Organic geochemistry of crude oils and Cretaceous source rocks in the Iranian sector of the Persian Gulf: An oil–oil and oil–source rock correlation study

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  The marls and argillaceous limestones of the Cretaceous Kazhdumi Formation, Ahmadi Member of the Sarvak Formation and Gurpi Formation are considered to be important source rock candidates in the Persian Gulf. To evaluate their source rock
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  Organic geochemistry of crude oils and Cretaceous source rocks in theIranian sector of the Persian Gulf: An oil – oil and oil – source rockcorrelation study Zahra Sadat Mashhadi, Ahmad Reza Rabbani ⁎ Petroleum Engineering Department, Amirkabir University of Technology, Hafez Street, Tehran 15875-4413, Iran a b s t r a c ta r t i c l e i n f o  Article history: Received 20 January 2015Received in revised form 5 May 2015Accepted 7 May 2015Available online 15 May 2015 Keywords: Persian Gulf Kazhdumi FormationAhmadi MemberGurpi FormationCrude oilsOil – source rock correlation The marls and argillaceous limestones of the Cretaceous Kazhdumi Formation, Ahmadi Member of the SarvakFormation and Gurpi Formation are considered to be important source rock candidates in the Persian Gulf. Toevaluate their source rock characteristics, 265 cutting samples of these rock units from 20  󿬁 elds in offshoreIran were analyzed using Rock-Eval pyrolysis, organic petrography, stable carbon isotope composition and bio-marker analysis. 1D basin modeling was also applied to analyze the burial and thermal history of these sourcerockcandidates.Basedontheresults,theKazhdumiandGurpiformations,whichhavefairsourcerockpotential,andtheAhmadiMember,whichhasfairtogoodsourcerockcharacteristics,weredepositedinamarinereducingenvironment with marine organic matter as the main input. Different maturity indicators showed that theAhmadiMemberandKazhdumiFormationarethermallyimmaturetomatureandtheGurpiFormationisimma-ture to early mature in the study area. The results of the modeling suggested that the Kazhdumi Formation andAhmadi Member in the southeastern parts of the study area are within the main oil window and that the GurpiFormation has just begun togenerate hydrocarbons. Allthreeformations are thermally immature intheeasternparts.InthewesternpartsofthePersianGulf,theKazhdumiFormationisearlymatureandtheotherformationsare immature. The biomarker and stable carbon isotope parameters of these rock units were compared with 20crude oil samples in the southeastern Persian Gulf. Based on the hierarchical cluster analysis (HCA), principalcomponentanalysis(PCA)andcarbonisotopecomposition,thestudiedcrudeoilsweredividedintotwogroups:Group-IoilsfromCretaceousreservoirswereprobablyderivedfromCretaceoussourcerocks,whileGroup-IIoilsfrom Jurassic and Cretaceous reservoirs with Jurassic rocks as the expected source rocks. The performed oil – source rock correlation shows a positive correlation between the Group-I oils and the Ahmadi Member.© 2015 Elsevier B.V. All rights reserved. 1. Introduction The Persian Gulf in southwest Asia is a marginal sea of the IndianOcean located between Iran and the Arabian Peninsula (Fig. 1). Thisgulf and its coastal areas contain the largest amounts of crude oil inthe world (Haghi et al., 2013), accounting for two-thirds of the world'sprovenoilreserves(715billionbarrels)andmorethanone-thirdofthetotal proven world gas reserves (2462 tcf) (Rabbani, 2007). The occur-renceofrepeatedandextensivesourcerockbeds,substantialcarbonateand some sandstone reservoirs, excellent regional caprocks, huge anti-clinal traps and continuous sedimentation are the major factors thatmake this region a remarkable area for hydrocarbon accumulations(Rabbani et al., 2014).According to Bordenave and Hegre (2010), the Middle Cretaceous – Early Miocene petroleum system is one of the  󿬁 ve petroleum systemsin the Zagros fold-belt and the Persian Gulf area. Two calcareousreservoirs, the Oligo-Miocene Asmari and the Cenomanian – TuronianBangestan, formed the dominant reservoirs of this petroleum system.The Albian Kazhdumi Formation, Ahmadi Member of the MiddleCenomanian Sarvak Formation and the Late Cretaceous Gurpi Forma-tion are importantCretaceoussourcerockcandidatesin this petroleumsystem (Bordenave and Hegre, 2010). The Kazhdumi Formation is anexcellent source rock and the Ahmadi Member and Gurpi Formationhavebeenidenti 󿬁 edasmarginalsourcerocksintheDezfulembayment(Alizadeh et al., 2012; Bordenave and Burwood, 1990; Bordenave andHegre, 2010; Bordenave and Huc, 1995; Opera et al., 2013; Rabbani,2007).Despitethesigni 󿬁 canthydrocarbonaccumulationintheMiddleCre-taceous – Early Miocene petroleumsystem within thePersian Gulf, littleisknownaboutthequalityandmaturityofthepotentialsourcerocksinthis petroleum system.Therefore, the present study aimson character-izing the Kazhdumi Formation, Ahmadi Member and Gurpi Formationwithrespecttothesourcerockrichness,typeoforganicmatter,deposi-tionalenvironmentandthermalmaturityin20 󿬁 eldsintheIraniansec-tor of the Persian Gulf. Rock-Eval pyrolysis, vitrinite re 󿬂 ectance International Journal of Coal Geology 146 (2015) 118 – 144 ⁎  Corresponding author. Tel.: +98 21 64543545; fax: +98 21 64543535. E-mail address:  rabbani@aut.ac.ir (A.R. Rabbani).http://dx.doi.org/10.1016/j.coal.2015.05.0030166-5162/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect International Journal of Coal Geology  journal homepage: www.elsevier.com/locate/ijcoalgeo  measurements, biomarker distribution and stable carbon isotopecomposition are applied for this purpose.Because the geochemistry and genetic relationships of producedcrudeoilsinthePersiaGulfhavebeeninvestigatedonlybylimitedstud-ies (Mobarakabad et al., 2011; Rabbani, 2008; Rabbani and Kamali,2005; Rabbani et al., 2014), this study also uses detailed organic geo-chemical data along with bulk geochemical parameters to characterize20 crude oil samples in the study area and perform an oil – oil correla-tion, an essential part of petroleum system studies. Crude oil sampleswere taken from the Jurassic Surmeh and Cretaceous carbonatereservoirs(Gadvan,Dariyan,SarvakandIlam Formations)inthesouth-eastern part of the Iranian sector of the Persian Gulf (Fig. 1). The depo-sitional environments, facies and thermal maturity of the crude oilssource rocks were investigated and the collected samples were classi- 󿬁 ed into genetic families based on the geochemical parameters andstatistical methods (HCA and PCA).Finally, the genetic relationships between the studied Cretaceoussource rocks and crude oil samples are investigated and the role of these rock units in charging the hydrocarbon reservoirs in the regionis de 󿬁 ned through an oil – source rock correlation study by biomarkeranalysis and stable carbon isotopic composition. Oil – source rock corre-lationwhichprovidesdetailedinformationaboutthepetroleumsystemand migration pattern helps to identify exploration prospects. 2. Geological setting  The PersianGulf is structurally a foreland basin 󿬁 lledbyterrigenousclastics transported from adjacent regions and carbonate sedimentsgenerated across the ramp surface (Alavi, 2004). During the Paleozoic,the Arabian Plate, including the Persian Gulf region, was located in thesouthern hemisphere with predominant clastic sedimentation (Konertet al., 2001). Afterwards, during the Mesozoic and Cenozoic, the studyarea was mainly in tropical regions where carbonate depositionprevailed (Murris, 1980; Ziegler, 2001). The Mesozoic carbonatesystems of the Persian Gulf contain most of the extensive reservoirrocks in this area and form one of the richest hydrocarbon provincesin the world.TheJurassic – Cretaceous sediments of thestudy area containimpor-tantproli 󿬁 cpetroleum-producingsystems.Sourcerocksofthesepetro-leum systems were deposited on passive continental-shelf margins inrelatively stable conditions with advantages of sea-level rises, anoxicenvironments and nutrient-rich upwelling sites off the coasts. At thistime, warm climate, high-stand seas and increases in the nitrogen – phosphorus – carbon contents of oceans, led to abundant radiation of plankton populations (a key factor in the organic richness of marinesediments). The sedimentary facies of the Jurassic – Cretaceous sourcerocksinthestudyareacomprisesofcarbonateshelforplatformcarbon-ates deposited under transgressive conditions of starved euxinic basins(Alsharhan and Nairn, 1997; Sharland, 2001).Fig. 2 shows the general lithostratigraphic column for the Iraniansector of the Persian Gulf. The sediments composing the Persian Gulf in 󿬁 ll are up to 10000 m thick, consisting of a nearly continuous se-quence from the Infra-Cambrian to the Pliocene except for Devonianand Carboniferous successions, which are missing. Sedimentationbegan with important Infra-Cambrian (Vendian) evaporites, followedby shallow marine carbonate and clastic deposits in the Lower Fig. 1.  Location of the studied  󿬁 elds in the Iranian sector of the Persian Gulf.119  Z.S. Mashhadi, A.R. Rabbani / International Journal of Coal Geology 146 (2015) 118 – 144  Paleozoic. Throughout most of the Mesozoic and up to the LowerMiocene, the area was part of a broad, shallow carbonate platform.Subsequently, thick evaporates followed by continental red beds char-acterized the Mio-Pliocene. Folding accompanied by syntectonic andposttectonicmolassesoccurredinthePlio-Pleistocene(Ghazban,2009).The reservoir rocks in this region are broadly distributed throughdifferentgeologicsectionswithgenerallyhighporositiesandperme-abilities improved by fracturing. Carbonates from the Late PermianDalan Formation, Early Triassic Kangan Formation, Early CretaceousFahliyan and Dariyan formations, Late Cretaceous Sarvak and Ilam Fig. 2.  Generalized stratigraphic column of the Iranian sector of the Persian Gulf (modi 󿬁 ed from Al-Husseini, 2008).120  Z.S. Mashhadi, A.R. Rabbani / International Journal of Coal Geology 146 (2015) 118 – 144  formations (known as the Bangestan reservoirs) and Oligo-MioceneAsmari Formation are the most signi 󿬁 cant reservoirs in the study re-gion (Alsharhan and Nairn, 1997). The massive carbonate anddolostones in the Jurassic Surmeh Formation can be considered asreservoir rocks in some parts (Rabbani, 2013). Among these reser-voirs, Jurassic and Cretaceous carbonates are of prime importancebecause commercial quantities of oil have accumulated in theserocks (Mobarakabad et al., 2011; Rabbani, 2008; Rabbani andKamali, 2005).Several potential source rock units with different geological ageswere deposited in the Persian Gulf and have charged several petro-leum reservoirs. Graptolite-bearing, organic-rich shales in theLower Silurian Sarchahan Formation, an anhydrite layer in themiddle part of the Permian Dalan Formation, marls and marly lime-stones in the Albian Kazhdumi Formation, Ahmadi Member of the Middle Cenomanian Sarvak Formation and Upper CretaceousGurpi Formation and calcareous shales, marls and limestones in thePaleocene to Early Oligocene Pabdeh Formation are commonly con-sidered as important source rock candidates in the study area(Ghazban, 2009; Rabbani, 2013).The Persian Gulf is situated at the junction of the Arabian andEurasian lithospheric plates. The Gotnia Trough in the north (Iraq andSyria), Arabian Trough in the central part (Saudi Arabia, northernbranch of the Persian Gulf and Bahrain) and Rub-Al-Khali in theUnitedArabEmirates(U.A.E.)areimportantrestrictedintra-shelfbasinsin the region (Rabbani et al., 2014). A series of events in the early LateCretaceous (Turonian) resulted in the closure of the Tethys Ocean atthe Mesozoic/Cenozoic boundary (Ziegler, 2001). These events led tothe formation of the Mesopotamian Foredeep, consisting of the ZagrosBasin, the Fars Block (Fars Platform) in the southwestern area of Iran,the northern slope of the Qatar Arch and most of the Persian Gulf area,including the Rub-Al-Khali Basin (Fig. 3). At the end of the EarlyMiocene during a later collision stage, the Zagros block was thrustover the eastern edge of the Arabian plate (Konyuhov and Maleki,2006).  2.1. Lithologic characterization of the Kazhdumi Formation Middle Cretaceous sedimentation started with a transgression andsealevel rise, whichresulted in thedepositionoftheKazhdumiForma-tion throughout the Albian (Alsharhan and Kendall, 1991). In theIranianoffshore 󿬁 elds,theKazhdumiFormationconsistsofdarkbitumi-nouslimestonewithsubordinateargillaceouslimestoneandcalcareousshale,whichformedmostlyinaneriticenvironment;additionally,somethin sandstone beds may be present (Ghazban, 2009). The Sarvak andDariyan formations overlie and underlie the Kazhdumi Formation, re-spectively.TheBurganandNahrUmrformationsaretheregionalequiv-alents of the Kazhdumi Formation in the Arabian parts of the Persian Fig. 3.  Tectonic scheme of the Persian Gulf region (modi 󿬁 ed after Ziegler (2001), and Konyuhov and Maleki (2006)). 121  Z.S. Mashhadi, A.R. Rabbani / International Journal of Coal Geology 146 (2015) 118 – 144   Table 1 Rock-Eval pyrolysis and vitrinite re 󿬂 ectance data for the Gurpi cutting samples.Field Well Lith. MD (m) Rock-Eval pyrolysis parameters and indices  a Ro (%)TOC S 1  S 1 /TOC S 2  S 1  + S 2  S 3  HI OI PI T max  Mean Num. of readingReshadat CR-10-H3 ShMl 1261 0.44 0.11 0.25 1.13 1.24 1.84 257 418 0.09 423 0.4 70Kish K-1 ShMl 1403 0.53 0.57 1.08 1.23 1.8 1.1 233 208 0.32 435 0.48 68ShMl 1393 0.47 0.54 1.15 2.21 2.75 0.88 470 187 0.20 430ShMl 1381 0.66 0.43 0.65 3.73 4.16 2 565 303 0.10 435K-2 ShMl 1396 0.93 0.16 0.17 1.27 1.43 0.72 137 77 0.11 429ShMl 1426 0.84 0.11 0.13 0.96 1.07 0.66 114 79 0.10 426Sirri-A SIA-DPG-1 Sh 2696 0.8 0.12 0.15 0.7 0.82 1.5 88 188 0.15 426 0.56 55ShMl 2660 1.83 0.77 0.42 6.92 7.69 3.67 378 201 0.10 424ShMl 2650 1.81 0.54 0.3 3.55 4.09 8.21 196 454 0.13Ml 2640 0.68 0.35 0.51 1.43 1.78 3.51 210 516 0.20 413ShMl 2625 0.5 0.12 0.24 1.38 1.5 2.13 276 426 0.08 432ShMl 2605 0.62 0.12 0.19 1.97 2.09 2.16 318 348 0.06 433ShMl 2595 0.47 0.11 0.23 1.33 1.44 1.94 283 413 0.08 433LimMl 2585 0.63 0.1 0.16 1.06 1.16 1.85 168 294 0.09 430Ml 2575 0.5 0.09 0.18 1.06 1.15 2 212 400 0.08 432Ml 2565 0.51 0.1 0.2 0.96 1.06 1.86 188 365 0.09 434Ml 2555 0.5 0.24 0.48 1.22 1.46 1.93 244 386 0.16 433Ml 2535 0.77 0.24 0.31 2.43 2.67 2.1 316 273 0.09 431Ml 2525 0.7 0.2 0.29 1.98 2.18 1.98 283 283 0.09 432Ml 2515 0.69 0.2 0.29 1.88 2.08 1.98 272 287 0.10 431Ml 2503 0.54 0.15 0.28 1.56 1.71 1.96 289 363 0.09 434Ml 2495 0.81 0.18 0.22 1.74 1.92 2.58 215 319 0.09 432Ml 2475 0.98 0.36 0.37 2.25 2.61 2.86 230 292 0.14 431Ml 2440 1.17 0.5 0.43 3.32 3.82 3.81 284 326 0.13 430Ml 2413 0.8 0.26 0.33 2.8 3.06 1.75 350 219 0.08 430Ml 2405 0.84 0.22 0.26 2.71 2.93 1.82 323 217 0.08 432Ml 2395 0.9 0.14 0.16 3 3.14 1.77 333 197 0.04 431Ml 2385 0.99 0.17 0.17 3.13 3.3 1.79 316 181 0.05 431Ml 2375 1.05 0.22 0.21 3.67 3.89 1.96 350 187 0.06 430SIA-1 Ml 1891 1.62 0.41 0.25 3.46 3.87 2.04 214 126 0.11 430Ml 1942 0.62 0.21 0.34 0.58 0.79 2.47 94 398 0.27 434Ml 1980 0.82 0.21 0.26 1.31 1.52 1.24 160 151 0.14 432Sirri-E SIE-E1-P1 ShMl 3523 1.91 2.07 1.08 7.72 9.79 1.58 404 83 0.21 434ShMl 3411 1.95 0.58 0.3 8.39 8.97 10.11 430 518 0.06 434Ml 3353 1.92 0.48 0.25 8.05 8.53 11.56 419 602 0.06 431Sh 3230 0.74 0.12 0.16 1.46 1.58 1.76 197 238 0.08 432Sh 3220 0.68 0.12 0.18 1.46 1.58 1.5 215 221 0.08 432Sh 3210 0.75 0.12 0.16 1.48 1.6 1.78 197 237 0.08 431Sh 3200 0.76 0.11 0.14 1.14 1.25 1.65 150 217 0.09 430Sh 3190 0.71 0.15 0.21 1.19 1.34 2.04 168 287 0.11 430Ml 3175 0.68 0.16 0.24 1.35 1.51 2.46 199 362 0.11 431Ml 3165 0.67 0.13 0.19 1.58 1.71 1.59 236 237 0.08 433Ml 3155 0.83 0.26 0.31 2.34 2.6 1.71 282 206 0.10 435Ml 3145 0.48 0.15 0.31 1.39 1.54 1.47 290 306 0.10 434Ml 3115 0.44 0.12 0.27 1.33 1.45 1.53 302 348 0.08 433Ml 3085 0.6 0.16 0.27 1.76 1.92 1.87 293 312 0.08 433Ml 3045 0.55 0.17 0.31 1.46 1.63 1.5 265 273 0.10 435Sirri-D SID-1 MlSh 2348 0.88 0.23 0.26 1.07 1.3 1.32 122 150 0.18 435MlSh 2345 0.9 0.31 0.35 1.14 1.45 1.43 127 158 0.21 433MlSh 2340 0.78 0.18 0.23 0.75 0.93 1.29 96 165 0.19 430MlSh 2335 0.96 0.29 0.3 1.31 1.6 1.53 136 159 0.18 433Nosrat NT-H1 MlSh 2916 0.77 0.82 1.06 1.56 2.38 1.46 202 189 0.34MlSh 2909 1.04 0.8 0.77 2.04 2.84 2.41 196 232 0.28 427MlSh 2896 0.87 0.91 1.04 2 2.91 1.83 229 211 0.31 428MlSh 2884 1.12 0.99 0.88 2.47 3.46 2.89 221 258 0.29 429MlSh 2877 1.09 1.28 1.17 2.71 3.99 2.12 249 194 0.32 430Sh 2848 1.18 1.76 1.49 3.47 5.23 3.32 294 281 0.34 432ShMl 2838 0.79 0.9 1.14 1.48 2.38 2.01 187 254 0.38ShMl 2831 0.85 0.62 0.73 2.41 3.03 2.74 284 322 0.20 432ShMl 2825 0.88 1.05 1.19 1.86 2.91 2.17 212 247 0.36 432ShMl 2819 0.83 0.83 1 2.41 3.24 2.94 290 354 0.26 432ShMl 2812 0.8 0.75 0.93 1.74 2.49 1.65 218 206 0.30 435ShMl 2805 0.76 0.76 1 1.62 2.38 1.66 213 218 0.32ShMl 2793 0.77 0.49 0.64 2.35 2.84 2.5 305 325 0.17 432ShMl 2782 1.06 1.08 1.02 2.71 3.79 2.02 257 191 0.28 433ShMl 2768 0.89 0.93 1.04 2.97 3.9 3.01 334 338 0.24 433ShMl 2756 1.09 1.46 1.33 2.93 4.39 2.41 270 222 0.33 432ShMl 2749 0.9 0.69 0.77 2.72 3.41 2.37 302 263 0.20 432ShMl 2738 1.01 1.06 1.05 3.22 4.28 3.26 319 323 0.25 428Ml 2701 0.87 0.97 1.11 2.03 3 1.95 233 224 0.32 430Ml 2689 0.79 0.69 0.87 2.53 3.22 2.79 320 353 0.21 431Ml 2677 0.8 0.8 1 2.28 3.08 3.06 285 382 0.26 430Ml 2649 1.23 1.05 0.85 3.9 4.95 2.84 317 231 0.21 432122  Z.S. Mashhadi, A.R. Rabbani / International Journal of Coal Geology 146 (2015) 118 – 144
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