Organic Geochemistry

Organic Geochemistry 42 (2011) 465 476 Contents lists available at ScienceDirect Organic Geochemistry journal homepage: www.elsevier.com/locate/orggeochem Organic geochemical characteristics of crude oils from the Masila Basin, eastern Yemen Mohammed Hail Hakimi a,b,, Wan Hasiah Abdullah a, Mohamed Ragab Shalaby a a Department of Geology, University of Malaya, 50603 Kuala Lumpur, Malaysia b Geology Department, Faculty of Applied Science, Taiz University, 6803 Taiz, Yemen article info abstract Article history: Received 22 January 2011 Received in revised form 2 March 2011 Accepted 13 March 2011 Available online 21 March 2011 The Masila Basin is an important hydrocarbon province in Yemen, but the origin of hydrocarbons and their generation history are not fully understood. In this regard, 10 crude oils from different petroleum reservoir sections in the Masila Basin were characterized by a variety of biomarker and non-biomarker parameters using GC, GC MS and stable carbon isotope techniques. Oils from the Masila Basin display pristane/phytane (Pr/Ph) ratios ranging from 1.7 to 2.0, low sulfur content, high C 35 homohopane index, relatively high C 27 sterane concentrations and relatively high tricyclic terpanes suggesting a marine clay source rock that was deposited in mildly anoxic to suboxic conditions with dominantly algal organic matter. C 29 20S/(20S + 20R) steranes and bb/(bb + aa) sterane ratios indicate that the Masila oils have reached peak oil window maturity. Another related feature of these oils is the absence of 18a (H)-oleanane, which suggests a source age older than Cretaceous. The carbon isotope compositions are similar to those of the potential source rocks, which range from 25.4 to 28.3, indicating a marine environment. The new data presented in this paper suggest that the Masila oils constitute one oil family and that the oil originated from the Upper Jurassic Madbi source rock in the basin. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Sedimentary organic matter and crude oils contain complex assemblages of biological marker compounds (biomarkers) that preserve the molecular structure of various compounds that constitute the organisms. Biomarkers are widely used in the petroleum industry to identify groups of genetically related oils, to correlate oils with source rocks and to describe the probable source rock depositional environments for migrated oil of uncertain origin (e.g., Moldowan et al., 1985; Peters and Moldowan, 1993; Peters et al., 2005). Many oilfields have been discovered in the Mesozoic Masila Basin, eastern Yemen, since oil was first discovered in the late 1990s. The majority of these oilfields are located in the central part of the basin (Fig. 1), but several are located north and south. Production from many wells in the basin is declining and some wells have been abandoned due to increasing water cuts. Despite the relatively long history of oil exploration and production, several important issues regarding aspects of the Masila Basin petroleum system still remain unanswered. For example, no oil source rock correlations have been published and geochemical studies reported on the Masila Basin are relatively few. Most published work Corresponding author at: Department of Geology, University of Malaya, 50603 Kuala Lumpur, Malaysia. E-mail address: ibnalhakimi@yahoo.com (M.H. Hakimi). address general characteristics of source rock (e.g., Hakimi et al., 2010) and very few evaluate the biomarker distributions (e.g., Hakimi et al., in press). We report the results of an investigation on crude oils and the potential source rock from this basin. The objective was to use biomarker distributions together with the bulk geochemical parameters to characterize the oil types and to assess the respective depositional environment, age and thermal maturity of their potential source rocks. Furthermore, the molecular composition results from both the oil and source rock samples allowed for an oil source rock correlation. 2. Geological background The Masila Basin is situated in eastern central Yemen and is one of the most productive sedimentary basins in the Republic of Yemen (Fig. 1). The stratigraphic section in the Masila Basin ranges in age from Proterozoic to Tertiary and can be subdivided into three megasequences: pre-rift (Proterozoic to mid-jurassic), synrift (mid-jurassic to earliest Cretaceous) and post-rift (earliest Cretaceous and Tertiary; Fig. 2). Mesozoic and Cenozoic units are widely exposed in the Masila Basin. The Jurassic and Lower Cretaceous strata in Yemen regionally, and locally at Masila Basin, reflect post-pangea breakup and basin creation formed by rifting (Beydoun et al., 1993, 1996; Redfern and Jones, 1995; Bosence, 1997; Beydoun and Al-Saruri, 1998). Rifting caused a series of NW and EW trending major basin-bounding faults, adjacent to 0146-6380/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2011.03.015

466 M.H. Hakimi et al. / Organic Geochemistry 42 (2011) 465 476 Fig. 1. Main sedimentary basins map in Yemen showing location of the Masila Basin and the study area. which are three main Jurassic-Cretaceous rift graben basins: the Sab atayn Basin (also known as the Marib-Shabwa-Hajar Basin), the Say un-masila Basin (also known as the Masila Basin), and the Jiza -Qamar Basin (Beydoun et al., 1998). To date, only Sab atayn and Masila basins contain proven commercial quantities of oil (Beydoun, 1991). Overprinted on the Jurassic rift is reworking and reactivation caused by the Tertiary rifting event in the Gulf of Aden to the south, where the Horn of Africa separated from the Arabian Plate (Redfern and Jones, 1995). The Jurassic units comprise clastic, carbonate and minor argillaceous sediments. Syn-rift sections of the Madbi and Naifa formations were deposited during the Jurassic in marine settings in the structurally lowest areas (Smewing, 1997; Smewing et al., 1998). The Madbi Formation is composed of porous lime grainstone to argillaceous lime mudstone (Beydoun et al., 1998). The lithofacies of this unit reflects an open marine environment. The upper part of Madbi Formation is composed of laminated organic rich shale, which is classified to be a prolific source rock in the Masila Basin (Mills, 1992; King et al., 1990; Hakimi et al., 2010). The Upper Jurassic Madbi sediments are characterized by high total organic carbon content of 1 8 wt% and have very good to excellent hydrocarbon potential (Hakimi et al., 2010). Organic matter is mainly algal Type II with minor Type I. The Upper Jurassic source rock has variable maturities across the basin ranging from early to peak maturity based on vitrinite reflectance measurements (Ro 0.69 0.91%; Hakimi et al., 2010). the Masila Basin. The geographic locations of the oilfields chosen are shown in Fig. 1. Stable carbon isotopic compositions (d 13 C) of bulk fractions (saturated and aromatic) have been used to determine whether a genetic relationship exists between crude oils and potential source rocks (e.g., Summons et al., 1992; Boreham et al., 2001). This analysis was conducted at Total Scientific and Technical Center (Total, 1993 1996 personal communication ). In this study, 10 oils and five Madbi Shale source rock samples were selected for further geochemical analyses. This source rock was studied by Hakimi et al. (2010). Samples were extracted using a Soxhlet apparatus for 72 h using an azeotropic mixture of dichloromethane and methanol (93:7 v:v). The samples of extracts and crude oils were fractionated into saturated and aromatic hydrocarbons and polar fractions. The saturated hydrocarbon fractions of the oils and source rock were then analysed by gas chromatography (GC) and gas chromatography mass spectrometry (GC MS). A FID gas chromatograph with HP-5MS column, helium carrier gas was used. A temperature program from 40 300 C at a rate of 4 C/min and then held for 30 min at 300 C was used for GC analysis. GC MS experiments were performed on a V 5975B inert MSD mass spectrometer with a gas chromatograph attached directly to the ion source (70 ev ionization voltage, 100 ma filament emission current, 230 C interface temperature). 4. Results and discussion 3. Samples and experimental The materials used in this study include 10 crude oil samples representing different petroleum reservoirs and drill cuttings samples selected from intervals of Madbi organic rich shale in 4.1. Bulk properties of crude oils The bulk crude oil properties and compositions for the studied oils are presented in Table 1. The similar range of bulk property values of the crude oils analysed indicates that only one oil type

M.H. Hakimi et al. / Organic Geochemistry 42 (2011) 465 476 467 Fig. 2. Regional stratigraphic nomenclature, Masila Basin, Republic of Yemen (modified after Beydoun et al., 1998; King et al., 1990). is present. Crude oils from the Masila Basin have high API gravity (30 41 ) (Table 1). Low API gravity is generally associated with either biodegraded oils or with immature sulfur rich oils (Baskin and Peters, 1992) as shown in Fig. 3. Sulfur content reflects the type of organic input and its depositional environment (Gransch et al., 1973; Moldowan et al., 1985). Carbonate source rocks deposited in a marine environment generally have high sulfur contents, whereas source rocks deposited in siliciclastic environment usually have low sulfur contents (Gransch et al., 1973). In this study, Masila Basin oils have low sulfur contents ranging from 0.25 to 0.45 wt% suggesting the generation from clay rich marine source rocks. The relative abundance of saturated hydrocarbons suggests that the oils are predominantly aliphatic. In addition, all the studied oils have saturated/aromatic ratios >1 and there is no sign of biodegradation among the samples (Table 1). Calculated ratio of P (C21 C 31 )/ P (C 15 C 20 ) indicate a low degree of waxiness (Table 1). The degree of waxiness is used to categorize the amount of land derived organic material in oil, assuming that terrigenous material contributes a high molecular weight normal paraffin component to the oil (Hedberg, 1968; Connan and Cassou, 1980).

468 M.H. Hakimi et al. / Organic Geochemistry 42 (2011) 465 476 Table 1 Bulk organic geochemical of crude oil samples from Masila Basin. Oilfields Crude Oils Reservoir age API gravity ( o ) Sulfur (wt%) Saturate HC (%) Aromatic HC (%) Resins + Asphaltenes (%) Saturate/ aromatic KHARIR-1 KCO1 Middle Jurassic 40.9 0.25 45.63 27.85 26.52 1.64 0.64 KHARIR-2 UCO1 L. Cretaceous 35.7 0.30 51.59 26.39 22.02 1.95 0.71 UCO2 L. Cretaceous 32.9 0.31 51.44 25.83 22.73 1.99 0.72 UCO3 L. Cretaceous 31.1 0.33 49.45 25.63 24.92 1.93 0.65 UCO4 L. Cretaceous 30.0 0.35 40.67 30.20 29.13 1.35 0.64 KHARIR-3 LCO1 L. Cretaceous 34.0 0.35 36.13 16.17 47.70 2.23 0.68 LCO2 L. Cretaceous 34.0 0.32 51.49 32.02 16.49 1.61 0.73 SCO1 L. Cretaceous 33.8 0.40 37.88 15.41 46.71 2.46 0.73 WADI TARIBAH BCO1 Pre-Cambrian (Basement) 38.6 0.45 54.32 28.57 17.11 1.90 0.64 SUNAH UCO5 L. Cretaceous 30.0 0.38 31.78 24.79 43.43 1.28 0.75 P ðc21! C 31 Þ= P ðc15! C 20 Þ Fig. 3. Plot of the API gravity versus the sulfur content (wt%) for crude oils from various reservoir rocks in the Masila Basin, showing good positive reverse correlation. Fig. 4. Relationship between sulfur content (wt%) and degree of the waxiness for Masila Basin crude oils.

M.H. Hakimi et al. / Organic Geochemistry 42 (2011) 465 476 469 Isotope analysis of organic carbon has been used to classify the environments of oils and source rock as marine or non-marine (terrigenous) by plotting the bulk values of the d 13 C saturated fractions versus those of their aromatic fractions (Sofer, 1984; Collister and Wavrek, 1996). The carbon isotope analysis was performed on crude oils and potential Madbi source rock from the Masila Basin and the results plotted on a bulk fraction isotope cross plot (Fig. 5). The d 13 C values of the saturated and aromatic hydrocarbon fractions range from 28.7 to 29.8 and 28.1 to 28.6, respectively, indicating a marine origin (Fig. 5). This is further supported by the biomarker environment indicators (Table 2). 4.3. Biomarker distributions Fig. 5. Plot of the d 13 C values of aromatic fractions versus of the d 13 C values of saturated fractions for oils samples, and source rock samples in the Masila Basin (Total, 1993 1996; personal communications ). The line represents the best fit separation for waxy and non-waxy oils and is described by the equation d 13 C Aromatic = 1.14 d 13 C saturated + 5.46 (after Sofer, 1984). The correlation between the degree of waxiness and sulfur content is shown in Fig. 4. The distribution of the points reflects the similarity of the oils, which have low sulfur contents and low degree of waxiness suggesting a marine origin (Fig. 4). 4.2. Carbon isotope composition 4.3.1. n-alkanes and isoprenoids Gas chromatograms of saturated fractions from representative oil samples are shown in Fig. 6b. The chromatograms indicate that saturated hydrocarbons are dominated by C 13 C 35 n-alkanes and isoprenoids pristane (Pr) and phytane (Ph). The presence of n-alkanes as the most abundant constituents (extending to C 35 ) is indicative of no or low levels of biodegradation (Volkman et al., 1984). The similarity in the distribution patterns of n-alkanes suggests that the studied oils are derived from one source and that no biodegradation has occurred. The n-alkanes show a unimodal distribution and abundant low to medium molecular weight compounds (n-c 14 n-c 18 ), suggesting a significant contribution of algal derived organic matter from a marine environment (Brooks et al., 1969; Tissot et al., 1978; Ebukanson and Kinghorn, 1986; Murray and Boreham, 1992). Acyclic isoprenoids pristane and phytane are present in all studied samples. Diagnostic biomarker ratios highlight similarities in the oil types are listed in Table 2. The pristane/phytane (Pr/Ph) ratio is one of the most commonly used geochemical parameters and has been widely invoked as an indicator of the redox conditions in the depositional environment and source of organic matter (Powell and McKirdy, 1973; Didyk et al., 1978; Tissot and Welte, 1984; Chandra et al., 1994; Large and Gize, 1996). Organic matter originating predominantly from land plants would be expected to contain high Pr/Ph > 3.0 (oxidizing conditions), low values of (Pr/Ph) ratio (<0.6) indicate anoxic conditions and values between 1.0 and 3.0 suggest intermediate conditions (suboxic conditions) (Powell, 1988; Amane and Hideki, 1997). The Pr/Ph ratios of oil samples from the Masila Basin range over 1.7 2.0 (Table 2), suggesting that these oils derived from source rocks deposited in marine under suboxic conditions (Amane and Hideki, 1997; Sarmiento and Rangel, 2004; Basent et al., 2005). In addition, redox potential of the source sediments inferred from the Pr/Ph ratios correlate well with the sulfur content and degree of waxiness. The graphical representation between Pr/Ph and sulfur content (Fig. 7) classifies the oils into one oil type generated from marine organic matter and the relationship between Pr/Ph and degree of waxiness gives the same interpretation (Fig. 8). Table 2 Summary of biomarker parameters of the crude oil samples from Masila Basin. Oilfields Samples ID n-alkanes and isoprenoids Triterpanes and terpanes (m/z 191) Steranes (m/z 217) Pr/Ph Pr/C 17 Ph/C 18 CPI C 29 /C 30 H index 22S/(22S + 22R)C 32 20S/(20S + 20R) bb(bb + aa) C 29 /C 27 Regular steranes (%) Dia Reg:ster: C 27 C 28 C 29 KHARIR-1 KCO1 1.7 0.57 0.35 0.96 0.34 3.94 0.62 0.50 0.58 1.2 41 19 40 1.5 KHARIR-2 UCO1 1.9 0.59 0.37 1.00 0.38 4.78 0.61 0.50 0.60 0.8 41 20 39 1.2 UCO2 1.9 0.62 0.38 0.97 0.34 5.53 0.61 0.54 0.62 0.8 41 17 42 1.3 UCO3 1.6 0.61 0.48 1.01 0.38 4.64 0.62 0.50 0.59 0.8 42 17 41 1.4 UCO4 2.0 0.63 0.38 1.00 KHARIR-3 LCO1 1.9 0.58 0.35 1.01 0.40 4.97 0.58 0.54 0.61 0.7 42 18 40 1.4 LCO2 2.0 0.64 0.38 0.98 0.35 4.70 0.58 0.52 0.60 1.0 40 18 42 1.2 SCO1 1.9 0.63 0.38 1.00 0.40 4.94 0.56 0.50 0.63 1.0 41 18 41 1.3 WADI TARIBAH BCO1 1.8 0.58 0.35 0.99 SUNAH UCO5 1.9 0.83 0.52 0.98 0.35 5.53 0.60 0.50 0.62 0.9 40 20 40 1.4 H index = Homohopane index [{C35 17a(H), 21 h(h)-30-pentahomohopane (22S + 22R)}/{C31 C35 17a(H), 21 h(h)-30-homohopanes (22S + 22R)}] 100. Dia. = Diasterane; Reg. ster. = Regular sterane.

470 M.H. Hakimi et al. / Organic Geochemistry 42 (2011) 465 476 Fig. 6. (A) Gas chromatograms for extracts of source rocks included in this study. (B) Gas chromatography traces for representative oil samples. The conclusion drawn above with respect to source rock deposition is further supported by analysis of the polycyclic saturated hydrocarbon biomarkers. 4.3.2. Terpane biomarkers Together with steranes, triterpanes are the most important petroleum hydrocarbons that retain the characteristic structure

M.H. Hakimi et al. / Organic Geochemistry 42 (2011) 465 476 471 Fig. 7. Relationship between sulfur content (wt%) and Pr/Ph ratio for studied crude oils from Masila Basin. Fig. 8. Relationship between degree of waxiness and Pr/Ph ratio for studied crude oils from Masila Basin. of the original biological compounds. Tricyclics, tetracyclics, hopanes and other compounds contribute to the terpane fingerprint (mass chromatogram m/z 191) commonly used to relate oils and source rocks (Seifert and Moldowan, 1979; Peters et al., 2005). The assignment of the peaks of steranes and triterpanes labelled in Figs. 9 and 10 are listed in Table 3. Individual components were identified by comparison of their retention times and mass spectra with published data (Philp, 1985; Peters and Moldowan, 1993). m/z 191 fragmentograms were used to detect the presence of triterpanes in the saturated hydrocarbon fractions of representative oil samples (Fig. 9b). The most distinct feature of Masila crude oils are that the relative abundance of the C 29 hopane is generally half, or less than that of the C 30 hopane (Fig. 9b) and C 29 /C 30 17a (H) hopane ratio (<1) suggests that the oil derived from clay rich, shale source rocks (Gürgey, 1999). Extended hopanes are dominated by the C 31 homohopane and decrease towards the C 35 homohopane (Fig. 9b). It shows elevated homohopane index (Table 2). This is thought to indicate that the oil was derived from marine source rock deposited under suboxic conditions (Peters and Moldowan, 1991). Tricyclic terpanes are generally below detection levels in oils derived from terrigenous organic material and normally associated with a marine source (Aquino Neto et al., 1983; Philp, 1985). Crude oil samples from Masila Basin have relatively high tricyclic terpane. The relatively high abundance of the C 23 tricyclic and low abundance of the C 24 tetracyclic supports the high contribution from marine material to the source rocks (Zumberge, 1987; Burwood et al., 1992; Hanson et al., 2000). 4.3.3. Sterane biomarkers The distribution of steranes is best studied by monitoring the ion m/z 217 which is the characteristic fragment in the sterane series. The resulting mass chromatograms for representative

472 M.H. Hakimi et al. / Organic Geochemistry 42 (2011) 465 476 Fig. 9. (A) m/z 191 mass-chromatograms for source rock samples. (B) m/z 191 traces for representative oil samples. samples are shown in Fig. 10. It is suggested that the relative amounts of C 27 C 29 steranes can be used to indicate depositional setting (Philp, 1985). The most abundant tetracyclic components in Masila crude oils are the C 27 steranes (Table 2), which suggests a dominance of marine organic matter. The occurrence of diasteranes in crude oils and source rocks has been ascribed to clay catalyzed rearrangement reactions (Rubinstein et al., 1975; Sieskind et al., 1979; Peakman and Maxwell, 1988; de

M.H. Hakimi et al. / Organic Geochemistry 42 (2011) 465 476 473 Fig. 10. (A) m/z 217 mass chromatograms for source rock samples. (B) Sterane ternary plot for oils, and Upper Jurassic Madbi source rock samples. (C) m/z 217 traces for representative oil samples. Leeuw et al., 1989). Thus, high diasterane concentrations, relative to regular steranes, can be expected to occur in clay rich source rocks and their hydrocarbon derivatives (Gürgey, 1999) and low ratios in oils indicate clay poor, carbonate source rocks deposited under anoxic conditions (McKirdy et al., 1983). Masila crude oils exhibit high diasterane/sterane ratios (Table 2), which suggests

474 M.H. Hakimi et al. / Organic Geochemistry 42 (2011) 465 476 Table 3 Peak assignments for alkane hydrocarbons (I) m/z 191 mass chromatograms and (II) m/z 217 mass chromatograms. Compound Abbreviation (I) Peak No. T s 18a(H),22,29,30-trisnorneohopane T s T m 17a(H),22,29,30-trisnorhopane T m 29 17a,21b(H)-nor-hopane C 29 hop 30 17a,21b(H)-hopane hopane 3M 17 b,21a (H)-moretane C 30 Mor 31S 17a,21b(H)-homohopane (22S) C 31 (22S) 31R 17a,21b(H)-homohopane (22R) C 31 (22R) 32S 17a,21b(H)-homohopane (22S) C 32 (22S) 32R 17a,21b(H)-homohopane (22R) C 32 (22R) 33S 17a,21b(H)-homohopane (22S) C 33 (22S) 33R 17a,21b(H)-homohopane (22R) C 33 (22R) 34S 17a,21b(H)-homohopane (22S) C 34 (22S) 34R 17a,21b(H)-homohopane (22R) C 34 (22R) 35S 17a,21b(H)-homohopane (22S) C 35 (22S) 35R 17a,21b(H)-homohopane (22R) C 35 (22R) (II) Peak No. a 13b,17a(H)-diasteranes 20S Diasteranes b 13b,17a(H)-diasteranes 20R Diasteranes c 13a,17b(H)-diasteranes 20S Diasteranes d 13a,17b(H)-diasteranes 20R Diasteranes e 5a,14a(H), 17a(H)-steranes 20S aaa20s f 5a,14b(H), 17b(H)-steranes 20R abb20r g 5a,14b(H), 17b(H)-steranes 20S abb20s h 5a,14a(H), 17a(H)-steranes 20R aaa20s 4.5. Oil source rock correlation To investigate the genetic link between the studied oils and potential source rocks, we analysed extracts from Upper Jurassic shale by GC and GC MS using the experimental conditions described above. Analytical results are presented in Figs. 6a, 9a and 10a. Overall, the oil data closely match the Upper Jurassic source rock data. Key factors include biomarker parameters and carbon isotope values (Table 4) and the similar positions on the sterane ternary plot (Fig. 10B). The Upper Jurassic Madbi samples have Ph/Ph ratios range from 1.89 2.20, abundance of C 27 steranes, C 29 /C 27 sterane ratios range of 0.91 1.20 and diasterane/sterane ratios relatively high indicating a marine clay source rock and suboxic conditions. These rocks also have C 29 sterane 20S/(20S + 20R) and bb/(bb + aa) ratios of 0.45 0.48 and 0.50 0.55, respectively. These data for the Upper Jurassic Madbi source rock in Masila Basin are relatively consistent with observations for the Masila oils described above and strongly suggest that these oils are derived from the Upper Jurassic source rock. This conclusion is further supported by the additional plots (Fig. 11). The 20S/(20S + 20R) and bb/(bb + aa) C 29 sterane plot indicates that the oil and Madbi source rock samples are thermally mature and the oil window has been reached. Therefore the Upper Jurassic Madbi source for the crude oil can be suggested (Fig. 11A). This is consistent with clay rich marine source rock deposited in anoxic to suboxic conditions with dominant algal organic matter (Fig. 11B). clay rich source rocks for these oils. In addition, these oils have 20S/(20S + 20R) regular steranes and bb/(bb + aa) for C 29 in the range of 0.50 0.54 and 0.58 0.63, respectively, suggesting that these oils are thermally mature and that the oil window has been reached (Table 2). 4.4. Summary of oil characterization There are no differences in the oil samples based on the biomarker results. All oils are grouped into one genetic family. All the oil samples contain mainly algal organic matter. The oil data generally point to a source rock that was deposited in a weakly reducing or suboxic setting. Oil samples have high diasterane concentrations relative to regular steranes (Fig. 10c), suggesting that clays were abundant in the source rock that generated the oils. All oil samples have no 18a(H)-oleanane suggesting a source age older than Cretaceous. All the oil samples cluster tightly on a sterane ratio ternary diagram (Fig. 10B), strongly suggesting that these oils are derived from one source. 5. Conclusions Using the GC and GC MS techniques made it possible to arrive at a clear characterization and classification of Masila Basin crude oils according to their sources. This has been achieved from the acyclic isoprenoid, terpane and sterane biomarkers. Geochemical parameters based upon these components coupled with the bulk geochemical parameters indicate that there is one oil family represented in the suite of analysed samples. These oil samples were derived from a source rock that was deposited in slightly anoxic to suboxic conditions. The source rock contained marine algal organic matter. The oil samples are indicative of a thermal maturity stage that is within the oil window. The best source rocks in the basin, with regards to liquid hydrocarbon generation, are the marine shales of the Upper Jurassic Madbi Formation. This source rock is near the early to peak stages of the oil window. Oil samples correlate with the Upper Jurassic shale source rocks, so exploration strategies should focus on the known location of Upper Jurassic Madbi strata for predicting the source kitchen. Table 4 Summary of typical geochemical parameter distributions for the Masila crude oils and potential source rock extracts. Samples Reservoir/source rock age CPI Pr/Ph Pr/C 17 Ph/C 18 C 29 /C 27 sterane MASILA OILS MASILA SOURCE ROCK Per-cambrian to lower cretaceous Upper Jurassic (5) CPI = Carbon Preference Index. Pr = Pristane. Ph = Phytane. Dia. = Diasterane; Reg. ster. = Regular sterane. Number of data analysed from this study. +Number of data from Total Oil Company (1993 1996). Same samples from Hakimi et al. (2010). C 27 (%) C 28 (%) C 29 (%) Dia Reg:ster: d 13 C values ( ) 0.96 1.01 1.7 2.0 0.57 0.83 0.35 0.52 0.7 1.2 39 47 15 21 35 46 1.2 1.5 28.6 to (10) (10) (10) (10) (8) (8) (8) (8) (8) 29.8 (9)+ SK1309 0.99 2.20 1.37 0.68 1.00 40 20 40 0.9 25.7 to SK2105 1.01 1.89 1.10 0.83 1.00 40 18 42 1.1 28.9 (9)+ SK12710 1.06 2.04 1.00 0.54 0.91 42 20 38 0.8 SK3083 1.06 1.92 1.24 0.60 1.20 40 18 42 0.9 SK2165 1.00 1.96 1.20 0.50 1.20 41 20 39 0.9

M.H. Hakimi et al. / Organic Geochemistry 42 (2011) 465 476 475 Fig. 11. (A) Pristane/phytane versus C 29 /C 27 steranes plot for the oils, and Upper Jurassic source rock extracts. (B) Sterane maturation parameters for the oils, and Upper Jurassic source rock extracts. Acknowledgements The authors wish to acknowledge the assistance of PEPA (Petroleum Exploration and Production Authority) and Total Oil Company, Yemen, for supplying the data and samples and University of Taiz for financial support. The authors are more grateful to the Department of Geology, University of Malaya for providing facilities to complete this research. Special thanks are offered to Peter Abolins for his helpful comments on an earlier version of the manuscript. We thank Paul Mankiewicz and an anonymous reviewer who provided useful comments that improved the revised manuscript. Associate Editor Kenneth E Peters References Amane, W., Hideki, N., 1997. Geochemical characteristics of terrigenous and marine sourced oils in Hokkaido, Japan. Organic Geochemistry 28, 27 41. Aquino Neto, F.R., Trendel, J.M., Restle, A., Connan, J., Albrecht, P.A., 1983. Occurrence and formation of tricyclic and tetracyclic terpanes in sediments and petroleums. In: Bjorøy, M., et al. (Eds.), Advances in Organic Geochemistry, 1981. John Wiley and Sons, pp. 659 667. Basent, G.G., Rajendra, S.B., Ashok, K.B., Dinesh, K., Kusum, L.P., Adarsh, K.M., Jagdish, P.G., Gaur, C.D., Nizhat, J.T., 2005. Geochemical characterization and source investigation of oils discovered in Khoraghat Nambar structures of the Assam Arakan Basin, India. Organic Geochemistry 36, 161 181. Baskin, D.K., Peters, K.E., 1992. Early generation and characteristics of a sulfur-rich Monterey kerogen. American Association of Petroleum Geologists Bulletin 76, 1 13. Beydoun, Z.R., Al-Saruri, M.A.L., 1998. The Phanerozoic depositional basins and inter-basinal highs of Yemen: their structural framework and sedimentary cover. Zeitschrift fur Geologische Wissenschaften 26, 517 529. Beydoun, Z.R., Al-Saruri, M., El-Nakhal, H., Al-Ganad, I.N., Baraba, R.S., Nani, A.S.O., Al-Aawah, M.H., 1998. International lexicon of stratigraphy, Volume III, Republic of Yemen, Second Edition. International Union of Geological Sciences and Ministry of Oil and Mineral Resources, Republic of Yemen, Publication 34, 245 p. Beydoun, Z.R., Al-Saruri, M.A.L., Baraba, R.S., 1996. Sedimentary basins of the Republic of Yemen: their structural evolution and geological characteristics. Revue de l Institute Français du Pétrole 51, 763 775. Beydoun, Z.R., Barmahmoud, M.O., Nani, S.O., 1993. The Qishn Formation, Yemen, lithofacies and hydrocarbon habitat. Marine and Petroleum Geology 10, 364 372.

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