organic petrography, geochemical behavior, and paleo...

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The 1 st International Applied Geological Congress, Department of Geology, Islamic Azad University - Mashad Branch, Iran, 26-28 April 2010

Organic Petrography, Geochemical Behavior, and Paleo Depositional Environment of Binak Oilfield, SW Iran

Fouladvand, Razieh and Alizadeh, Bahram

Department of Geology, Faculty of Earth Sciences, S. Chamran University of Ahvaz, Iran. Corresponding Author: [email protected]

Abstract In this study hydrocarbon generation potential and depositional environment of Pabdeh, Gurpi, Lafan and Kazhdumi Formatins is investigated using Rock Eval 6 pyrolysis and organic petrography. Based on Rock-Eval 6 results, organic matter in Pabdeh, Gurpi and Lafan Formations are mixed Kerogen Types II and III. Organic matter of Kazhdumi Formation demonstrates prominent Kerogen Type III. Liptinites, Vitrinites and Inertinites occurrence altogether, reveal kerogen mixture of Type II and III in Gurpi Formation. However, abundance of Vitrinite and Inertinit macerlas in Lafan Formation indicate that Kerogen Type III is prominent. HI vs OI diagram, demonstrates Pabdeh, Gurpi and Lafan Formations being deposited in three different facies (B, BC and C). This reveals that there are terrestrial as well as marine organic matters in the said formations. Kazhdumi Formation has CD facies representing an oxidant depositional condition being dominant. Genetic hydrocarbon potential of Pabdeh, Gurpi, Lafan and Kazhdumi Formations decreases in the central part of the oilfield due to activity of Kharg-Mish fault. On the other hand the studied source rocks towards eastern and western (moving away from its center) sides of Binak oilfield have a better genetic potential for hydrocarbon generation. Keywords: Organic petrography and geochemistry, Depositional Environment, Rock-Eval6.

1. Introduction During the last three decades the Rock-Eval pyrolysis has been routinely used in organic geochemistry for examining the oil and gas potential and maturity of different rock samples (Chang Ryu, 2008). In Iran also few researchers used this instrument, such as Alizadeh and Moradi in 2007 to geochemically evaluate Pabdeh Formation in Giant Oilfield of Ahwaz as well as Zeloi. Optical methods are also used for source rock characterization but are comparatively require more sample preparation time (Akinlua et al., 2005). 2. Geological Setting The study area located at in the vicinity of Persian Gulf at 22 km northwest of Genaveh port and south of Bibi hakimeh, Rag-e-Safied and Kilur Karim oilfields (Fig. 1A). In total, 18 wells were drilled along the axial line of this anticline (Fig. 1B). The source rocks of this oilfield are Kazhdumi, Lafan, Gurpi and Pabdeh Formations. During the Albian, Kazhdumi disconformably overlaid Dariyan Formation. This formation consists of dark and bituminous limestone interbeded with dark argillaceous limestone and calcareous shale. In this oilfield, late Certaceous began with deposition of Coniacian Laffan Shale and completed with deposition of the Gurpi marls of Campanian to Maastrichtian. Following the Late Certaceous tectonic activities, the Paleocene-Eocene transgression resulted in deposition of marls and argillaceous limestones of the Pabdeh Formation (Motiei, 2007).

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3. Methodology and work plan The 43 drill cuttings obtained from source rocks of wells numbers 2, 4, 6 and 18 were prepared for Rock Eval analysis by crushing, pulverizing and homogenizing. Then, 50 to 70 mg of sample was loaded in a crucible to perform pyrolysis with Rock-Eval 6 (Table 1). Later, 21 out of 43 samples were prepared for incident light microscopy by hand-picking and then placing cutting particles into a 2.5 cm diameter mold (3 cm deep) and impregnating with epoxy. After hardening, the sample was ground using carborundum grits, and finally polished using slurries of alumina and isopropyl alcohol. Maceral compositions were determined on polished pellets using a Zeiss Axioplan2 microscope with a 100 oil immersion objective under normal white light. 4. Discussion This study attempted to understand hydrocarbon potential and paleo depositional environment of source rocks of Binak oilfield. To determine the relative ability of source rocks, quantity, quality and maturation of organic matter was evaluated. 4-1. Quantity of Organic Matter The TOC content of the studied samples, are determined to be poor to very good according to Peters, 1986, classification. The results reveal that TOC of source rocks decrease toward the center of this oilfield due to the activity of Kharg-Mish fault (Fig. 2). 4-2. Quality evaluation of organic Matter by Rock Eval pyrolysis and organic petrography Rock-Eval pyrolysis data were plotted on a modified Van Krevelen diagram to evaluate kerogen type. The results indicate that kerogen of the source rocks in Binak oilfield range from type II to type III (Fig. 3). However, Organic matter of Kazhdumi Formation is dominated by Type III. Oxidant environments convert hydrogen-rich organic matter into organic matter with lower H/C (Jones, 1987). The low HI and TOC values and the high OI value in Kazhdumi Formation from well number 4 indicate that organic matter composition in this formation is not dependant only to the original input material, but also to the availability of oxygen during and after deposition. The abrupt decrease of HI values in Kazhdumi Formation is probable due to the remarkable activity of Kharg-Mish Fault during Albian. The Gurpi and Lafan Formations are distinguished suitable for organic petrography because of their abundant maceral content. Petrological observations show that kerogen macerals in Gurpi Formation are dominated by Type II and III. Organic matter of this formation is composed of Liptinite (Sporinite), Vitrinite and Inertinte (Fusinite, Semifusinite and Micrinite)(Fig. 4). The common macerals of Lafan Formation are Vitrinite and Inertinite (Fusinite and Semifusinite)(Fig. 4). Similarly, low amount of Hydrogen Index parameter (249-298 mg HC/g TOC) for this formation reveals that kerogen Type III is prominent. 4-3. Maturity of Organic Matter Rock-Eval Tmax values for total samples lies within the oil window (430 °C - 470 °C) with the exception of Kazhdumi Formation samples in well number 4. In this formation strong decomposition of organic material caused anomalies.

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4-4. Paleo Depositional Environment Jones (1987) defined seven types of organic facies (A, AB, B through D). The Pabdeh, Gurpi and Lafan Formations are composed of mixture of organic facies B, BC and C (Fig. 5). This reveals that there are terrestrial as well as marine organic matters in the said formations. The Kazhdumi Formation has organic facies CD representing an oxidant depositional condition being dominant (Fig. 5). 5. Conclusions The organic matter of Pabdeh and Gurpi Formations include kerogen type II/III in this oilfield. Integration of geochemical and petrographic analysis indicates that the common constituents of Lafan Formation are kerogen Type III. Diagram of HI vs. OI demonstrates that the Kazhdumi Formation has deposited in more oxidant depositional condition in compare with Pabdeh, Gurpi and Lafan Formations. The low HI and TOC values and the high OI value in Kazhdumi Formation from well number 4 indicate that organic matter compositions are not only dependant on the original input material, but also on the availability of oxygen during and after deposition. The drastic change in the geochemical data of Kazhdumi Formation can be due to remarkable increase of the activity of Kharg-Mish Fault and the flexure forming in basin during the Albian. It appears that the major factor in variations of genetic potential of source rocks is activity of Kharg-Mish Fault in this oilfield. The genetic potential for all source rocks decreases toward the Kharg-Mish Fault and center of the oilfield. Finally, this study indicates that the Pabdeh and Gurpi Formations have capability to generate oil and gas and the Lafan Formation has potential to generate gas and minor oil. References

[1] Akinlua, A., Ajayi, T. R., Jarvie, D. M. and Adeleke, B. B., 2005, A Re-appraisal of the application of Rock-Eval pyrolysis to source rock studies in the Niger delta, Journal of Petroleum Geology, v. 28, p.39-48.

[2] Alizadeh, B. and Moradi, M., 2007, Geochemical evaluation of Pabdeh Formation in oilfields of Zeloi and Ahwaz, Shahid Chamran University Journal of Science, No. 17, p.33-45.

[3] Chang Ryu, I., 2008, Source rock characterization and petroleum systems of Eocene Tyee basin, southern Oregon Coast Range, USA, Organic Geochemistry, v. 39, p.7590.

[4] Jones, R.W., 1987, Organic Facies. In: Brooks, J., Welte, D. (Eds.), Advances in Petroleum Geochemistry, Academic Press, New York, 1-90.

[5] Motiei, H., 2007, Petroleum Geology of the Persian Gulf, Tehran University and National Iranian Oil Company, 707p.

[6] Peters, K.E., 1986, Guidelines for evaluating petroleum source rock using programmed pyrolysis: AAPG Bulletin, v.70, pp. 318-329.

[7] Sherkati, S.H. and Letouzey, J., 2004, Variation of structural style and basin

evolution in the central Zagros (Izeh zone and Dezful Embayment), Iran, Marine and Petroleum Geology, v. 21, p.535554.

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Table 1. Rock-Eval data from Pabdeh (Pb), Gurpi (Gu), Lafan (La) and Kazhdumi (Kz) Formations

Pb 2657 672 540 0.12 435 Pb

2520 513 83 1.21 439 Gu 3007 372 272 0.4 434 2560 430 70 1.05 444

La 3211 287 67 2.94 439 2656 314 116 0.51 438 3241 200 93 0.43 435

Gu

2722 220 120 0.8 444 3251 250 92 0.38 438 2760 330 190 0.3 442

Kz

3930 136 252 0.58 422 2818 350 150 3.5 456 3980 64 318 0.44 411 2870 300 180 2.2 457 4005 84 334 0.5 416 2930 209 200 1.9 456 4025 100 586 0.42 421 2972 395 93 0.4 436 4045 110 540 0.3 417 3030 250 160 4.4 456 4060 148 490 0.21 305

La 3208 220 26 4 441

4185 88 676 0.33 411 3224 200 16 16 431 4205 141 539 0.41 426 3260 250 25 4 444

Pb 2621 222 550 0.18 433 Pb 3000 404 164 0.99 439 Gu 2717 289 236 0.53 436 3102 232 296 0.47 436

Table 1. Continued

Gu

2763 312 298 0.41 435

Gu

3402 408 265 0.65 435 2817 271 368 0.38 436 3456 424 209 0.75 434 2865 365 295 0.43 435 3504 412 382 0.4 436 2916 442 280 0.55 436 3562 344 312 0.34 436 2956 279 259 0.29 432 3604 577 131 0.91 438

La 3157 116 242 0.31 332 La

3840 185 107 0.64 447

3878 217 19 11.9 435

Figure 1. Location map of the Zagros oil fields (modified after Sherkati and Letouzey, 2004) (A) and Well locations on Binak underground contour map on top of Sarvak Formation (B).

B)(

(A)

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Figure 2. Column charts frequency (%) versus TOC (wt %) in Binak oilfield: Pabdeh Formation (A), Gurpi Formation (B), Lafan Formation (C) and Kazhdumi Formation (D).

Figure 3. Modified Van Krevelen diagram (HI vs. OI).

Figure 4. Digital images of maceral components in Gurpi and Lafan Formations using a 100 oil immersion objective and under normal white incident light. Gurpi Formation: Sporinite (A), Fusinite and Semifusinite (B) and Sporinite, Micrinite and Vitrinite (C). Lafan Formation: Vitrinite (D) and Fusinite (E) and (F).

10

(B

Semifusinit10µm

(A Sporinit

Vitrinite10 µm

(C)

(E)

10

1010

(D)

10 µm

10

(F)

(C)

(D

(B)

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Figure 5. plot of HI versus OI for determination of organic facie (modified after Jones, 1987). A: Strongly reducing lacustrine environment, AB: Reducing transgressive marine environment, B: relatively reducing lacustrine or marine environment, BC: The environments that consist of terrestrial and marine organic matters together and have a high sedimentation rate in suboxic environment, C: Environments with intermediate sedimentation rate in reducing conditions, CD: Deep environments adjacent to orogeny areas, D: Strongly oxidant continental environments.

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