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Marine and Petroleum Geology 61 (2015) 1e13

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Marine and Petroleum Geology

journal homepage: www.elsevier .com/locate/marpetgeo

Research paper

Modelling petroleum generation of Late Cretaceous Dabut Formationin the Jiza-Qamar Basin, Eastern Yemen

Mohammed Hail Hakimi a, *, Wan Hasiah Abdullah b

a Geology Department, Faculty of Applied Science, Taiz University, 6803 Taiz, Yemenb Department of Geology, University of Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e i n f o

Article history:Received 25 April 2014Received in revised form13 June 2014Accepted 26 November 2014Available online 11 December 2014

Keywords:Dabut source rocksHeat flowPetroleum modellingJiza-Qamar Basin

* Corresponding author.E-mail address: [email protected] (M.H. Ha

http://dx.doi.org/10.1016/j.marpetgeo.2014.11.0040264-8172/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

The Jiza-Qamar Basin in eastern Yemen is still undergoing hydrocarbon exploration and hydrocarbonpotential has not been assigned yet. In this study, subsurface samples from two onshore and offshorewells were collected to characterize the source rocks of Dabut Formation and to incorporate into basinmodelling in order to know and determine the timing of petroleum generation and expulsion of theDabut source rocks. Based on organic geochemical analysis, the Dabut sediments have variable TOCcontent in the range of 0.5e2.5 wt%, indicating fair to good source rock generative potential. The organicmatter of Dabut source rock is dominated by Type III kerogen with minor mixed IIIeII kerogen contri-butions, and is thus considered to be mainly gas-prone and some oil-prone. This is supported by HIvalues in the range of 50e212 mg HC/g TOC. Vitrinite reflectance in the range of 0.63e0.88%Ro andpyrolysis Tmax in the range of 427e454 �C, generally indicate that the Dabut source rocks contain matureorganic matter.

One-dimensional basin modelling was performed to analyse the hydrocarbon generation and expul-sion history of the Dabut source rocks in the Jiza-Qamar Basin based on the reconstruction of the burialand thermal maturity histories in order to improve our understanding of the hydrocarbon generationpotential. Calibration of the model with measured vitrinite reflectance (%Ro) and borehole temperature(BHT) data indicates that the paleo-heat-flow was constant during the Late CretaceousePaleogene butincreased during the Late PaleogeneeNeogene and then decreased exponentially from Neogene topresent-day.

The modelled maturity history predicts that the oil generation (0.5% Ro) in the Dabut source rockbegan from about 56 Ma to 42 Ma and the peak hydrocarbon generation (0.86% Ro) occurred approxi-mately from 14 Ma to 13 Ma. These models also indicate that the main phase of oil generation was at56 Ma to 25 Ma, whereas the main phase of gas generation began from about 25 Ma to 14 Ma and stillgenerated in the present-day.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The study area lies in the Jiza-Qamar Basin in the east of Yemenfocussing on the Qamar sector (Fig. 1). The Jiza-Qamar Basin isundergoing hydrocarbon exploration and research since the sig-nificant hydrocarbon potential still poorly known. Of the 5exploratory wells drilled in the Qamar sector of the Jiza-QamarBasin by the Nimir Agip Oil Companies, minor oil shows wereencountered in only two of the offshore wells. The poor knowledge

kimi).

of the evolution of the subsurface rocks in the Jiza-Qamar Basin,especially with respect to the characteristics and the depositionalconditions of the organic matter within the potential source rocksmay have been responsible for the unsuccessful exploration at-tempts within the basin. In this regards, the current study re-evaluates the source rock characteristics of the Late CretaceousDabut sediments in the Jiza-Qamar Basin, including the drilling oftwo offshore and onshore exploration wells (Wells 16/G-1 and 16/U-1, respectively) and acquisition of a comprehensive suite ofgeochemical data and incorporated into basin modelling, has pro-vided a large volume of new data. Although, several studies hadbeen undertaken on the Dabut source rocks (Alaug, 2011a,b),detailed modelling of petroleum generation and expulsion of Late

Figure 1. Main sedimentary basins in Republic of Yemen (modified after Beydoun et al., 1998) showing location map of the Jiza-Qamar Basin and the studied wells.

M.H. Hakimi, W.H. Abdullah / Marine and Petroleum Geology 61 (2015) 1e132

Cretaceous Dabut source rocks in the Jiza-Qamar Basin are scarce ormissing. Most of the previous studies have established the pre-dominantly gas- and oil-prone nature of the potential Dabut sourcerocks, but without adequate estimation of the timing generationand expulsion of the potential source rocks. However, these sourcerocks are yet to be studied in detail and their thermal maturity andburial histories, and the timing of petroleum generation andexpulsion have not been conducted yet. The basinmodelling resultsdemonstrate that the evolution of the petroleum generation andexpulsion of the Dabut source rocks is more complex than the'reconnaissance' studies published to date (e.g., Alaug, 2011a,b)might suggest. This study also aimed at re-appraising and vali-dating the potentials of the formation as an effective source rock,and hence ultimately provides further insight into the geology ofthe basin, for future petroleum exploration and resource assess-ment in the region.

2. Geologic setting

The Jiza-Qamar Basin is situated in eastern Yemen withextending into Oman and is one of the Mesozoic sedimentarybasins of Yemen (Fig. 1). The basin was formed as a rift-basinlinked to the Mesozoic breakup of Gondwanaland and the evolu-tion of the Indian Ocean during the Jurassic and Cretaceous(Redfern and Jones, 1995). The main stratigraphic succession of theJiza-Qamar Basin is presented in Figure 2, and is dominated by athick Mesozoic succession ranging in age from Jurassic to Paleo-gene. Pre-rift megasequences range in age from Proterozoic tomid-Jurassic. Rifting in the Mesozoic basins of Yemen began in theLate Jurassic (Redfern and Jones, 1995) and these basins rapidlyfilled with thick mixed clastic and carbonate deposits during theLate Jurassic and Early Cretaceous (Madbi, Naifa and SaarFormations).

Sedimentation was initiated during the mid-Jurassic, producingthe Kuhlan and Shuqra Formations (Fig. 2). The Kuhlan Formationrepresents a basal continental sandstone that grades up in themarine carbonates that comprise the Shuqra Formation. During theKimmeridgian, syn-rift sediments of the Madbi Formation weredeposited (Beydoun et al., 1998). This formation is composed oforganic-rich shales and limestones, which reflect an open marineenvironment (Beydoun et al., 1998). During latest Jurassic to EarlyCretaceous time, the rifting system of the Jiza-Qamar Basincontinued, but the subsidence became slower. It was accompaniedby the accumulation of carbonates in shallow-marine shelf deposits(Naifa Formation). Thick Early Cretaceous syn-rift carbonates andclastics of the Saar Formation were deposited within the rift whilstthin carbonates were deposited outside the basin margins (Elliset al., 1996).

The Late Cretaceous sedimentary fill of the Jiza-Qamar Basin isconsiderably thicker than in other basins of Yemen (Brannan et al.,1997). In the earliest Cretaceous to Late Cretaceous, post-rift sedi-ments accumulated in the basin producing Qishn, Fartaq, Mukalla,Fiqa and Sharwyn Formations. The Qishn Formation can be dividedinto two members, the Qishn Clastic Member and the Qishn Car-bonate Member (Beydoun et al., 1998). The Qishn Clastic Member isthe main reservoir rock for some of the oilfields in the Masila Basin(King et al., 2003; Hakimi et al., 2012) to the south. The QishnClastic consists mainly of sandstones, with shale and minor car-bonate interbeds, deposited in braided river channels, shorefaceand shallow-marine settings as predominantly post-rift sediments.The Qishn Carbonate Member was deposited in deep water underalternating open and closed marine conditions (Beydoun et al.,1998). The Fartaq Formation is composed of light grey slightlydolomitized limestones with intercalated mudstones (Fig. 2). TheMukalla Formation is composed of white to light grey, compacted,fine grained sandstones. These sandstones are intercalated with

Figure 2. Regional stratigraphy of the Jiza-Qamar Basin, Eastern Yemen.

M.H. Hakimi, W.H. Abdullah / Marine and Petroleum Geology 61 (2015) 1e13 3

grey siltstones, greenish grey to reddish brown mudstones withcoal layers, and carbonaceous shales (Fig. 3). The coals, shales andcarbonaceous shales of the Late Cretaceous Mukalla are consideredto be the important prolific oil and gas prone source rocks in thebasin (Alaug, 2011a,b; Hakimi and Abdullah, 2014; Hakimi et al.,2014). The Dabut Formation conformably overlies the MukallaFormation (Fig. 2). The Dabut Formation was deposited during LateCampanian time, and is mainly composed of limestone and sand-stones with shales (Brannan et al., 1997). The Sharwyn Formation iscomposed of limestones and marls that contain a shallow marinefauna and marks the resumption of carbonate deposition in the

basin (Brannan et al., 1997). The formation extends into southernOman, where it reaches a maximum thickness of 30 m (Roger et al.,1989). At the end of the Cretaceous sequence, a marine trans-gression flooded the whole area (Hadramaut Group), establishingwidespread shallow marine carbonate deposition. The Late Paleo-ceneeEarly Eocene units are composed of homogeneous argilla-ceous, detrital carbonates and hard, compacted, massive andbedded dolomitized fossiliferous limestone (Umm Er-RadhumaFormation) that transition to shallow marine carbonaterocks (Jeza Formation). The Jeza deposits are widespread in theMiddle Eocene, and are followed by the deposition of anhydrite

Figure 3. Simplified stratigraphy of the Late Cretaceous Mukalla Formation and the overlying Dabut Formation in the studied wells, Jiza-Qamar Basin.


beds (Rus Formation) and carbonate rock (Habshiyah Formation)(Fig. 2).

A regional hiatus spanning the Late Eocene to ?Late Oligoceneprobably represents regional uplift which preceded rifting andseafloor spreading in the Gulf of Aden and the Red Sea. Themainland of Yemen remained emergent from this time onwards,whilst in the Gulf of Aden and Qamar Bay renewed subsidenceallowed the accumulation of thick clastics and carbonates of Oli-goceneeMiocene age (Taqa and Sarar Formations).

3. Samples and methods

A total of 70 cutting and core samples from two onshore andoffshore deep exploration wells (16/U-1 and 16/G-1 Wells,respectively) in the Nimir's Qamar Bay (Block 16) concession (Fig. 1and Table 1) were used to determine vitrinite reflectance mea-surements and RockeEval pyrolysis analysis. The samples werecollected from limestone and shale intervals within the DabutFormation (Fig. 3). The geochemical and organic petrography ana-lyses of the Dabut sediments were carried out at Geochem GroupLimited Laboratories to investigate the characters of source rockswithin the Dabut Formation.

3.1. Basin modelling procedure

Basin modelling is widely used to study the burial and thermalhistories (e.g., Welte and Yukler, 1981; Waples, 1994; Burrus et al.,1991; Hermanrud, 1993; Littke et al., 1994; Yalcin et al., 1997). Ba-sin modelling has been successfully applied to some importantpetroliferous basins in different locations, such as the Masila Basinin Yemen by Hakimi et al. (2010), Shoushan Basin in Egypt byShalaby et al. (2011, 2013), the Jeanne D'Arc Basin, offshoreNewfoundland by Baur et al. (2011).

Present-day structure surfaces at the top of different formationsand two wells (16/U-1 and 16/G-1) located within Nimir's QamarBay (Block 16) concession in the Jiza-Qamar Basin (Fig. 1) were usedas representative sites to model the timing of hydrocarbon gener-ation. Thermal maturity history of this study area has been inves-tigated and reconstructed by means of one-dimensional modellingof single wells 16/U-1 and 16/G-1. The representative single wells16/U-1 and 16/G-1 are also used to simulate the petroleum gener-ation and expulsion of Dabut source rocks. The burial history wasestablished using the stratigraphic record of the region, thefollowing input data were defined from two exploration wells:event (e.g. deposition, erosion, hiatus or non-deposition), present-

Table 1Results of RockeEval pyrolysis and TOC content analyses with calculated parameters of the Dabut sediments in wells 16/G-1 and 16/U-1, Jiza-Qamar Basin.

Wells Sample type Lithology Depth (m) TOC wt.% RockeEval pyrolysis

S1 (mg/g) S2 (mg/g) Tmax (�C) HI (mg/g) PY (mg/g) PI (mg/g)

16/U-1 Well Cutting Limestone 2139 0.95 0.06 0.89 438 94 0.95 0.06Cutting Shale 2175 1.00 0.06 0.88 438 88 0.94 0.06Cutting Shale 2202 0.85 0.10 0.70 441 82 0.80 0.13Cutting Shaly sand 2238 0.74 0.14 0.70 456 95 0.84 0.17Cutting Shaly sand 2247 0.52 0.16 0.40 436 77 0.56 0.29Cutting Shale 2283 2.10 0.20 2.87 436 137 3.07 0.07Cutting Shaly sand 2292 1.03 0.16 1.01 448 98 1.17 0.14Cutting Shale 2310 1.81 0.21 2.58 443 143 2.79 0.08Cutting Shale 3337 1.58 0.14 2.40 437 152 2.54 0.06Cutting Shaly sand 2346 0.64 0.14 0.67 430 105 0.81 0.17Cutting Shaly sand 2355 0.57 0.16 0.46 440 81 0.62 0.26Cutting Shale 2391 1.26 0.17 1.22 447 97 1.39 0.12Cutting Shale 2400 1.51 0.15 1.36 440 90 1.51 0.10Cutting Shale 2409 1.41 0.08 1.41 435 100 1.49 0.05Cutting Shale 2418 1.13 0.31 1.00 443 89 1.31 0.24Cutting Shale 2445 1.26 0.24 1.01 441 80 1.25 0.19Cutting Shale 2454 1.50 0.31 1.39 444 93 1.70 0.18Cutting Shale 2463 1.19 0.16 1.09 442 92 1.25 0.13Cutting Shale 2472 1.13 0.17 1.11 443 98 1.28 0.13Cutting Shale 2481 1.07 0.09 1.05 442 98 1.14 0.08Cutting Shale 2490 1.49 0.13 1.51 431 101 1.64 0.09Cutting Shale 2499 1.25 0.06 1.28 442 102 1.34 0.04Cutting Shale 2508 1.44 0.07 1.50 445 104 1.57 0.05Cutting Shale 2517 1.25 0.07 1.33 443 106 1.40 0.05Cutting Shale 2526 1.19 0.06 1.04 443 87 1.10 0.05Cutting Shale 2535 1.27 0.06 1.42 446 112 1.48 0.05Cutting Shale 2544 1.85 0.08 2.01 442 109 2.09 0.04Cutting Shale 2553 0.93 0.09 0.88 444 95 0.97 0.09Cutting Shale 2562 1.03 0.21 0.91 442 88 1.12 0.19Cutting Shale 2571 0.98 0.06 0.80 441 82 0.86 0.07Cutting Shale 2580 1.16 0.11 0.97 440 84 1.08 0.10Cutting Shale 2589 0.71 0.07 0.69 436 97 0.76 0.09Cutting Shale 2598 0.83 0.07 0.72 444 87 0.79 0.09Cutting Limestone 2607 1.22 0.08 1.28 446 105 1.36 0.06Cutting Limestone 2625 1.14 0.06 1.26 444 111 1.32 0.05Cutting Limestone 2634 1.44 0.08 1.56 445 108 1.64 0.05Cutting Limestone 2643 1.50 0.08 1.72 443 115 1.80 0.05Cutting Limestone 2652 1.51 0.09 1.69 445 112 1.78 0.05Cutting Limestone 2661 1.29 0.06 1.32 439 102 1.38 0.04Cutting Limestone 2670 1.23 0.05 1.37 431 111 1.42 0.05Cutting Limestone 2697 1.20 0.13 1.25 442 104 1.38 0.09Cutting Limestone 2715 1.10 0.12 1.38 444 126 1.50 0.08Cutting Limestone 2733 1.05 0.08 1.11 438 106 1.19 0.07Cutting Limestone 2751 1.15 0.05 1.09 437 95 1.14 0.05Cutting Limestone 2760 1.13 0.12 1.08 441 96 1.20 0.10Cutting Limestone 2778 0.84 0.11 0.86 441 102 0.97 0.11Cutting Limestone 2796 1.05 0.09 1.06 444 101 1.15 0.08Cutting Limestone 2859 2.01 0.17 2.40 440 119 2.57 0.07Cutting Limestone 2868 1.29 0.12 1.22 439 95 1.34 0.09Cutting Limestone 2877 1.34 0.09 1.51 440 113 1.60 0.06

16/G-1 Well Core Limestone 2437 0.51 0.17 1.06 436 208 1.23 0.14Core Limestone 2460 0.60 0.36 0.87 432 145 1.23 0.29Core Limestone 2462 0.51 0.34 0.93 433 182 1.27 0.27Core Sandstone 2464 0.71 1.16 1.51 430 212 2.67 0.43Core Limestone 2465 0.66 0.43 1.00 429 152 1.43 0.30Core Limestone 2470 0.70 0.14 0.52 428 74 0.66 0.21Cutting Limestone 2468 0.70 0.34 1.12 438 160 1.46 0.23Cutting Limestone 2499 0.54 0.19 0.84 435 156 1.03 0.18Core Shale 2619 0.76 0.20 0.34 431 45 0.54 0.37Cutting Shale 2672 0.95 0.15 0.78 436 82 0.93 0.16Cutting Shale 2709 0.99 0.22 0.62 434 63 0.84 0.26Core Shale 2739 1.18 0.25 0.84 439 71 1.09 0.23Cutting Shale 2775 0.54 0.15 0.39 432 72 0.54 0.28Core Shale 2769 1.04 0.29 0.64 431 62 0.93 0.31Cutting Shaly sand 2785 0.50 0.20 0.39 437 78 0.59 0.34Cutting Shale 2799 1.07 0.39 0.60 439 56 0.99 0.39Core Shale 2829 0.77 0.21 0.44 431 57 0.65 0.32Cutting Shale 2854 0.59 0.14 0.39 429 66 0.53 0.26Cutting Shale 2859 0.81 0.04 0.50 437 62 0.54 0.07Core Shale 3129 0.67 0.08 0.34 427 51 0.42 0.19

S1: Volatile hydrocarbon (HC) content, mg HC/g rock; TOC: Total organic Carbon, wt. %.S2: Remaining HC generative potential, mg HC/g rock; PI: Production Index ¼ S1/(S1 + S2).Tmax: Temperature at maximum of S2peak; PY: Potential Yield ¼ S1 + S2 (mg/g).HI: Hydrogen Index ¼ S2 � 100/TOC, mg HC/g TOC.


Table 2Vitrinite reflectance measurements of five stratigraphic levels (Upper Creta-ceouseTertiary) including Dabut Formation in wells 16/G-1 and 16/U-1, Jiza-QamarBasin.

Formation 16/U-1 well 16/G-1 well

Depth (m) VR % Depth (m) VR %

Habashiya 304 0.40 674 0.35687 0.40 734 0.40

Umm Er Radhuma 1003 0.30 1048 0.421123 0.40 1168 0.401326 0.41 1383 0.451421 0.42 1893 0.481484 0.41 1950 0.57

Sharwayn 1919 0.58 e e

Dabut 2145 0.63 2230 0.642206 0.65 2340 0.652301 0.66 2395 0.672355 0.68 2450 0.722435 0.71 2505 0.732510 0.73 2529 0.742573 0.74 2769 0.792636 0.77 2927 0.832706 0.78 3098 0.882782 0.80 e e

2826 0.81 e e

Mukalla 2921 0.84 3288 0.952984 0.87 3315 0.963054 0.88 3380 0.983123 0.91 3390 0.983193 0.93 3410 1.023256 0.98 3415 1.013307 1.01 3543 1.10

Figure 4. Organic geochemical log of the Dabut source rock samples in the studied wells


day thicknesses, lithology of strata, and present-day depth(Table 2). The burial/thermal histories of the Dabut source rockswere reconstructed using one-dimensional basin modelling soft-ware (PetroMod; version 10.0 SP1) to calculate the levels of thermalmaturity and timing of hydrocarbon generation and expulsionbased on calibration of measured vitrinite reflectance (VR) andborehole temperatures in the modelling of single wells. Moreover,the hydrocarbon generation and expulsion stages were calculatedassuming Type III kerogen and using reaction kinetics data basedon Burnham (1989).

4. Results and discussion

4.1. Source rock characteristics

Appropriate source rock properties such as Total Organic CarbonContent (TOC), hydrogen index (HI), as well as kinetic parametersare necessary for hydrocarbon generation simulation in basinmodelling. Source rock evaluation of the limestone and shale sed-iments within the Dabut Formation was investigated for the pur-pose of characterizing the organic richness, hydrocarbon potentialof the organic matter and its thermal maturity level (Fig. 4).

The Dabut sediments displays widely variable organic-matterrichness with TOC contents ranging from less than 0.6% to 2.5%.The average TOC value is 1.07. In contrast, the amount of hydro-carbon yield (S2) generated during pyrolysis is a useful parameter toevaluate the generative potential of source rocks (Peters, 1986;Bordenave, 1993). The Dabut samples have pyrolysis S2 yieldvalues in the range of 0.34e2.87 mg HC/g rock (Table 1), with an

(16/G-1 and 16/U-1 wells) according to RockeEval pyrolysis and TOC content results.

Figure 5. RockeEval pyrolysis S2 versus total organic carbon (TOC), showing genera-tive source rock potential of Dabut source rock samples.

Figure 7. RockeEval pyrolysis Tmax versus production index (PI), showing the matu-ration and nature of the hydrocarbon products of the Dabut samples.


average S2 value of 1.09%. The TOC content and pyrolysis S2 yieldvalues indicate that the Dabut sediments meet the accepted stan-dards of a source with fair to good source rock generative potentialas classified by Peters and Cassa (1994) (see Fig. 5). The RockeEvalhydrogen index (HI) for the Dabut samples ranges from about 50 to212 mg HC/g TOC (Table 1) and the average hydrogen index (HI) is102 mg HC/g TOC. Based on RockeEval data, kerogen classificationdiagrams were constructed using the HI vs. pyrolysis Tmax plot ascarried out by previous workers (e.g. Mukhopadhyay et al., 1995). Ingeneral, the results show that the analysed Dabut samples aredominated by Type III kerogen (gas-prone) with minor mixed IIIeII

Figure 6. RockeEval Hydrogen Index (HI) versus Tmax, showing kerogen quality andthermal maturity stages of the Dabut source rocks.

kerogen contributions (gas-and oil-prone) and generally plotted inthe mature zone (Fig. 6).

The pyrolysis Tmax and PI are available for all analysed Dabutsamples (Table 1), which can be also used as thermal maturity in-dicators (Espitalie et al., 1977; Hunt, 1995). The Dabut source rockshave pyrolysis Tmax and PI in the range of 427e456 �C and0.05e0.43 respectively, indicating that most of the Dabut sourcerocks are thermally mature and within the main stage of hydro-carbon generation (Figs. 6 and 7). The PI values are also internallyconsistent with the HCs being indigenous to a mature source rock,with the exceptions of only three samples having relatively highvalues of PI (>0.37; Fig. 7). This is may be probably due to expelled

Figure 8. Plot of vitrinite reflectance data (Ro) versus depths showing thermalmaturity stages of the Dabut source rock in different studied wells in the Jiza-QamarBasin.

Table 3Input data used for burial, thermal maturity and hydrocarbon generation and expulsion modelling of Dabut source rocks in the two studied wells (16/G-1 and 16/U-1).

Formation Deposition age Lithology 16/G-1 well 16/U-1 well

From (Ma) To (Ma) Top (m) Bottom (m) Thick (m) Top (m) Bottom (m) Thick (m)

Sediments surface 13.65 0.0 Recent Sediments 0.0 100 100 0.0 30 30Taqa 24.9 13.65 Limestone & check & Shale 100 650 550 30 120 90Habashiya 40.6 24.9 Limestone & Shale & Dolomite 650 800 150 120 700 580Rus 43.2 40.6 Evaporite & Dolomite 800 900 100 700 850 150Jeza 58.7 43.2 Dolomite & Shale 900 1100 200 850 1000 150Umm er Radhuma 65.5 58.7 Limestone & Marl 1100 2000 900 1000 1900 900Sharwayn 70.6 65.5 Limestone & Shale 2000 2300 300 1900 2100 200Dabut 80.5 70.6 Sandstone & Limestone & Shale 2300 3100 800 2100 2900 800Mukalla 85.0 80.5 Sandstone & Shale & Coal 3100 3560 460 2900 3400 500Pre Mukalla 89.2 85.0 Clastics & Carbonates 3560 3700 250 3400 3600 300


and migrated hydrocarbon from source rock intervals within DabutFormation. On the basis of these data, the Dabut sediments undercurrent investigation are likely to be prolific petroleum sources andcan generate oil although their main generation products are gases.

Figure 9. Burial history modelling in wells 16/G-1 and 16/U-1 in the Jiza-Qamar Bas

4.2. Vitrinite reflectance measurements

Vitrinite reflectance measurements (%VR) are widely acceptedby exploration geologists as a technique for measuring the thermal

in. Notice that the bold lines represent the top and bottom of Dabut Formation.


maturity of sedimentary sequences. A variety of physical andchemical methods have been used for the evaluation of organicmatter thermal maturity and for the interpretation of coal andsource rock thermal history, with vitrinite reflectance (%VR) as themost widely used tool (Tissot andWelte,1984; Bustin,1987; Lerche,1990; Zhao and Lerche, 1993; Hunt, 1995). Thermogenic oil isthought to be generated frommarine and lacustrine source rocks atvitrinite reflectance values between 0.5% and 1.3%, suggest oil-generation window (Tissot and Welte, 1984; Sweeney andBurnham, 1990; Bordenave, 1993), while samples with values lessthan 0.5% are considered thermally immature. Vitrinite reflectancegreater than 1.3% indicates gas windowmaturity (Tissot and Welte,1984). In present study, vitrinite reflectance was derived frommaturity measurements of five stratigraphic levels (Upper Creta-ceouseTertiary) including Dabut Formation (Table 2 and Fig. 8), andused to predict past thermal conditions. Mean vitrinite reflectancevalues range from 0.40% to 1.10 %. These values correspond toimmature to late mature for hydrocarbon generation. The lowest

Figure 10. Palaeo-temperature modelling in wells 16/G-1

VRs are from near the bottom of Habashiya Formation and arethermally immature with respect to hydrocarbon generation. Thehighest values are from the Dabut and Mukalla Formations and arenear the value associated with peak oil window (0.63%-1.10 % Ro)(Fig. 8).

4.3. Basin modelling

4.3.1. Subsidence history and conceptual modelUsing the basin-modelling procedures described previously,

predictions on timing of hydrocarbon generation were made. Thetectonic evolution has significantly influenced burial and thermalhistory of the basins. To describe the resulting models clearly, wereview first the results of our reconstruction of the burial andthermal histories. An understanding of local subsidence and ther-mal history is necessary in order to predict timing of hydrocarbongeneration and expulsion. Based on well profiles, sedimentationrate can be estimated using the depositional age and thickness (the

and 16/U-1 calibrated using borehole temperature.

Figure 11. Calibration of the thermal and maturity modelling in wells 16/G-1 and 16/U-1. Notice that the good correlation between measured data and calculated curves oftemperature and vitrinite reflectance.


present thickness) for the formations, which are penetrated by thestudied wells (Table 3). The burial history of the studied wells isvery similar (Fig. 9), and these similar burial histories should pro-duce a much more similar observed maturity profile if subjected tothe same regional heat flow history. During the Late Cretaceous(89.2e65.5 Ma), the Pre-Mukalla sediments and Mukalla, Dabutand Sharwayn Formations were deposited and characterized byhigher burial rates of about 71.7 m per million years leading to apresent thickness of about 1700 m. The Late Cretaceous subsidenceis regarded as thermal subsidence associated with late Cretaceousrifting. However, the Dabut Formation during that time was burieddeeply for begin petroleum generation. The Dabut Formation un-derwent continuous burial as the Tertiary sediments accumulated,

Figure 12. Heat-flow histories of wells 16/G-1 and 16/U-1 were used to model

and the Dabut source rocks reached maximum burial depth duringOligocene time. The early Tertiary subsidence (65.5e24.9 Ma) wascharacterized by relatively high burial rates and decreased to about42.2 m per million years with present thickness of about 1250 m.The overlying late Tertiary time during Oligocene (24.9e13.65 Ma)was characterized by relatively low average subsidence rates ofabout 14.4 m per million years. A total thickness of 320 m of sedi-ment was deposited during Oligocene time.

4.3.2. Calibration data and paleo-heat flowThe thermal history of sedimentary basins can be evaluated

based not only on the deposition and erosion history but also on theheat-flow evolution (Allen and Allen, 1990; Lachenbruch, 1970).

the most probable scenario for hydrocarbon generation in the study area.


The borehole temperatures were used to calibrate the present dayheat flow regime. Temperatures increase with depth in the Earth,indicating that heat is transferred through sediment layers to thesurface. The transfer of heat is mainly controlled by thermal con-ductivity of the formations and geothermal gradient. Therefore, thethermal conductivity and geothermal gradient need to be deter-mined to estimate the heat-flow history (Shalaby et al., 2011;Xiaowen et al., 2011; Frielingsdorf et al., 2008). In the modelsonly the subsurface heat flow was varied and thermal conductivityof the rock formations were kept constant until a fit with presentday temperature (BHT) was achieved. Furthermore, the present-day geothermal gradient of each borehole location was calculatedusing BHTs that were corrected for the circulation of drilling fluids.The values increase systematically with depth from a surfacetemperature and the maximum temperatures were reached at

Figure 13. Thermal and maturity history for the modelled wells

Neogene time (Fig. 10). The heat flow is a vital input parameter inbasin modelling, but it is commonly difficult to define this value forthe geological past. Therefore, the reconstruction of the thermalhistory of the basin is always simplified and is usually calibratedwith thermal maturity measurements (e.g., vitrinite reflectance)and temperatures (Fig. 11). Vitrinite reflectance was derived frommaturity measurements of five stratigraphic levels (Upper Creta-ceouseTertiary) including Dabut Formation (Table 2), and used topredict paleo-heat flow. Measured VRs were also used to calibratethe thermal maturity and burial history models.

In the present study, the models assume that the heat-flowvalues range between 45 mW/m2 and 70 mW/m2 (Fig. 12) andthe paleo-heat flow increased from 85 Ma to 25 Ma, reached a peakheat-flow value (70.0 mW/m2) at 25 Ma and then decreasedexponentially from 25 Ma to present-day (Fig. 12). The applied

16/G-1 and 16/U-1 showing the positions of the oil window.


heat-flow values lead to a good match between optically measured(symbols) and calculated (lines) vitrinite reflectance (Fig. 11). Thiscalibration data helped us to reconstruct and validate the thermalhistory in the study area. According to the vitrinite reflectance (%VR) data and its best-fit calibration with the calculated maturity inthe one-dimensional model, normal heat flow similar to themodern-day values and the Dabut source rocks reached oil windowof petroleum generation (Fig. 11).

4.3.3. Source rock maturity historyIn thermal history reconstructions of the study area, the influ-

ence of the tectonic evolution on the heat-flow distributionthrough time was applied using one-dimensional modelling.Thermalmaturity levels of the Dabut source rocks is calculate basedon the Easy% Ro routine (Sweeney and Burnham, 1990) using one-dimensional modelling of single wells. The modelling present-daymaturity of the studied wells is shown in Figure 13. The oil gen-eration (0.5% Ro) from Dabut source rocks begins at the depth of2110e2120 m and reaching peak hydrocarbon generation(0.84e0.86% Ro) at the depth of 2890e3020 m (Fig. 13). Thedetailed maturity history of Dabut source rocks is modelled for therepresentative wells 16/U-1 and 16/G-1 in the different locations ofJiza-Qamar Basin (Fig. 1), helping us to determine the time whensource rocks passed through the oil window. The Modelling resultsof the studied wells in the study area show there are similar insource rockmaturity and hydrocarbon generation histories becauseof similarity in buried history (Fig. 9). Oil generation (0.50% Ro) of

Figure 14. Cumulative hydrocarbon generation potential from Dabut source rocks with trstudy area.

the Dabut source rocks began from about 56 Ma to 42 Ma with thetemperature values in the range of 87e91 �C (Fig. 13). The maturityof the Dabut source rocks reached 0.70% Ro at 31 Ma and thetemperature was 112 �C (Fig. 13). Peak hydrocarbon generationoccurred approximately from14Ma to 13Ma for the temperature ofapproximately 117e128 �C (Fig. 13).

4.3.4. Timing of petroleum generation of Dabut source rocksThe Dabut sediments (limestones and shales) in the Jiza-Qamar

Basin have a TOC value of 1.07 at averages and up to 2.5% atmaximum while the maturity ranges from 0.63% to 0.88% VR,indicating sufficient levels for hydrocarbon generation are present.The generated hydrocarbon (oil and gas) per unit weight andtiming of the Dabut source rocks was evaluated using rift heat flowmodel (Fig. 12). For the modelled hydrocarbon generation, the hy-drocarbon generated from the Dabut source rocks is mainly gaswith some oil (Fig. 14). The hydrocarbon generation evolution for16/U-1 and 16/G-1wells are slightly similar and can be summarizedinto tree stages: the first stage from approximately 56 Ma to 25 Ma,the second stage from approximately 25 Ma to 14 Ma and the thirdstage from 14 Ma to the present-day (Fig. 14). The first stage is thepredominant phase of hydrocarbon generation. The transformationratio reaches 1e37% at 56e25Ma, with oil as the main product. Thesecond stage is the predominant phase of gas generation withsignificant of oil generation. The transformation ratio of the Dabutsource rocks ranged from 37% to 61% during this stage. The thirdstage (approximately 14e0 Ma) is still the phase of gas generation

ansformation ratios in wells 16/G-1 and 16/U-1 using the rift heat flow model in the


with little oil generated during this stage and the transformationratio increased and ranging from 43% to 65% (Fig. 14).

5. Conclusions

Investigation of sediments from Dabut Formation in the Nimir'sQamar Bay (Block 16) concession, Jiza-Qamar Basin based onorganic geochemical characteristics has revealed that the Dabutsediments possess fair to good source rock-generative potential assuggested by TOC content in the range 0.50e2.5 wet%. Kerogentype from RockeEval pyrolysis indicate that the organic matter ofDabut source rocks is dominated by Type III kerogen with minormixed IIIeII kerogen contributions with HI values in the range of50e212 mg HC/g TOC and is thus considered to be generatedmainly gas and some contributions of oil. Vitrinite reflectance datashow that the Dabut source rocks have reached the mid-maturestage for hydrocarbon generation, consistent with RockeEval py-rolysis Tmax values.

Numerical modelling of two wells cross in the offshore andonshore Jiza-Qamar Basin indicates that the Dabut source rocksentered the mid-mature stage for significant of hydrocarbon gen-eration during Tertiary age. The onset of oil generation startedduring early Paleogene (56 Ma), and the main phase of oil gener-ation was at 56 Ma to 25 Ma. The main phase of gas generationbegan from about 25 Ma to 14 Ma, and maximum rates of gasgeneration were reached from 14 Ma to present-day. Overall, theresults indicate that the Dabut sediments are an effective sourcerocks in the Jiza-Qamar Basin, and there are significant amount ofgas with oil contributions to be generated in the basin.

Acknowledgements

The authors thank the Petroleum Exploration and ProductionAuthority (PEPA), Yemen for supplying the data for this study.Schlumberger is acknowledged for providing the PetroMod BasinModelling software. We would like to sincerely thank an AssociateEditor Pedro R Kress for his careful review and useful commentsthat improved the revised manuscript.

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