. No mixed layer illite/smectite was detected despite low maturity. Some hematite was detected in analysis of MDR-GS-06BI-1B, consistent with weathering at outcrop. Both marl samples showed ~10 wt.% loss on ignition from low temperature ashing (Table 2), also consistent with high kerogen content. 4.2. Petrography In thin sections the two marl samples are comprised of clay, quartz, and carbonate, with dispersed pyrite framboids (some altered to hydrated Fe-oxide) and abundant (approximately 10e20 vol.%) organic matter. Organic matter is dominated by brightly fluorescent amorphous bituminite grading to sparse lamellar alginite (Fig. 6AeD). The bituminite is finely disseminated and degraded and is interpreted to represent accumulations of algae (original kerogen), microbial biomass, solid hydrocarbon (solid bitumen), and some live oil. Bituminite is present as mineral coatings and filling interstices between mineral grains. Terrestrial vitrinite and inertinite are present in moderate concentrations (Fig. 6B) but subordinate to bituminite. Solid asphaltic bitumens are present as fluorescent void-filling materials, commonly occurring in the primary porosity of planktic foraminifera tests (Fig. 6EeF) as previously noted by Schmitz and Weippert (1966). A continuum of live oil to solid bitumen is present in marls and limestones. The solid bitumens are asphaltites with low reflectance (grading to live oil with no reflecting surface) and variable fluorescence (Fig. 6GeH). Solid bitumens are presumed to derive from microbial biodegradation and water washing/oxidation of live oil behind outcrop (e.g., Curiale, 1986). Apart from the presence of solid bitumens, the Madr marl samples are similar in organic petrographic

Fig. 1. Mapped exposures of Eocene sedimentary rocks which presumably include the Suzakian bituminous interval on the North Afghan platform and surrounding areas. Rocks of waz Basin (inset), but are not shown due to uncertain provenance. The interval has similar age and provenance exist in the Qal'eh-ye Now block, Khorosan, and possibly the Kata been eroded or is limited to shallow subcrops between most exposures on the platform; we tentatively estimate its present-day depth approaches 5000 m in the deepest areas of the Afghan-Tajik basin west of Kholm, and if present, 2400 m in the Amu Darya Basin center (~37
N, 63
E), 3100 m in the Kopet Dag foredeep, and 8000 m in the Badakshan foredeep.

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Fig. 2. Schematic and generalized stratigraphic column for the Amu Darya and Afghan-Tajik basins. From Klett et al. (2006).

characteristics to immature high sulfur (1e4 wt.% total S) Cenomanian-Turonian marine black shales from the Tarfaya Basin of Morocco where Kolonic et al. (2002) described unstructured organic aggregates of bituminite as the dominant organic maceral, suggesting its formation from “[restructuring of labile biopolymers (lipids and/or carbohydrates), with the incorporation of sulfur into the kerogen during early diagenesis].” Madr marl samples also are

similar to Kimmeridge Clay source rocks where Boussafir et al. (1995) documented enrichment of organic sulfur in bituminite and proposed its preservation occurred due to vulcanization of lipids. Microfossils are present in Madr marls, including planktic chambered foraminifera globigerinida(?) (Fig. 6EeF), and benthic discocyclina(?) (Fig. 7A) and nummulites(?) (Fig. 7B). Calcispheres of

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Fig. 3. Stratigraphic designation and sample location overlaid on outcrop photographs. A) looking northeast in Qol Gogerdak with sampled outcrops on left. Location of samples BI1 obscured behind colluvium. Approximate equivalent to USGS section measured at BI-1 (12.25 m) indicated by white lines in far-ground, along with speculative equivalent DGMA section of Schmitz and Weippert (1966, their Fig. 2) shown for comparison, base obscured and marked by question mark. Arrow on right sight of photograph indicates ISAF soldier for scale. B) Looking northwest at location of samples BI-1, showing oil seep.

Table 1 Rock-Eval (Rock-Eval II), Leco total organic carbon (TOC), and vitrinite/solid bitumen reflectance data. Sample ID

Leco TOC

LOI, LTA

S1

RE S2

S3

Tmax (
C)

HI

OI

MDR-GS-06-BI-1As MDR-GS-06-BI-1Ah MDR-GS-06-BI-1B MDR-GS-06-BI-1R MDR-GS-06-BI-1F MDR-GS-06-BI-2n MDR-GS-06-BI-2m MDR-GS-06-BI-2F

5.77 0.23 6.30 0.38 0.10 0.31 0.15 0.13

10.06 0.28 10.05 0.80 0.45 0.00 0.00 0.00

1.30 0.00 1.30 0.00 0.02 0.74 0.03 0.03

26.44 0.05 35.94 0.18 0.07 0.71 0.26 0.07

5.21 0.52 4.67 0.81 0.67 0.41 0.25 0.09

423 425 421 427 316 426 307

459 22 571 48 70 226 174 53

90 227 74 216 670 131 168 69

S1/TOC *100

PI

Ro (vit)

s.d.

No.

23

0.05

0.39

0.03

8

21

0.03

0.35

0.04

18

19 235 20 23

0.22 0.51 0.10 0.30

Ro (bit)

s.d.

No.

0.22 0.63 0.21

0.03 0.11 0.03

24 100 34

TOC, LOI in wt.%; S1, S2 in mg hydrocarbon/g rock; S3 in mg CO2/g rock; HI in mg hydrocarbon/g TOC; OI in mg CO2/g TOC; PI, Production Index ¼ S1/(S1þS2); Ro in %; vit, vitrinite; bit, solid bitumen; s.d., standard deviation; no., number.

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foraminifera chambers are homogeneous and contain little or no entrained inorganic phases. SEM/EDX shows non-stoichiometric S content in some pyrite (Fig. 8), indicating weathering at outcrop consistent with high S3 values. Limestone samples contained little organic matter (<1 wt.%) and displayed little mineralogical variation (Table 2). Sample MDR-GS06-BI-1Ah is most variable (Fig. 9A), containing 1e4 mm equant- to tabular-shaped re-sedimented clasts (identified by bedding discontinuities and higher(?) clay content), 0.5e1 mm irregularlyshaped carbonate clasts, and fractures filled with fluorescent solid hydrocarbons (Fig. 9B) ranging to live oil (no reflecting surface). Solid bitumen also occurs as isolated rounded blebs. The presence of a continuum of variously oxidized solid bitumen occurrences ranging to live oil suggests a nearby effective source rock (i.e., the adjacent bituminous marl) which is continuously sending oil to the outcrop; the more recent the less oxidized. However, a mature downdip source cannot be ruled out. In thin sections, solid bitumen shows a range of translucency with the more opaque solid bitumen containing more entrained carbonate. Rare dispersed vitrinite,

Fig. 4. Hydrogen Index vs. Oxygen Index pseudo-Van Krevelen plot.

Table 2 X-Ray diffraction mineralogy data (wt.%) from low temperature ash. Sample ID

QTZ

FLD

CARB

I/S

ILLITE

KAOL

CHLR

PY

OTHR

SCLY

MDR-GS-06-BI-1As MDR-GS-06-BI-1Ah MDR-GS-06-BI-1B MDR-GS-06-BI-1R MDR-GS-06-BI-1F MDR-GS-06-BI-2n MDR-GS-06-BI-2m MDR-GS-06-BI-2F

26 0 24 18 9 7 0 0

0 0 2 0 0 0 0 0

24 100 26 82 91 87 100 100

0 0 0 0 0 0 0 0

36 0 29 0 0 4 0 0

10 0 10 0 0 2 0 0

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

3 0 10 0 0 0 0 0

46 0 38 0 0 6 0 0

QTZ, quartz; FLD, total feldspar; CARB, total carbonate; I/S, mixed layer clay; KAOL, kaolinite; CHLR, chlorite; PY, pyrite; OTHR, other; SCLY, sum of clays.

inertinite and AOM are present. Calcispheres and planktic microfossils are present in limestone sample MDR-GS-06-BI-2n but in lower concentrations than in marl samples. Bedding is clearly defined by layers of slightly coarser carbonate, with finer-grained layers (<10 mm carbonate grains) containing more clays(?) along carbonate grain boundaries. Carbonate fluorescence is present but dim. Authigenic dolomite is present in some samples, identified by euhedral crystal shape. Hydrated Fe-oxide is estimated to be the second most abundant phase behind carbonate in the limestone samples. 4.3. Reflectance

Fig. 5. Ternary-normalized XRD-mineralogy.

unknown origin and layered, elongate calcite bioclasts also are common in the marl samples. Bedding lamination is represented by differences in relative proportions of clay vs. carbonate and/or slight variations in carbonate grain size. Traces of detrital zircon, ilmenite, rutile, apatite, and potassium feldspar are present; mmscale lenses of authigenic barite and phosphate are present parallel to bedding laminae. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX) shows fracture-filling solid bitumens contain sub-micron entrained mineral phases including calcite, clays, quartz, and sulfides (pyrite and sphalerite), which are incorporated as the oil/solid bitumen migrates through the fracture network. Solid bitumens present in the primary porosity of

Reflectance measurements performed on the two marl samples indicated mean random vitrinite reflectance (Ro) values of 0.35e0.39%, with mean solid bitumen reflectance (BRo) of 0.21e0.22% (Table 1). These values are consistent with the immature nature of the kerogen determined via Rock-Eval (Tmax and PI). Inertinite macerals had reflectance values >0.45%. Solid bitumen reflectance in limestone samples was highly variable, ranging 0.33e0.88% in sample 1Ah (live oil in limestone samples does not have a reflecting surface). The low Ro values suggest Suzak bituminous marls at Madr are ‘immature’ for hydrocarbon generation, considering the typically accepted start of the oil window in marine source rocks at Ro values of about 0.6% (e.g., Robert, 1988). However, as described below, high S concentrations in rock extracts suggest a Type IIs kerogen is present which can generate hydrocarbons at lower Ro values. Ro values for Suzak outcrop samples described by Klett et al. (2006) from 200 km north-northeast of Madr are >0.9% which does not correspond to multiple geochemical proxies from the same rocks (e.g., high TOC, S2, HI and low Tmax, PI, saturate/

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Fig. 6. Organic petrology photomicrographs: A) sample MDR-GS-06-BI-1B, blue incident light, oil immersion; B) same field as A in white incident light; C) sample MDR-GS-06-BI1As, blue incident light, oil immersion; D) same field as C in white incident light; E) sample MDR-GS-06-BI-1As, blue incident light, oil immersion, arrow points to globigerinida(?) foraminifera; F) same field as C in white incident light, arrow points to globigerinida(?) foraminifera; G) sample MDR-GS-06-BI-1Ah, blue incident light, oil immersion; H) same field as C in white incident light.

aromatic, Ts/Tm, MPI-1 etc., data in USGS, 2015) that suggest their outcrop samples are similar in maturity to Madr samples. We speculate inert components may have been misidentified as vitrinite in the previous work, resulting in the unreasonably high reported Ro values. 4.4. Inorganic geochemistry Major and trace element analyses are compiled in Table 3. High LOI values (31e43 wt.%) are consistent with volatilization of CO2 from carbonate and marl samples, and volatilization of clay hydroxides and organic material in marls. Whole rock sulfur

values were 2.2e2.3 wt.% and the inorganic S is presumed to reside in trace pyrite and sphalerite observed via optical and SEM petrography. Concentrations of other chalcophile elements were low. Total sulfur values determined by titration also were 2.3 wt.%. Trace metal analysis of the Suzak marl samples was undertaken to ascertain paleoredox conditions. Redox-sensitive trace metals (U, V, Mo, Cr, Co, Ni, Cu, Zn and Cd) are less soluble under reducing conditions, resulting in authigenic enrichment in anoxic sedimentary environments (Tribovillard et al., 2006; Lyons et al., 2009), and metal covariations frequently are used to assess benthic redox conditions (e.g., Algeo and Tribovillard, 2009). Here, we use Al-

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Fig. 7. Microfossil photomicrographs: A) Discocyclina(?) in sample MDR-GS-06-BI-1B, thin section, plane-polarized transmitted light, arrow points to microfossil; B) Nummulites(?) in sample MDR-GS-06-BI-1As, thin section, plane-polarized transmitted light, arrow points to microfossil.

Fig. 8. Scanning election micrograph of pyrite framboid in sample MDR-GS-06-BI-1As with energy dispersive spectra from weathered (loss of S) and un-weathered pyrite.

Fig. 9. A) Scan of thin section of sample MDR-GS-06-BI-1Ah showing re-sedimented clasts. Rectangle in (A) shows location of photograph (B), a low magnification thin section photomicrograph in plane-polarized transmitted light showing fracture-filling solid bitumen.

normalized metal concentrations (Calvert and Pedersen, 1993) to calculate enrichment factors (EF) for Suzak marls compared to average shale (Wedepohl, 1971, 1991; compiled in Tribovillard et al., 2006) according to the equation: EFelement X ¼ (X/Al)sample/(X/

Al)average shale. A calculated EF > 1.0 is enriched relative to average shale. With the exception of Pb and Co, all of the redox-sensitive elements are enriched in Suzak marls with significant enrichment of Cd, U, V, and Mo (Table 4).

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Table 3 Inorganic geochemistry of Madr marl and limestone samples. Analyte

SiO2

Al2O3

Fe2O3(T)

MnO

MgO

CaO

Na2O

K2O

TiO2

P2O5

LOI

Total

S

S

Unit Symbol

%

%

%

%

%

%

%

%

%

%

%

%

%

%

Detection limit

0.01

0.01

0.01

0.001

0.01

0.01

0.01

0.01

0.001

0.01

Analysis method

F-ICP

F-ICP

F-ICP

F-ICP

F-ICP

F-ICP

F-ICP

F-ICP

F-ICP

F-ICP

F-ICP

F-ICP

TD-ICP

0.001 titr.

Sample ID MDR-GS-06-BI-1As MDR-GS-06-BI-1Ah MDR-GS-06-BI-1B MDR-GS-06-BI-1R MDR-GS-06-BI-1F MDR-GS-06-BI-2n MDR-GS-06-BI-2m MDR-GS-06-BI-2F

28.03 2.53 23.66 9.56 3.99 10.96 3.90 3.45

6.46 0.28 5.46 1.38 0.27 3.15 1.09 0.91

2.30 0.15 1.91 0.55 0.12 1.23 0.43 0.39

0.018 0.006 0.018 0.036 0.007 0.019 0.018 0.010

1.14 0.84 1.05 1.58 0.52 0.86 0.79 0.73

21.76 52.26 25.12 46.25 51.61 44.08 51.13 51.04

0.65 0.09 0.50 0.13 0.07 0.08 0.06 0.06

1.33 0.05 1.10 0.26 0.05 0.56 0.22 0.18

0.268 0.009 0.239 0.056 0.010 0.122 0.044 0.037

0.17 0.03 0.22 0.05 0.02 0.09 0.04 0.04

31.17 42.81 33.70 38.90 41.96 37.29 41.72 41.77

93.29 99.05 92.97 98.78 98.63 98.45 99.45 98.60

2.33 0.131 2.25 0.544 0.250 0.244 0.091 0.221

2.36 2.36

Analyte

Ba

Zr

Be

Rb

Sr

V

Cr

Nb

Cs

Hf

Ta

W

Th

U

Unit Symbol

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

Detection limit

2 (3)

2 (1)

1

1

2

5

20

0.2

0.1

0.1

0.01

0.5

0.05

0.01

Analysis method

F-ICP

F-ICP

F-ICP

F-MS

F-ICP

F-ICP

F-MS

F-MS

F-MS

F-MS

F-MS

F-MS

F-MS

F-MS

Sample ID MDR-GS-06-BI-1As MDR-GS-06-BI-1Ah MDR-GS-06-BI-1B MDR-GS-06-BI-1R MDR-GS-06-BI-1F MDR-GS-06-BI-2n MDR-GS-06-BI-2m MDR-GS-06-BI-2F

371 9 156 39 11 62 25 29

68 8 66 16 7 26 12 11

1 <1 1 <1 <1 <1 <1 <1

57

2588 528 3535 348 320 334 278 548

741 27 736 51 28 221 48 62

90

3.1

3.8

1.8

0.41

0.8

4.27

10.1

70

2.6

3.5

1.5

0.36

1.3

3.89

13.3

44

Analyte

Co

Mo

Ni

Cd

Ga

Ge

As

In

Ag

Cu

Pb

Zn

Bi

Sn

Sb

Tl

Unit Symbol

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

Detection limit

1

2

1

0.5

1

0.5

5

0.1

0.5 (0.3)

1

5 (3)

1

10 (0.1)

1

0.2

0.05

Analysis method

F-MS

F-MS

TD-ICP

TD-ICP

F-MS

F-MS

F-MS

F-MS

TD-ICP

TD-ICP

TD-ICP

TD-ICP

TD-ICP

F-MS

F-MS

F-MS

<1

134

<0.5

7

<0.1

9

<0.1

10 <5 8 <5 <5 <5 <5 <5

71 4 68 9 4 20 4 7

<0.1 <10 <0.1 <10 <10 <10 <10 <10

2.66

1.3

57 2 59 7 4 11 5 4

0.8

7

<0.3 <0.5 <0.3 <0.5 <0.5 <0.5 <0.5 <0.5

<1

142

13.4 <0.5 11.8 0.8 1 32.1 <0.5 1.6

9

4

54 3 55 6 5 29 6 12

<1

0.9

3.35

Sample ID MDR-GS-06-BI-1As MDR-GS-06-BI-1Ah MDR-GS-06-BI-1B MDR-GS-06-BI-1R MDR-GS-06-BI-1F MDR-GS-06-BI-2n MDR-GS-06-BI-2m MDR-GS-06-BI-2F Analyte

Y

Sc

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Unit Symbol

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

Detection limit

1 (0.5)

1

0.05

0.05

0.01

0.05

0.01

0.005

0.01

0.01

0.01

0.01

0.01

0.005

0.01

0.002

Analysis method

F-ICP

F-ICP

F-MS

F-MS

F-MS

F-MS

F-MS

F-MS

F-MS

F-MS

F-MS

F-MS

F-MS

F-MS

F-MS

F-MS

Sample ID MDR-GS-06-BI-1As MDR-GS-06-BI-1Ah MDR-GS-06-BI-1B MDR-GS-06-BI-1R MDR-GS-06-BI-1F MDR-GS-06-BI-2n MDR-GS-06-BI-2m MDR-GS-06-BI-2F

9.8 7 9.3 13 8 11 6 11

6 <1 5 2 <1 4 1 <1

12.60

24.4

2.81

10.40

2.08

0.439

1.83

0.29

1.72

0.35

1.04

0.173

1.13

0.175

9.26

18.2

2.18

8.46

1.73

0.359

1.43

0.26

1.59

0.31

0.90

0.144

0.94

0.150

F-ICP, lithium metaborate/tetraborate fusion optical emission spectrometry; TD-ICP, total acid digestion optical emission spectrometry; titr., titration. F-MS, lithium metaborate/tetraborate fusion mass spectrometry. Detection limits in parentheses are for marl samples 1As and 1B due to slight differences in analytical approach.

We cannot examine covariance of metal concentrations with TOC due to the limited number of samples. However, low amounts of sulfide present in Suzak marls from Madr (as determined via XRD and petrography) may suggest anoxic, rather than euxinic, conditions despite strong metal enrichments. Ni/(Ni þ V) of ~0.07 and Al2O3/TOC ratios of 0.9e1.1 are consistent with anoxia (Lewan, 1984; Isaksen and Bohacs, 1995).

4.5. Organic geochemistry and implications to source rock potential Extractable organic matter (EOM) content was similar in the two marl samples at 0.44e0.46 wt.% (Table 5), also indicative of good source potential, but lower than previously reported (0.7%) by Schmitz and Weippert (1966) for samples collected from the Madr locality. Oxidation at outcrop may have reduced the total pool of

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Table 4 Enrichment factors. Element

Conc. in avg. shale (ppm)

EF in Madr marls

Al V Cr Co Ni Cu Mo Cd Pb Zn U

88,900 130 90 19 68 45 1.3 0.3 22 95 3

15e17 2.4e2.6 <1 2.1e2.5 3.3e4.0 268e336 116e121 1.2 1.9e2.2 9e14

Conc., concentration (in parts per million); EF, enrichment factor.

organic material available to extraction in our samples or lower EOM values may reflect natural variability. Klett et al. (2006) reported EOM values of 1.1e2.2% for Suzak outcrop samples 200 km northeast of Madr, which we presume are less oxidized as indicated by lower OI values. Total S analysis of whole extracts (by titration, Table 5) indicated high S content of 5.07e5.66 wt.% (relative to whole rock concentrations of 2.2e2.3 wt.%, Table 3). These S concentrations from whole extracts are similar to S concentrations reported from Paleogene oils produced from the Afghan-Tajik Basin in Tajikistan about 240e245 km north of Madr (Klett et al., 2006; data from USGS, 2015). Fractionation of Madr extracts showed low saturate and aromatic fractions with the bulk of the extract constituted by polars and asphaltenes (Table 6), similar to fractionation analyses reported by Klett et al. (2006) although the methods and results may not be strictly comparable. The preponderance of S-rich fractions in Suzak extracts is consistent with high S content from a low maturity Type IIs kerogen. Type IIs kerogens such as those present in the Monterey Formation of California (Orr, 1986) generate hydrocarbons at low thermal maturities due to the weaker CeS bond (e.g., Tissot et al., 1987; Lewan, 1998), consistent with presence of a continuum of live oil to oxidized solid bitumen present in Madr carbonate samples. These data and observations suggest marls at the Madr locality are mature for local generation of a high S, highviscosity polar-rich oil although a downdip mature source cannot be ruled out. High S concentrations in extracts from the Madr marl samples warrant discussion of sulfurizationdthe incorporation of reduced inorganic S into sedimentary organic matter, increasing original biomass S (0.5e1.0 wt.%; Amrani, 2014) to as much as 14 wt.% in oil (Orr, 1986). Sulfurization occurs via inter- and intramolecular addition and requires reactive reduced S (from sulfate reducing bacteria), low Fe concentrations (Fe2þ scavenges reduced S to form sulfide mineral pyrite, FeS2), and organic compounds with functions receptive to reduced S addition (Amrani, 2014). In sedimentary organic matter, S is present in biological precursors at specific locations where it has replaced functional groups such as C]C or OH, and other functions receptive to reduced S (Werne et al., 2004). In intermolecular addition, S is cross-linked to multiple C positions, suggesting multiple functions from the precursor molecules

participate in bonding (Koopmans et al., 1996; Adam et al., 2000). In intramolecular addition, incorporated S is cyclized, requiring at least two receptive functionalities in the precursor compound (Amrani, 2014). Koopmans et al. (1996) documented diagenetic alteration of Chlorobiaceae isorenieratene through intramolecular sulfur incorporation, to yield thiophenes and thiolanes with the isorenieratane skeleton. Isorenieratene is highly functionalized with nine conjugated double bonds, and a large portion is bound to sulfur in Black Sea sediments where the proportion sequestered in high molecular weight S-rich compounds increases with depth, indicating diagenetic incorporation (Repeta, 1993; Sinninghe  et al., 1993; Wakeham et al., 1995). Thus, sulfurization of Damste organic compounds effectively ‘quenches’ and preserves functionalized and reactive lipids that would otherwise be mineralized by  et al., 1989; microbiota during early diagenesis (Sinninghe Damste  and de Leeuw, 1990). Sinninghe Damste High S concentrations in Madr bituminous marl extracts are presumed due to incorporation of inorganic reduced S into organic  matter in the early diagenetic environment (e.g., Sinninghe Damste and de Leeuw, 1990). In general, the model for S incorporation presumes an anoxic environment favorable to sulfate-reducing bacteria and the presence of abundant sulfate in a marine setting. These conditions occur primarily in settings of chemical sediment deposition where clastic supply of reactive Fe species to scavenge S is limited as suggested by low sulfide contents in Madr marl samples. Isoprenoid and n-alkane data from gas chromatography (GC) of marls are provided in Table 5. Whole extract GC yielded low Pr/Ph ratios of 0.44e0.47 suggestive of deposition in an anoxic reducing environment (Didyk et al., 1978). Low Pr/Ph ratios may suggest hypersaline conditions (ten Haven et al., 1987, 1988) and are consistent with a possible archaea input to organic matter (Philp and Lewis, 1987). Klett et al. (2006) reported similar Pr/Ph ratios from the C8þ saturate fraction of Suzak extracts. Whole extract Pr/ n-C17 and Ph/n-C18 ratios also suggest an anoxic marine origin with dominantly algal input on the discriminant cross-plot (Fig. 10). However, whole extract chromatograms exhibit a strong component of unresolved complex mixture (Fig. 11A and B) presumed due to a high proportion of S-containing alkylated compounds (e.g., Adam et al., 2000; van Dongen et al., 2006). This feature of the whole extract chromatogram makes integration and interpretation of peak areas and heights more difficult and therefore prompted reevaluation of the saturate fraction by GC (Fig. 11CeD). Saturate fraction chromatograms showed higher Pr/Ph ratios (2.3e3.1), which may suggest preferential sulfurization of phytane precursors (Kohnen et al., 1991; De Graaf et al., 1992; Kenig et al., 1995) and retention of the S-bonded phytane in polar and asphaltene extract fractions. Kolonic et al. (2002) argued that selective sulfurization therefore invalidated use of the Pr/Ph ratio as a proxy for redox conditions in high S systems such as the Madr marls. Low dibenzothiophene/phenanthrene (DBT/Phen) ratios (0.16e0.34; Table 7) from aromatic extract fractions also suggest S compounds are retained in the heavier fractions. Although higher in saturate fractions, Pr/Ph ratios (2.3e3.1) are not inconsistent with deposition in anoxic marine marl. Pr/n-C17 and Ph/n-C18 ratios also were higher in saturate GC chromatograms compared to whole extracts (Fig. 10;

Table 5 Extract gas chromatograph data. Sample ID

Type

EOM (ppm)

S (wt.%)

Pr/Ph

Pr/n-C17

Ph/n-C18

Iso/n-alkanes

CPI

MDR-GS-06-BI-1As MDR-GS-06-BI-1B MDR-GS-06-BI-1As MDR-GS-06-BI-1B

Whole extract Whole extract Saturate fraction Saturate fraction

4407 4610

5.07 5.66

0.44 0.47 3.08 2.25

0.20 0.30 1.71 1.55

0.53 0.71 0.95 1.23

0.15 0.14 0.52 0.29

0.89 1.24 1.42 1.61

P.C. Hackley, J.R. SanFilipo / Marine and Petroleum Geology 73 (2016) 572e589

583

Table 6 SARA fractions and C isotope data. Sample ID

%Sat

%Aro

%NSO

%Asph

d13C Sat

d13C Aro

d13CNSO

d13C Asph

MDR-GS-06-BI-1As MDR-GS-06-BI-1B

14.89 3.42

11.70 2.05

31.92 20.49

40.43 49.45

26.52 26.86

26.30 26.25

25.28 26.24

24.94 25.02

Sat, saturates; Aro, aromatics; NSO, polars; Asph, asphaltenes. C isotope values reported relative to PDB.

Fig. 10. Isoprenoid Pr/n-C17 vs. Ph/n-C18 cross-plot. After Hunt (1995).

Table 5); Klett et al. (2006) reported similar Pr/n-C17 and somewhat higher Ph/n-C18 ratios (Fig. 10), but did not provide a rationale for their observed isoprenoid/alkane ratios. The Madr saturate GC chromatograms reflect the presence of minor terrestrial organic matter input in longer chain n-alkanes (C29eC31) with odd-overeven predominance (CPI 1.4e1.6) (as confirmed by organic petrography, reported above). Short chain n-alkanes peak at C16 (nC17/n-C29 > 1) indicating the dominant organic input was from marine algae and/or bacterial biomass (also consistent with organic petrography). Stable C isotopes ratios for aromatic vs. saturated hydrocarbons are consistent with a marine origin on the Sofer (1984) discriminant plot (Fig. 12; Table 6). Isotopic values are similar to average marine carbonates (n ¼ 1007; data from GeoMark, 2015) and high

Fig. 11. AeB) Whole extract gas chromatograms. CeD) Saturate fraction chromatograms.

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Table 7 Biomarker data and ratios. Sample ID

%C27 aaaR (217)

MDR-GS-06-BI21 1As MDR-GS-06-BI-1B 23

%C28 aaaR (217)

%C29 aaaR (217)

Steranes/ hopanes

C28/C29 (abbS) (218)

Gamma/ Hopane

Tri-terp/ Hopane

Tri-terp/ Sterane

DBT/ Phen

19

60

0.12

1.35

0.93

0.03

0.29

0.16

28

49

0.14

1.34

0.83

0.01

0.10

0.34

Selected ion monitoring m/z ratio listed in parentheses; Gamma, gammacerane; Tri-terp, tricyclic terpane.

and low sulfur Paleogene Tajikistan crude oils (Klett et al., 2006; data from USGS, 2015). C isotopic values are dissimilar to Suzak source rock extracts analyzed by Klett et al. (2006) where lighter 13 C values may suggest CO2 recycling (e.g., Wenger et al., 1988) in a closed lagoonal setting. Sterane distributions (aaaR, m/z 217) show a high proportion of C29 relative to C27 and C28 (Fig. 13; Table 7), suggesting input from terrigenous higher plant material (e.g., Moldowan et al., 1985). However, other workers (e.g., Volkman, 1986) have measured high C29 sterane proportions in rocks with major organic input from marine organisms and some workers discount interpretations of depositional environment from sterane distributions (Peters et al., 2005b, p. 525). Peters et al. (2005b) suggested sterane distributions are useful for oil-source rock correlation. Here we note Madr sterane distributions are similar to sterane distributions in high and low sulfur oils produced from Tajikistan Paleogene reservoirs located approximately 240 and 310e370 km northeast, respectively. Suzak rock extracts analyzed by Klett et al. (2006) contained significantly less C27 steranes relative to Madr extracts and Tajikistan Paleogene crude oils. Monoaromatic steroids (m/z 253; aromatic fraction) were dominated by C28 in the Madr extracts (Fig. 14AeB), also suggesting a marine organic matter source (Moldowan et al., 1985). Sterane/hopane ratios (0.12e0.14) are low, more typical of lacustrine settings (GeoMark, 2015), but along with the presence of abundant amorphous organic matter, consistent with a primarily bacterial source of organic matter (e.g., Moldowan et al., 1985; Peters et al., 2005b). C30 steranes and diasteranes were not resolved in Madr m/z 217 and 218 fragmentograms. Moderately elevated gammacerane index values of 0.83e0.93 (Fig. 14CeD; Table 7) may suggest intermittent hypersaline condi et al., 1995). tions and a stratified water column (Sinninghe Damste Gammacerane index values are lower in Suzak rock extracts and

Fig. 12. Stable C isotope ratios for aromatic vs. saturated hydrocarbon fractions. Marine vs. terrestrial fields from Sofer (1984). Average marine carbonate from GeoMark (2015) filtered by using data only with known depositional environment, non-biodegraded and low to moderate thermal maturity.

Fig. 13. C27, C28, C29 aaaR (m/z 217) sterane distributions (after Moldowan et al., 1985).

Paleogene Tajikistan oils to the north (Klett et al., 2006), suggesting less saline environments or less developed water column stratification (Fig. 15). Low tricyclic terpane/hopane ratios (0.01e0.03) in Madr saturate extract fractions are similar to Suzak extracts (0.02) and high S crude oils from Tajikistan (0.05) analyzed by Klett et al. (2006), potentially suggesting a correlation, and dissimilar to low S Tajikistan crude oils (0.07e0.22) which are more mature (as observed by gravity, saturate/aromatic ratios, Ts/Tm). Tricyclic terpane concentrations are low overall and tricyclic terpane/sterane ratios are 0.10e0.29 in Madr marls. We point out that tricyclic terpane concentrations and ratios derived from the saturate fraction may be impacted by sulfurization of these compounds and retention of their precursors in heavier fractions of rock extracts (e.g., McCaffrey et al., 1994; Kolonic et al., 2002). Discriminant plots of tricyclic terpane ratios confirm a marine marl depositional signature as identified by microfossils (described above). Low C26/C25 tricyclic terpane ratios (0.79e0.96) (Fig. 16A) suggest a marine (non-lacustrine) environment (Peters et al., 2005b, p. 558e559) and are similar to C26/C25 ratios in high S (0.74) and low S (0.69e0.98) Tajikistan Paleogene crude oils (Klett et al., 2006) and the average values for marine marl and carbonate (GeoMark, 2015). Suzak extracts reported by Klett et al. (2006) show more variable C26/C25 ratios (0.25e1.17). Low C22/C21 tricyclic terpane ratios (0.11e0.21) suggest a non-carbonate source (Peters et al., 2005b, p. 558e559) but are not dissimilar to these ratios in average marine marl and Tajikistan Paleogene crude oils (Fig. 16B). C35/C34 hopane ratios of about 0.4e0.6 are consistent with anoxia and presence of high sulfate at deposition although homohopane concentrations may be lowered through sulfurization, which in€ster et al., 1997). On the C35/ creases with increasing C number (Ko C34 vs. C29/C30 hopane crossplot (Fig. 16C) Madr marls are similar to high and low sulfur Tajikistan oils although slightly lower than the average marl, presumably due to diagenetic sulfurization.

P.C. Hackley, J.R. SanFilipo / Marine and Petroleum Geology 73 (2016) 572e589

585

Fig. 14. AeB) m/z 253 fragmentograms. CeD) m/z 191 fragmentograms.

Fig. 15. Gammacerane index vs. Pr/Ph.

Sulfurization also may be reflected in the relative distribution of

C31eC35 homohopanes (Fig. 17) which shows a pronounced concentration of the C31 homolog and low abundances of C35. 2a-methylhopane, a biomarker for photosynthetic cyanobacteria (Summons and Jahnke, 1992; Summons et al., 1999), was resolved in low abundances by monitoring m/z 205. The presence of this biomarker implies shallow photic zone deposition of Madr marls. 3b-methylhopane also was detected in low abundance, suggesting the presence of methanotrophic archaea in the microbial consortium (Summons et al., 1988; Brocks et al., 2005; Peters et al., 2005b, p. 571). Common saturate and aromatic biomarker proxies for thermal maturity determination confirm petrographic evidence to indicate Madr marls are immature for oil generation according to the normally accepted criteria of Ro  0.6% (e.g., Robert, 1988). For example, C29 aaa S/S þ R ratios 0.12, C29 bbS/bbSþaaR ratios 0.10, non-resolution of diasteranes, moretane/hopane ratios of 0.13 (Seifert and Moldowan, 1980), Ts/Ts þ Tm ratios of 0.02 (Moldowan et al., 1986), and C32 a S/S þ R ratios of 0.41e0.43 (Ensminger et al., 1977) all correspond to Ro values <0.6%. Mono-

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Fig. 17. Relative distributions of the C31eC35 homohopanes. Average marine marl and carbonate (GeoMark, 2015) shown for comparison. Average depositional facies derived from GeoMark (2015) filtering by using data only with known depositional environment, non-biodegraded and low to moderate thermal maturity.

Fig. 16. Tricyclic terpane discriminant plots. A) C31R vs. C26/C25, B) C24/C23 vs. C22/C21, C) C35/C34 vs. C29/C30. Average marine marl and carbonate, lacustrine fresh and saline (GeoMark, 2015) shown for comparison. Average depositional facies derived from GeoMark (2015) filtering by using data only with known depositional environment, non-biodegraded and low to moderate thermal maturity.

and triaromatic steroid ratios also confirm low maturity: (C20þC21)/ S TAS ratios of 0.08e0.11, (C21þC22/S MAS ratios of 0.03e0.04, and TA28/(TA28þMA29) ratios of 0.10e0.13 (Peters et al., 2005b, p. 631e634). However, the presence of locally-generated(?) solid bitumen ranging to live oil, and high S in Madr extracts suggests a Type IIs kerogen presently is generating a high S, high-viscosity polar-rich oil despite low thermal maturity. 4.6. Conditions of the depositional environment Suzak marl samples from Madr were deposited in an incipient proto-Paratethys epicontinental basin. According to Gavrilov et al. (2003) the peri-Tethys occupied the area from the modern Black Sea to Tajikistan with eastern areas (e.g., Kurpai, Tajikistan, Fig. 1) containing mineral proxies for arid conditions (palygorskite). Gavrilov et al. (2003) also noted the absence of terrestrial organic matter in the Kurpai section, which they interpreted as consistent

with a poorly vegetated arid setting, and identified microfossil assemblages consistent with warm water conditions. Kodina et al. (1995) identified biomarkers for anaerobic green sulfur bacteria (Chlorobiaceae) in the Kurpai samples, indicating anoxia and the presence of H2S in the photic zone at deposition. Biomarkers for Chlorobiaceae, e.g., isorenieratane, were not resolved in Madr samples suggesting H2S-rich euxinic conditions required for Chlorobiaceae were not present. Nonetheless, preservation of organic C in laminated Suzak sediments along with trace metal enrichment and high S content indicates generally anoxic redox conditions prevailed at deposition. The presence of fine-grained quartz and clays in the Madr marl samples is indicative of clastic dilution to the overall carbonate setting. Fine-grained terrigenous humic fragments (vitrinite and inertinite) in marls and their absence in adjoining carbonates suggest clastic dilution is due to increased terrestrial run-off. Phosphate is not an important component of the Madr bituminous beds (Table 3) but some micro-scale phosphate lenses were present in thin sections. Gavrilov et al. (2003) reported phosphate concretions in the lower Suzak at the Kurpai section and suggested transgressive sea-level rise in the peri-Tethys liberated P from soils into the basin, triggering phytoplankton blooms which contributed organic matter to Suzak bituminous beds. A more plausible scenario for the Suzak beds at Madr could be restriction of an isolated portion of the peri-Tethys basin, e.g., shallow estuary or bay associated with a sheltered archipelago (e.g., Kopt Dag, Fig. 1) where periodic terrestrial sediment influx brought P and other nutrients contributing to organic productivity. Dickson et al. (2014) also attributed evidence for anoxia in Paleocene-Eocene peri-Tethys sediments to high marine productivity stimulated by delivery of terrestrial nutrients. High concentrations of gammacerane in Madr samples are consistent with salinity or temperature stratification as would develop with freshwater influx to a restricted basin in arid settings. The absence of salt or salt replacement textures is inconsistent with evaporite deposition, however, these units could be present at or near the Madr locality and not identified during our limited field examination. Abed et al. (2005) documented Tethys phosphorites associated with bituminous marl (oil shale) in the Upper CretaceousPaleocene of Jordan, where they ascribed high bio-productivity to upwelling on a shallow epicontinental shelf. Although the lithology associations are similar, this depositional scenario is substantially different than envisioned for Suzak marls at Madr. For one, oil shale deposition in Jordan occurred on a broad, open shelf where

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prevailing east and northeast winds created an upwelling regime from deeper waters, bringing nutrients and stimulating marine plankton growth. For the interval sampled at Madr, a more restricted estuarine-type setting is indicated by high gammacerane concentrations and suggests P probably was derived from overland flow. Commercial Paleogene phosphates have been described in nearby areas of Central Asia (Orris et al., 2015), and given their potential importance to the Afghan Economy (Afghan Ministry of Mines, written communication, 2012) the entire stratigraphic section exposed at Madr should be investigated in more detail. 5. Summary and conclusions Organic-rich marine bituminous marl from Madr, northern Afghanistan, was analyzed via an integrated analytical approach. Petrographic and geochemical results indicate marls are immature for oil generation; oil seeps at outcrop suggest local generation but a downdip mature source cannot be ruled out and oil seep-rock extract correlation studies are warranted. High S content in marl extracts suggests diagenetic sulfurization of organic matter created a type IIs kerogen which is now yielding heavy oil at immature conditions. Trace metal enrichments are indicative of anoxiceuxinic conditions during sediment deposition; sulfurization may have obscured biomarker indicators of anoxia but C34 and C35 homohopanes are preserved in rock extracts. Organic matter is dominated by bituminite, and presence of the biomarker 2amethylhopane for photosynthetic cyanobacteria implies shallow photic zone deposition. 3b-methylhopane, a biomarker for methanotrophic archaea, may indicate bituminite represents a microbial mat facies relict from a diverse benthic consortium, which included rare discocyclina(?) and nummulites(?). Normal alkanes peak at C14eC16 indicating organic input from marine algae and/or bacterial biomass with longer chain n-alkanes (C29eC31) showing odd-overeven predominance and confirming presence of minor terrestrial vitrinite and inertinite content observed via petrography. High gammacerane is consistent with anoxia and intermittent water column stratification. Suzak bituminous marls were probably deposited in a restricted marine basin in the southeastern incipient or proto-Paratethys. Some geochemical proxies (aaaR normal sterane distributions, C isotopes) from Suzak rock extracts are similar to extant data from Paleogene-reservoired oils northward in the Afghan-Tajik Basin. This observation may indicate laterally equivalent strata are effective source rocks where buried more deeply; however, other proxies are dissimilar or inconclusive and further work is needed to strengthen a sourceeoil correlation. Acknowledgments Comments from journal editor Barry Katz and technical reviews by Palma Jarboe (USGS) and journal reviewers Michael Hsieh and David Curry improved this paper. Frank Dulong (USGS) provided XRD analyses. Vicky Rocha (Weatherford Laboratories) coordinated geochemical analyses. Scott Wagner (Wagner Petrographic) prepared thin sections. Harvey Belkin (USGS) helped with SEM/EDX analyses. Steve Suitt (USGS) helped with preparation of Fig. 1. Philip Davis (USGS) helped with sampling and Anya Bogdanow (USGS) assisted in translations from the German. Sincere appreciation is expressed for Maj. Kendall Peacock, Patrol Commander and the soldiers of NZPRT S2 ISAF forces for providing logistics and force protection for SanFilipo and Davis in the field. This research was funded by the U.S. Geological Survey Energy Resources Program and completed in coordination with other U.S. government organizations including the Departments of State and Interior, the U.S. Embassy in Kabul, and the U.S. Agency for International Development (USAID). Any use of trade, firm, or product names is for

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