ORIGINAL PAPER

Sedimentary and organic facies investigation of the GurpiFormation (Campanian–Paleocene) in southwestof Zagros, Iran

Elahe Zarei & Ebrahim Ghasemi-Nejad

Received: 20 March 2013 /Accepted: 10 June 2013# Saudi Society for Geosciences 2013

Abstract The Gurpi Formation in the southwest of Iranhas been studied for microfacies and bulk organic geo-chemistry in order to elucidate its depositional environ-ment and petroleum source rock characteristics. Theobtained results ended up with four types of organic faciesand three types of microfacies through the formation.Three microfacies types differentiated including Pelagic mud-stone Wackstone, microbioclast Packstone and bioclasticPackstone reflect a distal outer ramp or basinal environment.Combination of palynofacies and organic geochemistryresulted in differentiation of four organic facies correspondingto organic facies B, BC, C and CD of Jones 1987. Detailedorganic facies shows that the formation is characterized bylow values of TOC, high percentages of amorphous organicmatter and black phytoclasts, rare marine algae therebyrepresenting a mixture of terrestrial and marine kerogen thatconfirm the formation was deposited in a distal anoxic to oxiccondition. The formation shallows upward to theMicrobioclastPackstone facies below the Lopha Member reaching its mini-mum depth in boundstones of this member in uppermostCampanian and then is followed by stagnant condition andhigh contents of organic matter in suboxic to anoxic conditionthat favoured accumulation of organic matter in earlyMaastrichtian. Organic geochemical and petrographical dataindicate that the formation is not potentially suitable forpetroleum production except for the minor interval (organicfacies 2) in early Maastrichtian. Tmax values vary between340 and 440 °C confirming immaturity trends indicated byRock-Eval data.

Keywords Organic facies . Depositional environment .

Campanian–Paleocene . Zagros

Introduction

The Zagros basin as one of the largest and most importantpetroleum-bearing basins of the world (Alavi 1994) contains8.6 % of the oil and 15 % of the gas prone world reserves.The Zagros Mountains stretch over four provinces: Lurestan,Khuzestan, Internal Fars and Coastal Fars (Fig. 1), and theGurpi Formation constitutes one of the source rocks forpetroleum within the basin cropping out extensively in allthe four provinces (Alla et al. 1980; James and Wynd 1965).Two outcrop sections of the Gurpi Formation in Lurestanprovince perpendicular to the Zagros trend (Northeast tosouthwest) were measured and studied. The first one,Farhadabad section, with grid coordinates of N 33° 20′ 04″and E 47° 20′ 32″, is located in northeastern flank of theKabirkuh anticline in NW of Zagros. The second sectionnaming Kavar, with coordinates of N 32° 56′ 13.4″ and E:46° 48′ 31.8″, is located in southwest flank of Anaran anti-cline in the west of the Farhadabad section (Fig. 1).

The Gurpi Formation at the two sections studied is com-posed of 205 and 158m of grey to bluemarl and shale beds andoccasionally thin beds of argillaceous limestones with twoformal member of Seymareh (Lopha) and Emam-Hassan.The Lopha Member is made up of three medium-bedded unitsof few meters thick each, separated by marl layers, and it ispresent only in the Lurestan province (James and Wynd 1965;Motiei 2003), where it is a shallow-water shalely limestonewith abundant megafauna. The thin-bedded pelagic limestonesof the Emam-Hassan Member alternated with thin marly in-terbeds occur in the uppermost levels of the formation (Figs. 2and 3). The Gurpi Formation overlies the Ilam Formation and is

E. Zarei (*) : E. Ghasemi-NejadDepartment of Geology, Faculty of Sciences, University of Tehran,Enghelab Avenue, Tehran, Irane-mail: zarei@khayam.ut.ac.ir

Arab J GeosciDOI 10.1007/s12517-013-0993-3

gradationally overlain by the purple shales of the basal part ofthe Pabdeh Formation of Paleocene–Oligocene age.

The basic aim of this paper is to reconstruct the depositionalenvironment and evaluate petroleum production potential ofthe Gurpi Formation in the two stratigraphic sections inLurestan province on the basis of organic facies assemblagesand microfacies variations.

Materials and methods

For geochemical analysis and microfacies studies, the sampleselection was based primarily upon changes in colour, lithologyand other sedimentary characteristics. For microfacies studies,145 samples from the two sections were selected, and thinsections were prepared. In order to determine the nature of

0 20 km

study area

Ilam

Kaver

Mehran

Zarin-abad

Dareshahr

Abdanan

Kabir kuh

Kuhe-Anaran

Farhad abad

Dehloran

47º 30´ 47º46º 30´

33º33º 30´ N

IRAQ

N

48°E

Poldokhtar

Mormori

Sarabbagh

Persian Gulf

Caspian Sea

Tehran

IRANAhvaz

TurkmenistanUZ

Afg

han

istan

Pakistan

OM

U.A.E.

BH

QA

Saudi Arabia

KW

IRAQ

60°E55°50°45°

35°N

30°

AM AZ

Tyekr

u

0 200 km

Ilam

Lurestan

Khuzestan

Internal Fars

Coastal Fars

Fig. 1 Location map of thestudied areas

Arab J Geosci

the organic matter, a number of petrographic and geochemicalanalyses were made on the organic matter isolated from thetwo outcrop samples. For palynofacies studies, sampleswere collected every 5 m (47 samples from Kavar sectionand 32 samples from Farhadabad section) and prepared inthe palynology laboratory of the Department of Geology ofthe University of Tehran using standard preparation methoddescribed by Traverse (2007). The mineral matrix, carbonateand silicates of the samples were removed using dilutehydrochloric acid (HCL 30 %) and cold hydrofluoric acid(HF 40 %), respectively. The remaining residue was thensieved through a 20-μm nylon sieve prior to mounting on

slides. A count of 500 particles were recorded for each sampleusing transmitted light microscopy as is necessary for differ-entiation of kerogen type (Tyson 1995).

Total organic carbon (TOC) and Rock-Eval pyrolysiswere undertaken in the laboratories of the Institute ofGeology and Geochemistry of petroleum and coal, RWTHAachen University. Thirty-four samples from the twooutcrop sections were selected for geochemical analysis.These samples were representatives of the lithologic unitsconstituting the likely hydrocarbon source rock facies inthe Gurpi Formation. TOC was measured with a LecoRC-412 apparatus based on IR-absorption of CO2 bands.

Form

atio

nipru

Gra

myeSeh

Mem

ber

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richt

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pani

anSelandian

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Py

Py

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Thanetian

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knes

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)seitreporP

Gp

Gp

Gp

Gp

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PyGp

PyGp

Py

PyGp

Py

Py

Gl

Gl

Gl

Gl

Pabdeh

nasaH

mam

EM

embe

r

Ilam

GlGl

PoPoDanian

PoPo

PoPo

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2A

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606264

727678

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I

68

elpmaS

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A

Bivalves

Shell debris(undifferentiated)

Gl Glauconite

Limestone

Marly Limestone

Marl

Limey Marl

SandstoneGp Gypsum

Py Pyrite

Po Phosphate

B C D

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--

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AOMMP PHY

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%100

PFII

PFII

PF

IIP

FIII

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Microfacies

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agic

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Organic facies

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----

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15

1 2 3 4

-

1500500

--

HI

F G

Fig. 2 Presentation of data measured and calculated for Kavar section.a Microfacies (pelagic mudstone wackstone, microbioclast packstoneand bioclastic packstone). b Palynomorphs (AOM amorphous organicmatter, MP marine palynomorphs, PHY phytoclast). c Associated

palynofacies types (PF-1-4). d TOC total organic carbon content. eTIC total inorganic carbon. f Hydrogen index. g Organic facies (1, 2,3 and 4; stratigraphy column for Hemmati-Nasab 2008)

Arab J Geosci

It is a two-stage measurement process in oxygen–atmo-sphere at variable temperatures. During the first stage, organiccarbon is converted to CO2 at temperatures ranging from 350to 520 (TOC). During the second stage, carbonates areconverted to CO2 (TIC) in a temperature range from 520 to1050 °C. The CO2 concentrations were quantified by IRabsorption. Two standards were used for calibration contain-ing 1.01±0.01%C (Leco synthetic carbon) and 12%C (Lecocalcium carbonate). Standard deviations are less than 0.5.

Rock-Eval pyrolysis provides information on the quantityand type of organic matter in a sedimentary rock, in additionto the level of organic maturation (Espitalie et al. 1985,1977). The Rock-Eval pyrolysis data should be interpretedwith caution, in view of the low TOC values. Fourteensamples from the two outcrop sections with TOC valuesmore than 0.2 % were selected for Rock-Eval pyrolysis.Samples from each interval were pulverized, and about100 mg from each sample was analysed using a Rock-

Limestone

Limey Shale

Shale purple

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Ilam

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454647484950

108107106105104103102101100979485817875

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agic

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udst

one

wac

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icro

bioc

last

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ocla

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pack

ston

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Fig. 3 Presentation of data measured and calculated for Farhadabadsection. a Microfacies (pelagic mudstone wackstone, microbioclastpackstone and bioclastic packstone). b Palynomorphs (AOM amor-phous organic matter, MP marine palynomorphs, PHY phytoclast). c

Associated palynofacies types (PF-1-4); d TOC total organic carboncontent. e TIC total inorganic carbon. f Hydrogen index. g Organicfacies (1, 2, 3 and 4)

Arab J Geosci

Table 1 Measured parameters based on palynofacies and Rock-Eval pyrolysis for the studied samples of the Gurpi Formation

S.N AOM MP P TOC (%) TIC S1 (mg/g rock) S2 (mg/g rock) S3 (mg/g rock) Tmax (C) OI HI PI

A

1 95 2 3 0.7 8 0.06 1.39 0.96 433 135.4 196 6.13

3 93 2 5

6 93 2 5

8 93 2 5

9 93 5 2

10 10 30 60

11 95 2 3

12 4 5 91

14 50 20 30

15 85 5 10

16 90 5 5

17 95 1 4

18 90 7 3

19 35 3 62 0.3 3.8 0.05 0.12 1.57 424 464.4 35.5 29.4

20 40 20 40

21 40 25 35

22 68 7 25

24 45 10 45

25 60 5 35 2.95 4.62 0.83 424 2507 1395 38.9

26 80 5 15 0.23 3.07 0.03 0.07 0.15 436 37.7 17.6 22.2

27 5 10 85

28 1 0 99

33 10 0 90

36 5 0 95

39 10 0 90 0.09 10.5

41 99 0 1

42 99 0 1 0.5 9.8

43 100 0 0

44 100 0 0 2.1 9.7 0.24 13 0.43 421 20.3 614.7 1.8

47 100 0 0 0.43 8.2

48 90 3 7 0.5 10.3 0.03 0.01 0 340 0 4.76 75

50 5 0 95 0.08 10.1

52 5 0 95 0.15 10.5

53 10 0 90 0.12 10.1

54 30 10 60 0.12 9.4

55 95 0 5 0.1 9.6

57 15 5 80

58 55 25 20 0.19 9.1

59 55 20 25 0.25 7.3

61 80 15 5 0.19 8.9

64 95 3 2

67 15 10 75 0.22 8.44

69 30 40 30 0.17 8.65

70 20 20 60

71 47 7 46 0.21 8.1

73 70 5 25

74 88 7 5 0.13 6.1 0.03 0.01 0 340 0 4.76 75

Arab J Geosci

Eval/TOC analyser. Thus, S1, S2, S3 and Tmax values wereobtained. The S1 parameter is a measure of the amount ofhydrocarbon liberated at 300 °C. Peak S2 is the amount ofhydrocarbon released during temperature-programmedpyrolysis (300–600 °C). The hydrogen index (HI) is thenormalized S2 value (S2/TOC), expressed as milligrams HCper gram of TOC. The oxygen index (OI) is the normalized S3value (S3/TOC), expressed as milligrams CO2 per gram ofTOC. The production index (PI) shows the level of thermalmaturation (Leckie et al. 1988). The S2/S3 values indicate thetype of organic matter for low to moderately mature samples(a, b of Table 1). Vitrinite reflectance (VR) was measuredon a Zeiss Axiophot microscope following the proceduresdescribed in Schoenherr et al. (2007). Another ZiessAxiophot microscope equipped with an ultraviolet/violet

light source was used to recognize organic matter type,in fluorescent light.

Result

Microfacies analysis

Interpretation of depositional processes and sedimentarypaleoenvironment is usually done via using microfaciesand lithofacies. Three microfacies types (Figs. 1, 2, 3, 4, 5and 6 of Plate 1) were identified according to microscopiccharacteristics of the Gurpi Formation. Classification of theMFTs was based on Dunham (1962) and Folk (1974). Thedominant microfacies in this formation are:

Table 1 (continued)

S.N AOM MP P TOC (%) TIC S1 (mg/g rock) S2 (mg/g rock) S3 (mg/g rock) Tmax (C) OI HI PI

B

1 95 2 3 0.40 9 0.03 0.039 0.32 434 78.43 95.58 7.14

3 93 2 5

6 93 2 5

7 40 20 4 0.30 3.2 0.03 0.04 0.94 430 309.2 13.15 42.85

10 20 10 70

11 33 7 60 0.31 208 0.04 1.55 0.6 431 188.08 485.89 2.51

14 15 5 80

16 70 0 30

18 0 0 100

23 10 5 85

26 20 20 60 0.11 8.3

28 100 0 0

31 100 0 0

37 100 0 0 1.5 9.2 0.44 7.52 0.24 432 15.78 494.7 5.52

38 60 10 30 0.14 8.9

41 30 0 70

44 40 0 60 0.13 8.9

53 50 30 20 0.29 7.9

57 20 0 80

60 10 0 90

65 7 3 90

69 35 30 35

70 45 10 45 0.43 8.2 0.02 0.23 0.24 433 54.66 52.39 8

71 85 5 10 0.93 7.35 0.04 1.12 0.92 440 110.44 134.45 3.44

81 50 0 50

86 85 5 10 0.30 6.33 0.02 0.32 0.11 437 36.30 105.6 5.88

91 91 2 7

94 50 10 40

103 80 3 17

108 10 1 89

115 7 3 90

Arab J Geosci

Pelagic mudstone wackstone

About 77–85 % of the volume of this formation is composedof this facies. This lithofacies consists of calcareous,slightly phosphatic and finely laminated shales andclaystones, with abundant planktonic foraminifera.Allochems consist mainly of planktonic foraminifers(Globotruncana, Rugoglobigerina, Heterohelix; 80–95 %)with rare benthic foraminifera (5–10 %) that suggest abasinal environment (Flügel 1982, 2004; Jenkyns 1986;Einsele 2000; Raymond 2002; Cojan and Renard 2002;Heldt et al. 2008). This microfacies is developed in lowerand upper parts of both sections of the Gurpi Formation(Plate 1 and Fig. 4).

Microbioclast packstone

About 4–5 % of the samples of the Gurpi Formation (lime-stone successions below the Lopha Member) are dominatedby this microfacies. This facies is mainly of planktonic andbenthic foraminifera, echinoid fragments and spines. Smallsize of bivalve shell debris, sponge spicules and scarcenessof benthic macrofossils, as well as existence of pyrite tracesreflect a distal outer-ramp depositional environment (Flügel1982; Einsele 2000; Bromley 1990; Heldt et al. 2008; Plate 1and Fig. 4).

Bioclastic packstone

About 12–15 % of the samples specifically taken from theLopha Member are categorized in this microfacies. Large

fragmented shells of oyster (bivalves), echinoderm fragmentsand echinoid spines are common, and benthic microfossilssuch as benthic foraminifers, bryozoans and green algal debrisare rare in the whole succession. In the first 23 m of the LophaMember of the Kavar section and first 18 m of Farhadabadsection, macrofossil assemblages are common. The upper 7 mof the Kavar section and 5 m of Farhadabad section are devoidof large macrofossils, while small brachiopod fragmentsare abundant. Bioclasts in the thin sections are mainlyconsist of fine-grained skeletal detritus and smaller-sizedfragmented shells (Plate 1). The diverse normal marinemacro-and microfossil assemblages that include planktonicorganisms and the muddy textures of the deposits suggestan open marine condition. A mixed benthic/pelagic micro-fossil association with a macrofossil assemblage dominatedby bivalves, oysters, Exogyra type and echinoid debrischaracterizes the outer ramp microfacies (Flugel 2004;Einsele 2000; Bromley 1990; Eberli 1987; Fig. 4). Theintergranular space is filled with a micrite matrix; Principalgrains are bivalves, rare echinoids, gastropods, brachiopodsand benthic foraminifera. More internal benthic fauna canoccasionally be encountered and was probably brought inby storms.

Organic facies

Organic matter is viewed as sedimentary components thatreflect original conditions in the source area and the deposi-tional environments. The organic matter content of sediments(organic facies) is defined by various parameters of palynology,organic petrography and geochemistry.

FWWB

SHOAL/BARRIERLAGOONLimit of photic zone

SWB

FWWBMLT

MHT

ContinentalTidal flatInner ramp/platformMidOuterpelagic

Lower offshore Upper offshoreSubtidal

Shoreface Foreshore

MHT: Mean High Tides

MLT: Mean Low TidesFWWB:Fair Weather Wave BaseSWB: Storm Wave Base

Zonation of depositional enviroment Intertidal SupratidalBathyal/Abyssal

Backshore

lan

oitcerid

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low

deni

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250m 90m 40mBasinDeep basin25m 10m 0

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Rugoglobigerinidae

Heterohelicidae

Globotuncanidae

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Fig. 4 Zonation of depositional environment of Gurpi Formation based on benthic and planktonic foraminifera and benthic fauna

Arab J Geosci

(b)

2

1

Phytoclast

AOM Palynomorph

95

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60 35

V

IIIIVa

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IXVIII

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AOM Palynomorph

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Mud-dominated oxic shelf (distal shelf)

I:II:III:IV:

Highly proximal shelf or basinMarginal dysoxic-anoxic basinHeterolithic oxic shelf (proximal shelf)Shelf to basin transition

V:

Distal dysoxic-anoxic shelfVI: Proximal suboxic-anoxic shelfVII:VIII:IX:

Distal anoxic shelfDistal suboxic-anoxic basin

F i g . 5 AOM-phy t o c l a s t -palynomorph kerogen plots(Tyson 1993–1995) of sampledata from two sections ofGurpi formation. Numbersfrom 1 to 4 [(1) organic facies1, (2) organic facies 2, (3) or-ganic facies 3, (4) organic fa-cies 4] are indicated by a circleon APP ternary diagrams. aThe Farhadabad section in-variably plotted in the upperleft side of the diagram is ahighly proximal, oxic to dyoxicsetting, and b Kavar section withhigher relative abundances ofmarin palynomorphs and AOMwas deposited in open marin,distal and anoxic to suboxicenvironment

Arab J Geosci

Palynofacies

Relative proportions of the three main groups of palyno-logical elements including marine palynomorphs (dinofla-gellate cyst, rare acritarchs, foraminiferal test linings),terrestrial elements (phytoclast, spore, pollen) and amor-phous organic matter (AOM) were determined and count-ed for all the samples studies to differentiate palynofaciestypes (Traverse 2007; Tyson 1993; Batten 1996; Battenand Stead 2005).

In this study, four types of palynofacies have beenidentified depending on the estimated ratio of the existingorganic matter components as been mentioned by Pittetand Gorin (1997). Statistical analysis and visual observa-tions were used to identify palynofacies groups (Plate 1and Table 1). The details of palynofacies differentiatedare as follows:

Palynofacies type I: dominated by fluorescent AOM(90–95 %) with minor terrestrial elements (1–5 %) andno marine palynomorphs.Palynofacies type II: dominated by terrestrial elements(48–88 %) with subsidiary AOM (10–46 %) and marinepalynomorphs (1–25 %).Palynofacies type III: dominated by black terrestrialelements (65–88 %) with subsidiary AOM (12–35 %)and no marine palynomorphs.Palynofacies type IV: dominated by non-fluorescentAOM (43–98%) with subsidiary black terrestrial element(0–25 %) and marine palynomorphs (0–10 %)

The estimated ratios of the sedimentary organic mattercomponents for the two sections of the Gurpi Formation areplotted on the Tyson type ternary diagrams (Fig. 5a, b).The Kavar section samples with higher relative abun-dance of marin palynomorphs and AOM and less amountof phytoclast are plotted in the center of the diagrams 5bshowing deposition in an open marine, distal and anoxic tosuboxic environment, whereas those of Farhadabad are plottedalong the upper left side of the diagram 5a, showing a highlyproximal, oxic to dyoxic setting.

Organic facies types

Organic facies investigation uses techniques from microscopy(palynofacies analysis) and organic geochemistry (TOC,Rock-Eval pyrolysis, etc.) to the application of these tech-niques mainly in the palaeoenvironmental characterization(Batten and Stead 2005; Tissot and Welte 1984; Espitalieet al. 1985; Tyson 1995). Jones (1987) in a study developedthe organic facies concept and defined seven organic faciesusing geochemical, and to a lesser extent optical data(microscopy). The seven organic facies defined by Jones

(1987) are: A, AB, B, BC, C, CD and D. According toTyson (1995), the three main divisions are into anoxic–dysoxic (A, AB, B, BC), proximal fluvio-deltaic toprodeltaic-oxic shelf (C, CD) and distal deposited, oxicfacies (D; Table 2). The main characteristics of Jones’sfacies (1987) were discussed by Jacobson (1991), Jonesand Demaison (1982) and Tyson (1995, 1996). This clas-sification system is used in the present study forinterpreting depositional environment. However, to deter-mine organic facies of the Gurpi Formation and compare tothe organic facies of Jones (1987), study of total organiccarbon (TOC), Rock-Eval pyrolysis data (HI, S1, type ofkerogen) and palynofacies types is nessesary (Figs. 2, 3and 6).

Table 1 illustrates the TOC and Rock-Eval pyrolysisvalues for the samples studied. The obtained data show thatthe TOC values are between 0.08 and 2.3, with an average ofabout 0.64 % for the Kavar section and of 0.58 % for theFarhadabad section. These results indicate that none of thesections contain sufficient organic matter to be hydrocar-bon source rock except for the early Maastrichtian intervalwhere the TOC content gradually rises and reaches 1.5 atFarhadabad and 2.3 at Kavar section (Figs. 2 and 3). Also,kerogen types are distinguishable under a light microscopeand by geochemical analysis via obtaining HI and OIvalues (Plate 1). A modified van Kervelen plot of HI vsOI for the Gurpi Formation shows most of samples plotting inthe region between the type II (marine) and type III (terrestrial)fields (Fig. 6). This is consistent with the mixed terrestrial(non-marine) and marine organic matter assemblages identi-fied in the palynofacies analyses (discussed above).

Type I

Type II

Type III

Type IV

0 100 200 400300 5000

200

400

600

800

1000

1200

Kavar section

Farhadabad section

OI (mg CO2/g TOC)

HI (

mg

HC

/g T

OC

)

1400

1600

Fig. 6 Classification of kerogen recorded in two sections on the HI-OIdiagram

Arab J Geosci

Most of the geochemical parameters and palynofaciesdata of the Gurpi Formation in the study area display arelatively wide range. However, the geochemical andpalynofacies variations make it possible to identify differentorganic facies (Figs. 2 and 3). According to the classificationscheme proposed by Jones (1987), four different organicfacies types were recognized within the Gurpi Formation inboth sections (Table 2).

Organic facies 1. This is characterized by a mean TOCof below 0.5 and kerogen type II±III and palynofaciestype II. Organic facies types were plotted on APP plotsfor determination of palaeoenvironment (Fig. 5). Samplesfrom organic facies 1 were located along the left side ofthe Tyson diagram (APP) as a result of higher relativeabundance of amorphous organic matter and rare marinepalynomorphs. Distribution of this organic facies on APP

1 2 3

5 6

7 8 9

10 11 12

4

GLO

Hetero

Rugo

Gl

Po

BIV

Echino

Omph

BF

OPPHY

OPAOM

TAOM

MP

MP

OPAOM

OPPHY

OPPHY

TAOM

100 µm 100 µm 100 µm

100 µm 100 µm 100 µm

50 µm50 µm

50 µm50 µm 50 µm50 µm50 µm50 µm

50 µm50 µm 25 µm

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plots (IV,VI and VII field) shows that this facies formedin the marine, shelf to basin transition environment inagreement with general sedimentological observations.This facies is the most widespread organofacies in theGurpi succession (representing about 54 % of the

samples) and corresponds to organic facies BC ofJones (1987; Tyson 1995, 1996; Jacobson 1991;Table 2). Depositional environment of this organicfacies is in accord with the results gained frommicrofacies studies (pelagic mudstone wackstone andmicrobioclast packstone).Organic facies 2: About 9 % of the volume of the forma-tion is composed of organic-rich shales as shown byhydrogen index ranging from 400 to 614 mg HC/g TOC,TOC 0.5±2.3 with kerogen type I, II and palynofacies Ithat corresponds to organic facies B of Jones (1987) anddistal environment (Tyson 1995, 1996; Jacobson 1991;Table 2). The APP plots (Fig. 5a, b) clearly show adominance of amorphous organic matter (AOM) for sam-ples of organic facies 2 (IX field) that indicate this organicfacies formed in distal anoxic–suboxic basin. Abundanceof fluorescent AOM coinciding with highest values ofTOC (up to 1.5), HI and organic matter type I as well asthe presence of glauconite and phosphates are evidence ofhigh productivity and sub-oxic to anoxic condition,which favoured accumulation and preservation of or-ganic matter in early Maastrichtian ( Plate 1). Thisorganic facies occurs in pelagic mudstone, wackstonemicrofacies which reflects open marine, distal anoxicenvironment for early Maastrichtian.Organic facies 3: is typically marked by low TOC(TOC ∼0) and palynofacies III (Plate 1). Organic facies 4with increased percentage of opaque and brownphytoclast values and very low values of TOC relate tobioclast packstone microfacies of Loph Member that sug-gest a proximal outer ramp depositional environment witha strong detrital influx. A mixture of reworked oyster

Table 2 Relationship between some selected palynofacies kerogen parameters and organic facies sensu Jones (1987) in idealized marine siliciclasticfacies (generalized relative trends only; Tyson 1995, 1996)

Organic facies A AB B BC C CD D

Palynofacies characteristics

% AOM of kerogen Dominant Mod Usually low/absent

AOM matrix fluorescence Highest Mod-weak Weak Usually absent

% prasiophytes of plankton Highest Mod Rare Usually very rare

% phytoclasts of kerogen Low (dilution) Mod Usually dominant

opaque: translucent phytoclats often high usually low increase

Geochemical characteristics (for immature sediments)

Hydrogen index >850 >650 >400 >250 >125 50–125 <50

Kerogen type I I/II II II/III III III/IV IV

TOC% 5–20+ 3–10+ 3–3+ <3 <0.5

Environmental factors

Proximal–distal trend Distal Proximal Distal

Oxygen regime Anoxic Anoxic–dysoxic Oxic V.Oxic

Sediment accumulation rate Low Varies High mod Low

Organic facies A AB B BC C CD D

�Plate 1 1–6 Microfacies from the Gurpi Formation. All magnifications×32. 1 Pelagic mudstone wackstone microfacies with abundance ofplanktonic foraminifers (Glo Globotruncana, Rugo Rugoglobigerina,Hetero Heterohelix) (arrow), sample 4, in 7 m of of Kavar section relateto organic facies 1. 2 Pelagic mudstone wackstonemicrofacies, glaucony-rich calcareous shale, one of several glaucony and phosphate grains isindicated by an arrow, sample 37, in 85 m of Farhadabad section relate toorganic facies 2. 3 Pelagic mudstone wackstone microfacies, sample 50of Emam-Hassan member in 166 m of Kavar section relates to organicfacies 4. 4 Bioclast packstone microfacies, larger fragmented shells ofoyster (bivalves) (arrow), sample 50 of the Lopha Member in 166 m ofKavar section relates to organic facies 3. 5 Bioclast packstonemicrofacies, larger fragmented shells of oyster (bivalves), sample 30 ofthe Lopha Member member in 90 m of Kavar section relate to organicfacies 3. 6 Microbioclast packstone microfacies, planktonic and benthicforaminifera, echinoid fragments and small size of bivalve shell debris(arrow), sample 11 in 42m of Farhadabad section relates to organic facies1. 7–10 Palynofacies from the Gurpi Formation Seen in transmitted whitelight.(OP PHY opaque palynomacerals, MP marine palynomorphs, OPAOM opaque AOM, T AOM transparent AOM) 7 Palynofacies type 3(PF-III), showing dominance of opaque degraded terrestrial elements,sample no. 50 of Emam-Hassan member in Kavar section, slide 1.8 Palynofacies type 4 (PF-IV), showing dominance of degraded terrestrialelements, with subsidiary AOM, and a sparse marine element. Sampleno. 61 of Farhadabad section, slide 1. 9 Palynofacies type 2 (PF-II),showing dominance of terrestrial elements with subsidiary AOM andmarine palynomorphs. Sample no. 53 of Farhadabad section, slide 5.10 Palynofacies type 1 (PF-I), showing dominance of transparentAOM. Sample no. 44, slide 1. 11 Dinoflagellate cyst seen under bluelight fluorescence 15 μm, sample 37 of Farhadabad section. 12Vitrinite maceral, 25 μm in vitrinite reflectance, sample 37 ofFarhadabad section

Arab J Geosci

(bivalves) clasts, echinoderm fragments, benthic forami-nifera inside high percentage of opaque and brownphytoclast suggests an influence of bottom currents attimes and was probably brought in by storms. The max-imum recorded diameter of the black equant phytoclastsconfirms the proximal character of the organic facies C ofJones (1987) and is essentially nongenerative (Table 2).Organic facies 4: presents a mean TOC of below 0.1 %and is clearly distinguished by its black phytoclast-dominated palynofacies. The high percentages of blackphytoclast and non-fluorescent AOM indicate oxic condi-tion unsuitable for preservation of organic matter. Samplesof organic facies 3 and 4 show a dominance of phytoclaston the ternary diagram (APP; fields I and II) and deposi-tion in highly proximal shelf or basin (Fig. 5a, b). Highratio of small equidimensional opaque phytoclasts in car-bonate sequence of organic facies 3 generally indicatesdistal setting under suboxic to oxic bottom environment.This facies occurs in pelagic mudstone and wackstone ofEmam-Hassan Member and limestone successions belowthe Lopha Member with high total inorganic carbon (TIC)that corresponds to the environment of organic facies CDof Jones (1987; Figs. 2 and 3; Table 2).

Discussion

The obtained results indicate how organic facies in combi-nation with microfacies data may contribute to the evaluationof environmental changes. Microfacies, organic facies andpalynofacies analyses make it possible to identify different

depositional settings and obtain information on source rockpotential. Figures 2 and 3 show the ranges of the faciesassociations in our sections. The following microfacies andorganic geochemistry criteria of the two sections show:

The lower part of the Gurpi Formation with a middleCampanian age is composed of pelagic mudstone wackstonewith abundant planktonic foraminifers such as Globotruncana,Rugoglobigerina, Heterohelix (indication of low energy envi-ronment) which cover more than 50 % of the thin sections. Theminimal size of the black equant phytoclasts and planktonicorganic matter with subsidiary fluorescence AOM (PF I and II)and TOC below 0.5 and kerogen type II±III (organic facies 1)show that this interval was deposited under euxinic condition,water column circulation was sluggish and recycling of organicmatter limited. This interpretation is supported by microfaciesand organic facies data which indicate that the laminated darkwackstone of the Gurpi Formation were deposited in a quitewater environment under anoxic conditions in pelagic environ-ment. This formation shallows upward to the microbioclastpackstone facies below the Lopha Member with abundanceof benthic foraminifera, echinoid fragments, small size of bi-valve shell debris and terrestrial elements (PF II) that reflect adistal outer-ramp depositional environment. Depositional en-vironment of this microfacies is in agreement with theresults gained from palynofacies, organic facies studiesand general sedimentological observations. Sediments ofpelagic mudstone wackstone and microbioclast packstonefacies in lower parts of the formation are characterized bylow amounts of organic matter and kerogen type II±III(organic facies 1) that probably suggest poor to fair gas-prone potential. Gurpi Formation then reaches its minimum

Table 3 Guidelines for pyrolysisparameters of quality, quantityand thermal maturity (after Petersand Cassa 1994)

Quantity TOC (wt%) S1 (mg HC/g rock) S1 (mg HC/g rock)

Poor 0–0.5 0–0.5 0–2.5

Fair 0.5–1 0.5–1 2.5–5

Good 1–2 1–2 5–10

Very good 2–4 2–4 10–20

Excellent >4 >4 >20

Quality HI (mh HC/g TOC) S2/S3 Kerogen type

None <50 <1 IV

Gas 50–200 1–5 III

Gas and oil 200–300 5–10 II/III

Oil 300–600 10–15 II

Oil >600 >15 I

Maturation R0 (%) Tmax (C) TAI

Immature 0.2–0.6 <435 1.5–2.6

Mature

Early 0.6–0.65 435–445 2.6–2.7

Peak 0.65–0.9 445–450 2.7–2.9

Late 0.9–1.35 450–470 2.9–3.3

Postmature >1.35 >470 >3.3

Arab J Geosci

depth in boundstones of the Lopha Member in UppermostCampanian. The Lopha limestone facies is categorized inbioclast wackstone that contains a mixed benthic/pelagicmicrofossil association with a macrofossil assemblage dom-inated by oysters (Exogyra type) and echinoids suggestingthat the Lopha Member was deposited in an outer rampenvironment in the uppermost Campanian (Fig 4). Highrelative abundance of black terrestrial-derived organic matterand very low values of organic matter (organic facies 3)and faunal content of the Lopha limestone indicate oxygen-ated conditions where circulation in the water body wasstrong and an open platform fauna dominated by the largeLopha oyster. This is followed by an abrupt reducing andstagnant condition and the deposition of pelagic mudstonewackstone microfacies. The high contents of organic matteras well as the presence of pyrite and phosphates in organicfacies 2 are evidence of surface water productivity andsuboxic to anoxic condition that favoured accumulationand preservation of organic matter in pelagic environmentin early Maastrichtian. The type of kerogen content (typeI), abundance of transparent AOM, highest TOC and HIvalues in this interval (organic facies 2) suggest high sourcepotential for oil at maturity.

This was shortly followed by further influx of mud anddevelopment of calcareous unit in the middle of Maastrichtianto form the argillaceous limestones of the Emam-HassanMember. The Emam-Hassan limestone with abundant plank-tonic foraminifera (an indication of low energy environment)represents a more carbonate-rich pelagic deep water setting(high TIC) and pelagic environment. A TOC value of below0.1 % and high percentages of non-fluorescent and blackphytoclast of organic facies 4 indicate oxic conditions in pe-lagic environment that were unsuitable for preservation oforganic matter and is essentially nongenerative. The memberis overlain by the marls of Late Maastrichtian with dominatedblack terrestrial elements and subsidiary AOM (PF III).Microbiostratigraphy, sedimentology and palynofacies obser-vation indicate sea level fall and a 3.5 million year hiatus at theK/P boundary (Hemmati-Nasab 2008). Presence of LatePaleocene planktonic foraminifera, such as Globigerina andGloborotalia, coincides with glauconite in upper parts of thesections in purple shales of the Pabdeh Formation are referredto this depth decreasing. Deposition of Pabdehmarl and shale isan indication of re-deepening of the sea from Upper Paleocene.

Assessment of source rock potential

The three main factors for evaluating petroleum potentialproduction of a rock are:

1. Potential quantity of produced hydrocarbon which isbased on S1, S2, TOC

2. Type of produced hydrocarbon which is based on the HIand S2/S3 ratio

3. Level of thermalmaturity which is based on Tmax andVR

The parameters measured are presented in Table 1. Thestandard guidelines for interpreting these values are given inTable 3. Comparing the values of S1, S2, Tmax and TOCbased on Rock-Eval pyrolysis of the samples studied withstandard guidelines (Peters and Cassa 1994; Tables 1 and 3),it becomes evident that the Gurpi Formation at kavar section(S1, 0.4; S2, 2.6; TOC, 0.64) and at Farhadabad section(S1, 0.08; S2, 1.73; TOC, 0.58) is poor to fair in terms ofproduction potential and therefore could not have producedhydrocarbons in commercial quantities. Average Tmax, S1Rock-Eval pyrolysis values (between 340 and 440 °C) andvitrinite reflectance (0.5 to 0.7) suggest that the formation isimmature to early mature (Fig. 12 of Plate 1).

Conclusions

1. Two outcrops of the Gurpi Formation in the Zagros basin,southwest Iran were studied in detail with regard tomicrofacies, palynofacies and organic geochemistry. Ourdata provide insight into the paleoenviromental evaluationand assessment of petroleum potential. The GurpiFormation consist of pelagic mudstone/wackstone,microbioclast packstone and bioclastic packstone reflectdistal outer ramp or basin. Combination of palynofaciesand organic geochemistry resulted in differentiation offour organic facies corresponding to facies B, BC, C andCD of Jones (1987). Detailed organic facies shows thatthe formation is characterized by low values of TOC, highpercentages of amorphous organic matter and blackphytoclasts, rare marine algae thereby representing amix of terrestrial and marine kerogen.

2. Organic facies 1 which is the most widespread typethroughout the succession (representing about 54 % ofthe samples) and corresponding to organic facies BC ofJones (1987) probably suggests shelf to basin transitionand gas-prone nature of the host layers. Organic facies 2(about 9 % of the volume of this formation) correspondsto organic facies B of Jones (1987) and contains highvalues of TOC, S2 and HI, indicating a good potentialand of being petroleum source corresponding to distallower energy anoxic condition in Early Maastrichtian.Samples of organic facies 3 and 4 corresponding toorganic facies C and CD of Jones (1987) occur in lime-stone succession below the Lopha, Lopha and Emam-Hassan Members with low values of TOC and highpercentages of non-fluorescent and black phytoclast thatwere unsuitable for preservation of organic matter and isessentially nongenerative.

Arab J Geosci

3. A gradual sea-level fall from Late Campanian to EarlyMaastrichtian was presumably responsible for the decreasein the frequency of planktonic foraminifera inmicrobioclastpackstone and increase in percentages of black phytoclastand abundance of macrofossil assemblage dominated bybivalves, oysters (Exogyra type) and echinoids in bioclasticpackstone of the Lopha Member.

4. The APP plots of Farhadabad section indicate that thesection was deposited in a marginally marine proximalsetting with a strong influx of terrestrial plant matter,while samples from the Kavar section in the west ofFarhadabad have elevated values of transparent AOMand marine algal cysts indicating deposition in anopen marine, distal, anoxic to oxic environment. Thisdifference reflects changes in depositional setting alonga distal–proximal transect and confirms more marineenvironmental condition for the Kavar section.

5. Both Kavar and Farhadabad sections contain low valuesof total organic matter (TOC), S2, and hydrogen index(HI), indicating that the formation does not present a goodsource for petroleum production except for minor intervals(organic facies 2) in Early Maastrichtian. Tmax values(vary between 340 and 440 °C) and VR (0.5 to 0.7)confirm immaturity trends indicated by Rock-Eval data.

Acknowledgments We are grateful to Professor Ralf Littke andDr. Sabine Heim for providing a sabbatical research period at theInstitute for Petroleum and Organic Geochemistry in Aachen for the firstauthor. Thanks are also directed to the staff of this Departments for theirhelp provided in organic geochemical analysis.

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