Diamondoid Hydrocarbons as Maturity Indicators for Condensates from Southern Indus Basin , Pakistan

Diamondoid hydrocarbons have been examined in condensates reservoired in the Southern Indus Basin using GC-MS. Bulk properties reveal that samples are waxy and low sulfur with the exception of Pakhro and Gopang which are nonwaxy. TIC show bimodal distribution of n-alkanes along with high abundance of C 20+ n-alkanes indicating substantial contribution of terrigeneous OM in these samples. CPI close to one is consistent with mature nature of oils. The samples show two ranges of Pr/Ph ratios. Those within the range of 2.2–2.7 reflect marine depositional settings for OM while others with Pr/Ph >3 may have originated from terrestrial OM deposited under marine oxic conditions. The cross plot of Pr/n-C 17 versus Ph/n-C 18 indicate type III kerogen as main source of OM deposited under marine to marine oxic conditions. The values of diamondoid based maturity parameters, like methyladamantane index 54.1–75.8% and methyldiamantane index 34.9–56.3% indicate high level of thermal maturity corresponding to vitrinite reflectance 1.1–1.6%. No biodegradation is observed in any of these samples as shown by methyladamantanes/adamantane 3.99–5.52 and methyldiamantanes/diamantane 2.16–2.99 and supported by high values of API gravity (45.13–60.02) and absence of UCM.


Introduction
Diamondoids are a group of three-dimensional cyclohexane ring alkanes that have highly symmetrical and strain-free diamond like fused ring structures in chair conformation (Figure 1) [1][2][3].This name originated from the word "adames, " the Greek letter for diamond, after the discovery of these compounds from Czechoslovakian petroleum [4].The simplest of these polycyclic diamondoids is adamantane followed by its homologues diamantanes, trimantanes, tetramantanes, pentamantanes, and hexamantanes [5].Diamondoids in petroleum and sediment extracts are extremely stable compounds.These are generally more stable than any other hydrocarbon class in geological samples, hence more resistant to alteration processes like biodegradation and maturation [3,[5][6][7][8][9][10][11].Because of these properties diamondoids have been used in evaluating geochemistry of source rocks and crude oils including biodegradation and thermal maturity of high maturity crude oils and condensates [9][10][11][12][13][14]. Sassen and Post used  13 C values of diamondoids for establishing oil to source correlation of condensates [15].Jalees et al. used diamondoids and biomarkers to study the effect of maturity and biodegradation on crude oils and condensates [16].Dahl et al. proposed diamondoid hydrocarbons as indicators of oil cracking [12].Others researchers used methyladamantane and methyldiamantane indices as maturity parameters for high maturity oils [9,16,17].The most commonly used technique to analyze diamondoids is gas chromatographymass spectrometry (GC-MS).Recently, Liang et al. used GC-MS-MS and Li et al. used two-dimensional gas chromatography with time of flight mass spectroscopy (GC × GC-TOFMS) for identification and characterization of diamondoids in condensates [18,19].These techniques enabled these authors to identify di-and tri-methyladamantanes more effectively.
In this study we have used diamondoids along with nalkanes and isoprenoid parameters to evaluate geochemistry and thermal maturity of condensates from the Southern Indus Basin which is difficult to find otherwise because of extremely low concentration of biomarkers in these samples.

Experimental
2.1.Samples Details.Eight condensates samples collected from different oil fields of the Southern Indus Basin were analyzed in this study.The location of sample wells is shown in Figure 2, while geological information is listed in Table 1.A brief geological description of the study area is outlined below and described in more detail by Wandrey et al. [20].The basin is bounded to the north by the Middle Indus Basin, to the northwest by the Sulaiman Fold Belt, and the Kirthar Fold Belt in the southwest (Figure 2).The hydrocarbon expulsion, migration, and entrapment occurred mainly during Eocene to Miocene time.Source rocks have been identified in Cretaceous and Eocene successions.The Early Cretaceous shales of Sembar and Goru Formations have been recognized as source for the bulk of hydrocarbons charging several oil and gas fields in the Southern Indus Basin.Sembar shales contain mixed type II and type III kerogen and are thought to be deposited under shallow marine depositional conditions [21].Upper Cretaceous Goru sands and Eocene carbonates are the main reservoirs in the Southern Indus Basin [20][21][22][23].The condensate samples used in this study belong to the Cretaceous reservoirs of Lower Goru Formation.

Separation of Compound
Classes by Liquid Chromatography.Condensate samples were fractionated into saturates, aromatics, and polar fractions using liquid column chromatography employing a standard method described elsewhere [28].In brief, sample (30 mg) dissolved in n-hexane (500 L) was introduced onto the top of activated silica gel column that has been activated at 250 ∘ C for overnight.Saturated hydrocarbon fractions were eluted with three bed volumes of n-hexane, aromatic fractions with three bed volumes of mixture of n-hexane: dichloromethane (9 : 1), and the polars with three bed volumes of 1 : 1 mixture of dichloromethane and methanol.Saturated fractions were analysed by gas chromatography-mass spectrometry (GC-MS).

Gas Chromatography-Mass Spectrometry. GC-MS analysis was performed using a Hewlett-Packard (HP) 5973 Mass
Selective Detector Interfaced to a HP6890 gas chromatograph, which was fitted with a DB-5 capillary column (J and W Scientific, 60 m, 0.25 mm internal diameter, 0.25 m).The GC oven was programmed from 40 ∘ C to 310 ∘ C at a heating rate of 3 ∘ C/min with initial and final hold times of 1 and 30 minutes, respectively.The MS source and quadruple temperatures were at 230 ∘ C and 106 ∘ C, respectively.Samples were dissolved in n-hexane and injected in a pulsed splitless mode using an autosampler.Helium was used as the carrier gas at a linear velocity of 27 cm/s with the injector operating at constant flow.The MS was operating with ionization energy 70 eV, source temperature 180 ∘ C, and electron multiplier voltage 1800 V. Data was acquired in a full scan mode from mass range of 50 to 550 amu.  .The samples analyzed in this study have API gravity values between 46 and 60 ∘ (Table 1) and are classified as condensates.

Results and Discussion
Pour point (PP) is generally associated with paraffinicity or waxy nature of crude oils.The term high paraffinic reflects high concentration of C 20+ n-alkanes in petroleum.Crude oils derived from terrigenous OM generally have high paraffinic content and high PP. Figure 3 and data listed in Table 1 show that the analyzed samples having high concentration of C 20+ n-alkanes have high values of PP up to 4 ∘ C and are waxy in nature, so all the samples except Pakhro and Gopang are waxy condensates, while those having low concentration of C 20+ n-alkanes show low PP and are nonwaxy in nature, for example, Pakhro and Gopang.
Sulfur compounds are undesirable in crude oils and its products because of extra cost incurred on sulfur removal and environmental problems associated with sulfur compounds.The crude oils are termed as sour or sweet on the basis of sulfur content >1% and <1%.The sulfur content of analyzed samples is extremely low (<0.1%)rendering these samples as sweet and commercially valuable.

n-Alkanes and Isoprenoids
Distributions.The distribution pattern of n-alkanes provides useful information on the source of OM, thermal maturity, and biodegradation.Total ion gas chromatograms of waxy condensates described above show bimodal distribution of n-alkanes (Figure 3), thereby reflecting that source generating these oils has high contribution of terrestrial OM.The absence of UCM indicates that the samples have not undergone biodegradation.Carbon preference index (CPI) of samples is close to one indicating that these samples are high maturity condensates [30] (Table 1).Oil derived from terrigenous OM show marked predominance of C 27 , C 29 , and C 31 n-alkanes which is lost at high maturity level.The analyzed samples do not show odd or even predominance of n-alkanes and exhibit uniform distribution.This feature is attributed to high thermal maturity of these samples.
Pristane to phytane (Pr/Ph) ratios in crude oil and source rocks within oil window reflect redox potential in the depositional environment and nature of OM [31].Most marine and organic rich sediments have Pr/Ph ratios in the range of 0.8-2.5, which show a gradual increase with increasing maturity within this range, although marine OM usually has Pr/Ph ratio < 1.5 at high maturity levels, while terrigenous OM input under oxic conditions shows ratios >3 [29].Lower values (<0.8) typify anoxic commonly hypersaline or carbonate environment, particularly if it is accompanied by high sulfur content [32][33][34]  Higher value of 3.95 has been observed for Gopang, which is 60 ∘ API and < −10 ∘ C PP condensate, and high value of Pr/Ph reflects very high thermal maturity as well as substantial contribution from terrigenous source of OM input under oxic conditions for this sample.Similarly high Pr/Ph ratio (3.98) for Dhamrkhi is consistent with an origin from source rocks having substantial contribution of terrestrial OM and deposited under marine oxic conditions.
The Pr/n-C 17 ratios have been used to differentiate OM under swamp environment (<1.5) from those found under marine environments (<0.5) although this ratio is affected by maturity [35][36][37].The Pr/n-C 17 ratios of analyzed samples are in a close range of 0.11-0.and depositional conditions of OM [35].Increased thermal maturity results in decreased isoprenoid/n-alkane ratio due to fast release of n-alkanes compared to isoprenoids.On the other hand biodegradation results in increased isoprenoid/n-alkane ratios due to fast removal of n-alkanes prior to isoprenoids.The Southern Indus Basin condensates show extremely low values of Pr/n-C 17 and Ph/n-C 18 in the range of 0.11-0.34and 0.04-0.11,respectively (Table 1).
The samples have gathered towards the lower left corner of the plot suggesting high thermal maturity for the samples (Figure 4).The source of OM is mainly type III kerogen deposited under marine oxic conditions which is likely to generate waxy oils or condensates under high thermal conditions.

Assignment of Diamondoid Hydrocarbons.
Diamondoids in the condensate samples have been analyzed and identified by GC-MS.These compounds were identified by comparing their mass spectra and relative retention time (Figures 5 and  6) with the previous studies [2, 3, 9-11, 13, 38].The structure and numbering of adamantane and diamantane are shown in Figure 1.Adamantanes and diamantanes were examined using m/z 135, 136, 149, 163, and 177 and m/z 187, 188, and 201, respectively.Figure 6 shows the distribution and elution order of adamantanes and diamantanes in a representative sample, while Table 2 enlists identity and mass spectral properties of different isomers.guidelines for maturity determination particularly for high maturity samples including condensates.The distribution and relative abundances of adamantane, methyladamantanes, diamantine, and methyldiamantanes in a representative sample (Gopang) are shown in Figure 6, while values of MAI and MDI are listed in  [11].The values of MA/A and MD/D for the analyzed samples are listed in Table 2 which range from 3.99 to 5.52 and 2.16 to 2.99, respectively.These values are lower than the limits described for biodegradation suggesting that no biodegradation has taken place in the Southern Indus Basin condensates.Moreover, n-alkanes profile and absence of UCM also support nonbiodegraded nature of these samples.
(ii) Bimodal distribution of n-alkanes and high abundance of C 20+ n-alkanes suggests terrigenous source of OM for the condensates, while Pr/Ph ratios 2.2-2.77and 3.95-3.98suggest marine to marine oxic environment of deposition for the samples.
(iii) Isoprenoid/n-alkane ratios and Pr/n-C 17 versus Ph/n-C 18 plot suggest marine to marine oxic depositional conditions and type III kerogen as main source of OM which is likely to generate waxy oils or condensates under high thermal conditions.
(iv) Diamondoid-based maturity parameters (MAI and MDI) have indicated that samples have attained high levels of thermal maturity corresponding to vitrinite reflectance 1.1-1.6% (v) Absence of UCM and ratios of MA/A and MD/D indicate that samples are nonbiodegraded.

3. 1 .
Bulk Properties.The crude oils are classified as heavy, medium, and light on the basis of API gravity values ≤ 20 ∘ ,

Figure 2 :
Figure 2: Map showing location of condensates wells within the Southern Indus Basin.

Figure 3 :
Figure 3: Total ion chromatograms from GC-MS analysis showing distribution of n-alkanes and isoprenoids (Pr and Ph) in condensates from the Southern Indus Basin.C 8 -C 33 represents n-alkanes with carbon atoms 8-33.Pr and Ph = Pristane (Ph) and Phytane (Ph) are C 19 and C 20 regular acyclic isoprenoids which elute after C 17 and C 18 n-alkanes.

R
34 and indicate marine depositional conditions of OM.The Pr/n-C 17 versus Ph/n-C 18 plot proposed by Shanmugam provides useful information regarding thermal maturity, biodegradation, e I I -I I I k e r o g e n T y p e I I I k e r o g e n a n d h u m i c c o a ls T e r r e s t r i a l B i o d e g r a d a t i o n

Figure 4 :
Figure 4: Pr/n-C 17 versus Ph/n-C 18 plot showing high maturity and substantial contribution of type III organic matter in source rocks of these samples.
. Pr/Ph ratios of six samples (Sr.number 1-6) are 2.2 to 2.77 and indicate significant contribution from terrigenous OM input under marine depositional settings.

Table 3 .
Among the samples suit Dhamrkhi with MAI 58.7% and MDI 34.9% is the least mature and Pakhro with MAI 75.8% and MDI 52% is the most mature (

Table 3
[11]ssess biodegradation of crude oil[11].Higher values of MA/A up to 16 indicate progressive level of microbial activity and in-reservoir biodegradation.MA/A is sensitive to even initial stages of biodegradation, and values > 6.3 have been observed for slightly biodegraded oils.However the value increases with progressive increase in level of biodegradation.On the other hand MD/D values are not affected by slight to moderate biodegradation; however slight change in MD/D ratio (>3.5) has been observed at severe levels of biodegradation (level 8) 3.5.Biodegradation.Diamondoids are highly resistant to biodegradation.The stability of these compounds allows to use these compounds as fingerprints for the assessment of biodegradation.The ratios methyladamantanes/adamantane (MA/A) and methyldiamantanes/diamantane (MD/D) have been applied