D. Mani+*, D. J. Patil*, M. S. Kalpana* and A. M. Dayal*
The Saurashtra Basin in western India is considered to have significant hydrocarbon potential.However conventional exploration methods, particularly for Mesozoic prospects, have beenhampered by the thick basalt cover. In this study, near-surface geochemical methods are usedto investigate the generation of thermogenic gaseous hydrocarbons in the basin. Shallow soilsamples were collected from favourable locations identified by integrated geophysical andgeochemical studies. The compositional and isotopic signatures of adsorbed gaseous hydrocarbons(methane through pentane) together with soil iodine concentrations were used as surfaceindicators of petroleum micro-seepages. High concentrations of adsorbed thermogenic methane(C1= 518 ppb) and ethane plus higher hydrocarbons (ΣC
2+=977 ppb) along with iodine
concentrations up to 68.5 ppm were observed. Total organic and inorganic carbon (TOC andTIC) measurements, fluorescence and X-ray diffraction (XRD) studies showed that the near-surface hydrocarbon occurrences were seepage related. Elevated hydrocarbon and iodineconcentrations were coincident with dykes and lineaments in the study area, which probablyserved as conduits for the micro-seepage of hydrocarbons.
* National Geophysical Research Institute (CSIR), UppalRoad, Hyderabad 500007, Andhra Pradesh, India.+ Corresponding author, email:
Western India includes prolific oil and gas provincesincluding the broad shelf area offshore Mumbai andthe onshore Cambay graben. Mesozoic and Cenozoicreservoirs in the Jaisalmer, Bikaner-Nagaur and BarmerBasins in Rajasthan (NW India) produce commercialvolumes of hydrocarbons, and the Kutch and
Saurashtra Basins in the north (DGH, 2010) and theKerala-Konkan Basin in the south also have fairexploration potential.
This study focuses on the Saurashtra Basin, whichis located north of the commercially proven Mumbaioffshore and west of the petroleum-bearing CambayBasin (Fig. 1a). The basin is considered to have fairhydrocarbon prospects in Mesozoic reservoirs (DGH,2010; Infraline, 2002). However, the resourcepotential of the basin is little known owing to the
68 Near surface geochemical data, Saurashtra Basin, India
presence of thick Deccan basalts of Late Cretaceousage, which have hindered studies of the underlyingMesozoic hydrocarbon targets by conventionalgeophysical methods.
Surface geochemical prospecting methods havetherefore been applied in the Saurashtra Basin.Geochemical methods seek to demonstrate thepresence of adsorbed and/or free hydrocarbons innear-surface soils and sediments (Schumacher andLeShack, 2002). These hydrocarbons have migratedeither from source rocks or from breached oil andgas reservoirs, and will have moved along faults and
fractures by a variety of processes to the near-surfaceenvironment (e.g. Abrams, 1996a,b; 2005, 2007;Jones and Drozd, 1983; Horvitz, 1985; Kartsev etal., 1959; Klusman, 1993; Mani, 2008; Schumacher,1996; Sechman, 2011; Whiticar, 2002). Other indirectgeochemical indicators for hydrocarbon micro-seepage include the near-surface soil iodineconcentration and the presence of hydrocarbon-oxidising microbial populations.
Integrated geophysical studies in the SaurashtraPeninsula have indicated that thick Mesozoicsedimentary successions are present in two areas:
Fig. 1. (a) Regional location map of the Saurashtra Peninsula in western India (after Mehr, 1995). Box indicatesthe study area. (b) Location map of the study area in the Saurashtra Peninsula with sediment thicknesses andhydrocarbon concentrations (modified after Satpal et al., 2002; Harinarayana, 2008; Kumar et al., 2004; Kumar
et al., 2009). (c) Detailed map showing sample collection points for adsorbed hydrocarbons and soil iodineconcentration measurements in the study area.
(a)
(b) (c)
69.0° 69.5° 70.0° 70.5° 71.0° 71.5° 72.0°
69.0° 69.5° 70.0° 70.5° 71.0° 71.5° 72.0°
23.0°
22.5°
22.0°
21.5°
21.0°
20.5°
23.0°
22.5°
22.0°
21.5°
21.0°
20.5°
Recent / QuaternaryTertiaryDeccan TrapCretaceous
LegendVeraval
JunagadhPorbander
Dwarka
JamnagarRajkot
AmreliBhavnagar
Surendranagar
Kandla
StudyArea
C2+ Conc (ppb)
0 to 50
50 to 100
100 to 766
Jamnagar
Ialpur
Matwa
Khandera
Laloi Kalavad
Lodhika
Gulf of Kutch
+
+
++
+ +
SedimentThickness (m)
2500
2000
1500
1000
500
0
22.8°
22.6°
22.4°
22.2°
22.0°
21.8°69.8° 70.0° 70.2° 70.4° 70.6° 70.8°
Khandera
LaloiKalavad
Matwa
70.20° 70.25° 70.30° 70.35° 70.40°
70.20° 70.25° 70.30° 70.35° 70.40°
22.34°
22.29°
22.24°
22.19°
22.14°
22.34°
22.29°
22.24°
22.19°
22.14°
Deccan Trap Soil Iodine AdsorbedHydrocarbons+
69D. Mani et al.
around Jamnagar in the NW and near Dwarka to thewest (Fig. 1) (Harinarayana, 2008; Rao et al., 2004;Sarma et al., 2004; Satpal et al., 2006). The few wellswhich have been drilled (e.g. Viramgaom-1,Dhandhuka-1 and Lodhika-1) have indicated varyingthickness of basalt (350m-1300m) and underlyingMesozoic sediments (175m-2100m+) (Singh et al.,1997). Mesozoic sediments penetrated in the Lodhika-1 well near Rajkot (Fig. 1b) showed fair hydrocarbonpotential (Singh et al., 1997).
Preliminary surface geochemical studies in the NWof the peninsula showed high light hydrocarbonconcentrations (methane through butane) in areasadjoining Jamnagar (Kumar et al., 2004; 2009). Lowconcentrations of C
1-C
4 hydrocarbons characterized
the soils around Dwarka.In this study, these preliminary geochemical
surveys were extended to a detailed grid of closelyspaced samples from the Jamnagar area. Theobjectives of the study were two-fold; firstly, to assessthe use of geochemical indicators as a tool for therapid evaluation of hydrocarbon prospects in thisfrontier basin; and secondly, to provide an improvedunderstanding of the occurrence of thermogenichydrocarbons in the Saurashtra Basin by integratingthe results of the present study with the availablegeophysical and geological data.
In addition to analysing adsorbed light gaseoushydrocarbons, other data were derived from studiesof the stable carbon isotope ratios of the hydrocarbongases, and soil iodine concentrations which are anindirect indicator of hydrocarbon seepage (Allexan etal., 1986; Goudge, 2007; 2010; Kartsev, 1959; Leaverand Thomasson, 2002; Mani et al., 2011a; 2011b;Moran, 1998; Tedesco et al., 1987; Xie Xuejing andYang Binzhong, 1989). Analyses of TOC and TICcontent and fluorescence and XRD studies wereperformed on selected samples.
GEOLOGIC SETTING AND STRATIGRAPHY
The Saurashtra Basin is a rifted passive margin basinin western India (Fig. 1a), comprising an onshorearea of 52,000 km2 and a 20,000 km2 area extendingoffshore into the Arabian Sea (Biswas, 1982; Merh,1995). The onland part of the basin, known as theSaurashtra Peninsula, developed by rifting along threeintersecting Precambrian orogenic trends: the ENE-WSW Narmada Son lineament; the west coast faultwith the NNW-SSE Dharwar trend; and the NE-SWDelhi Aravalli trend (Biswas 1980; 1987). In terms ofgeo-morphological features, the Saurashtra Peninsulais dominated by a central plateau formed of DeccanTrap lavas (Fig. 1a). Post Trappean intrusives of plugsand dykes with faults and lineaments occur in thepeninsula (Auden, 1949; Karanth and Sant, 1995;Misra, 1999).
Geological and magneto-telluric surveys in theSaurashtra Peninsula have shown the presence ofthick Mesozoic sedimentary sequences in the westernpart (Harinarayana, 2008; Rao and Reddy, 2005), andled to the drilling of the Lodhika-1 well near Rajkot(Fig. 2a) which confirmed the presence of a Mesozoicsection beneath the Deccan Traps (Fig. 2b). TheJurassic Lodhika Formation is composed of volcanicrocks with a thickness of more than 535 m. OverlyingCretaceous sedimentary rocks are exposed over anarea of approximately 5000 km2 in the NE part of theSaurashtra Peninsula and comprise the LowerCretaceous Dhrangadhra and Wadhwan Formations.The Dhrangdhra Formation, which crops out in theNE Saurashtra Peninsula, is divisible into Lower andUpper Members. The Lower Dhrangadhra Memberhas a thickness of 675 m and is dominantly composedof rudaceous sediments with thin claystones andminor siltstone/sandstones. Fossils are absent and thedepositional characteristics indicate a non-marine
0
-1
-2
-3
-4
-5
Dep
th (k
m.)
Distance (km.)0 10 20 30 40 50 60 70 80 90
W E
Lodhika Well
Deccan Trap
Mesozoic Sediments
Basement
Lochika
Saurashtra
Fig. 2a. An east-west profile across the Saurashtra Peninsula showing the occurrence of a relatively thick
sedimentary succession towards the western part of the basin (after Harinarayana, 2008).
70 Near surface geochemical data, Saurashtra Basin, India
alluvial fan system. The Upper Member is characterizedby sandstones and siltstones with shales containingorganic matter, pyrite and siderite. Pollen, spores, plantfossils and gastropod shells have been reported(Bhandari and Kumar, 1970). The presence of deltaicchannel, flood plain and levee deposits indicate a fluvialto deltaic depositional environment. The fair to goodorganic content (Singh et al., 1997) suggests that theUpper Dhrangadhra Member has source rock potential.
Deltaic deposition was followed by a marinetransgressive phase resulting in deposition of the Aptian-Albian Wadhwan Formation (Biswas and Deshpande,1983), which comprises claystones and sandstones.The formation includes bivalves, echinoids, gastropods,bryozoa and fragments of ammonites and corals,suggesting a shallow marine origin. Regional uplift tookplace during the Late Cretaceous and was followed byDeccan Trap volcanism (Bose, 1972; Biswas, 1980).
The Cenozoic consist dominantly of shallow-marineclays and limestones (Biswas and Deshpande, 1983).Phases of tectonic deformation in the Cenozoic areindicated by extensive unconformities. Post-Trapcarbonates and fine-grained clastics are prospectiveoffshore targets.
Petroleum geologyThe sub-Trappean Mesozoic succession in theSaurashtra Basin is thought to contain about 40 MMtonnes of undiscovered hydrocarbon resources(Singh et al., 1997). Structures include regional andlocal horsts and grabens along the dominant basementtrends (NNW-SSE) (Biswas, 1982; Gombos et al.,1995; Schutter, 2003; Zutshi, 1991).
The Lower Cretaceous Dhrangdhra and WadhwanFormations can be correlated with the BhujFormation and Ukra Member, respectively, of thehighly prospective Kutch Basin (DGH, 2010). TheDeccan Trap volcanics may have protectedunderlying hydrocarbon accumulations (Zutshi,1989; 1991).
Potential play types in the offshore SaurashtraBasin include carbonate build-ups, transpressionalroll-overs, pinch-outs and wedge-outs and thecanyon complex in the Indus fan system (DGH,2010). Possible reservoir rocks are Cretaceoussandstones, Eocene-Miocene shelfal carbonates /reefal build-ups, and Tertiary deep-water sands(Indus Fan). Eocene to Miocene shales andPaleocene to Oligocene shales are possible source
Light grey to brownsandstone with clayalterations.
Grey, brownish greysandstone, hard andcompact with darkgrey to brownish greyclaystone, in placescarbonaceous.
Dominantly polymicticconglomerate with redclaystone. Brownsandstone with alterationsof siltstone / claystone.
Basalt / dolerite.
Basalt with red claystone.
Tuff.
Upp
er C
reta
ceou
sP
aleo
cene
Dec
can
Trap
Dhr
anga
dhra
Upp
erLo
wer
Lodh
ika Upp
erLo
wer
Upp
er J
uras
sic
to L
ower
Cre
tace
ous
Jura
ssic
Wadhwan
Age Formation Depth(m)
Litho-log Lithology
1000
0
1200
1400
1600
1800
2400
2600
2800
3200
3000
3400
2000
2200
3500
Fig. 2b. Generalized lithostratigraphy of the
Lodhika-1 well (after Singh et al., 1997).Well location in Fig. 2a.
71D. Mani et al.
and cap rocks, respectively for the offshore area. Forthe onshore basin, limited information shows thatsource rocks are present in the Upper DhrangadhraMember (Singh et al., 1997).
The Rajkot depression has been suggested inunpublished reports to be a probable location ofhydrocarbon generation, based on data from theLodhika-1 well. Gravity “lows”, one around Jamnagarin the north and the other around Dwarka to the west,separated by a basement ridge are consideredfavourable locales for hydrocarbon entrapment (Satpalet al., 2006). A thicker marine sequence may bepresent to the west of Lodhika (Srivastava et al.,1995). The burial depth and timing of hydrocarbongeneration has been inferred on the basis of thermalmaturity and vitrinite reflectance measurements fromcore/well data (Singh et al., 1997). Initiation ofhydrocarbon generation from the Upper Dhrangadhrasediments is inferred to have taken place during theLate Cretaceous, with peak hydrocarbon generationduring the Paleocene to Recent.
MATERIALS AND METHODS
SamplingA total of 150 soil samples, in grid of 1x1 km, werecollected for soil gas analyses from the Jamnagar area.They were collected from a depth of 1.5-3.5 m bymanual hammering of a hollow metal pipe. For thesoil iodine survey, a total of 73 samples from a depthof 2-6 in were collected at an interval of ~ 2 km, atalternate points to the soil gas sampling stations. Thesamples were packed separately in aluminium foil,sealed in polythene covers and marked with theirGlobal Positioning System (GPS) locations.
Precautions were taken to avoid the collection ofsamples from disturbed or excavated areas; or of soilscontaminated with hydrocarbons, chemicals or animalwaste; or samples from root zones, swamps and areasunder standing water. Fig. 1c shows the samplelocations. GPS locations are given in online AppendixS1 (see note on Supporting Information on page 83).
AnalysesAdsorbed gaseous hydrocarbonsAnalysis of adsorbed soil gases was based on acid-extractable hydrocarbons (Horvitz, 1985). One gramof sieved, <63 μ soil sample was treated with 50%ortho-phosphoric acid (H
3PO
4) under partial vacuum
conditions to desorb the light hydrocarbons. The gaseswere collected by water displacement in adegasification apparatus. The carbon dioxide evolveddue to the decomposition of carbonates in soil wastrapped in 20% potassium hydroxide in a round-bottom flask attached to the apparatus. The volumeof the displaced gases was recorded, and 500μl was
injected to a Varian CP-3380 gas chromatograph (GC)equipped with Porapak ‘Q’ column and flameionization detector (FID). The column oven wasprogrammed at 60o with a hold time of 3 minutes andincreased to 120oC at a rate of 20oC/min and held for18 min with a total time of 24 min. Nitrogen wasused as the carrier gas with a flow rate of 30 ml/min.The fuel gases were hydrogen and zero air with aflow of 300 ml/min. The temperature of the injectorand detector was maintained at 120oC and 200oC,respectively. A Star workstation was used for dataacquisition. Calibration of the gas chromatograph wasperformed using external standards on a peak areasbasis. The adsorbed gases were quantified in ppb pergram weight of the fine-grained fraction of thesediments with corrections for moisture applied.
Stable carbon isotope ratiosFifteen samples with high concentrations of adsorbedgases were measured for their stable carbon isotopecompositions. The δ13C analyses of desorbedhydrocarbon gases were carried out using a GasChromatograph-Combustion-Isotope Ratio MassSpectrometer (GC-C-IRMS). An Agilent 6890 GCcoupled to a Finnigan-Delta PlusXP IRMS via a GCcombustion III interface was used for thedetermination of carbon isotope ratios. One ml of thedesorbed gas was injected into the GC which wasequipped with a “Pora Plot Q” capillary column, 25m in length and with a diameter of 0.32 mm, in splitlessmode with helium as the carrier gas at a fixed oventemperature of 28oC. The chromatographicallyseparated hydrocarbon gases eluted from the GCcolumn entered a pre-oxidized Cu-Ni-Pt combustionreactor maintained at 960oC, where they wereconverted into carbon dioxide and water. Water wasremoved using a Nafion membrane tube prior to entryinto the mass spectrometer. The purified CO
2 after
combustion entered the mass spectrometer for 13C/12C ratio measurement of the individual hydrocarboncomponents. The GC-C-IRMS was calibrated usingNatural Gas Standard (NGS-1) mixture and reportedto the Vienna PeeDee Belemnite (VPDB). Replicateanalyses of this gas yielded values within 0.5 ‰ ofthe reference value.
Soil IodineFor the determination of iodine concentrations, 100mg of <63 μ soil sample was treated with 5 ml of10% tetra-methyl ammonium hydroxide in a Savillexpressure decomposition vessel at 80oC for 6 hrs. Aftercooling, the solution was diluted with Millipore waterand 1 ml of 250 ppb antimony was added to act as aninternal standard, and the volume was made up to 25ml. The final solution containing 2% TMAH and 10ppb antimony was centrifuged and filtered. The
72 Near surface geochemical data, Saurashtra Basin, India
analyses for samples, blank and soil standards wereperformed on a PerkinElmer Sciex DRC II InductivelyCoupled Plasma Mass Spectrometer (ICP-MS). In theprocedure mentioned, SO-1 (I=12 ppm), a matrix-matching international soil standard reference material(Canadian Certified Reference Material; CCRM), wasused to calibrate the ICP–MS. Online Appendix S2(see note on page 83) provides details of the soilreference materials, where the measured values are inclose agreement with the certified values. The relativestandard deviation (RSD) of selected samples was <1%and the limit of detection was 0.027 mg kg-1. Detailsof the analytical procedure were described by Balaramand Rao (2003) and Mani et al. (2007).
Total Organic Carbon (TOC)About 1.5 g of 63 μ soil sample was treated with 3-4drops of HCl to remove the inorganic carbon and keptovernight at 50oC in an oven. About 50 mg of thedried, HCl-treated sample was loaded onto a quartzboat and transferred to the furnace of a Soild Module1000°C, a modification of the Liqui TOC analyzerbasic unit (Elementar Analysensysteme GmbH) forTOC measurement. Boden Soil Standard (TOC =4.1%) was used to calibrate the instrument. The CO
2
released due to the chemical oxidation of the organiccarbon was measured by the Infra Red (IR) detectorand expressed in wt %. The relative standard deviation(% RSD) of the procedure was < 1%. The sampleswere also measured for their total carbon (TC) contentwith the untreated soil samples. The inorganic carbon(IC) was determined from IC = TC - TOC.
FluorescenceTen near-surface sediment samples with high valuesfor the adsorbed hydrocarbons were selected forfluorescence studies to measure naphthalene,phenanthrene and anthracene concentrations and alsosubjected to total scanning fluorescence (TSF), usinga Perkin Elmer LS-55 spectrophotometer. One gramof finely powdered soil sample was extracted with 15ml of hexane in an ultrasonic bath (Bandalin Sonorex)for 15 minutes. The sediment extract was centrifugedand the supernatant was used for fluorescencescanning. For the individual aromatic compounds, theinstrument was set using following parameters:
For the TSF method, the sample extracts wereirradiated with light scanning between 200 to 500 nmat an interval of 5 nm and corresponding emission
wavelengths were recorded. The excitation andemission split width were 5 nm each.
X-Ray DiffractionMineral identification for ten selected samples wascarried out on a Bruker AXS –D8 Advance X-RayPowder Diffractometer with Cu Kα radiation at theIndian Institute of Chemical Technology (IICT-CSIR,Hyderabad). The ground and sieved sample powderswere subjected to X-ray diffraction analyses. Theinstrument parameters were set as: start, 2000o; end,79.998o (diff. angle); step time, 13.6 s; anode, Cu-WL1:1.5406.
RESULTS
Table 1 provides a summary of the results of thegeochemical analyses and further details are presentedin online Appendix S1 (see note on page 83).
The statistical distribution of the data was evaluatedto determine whether different groups or populationsare present within the data-set and also to distinguishanomalous populations from the background (Beltand Rice, 2002; Koch and Link, 1971; Lepeltier, 1969;Sinclair, 1976). Log histograms, probability diagramsand cross-plots were the statistical techniques usedfor this purpose.
Adsorbed Soil GasHistogramsLog histograms for the adsorbed gases (C
1-C
5) show
the presence of different populations in the data (Fig.3a-e). In general, the frequency distribution patternshows more than one modal peak with positiveskewness in all the hydrocarbon components. Thelowest concentrations shift towards smaller valueswith an increase in carbon number, i.e. the pentaneconcentration was found to be the lowest (Fig. 3e).Also, the frequency range becomes narrower withan increase in carbon number from methane topentane. These characteristics are commonlyobserved for hydrocarbon gases derived fromthermogenic processes (Klusman, 1993; Dai Jinxinget al., 1992).
Cross-plotsA cross-plot illustrates the correlation between twocompositional variables and provides information ona hydrocarbon’s source and the effects of secondaryalteration. Cross-plots between light gaseoushydrocarbons (C
1-C
5) are plotted in Fig. 4a-c. The
plots of C1 versus C
2 (Fig. 4a) and C
2 versus C
3 (Fig.
4b) show linear correlations, suggesting athermogenic origin for the hydrocarbons. The plotof C
1 with ΣC
2+ (sum of ethane, propane, i-butane,
n-butane, i-pentane and n-pentane) indicates a
73D. Mani et al.
0 0.8 1.6 2.4 3.2Conc. of C3(log ppb)
0
10
20
30
40
No.
of S
ampl
es
0 0.8 1.6 2.4 3.2Conc. of C2(log ppb)
0
10
20
30
40
No.
of S
ampl
es
0 0.8 1.6 2.4 3.2Conc. of C4(log ppb)
0
10
20
30
40N
o. o
f Sam
ples
0 0.8 1.6 2.4 3.2Conc. of C5(log ppb)
0
10
20
30
40
No.
ofS
ampl
es
0 0.8 1.6 2.4 3.2Conc. of C1(log ppb)
0
10
20
30
40
No.
of S
ampl
esa b
c d
e
Fig. 3. Histogram showing the presence ofdifferent populations in the data set for:
Table 1. Summary of geochemical results from soil samples from the Jamnagar area.
bimodality (Fig. 4c), possibly indicating two differentsources for the hydrocarbons (Belt and Rice, 2002;Jones and Drozd, 1983).
Probability diagramsThreshold and anomaly estimates using probability plotsor cumulative frequency diagrams are widely used inthe interpretation of surface geochemical data (Abrams,2005; Sinclair; 1976; 1991). A probability plot betweenthe cumulative frequency (%) and the concentration ofdesorbed light gaseous hydrocarbons C
1 and ΣC
2+ is
shown in Figs. 5a and b, respectively. The changein slope at about 1.75 log ppb of methane wasdefined as the threshold value, which indicates that~27% of the samples with concentration above 56ppb are likely to be anomalous (Fig. 5a). The lowerportion of the plot represents the backgroundconcentration range of the methane. The thresholdfor ΣC
2+ is defined at a concentration of 2.25 log
ppb, and the anomalous samples above 142 ppbare represented by ~ 10% of the data set in theprobability diagram (Fig. 5b).
Min. 2315 34.40 25.23 9.08 Quartz, Feldspar, Carbonate,
Max. 10670 97.66 96.45 1.29 and Smectites
TOC1 = 1.2- 3.5 m depth; TOC2 = 2-6 inches depth
δ δ δ
74 Near surface geochemical data, Saurashtra Basin, India
Bernard plotsStable carbon isotopes yield information on the originof hydrocarbons from varied locations or sources(Fuex, 1977; Faber and Stahl, 1983; 1984; Schoell,1983; Stahl, 1977; Stahl et al., 1981; Whiticar, 1996;1999). A Bernard plot can be used to differentiate lighthydrocarbon gases derived from thermogenic orbacterial sources (Bernard et al., 1976). C
1/C
2+C
3
ratios <100 and δ13C1 heavier than -60 ‰ are
characteristic of thermogenic hydrocarbons; whereasC
1/C
2+C
3 ratios > 1000 and δ13C
1 lighter than -60 ‰
indicate an origin from biogenic sources (Bernard etal., 1976). A Bernard plot showing the relationshipbetween methane δ13C
1 and C
1/C
2+C
3 from the soil
samples from the Saurashtra Peninsula is shown inFig. 6. The plot indicates the likelihood of athermogenic source for the light hydrocarbons.
Soil iodine concentrationsA probability plot between the cumulative frequencyand the concentration of iodine in ppm is shown inFig. 7. The presence of multiple populations isindicated by the change in slope of the curve. Thepopulation with lower values is assumed to represent“background”; the middle set of values are transitional;and the highest values are considered to be anomalous.The inflection point at about 1.5 log ppm of iodinehas been defined as the threshold value (Fig. 7). About15% of the samples with concentrations above 28ppm of soil iodine are therefore likely to be associatedwith a seepage-related anomaly.
DISCUSSION
Light hydrocarbonsGases of thermogenic origin generally show a trendof decrease in concentration from methane to pentanei.e. C
1>C
2>C
3>C
4>C
5 (Klusman, 1993; Tedesco, 1995;
Sanez, 1984). The light hydrocarbon gases from theJamnagar soil samples are highly correlated and followa similar pattern as shown by the frequencydistribution charts (Fig. 3 a-e) and cross-plots (Fig.4a-c).
The isotopic signatures of a few samples, J-79,J-98, and J-100, also show a progressive depletion inδ13C from propane to ethane to methane (see onlineAppendix S1). However, an enrichment of δ13C
2
compared to δ13C3 is observed in a few samples which
may be due to mixing between thermal gases ofdifferent maturities (Berner and Faber, 1988; Chunget al., 1988; James, 1990; Mani et al., 2011b). Thesegases appear to have been derived from mixed sourcesas indicated by the bi-modality of the cross-plot inFig. 3c. Detailed isotopic studies of the adsorbed gasesfrom the Jamnagar area indicate that they are anadmixture of thermally-generated hydrocarbon gasesfrom humic and sapropelic kerogens (Mani et al.,2011 b).
Soil iodineThe concentration of soil iodine in samples from theJamnagar area shows relatively higher valuescompared to the average distribution of iodine insedimentary rocks. Shales, sandstones and carbonates
Fig. 4. Soil gas hydrocarbons cross-plot of (a)
methane (C1) versus ethane (C
2); (b) ethane (C
2)
versus propane (C3); and methane versus ethane
plus higher hydrocarbons (ΣΣΣΣΣC2+
).
R² = 0.9
200
300
400
500
of C
2 (p
pb) a
0
100
200
0 100 200 300 400 500 600
Con
c. o
Conc. of C1 (ppb)
R² = 0.9
200
300
400
of C
3 (p
pb) b
0
100
0 100 200 300 400 500
Con
c. o
Conc. of C2 (ppb)
R2 = 0.9
600
800
1000
1200
C2+
( pp
b)
c
R2 = 0.9
0
200
400
600
0 200 400 600
Con
c. o
f C
Conc. of C1 (ppb)
75D. Mani et al.
Fig. 5.Cumulative frequency diagrams showing the background and anomalous populations of (a) methane and
(b) ethane plus higher hydrocarbons in soil samples from the Saurashtra Peninsula.The anomaly threshold isdefined where the change of slope occurs. In Fig. 5a, the slope change occurs at 1.75 log ppb and matches withthe mean (56 ppb). In Fig. 5b, the change of slope occurs at 2.25 log ppb and matches with the mean plus half
of the standard deviation (60 + 150/2).
10
100
1.50
2.00
2.50
dine
(ppm
)
ne (l
og p
pm)
Anomalous Values
1
10
0.00
0.50
1.00
0% 20% 40% 60% 80% 100%
Con
c. o
f Iod
Con
c. o
f Iod
in
Cumulative Frequency
Background Values
AnomalyThreshold
Fig. 6. Log C1/(C
2+C
3) versus δ δ δ δ δ13C
1 (Bernard plot) for
adsorbed light hydrocarbon gases from the
Jamnagar area, Saurashtra Peninsula (modified afterMani et al., 2011).
Fig. 7. Cumulative frequency diagram for soil iodine concentrations in soil samples from the Saurashtra Basin.
-90-70-50-30δ13CH40/00
1
10
100
1000
10000
100000C
1/C
2+C
3
Biogenic
Thermogenic
2
2.5
3
3.5
100
1000
C1
(log
ppb)
C1
( ppb
)
Anomalous values
0
0.5
1
1.5
1
10
0% 20% 40% 60% 80% 100%
Con
c. o
f C
Con
c. o
f C
Cumulative Frequency
Background ValuesAnomaly Threshold
2
2.5
3
3.5
100
1000
C2+
(lo
g p
pb
)
C2+
(p
pb
)
AnomalousValues
AnomalyThreshold
0
0.5
1
1.5
1
10
0% 20% 40% 60% 80% 100%
Co
nc o
f
Co
nc o
f
Cumulative Frequency
Background Values
76 Near surface geochemical data, Saurashtra Basin, India
contain an average of 2.3, 0.8 and 2.7 ppm of iodine,respectively (Kebata-Pendias and Pendias, 1992; Fugeand Johnson, 1986), whereas iodine concentrationsin soil samples from the Jamnagar area are up to 68.4ppm. Organic matter is believed to be a majorconcentrator of iodine in petroliferous sedimentarybasins (Birkle, 2006; Collins and Egleson, 1969;Fabryka-Martin et al., 1985; Kartsev, 1959; Mani etal., 2011a; Moran et al., 1995; Moran, 1996; Fehn etal., 2007; Muramastu and Wedepohl, 1998; Sheppardet al., 1995; Vinogradov, 1959; Whitehead, 1978).Soil iodine geochemistry has been related to subsequentdrilling results, and iodine anomalies have beenobserved to exhibit a strong correlation withhydrocarbon accumulations (Leaver and Thomasson,2002). Fixing of iodine in soil particles due to theinteraction of seeping hydrocarbons with iodine inthe form of iodo-organic compounds (Allexan et al.,1986; Tedesco, 1995), or transport through verticalmigration of subsurface brines, have been suggestedas probable causes for elevated concentrations of soiliodine (Moran, 1996; Fehn et al., 2007; Land, 1991).The source of iodine in brines in basins with largeaccumulations of hydrocarbons may be the organicmatter which is the precursor of the crude oil.Elevated levels of iodine in the soil samples from thestudy area may result from the fixing of iodine by theorganic compounds derived from the micro-seepageof hydrocarbons in the near-surface environment.
The possibility of recent organic matter bindingwith soil iodine cannot be ignored; hence, soil sampleswere analyzed for their TOC content to investigatethe correlation of surficial TOC with the iodineconcentration. The TOC content of the soil samplesvaries between 2315 ppm and 1.067 %, and shows apoor correlation with soil iodine (r<<1). This suggeststhe possibility of a different organic component as asource of the interaction with iodine. This componentcould be the seeping hydrocarbons.
The lack of binding of iodine with surficial organicmatter is supported by the soil types occurring in thestudy area. Soils generally contain 0.01-6 mg/kg ofiodine (Kebata-Pendias and Pendias, 1984). Theaverage content of iodine in continental shales andlimestone is 1.8 ppm and 2.5 ppm, respectively,whereas greywackes and sandstones on averagecontain 150 ppb and 120 ppb, respectively(Muramastu and Wedephol, 1998). Most of theSaurashtra Peninsula is covered with red soils whichare weathering products of the basalts, together withalluvium and some black soil (Merh and Chamyal,1993). These soils are highly argillaceous and containhigh proportions of calcium and magnesiumcarbonates; however they are poor in organic matter.These soil lithologies have a low potential to fix iodine(Fuge, 1987).
In the sampled area, the geometric mean value (μg/g) of bound iodine in soil fractions is <2.2 (Fuge andJohnson, 1986). However, iodine values reported hereare significantly high, indicating the possibility of adifferent organic component source which may beseepage related. An increase in soil iodine is alsoobserved to occur with an increase of hydrocarbongas concentration at most locations in the study area(Fig. 8). A good correlation between the two variablescan be seen, and the pattern of iodine anomaliesmatches with the soil gas anomalies, especially aroundLaloi and Khandera.
Microseepage-related anomaliesThe near-surface expression of hydrocarbon micro-seepage may vary greatly but the mechanisms causingthese variations are poorly understood. Diversephysico-chemical variations can cause non seepage-related anomalies. To ascertain the occurrence of sub-surface seepage, total scanning fluorescence (TSF),TOC/TIC and XRD studies were carried out onselected samples.
Fig. 8. Composite map of iodine with desorbed
hydrocarbon gases (ΣΣΣΣΣC2+
). Symbols show therange of iodine concentration and the contoursare the concentration of adsorbed hydrocarbon
gases.
77D. Mani et al.
Fluorescence spetrometryUltraviolet fluorescence spectrometry can be used todetect and measure aromatic hydrocarbons in near-surface soils and sediments which may have migratedfrom underlying reservoirs (Abrams, 2005; Barwiseand Hay, 1996). Recent organic matter, oil spills andnon petroleum-related compounds also fluoresce, butthe maximum fluorescence of these compounds isobtained at different wavelengths and intensities,allowing them to be discriminated from migratedhydrocarbons. In total scanning fluorescence (TSF),the maximum fluorescence intensity (MFI) is recorded
along with the emission wavelength (Max_Em) andexcitation wavelength (Max_Ex) (Brooks et al., 1983;Abrams, 2005).
The results of fluorescence studies are summarizedin Table 1 and the details with MFI data are given inTable 2. It is notable that naphthalene, phenanthreneand anthracene occur in all the samples analysed byfluorescence spectrometry. Three-dimensionalfluorograms of two samples (J-79 and J-142) areshown in Figs. 9a and 9b. The MFI for these samplesis 480 and 705 nm, respectively. The overall patternof fluorescence intensities and concentrations are
78 Near surface geochemical data, Saurashtra Basin, India
consistent with other geochemical parametersmeasured, and indicate the possibility of seepage-related compounds in the near-surface soils.
TOC/ TIC contentsA check on false anomalies can be achieved from theTOC/TIC content (Abrams, 2010; Brekke, 1997).Traces of light hydrocarbons such as ethane, propane,butane and pentane, as well as methane, are producedby microbial activity on surficial organic matter andhave previously been reported (Hunt, 1980; 1981;1984; 1995; Klusman; 1996). If the light hydrocarbongases are assumed to be derived from deep-lying,organic-rich source rocks associated with oil and gasaccumulations, then the TOC of near-surface organicmatter should show a poor correlation with theconcentration of light hydrocarbon gases.
Mineralogical differences in soil samples can alsocause false anomalies (Brekke, 1997). The TOC
andTIC content of fifty-three soil samples from theSaurashtra Peninsula showed a very poor correlation(r2 << 1) with the acid-extracted ethane and higherhydrocarbon yield (ΣC
2+) (Fig 10). This suggests that
the adsorbed gases in the soil samples do not haveany correlation with the organic matter of the surficialsediments, supporting the likeliness of a seepage-induced anomaly.
To further discriminate carbonate-inducedanomalies in the near-surface sediments, ten sampleswith varying gaseous hydrocarbon concentrationswere subjected to X-ray diffraction studies. The XRDanalyses showed that clays, carbonates, quartz andfeldspar were the main mineral phases. Table 3 showsthe mineralogy and light gaseous hydrocarbonconcentrations of the selected soil samples, and theXRD spectra typical of high and low gas concentrationare represented in Figs. 11a and 11b, respectively. Atrend that is evident from the XRD observations is
R² (TIC) = 0.05
30
40
50
60
pp
mx
10
00
)
TOC TIC
R² (TOC) = 0.2
0
10
20
30
0 200 400 600 800 1000 1200
TO
C/T
IC (
p
∑C2+ (ppb)
Fig. 10. Variation of TOC and TIC with
C2+
hydrocarbons.
Table 3. Data of light hydrocarbon gases with minerals and clays identified using XRD in soil samples from theJamnagar area.
that higher gas yields are represented by a mineralassemblage in which quartz and clays are dominant.
The increased presence of carbonates is typical ofsamples with a comparatively lower light gaseoushydrocarbon concentration. Hence, it is unlikely thatthe occurrence of light hydrocarbon gases in the near-surface soils of the Jamnagar area is carbonateinduced.
INTEGRATION OF GEOCHEMICAL DATAWITH GEOLOGICAL AND GEOPHYSICALDATA
A model integrating the geochemical observations withgeological and geophysical data is shown in Fig. 12.Focussed fluid migration along faults, discontinuitiesor unconformities is more effective than non-focussedseepage through a sedimentary column (Abrams,1992; Brown, 2000). Intrusions, such as sills anddykes, represent preferential pathway for fluidmigration and may be associated with activehydrocarbon seepage (Gay et al., 2006; Rollet et al.,2006). A series of dolerite dykes mainly trending NE-SW and NW-SE occur in the Saurashtra Peninsula.
The Jamnagar area is almost entirely covered bythe Deccan Trap volcanics and numerous dykes arepresent. Two NNW-trending faults are identified 40km west and 30 km east of Jamnagar, respectively
(Reddy, 2005), and a major NNW trending dykeoccurs close to Khankotda (Chopra et al., 2008).Light hydrocarbons and soil iodine concentrationswere plotted on the map of dykes and lineaments(Fig. 12). High values of light hydrocarbons wereseen around the dykes and lineaments, particularly inthe south. In the NE where dykes and lineaments areabsent, soil hydrocarbon concentrations appear to below. Thus elevated hydrocarbon and iodineconcentrations are coincident with dykes andlineaments, which probably serve as preferentialconduits for micro-seepage of hydrocarbons.
Geophysical studies (Satpal et al., 2006) showthat thick Mesozoic sediments (about 2 km) occur inthe NW of the Saurashtra Peninsula around Jamnagar.The Deccan Traps are in general 1.5–3 km thick inthe south, but are thinner in the NW, possibly allowingthe migration of hydrocarbons towards the surfacehere. A map of light gaseous hydrocarbons ΣC
2+
contoured over the Mesozoic sediment thickness(Satpal et al., 2006) is shown in Fig.12. Theconcentrations of adsorbed light hydrocarbonscorrelate well with sediment thickness, and highconcentrations occur over areas with higher sedimentthicknesses, especially near Laloi. When theconcentrations of hydrocarbons are related to Trapthickness, it appears that the adsorbed gas anomaliesare located in areas where Trap thickness is less
Fig. 11a. XRD spectrum of soil sample J-79
showing that this sample with a high gasyield has a mineral assemblage dominatedby clays and quartz.
Fig. 11b. XRD spectrum of soil sample J-39showing that this sample with a low gas yield
has a mineral assemblage dominated bycarbonates.
J-79
J-39
80 Near surface geochemical data, Saurashtra Basin, India
compared to other regions. Thus the surfaceprospecting methods complement the geological andgeophysical results available for the area.
CONCLUSIONS
Using an integrated approach involving near-surfacegeochemical indicators, light gaseous hydrocarbonsand their isotopic compositions and soil iodineconcentrations, this study suggests that thermogenichydrocarbons occur in the subsurface of the basalt-covered onshore Saurashtra Basin. Elevatedconcentrations of adsorbed hydrocarbon gases wereobserved around Laloi and around Khandera. A fewpoint anomalies are scattered south of Kalavad andMatwa. Fluorescence studies showed the presenceof high molecular weight hydrocarbon, notindigenous to near-surface soils, and indicate that theoccurrence of near-surface PAHs (napthalene,anthracene and phenanthrene) is seepage related. Theconcentrations of light hydrocarbon gases correlatewell with soil iodine concentrations. Near-surfacelithology and mineralogy as indicated by the XRD,TOC, and TIC studies rule out the possibility of falsenear-surface anomalies, indicating the sources oftheses gases to be deep-seated. The geochemical
results are in accordance with the occurrence ofdykes, faults and lineaments which may be migrationconduits.
ACKNOWLEDGEMENTS
D.M. is grateful to the Council for Scientific andIndustrial Research (CSIR) for a ResearchAssociateship. B. Kumar is thanked for theencouragement to pursue the study. C. Vishnu Vardhanand M. A. Rasheed are acknowledged for theircontributions towards the fieldwork. K. Ravi Kumar(CSIR-IICT, Hyderabad) is acknowledged for XRDanalyses. The authors thank the Director, CSIR-NGRI, for permitting the publication of this work.The Oil Industry Development Board, New Delhi, isacknowledged for providing financial assistance forsetting up of geochemical facility. Journal reviews byP. K. Saraswasti and S. Dutta are acknowledged withthanks.
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Supporting informationAdditional supporting information may be found inthe online version of this article:
Appendix S1. Geochemical results of soil sampleanalyses and coordinates of sample locations.
Appendix S2. Certified and measuredconcentrations of iodine with standard deviations (%)using ICP–MS in soil reference materials.
Please note: Wiley-Blackwell are not responsiblefor the content or functionality of any supportinginformation supplied by the authors. Any queries (otherthan missing material) should be directed to thecorresponding author for the article.
