uAbstract

uFigure captions

uIntroduction

uGeology

uProcedures

uResults

uReferences

uAbstract

uFigure captions

uIntroduction

uGeology

uProcedures

uResults

uReferences

uAbstract

uFigure captions

uIntroduction

uGeology

uProcedures

uResults

uReferences

uAbstract

uFigure captions

uIntroduction

uGeology

uProcedures

uResults

uReferences

Figures Captions

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Introduction

Hydrocarbons generated and trapped beneath the surface seep or leak to the surface in varying but detectable quantities. These phenomena occur because processes and mechanisms such as diffusion, effusion, and buoyancy allow hydrocarbons to escape from reservoirs and migrate to the surface where they may be retained in the sediments and soils or diffuse into atmosphere or water columns (Klusman, 1993; Schumacher and Abrams, 1996). Based on these assumptions, various techniques of surface geochemical prospecting for hydrocarbons have been developed to identify the surface or near-surface occurrences of hydrocarbons. Surface geochemical prospecting for hydrocarbons consist of direct and indirect methods to identify the microseepage. These methods include adsorbed soil gas surveys, microbial techniques, soil salts, bitumen, and trace element techniques, helium emanometry, etc. In order to meet the challenges of India Hydrocarbon Vision 2025 for Exploration & Production sector, a National Facility for surface geochemical prospecting of hydrocarbons has been nucleated at NGRI with a grant from Oil Industry Development Board (OIDB), New Delhi (Kumar et al., 2002a). As a part of this National Facility and under its own Research & Development program, NGRI is carrying-out/planning to carry-out surface geochemical surveys for hydrocarbon prospecting in the frontier onland/offshore basins of India. Adsorbed soil gas surveys have been completed in Vindhyan basin (Chambal Valley) and Kutch onland basin, Gujarat (Kumar et al., 2002b, 2003), and the work in Saurashtra basin is in progress. The results of adsorbed soil gas analyses for light gaseous hydrocarbons in one of the frontier basin (Saurashtra basin) and the future strategies for geochemical surveys in other frontier basins of India are discussed.

Geology and Tectonics of Saurashtra Basin

Saurashtra basin, Gujarat, consists of mostly Mesozoic and Cenozoic rocks (Biswas, 1983; Merh, 1995), and stratigraphically the sequence begins with Cretaceous to be followed upward by the Deccan volcanics, Tertiary and the Quaternary (Figure 1a) The area is largely covered and prominently exposed by the Deccan Trap (basaltic rocks), whose thickness varies from few hundreds to thousands of meters. Traps are underlain by thick Mesozoic sediments (100- 4000 m), which can form potential source rocks. Deccan Trap volcanicity during Late Cretaceous may have generated the requisite thermal conditions and acted as a catalyst in Mesozoic hydrocarbon-generation process. Sedimentation in marine intertonguing environments is considered to have been favorable phenomena for hydrocarbon generation and entrapment. The basin is bounded by three intersecting rift trends, Delhi (NE-SW), Narmada (ENE-WSW) and Dharwar (NNW-SSE). Integrated geophysical studies show that two of the sub-basins of Saurashtra, namely Jamnagar and Dwarka, have significant sediment thicknesses below the Deccan traps and can be considered for future hydrocarbon research.

Soil Sampling and Analytical Procedure

Soil samples from Jamnagar and Dwarka sub basins of Gujarat have been collected in the depth range of 1.2 – 3.5 m using manual augers. The soil cores collected were wrapped in aluminum foils and sealed in poly-metal packs. A total of ~290 soil samples were collected in two phases of field work. The sample location map of the area is given in Figure 1b. In the first and second phase the samples were collected in intervals of 5km and 2 km intervals, respectively, along existing roads.

One gram of soil sample is reacted in vacuum with orthophosphoric acid to desorb the soil gases. The CO₂ released was trapped in KOH solution and the light gaseous hydrocarbons are collected by water displacement in a graduated tube fitted with rubber septa. The volume of desorbed gases is recorded, and 500 µl of desorbed gas sample is injected into the Nucon 5765 Gas Chromatograph fitted with ¼" glass-packed column (squalane), programmable temperature controller, and flame ionization detector. The GC was calibrated by using an external standard with known concentrations of methane, ethane, propane, i-butane and n-butane. The quantitative estimation of light gaseous hydrocarbon constituents in each sample was made using peak area measurement as a basis, and the correction for moisture content was applied. The accuracy of measurement of C₁ to C₄ components is < 1 ng/g.

Results and Discussion

The light gaseous hydrocarbon concentrations (CH₄, C₂H₆, C₃H₈, i-C₄H₁₀ and n-C₄H₁₀) in soil samples of Jamnagar sub basin vary from 3 to 766, 3 to 261, 9 to 173, 8 to 45 and 5 to 118, in ppb, respectively. The Dwarka sub basin soils are characterized by C₁-C₄ concentrations in the range of 9 to 145, 3 to 130, 2 to 69, 4 to 29 and 6 to 82, in ppb, respectively. Figure 2 depicts the crossplots between C₁-C₂, C₁-C₃, C₂-C₃ and C₁-ΣC₂₊, showing linear correlation. This indicates that the light hydrocarbon components may have migrated from a thermogenic source, and the effect of secondary alteration during their seepage toward the surface may be insignificant. Pixler (1969) proposed a variation diagram using the ratios of C₁/C₂ and C₁/C₃, etc. to distinguish the non-productive zone from that of oil/gas producing zone. The discrimination diagram of Pixler for samples of Jamnagar and Dwarka sub basins in which methane, ethane, and propane were present suggest that ~70% of these samples fall in the oil-producing zone and the rest in the gas zone. The concentration of ΣC₂₊ is plotted in Figure 3 into three groups (i.e., ΣC₂₊ >100 ppb, ΣC₂₊  = 50-100 ppb, and ΣC₂₊ <50 ppb) and shows that the samples around Jamnagar, Kalavad, and Lalpur are characterized by higher ΣC₂₊ values. This suggests that hydrocarbon generation has taken place in the basin and the area may prove to be warm for future hydrocarbon exploration. The present geochemical data support the earlier geological and geophysical findings.

The adsorbed soil gas surveys carried out by NGRI for Directorate General of Hydrocarbons, New Delhi in Vindhyan basin (Chambal Valley) and Kutch onland basin have also demarcated the warm areas for hydrocarbon research and exploration. The efforts to conduct detailed geochemical surveys in other frontier basins of India are in progress.

References

Biswas, S.K., and Deshpande, S.V., 1983, Geology and Hydrocarbon Prospects of Kutch, Saurashtra and Narmada Basins. In: Bhandari, L. L. et al. (Ed.), Petroliferous Basins of India, pp. 111-126.

Klusman, R.W., 1993, Soil Gas and Related Methods for Natural Resource Exploration. John Wiley & Sons, England, 473 pp.

Kumar, B., Patil, D.J. and Kalpana, G., 2002a, National Facility for studies on surface geochemical prospecting of hydrocarbons. 9th Annual India Oil and Gas Review Symposium, 9-10 Sept. 2002, Mumbai, pp. 274-278.

Kumar, B., Das Sharma, S., Patil, D.J., Sreenivas, B., Raju, S.V., Kalpana, G., and Madhavi, M., 2002b, Adsorbed Soil Gas Surveys for Hydrocarbon Research and Exploration in western part of Vindhyan Basin, Chambal valley.” Submitted to Directorate General of Hydrocarbons, New Delhi. Unpubl. Tech Rep. No. NGRI- 2002-Exp-361.

Kumar, B., Das Sharma, S., Patil, D.J., Sreenivas, B., Raju, S.V., Kalpana, G., Panicker, S.K., and Vishnu Vardhan, C., 2003, Adsorbed Soil Gas Surveys for Hydrocarbon Research and Exploration in parts of Kutch onland basin, Gujarat. Submitted to Directorate General of Hydrocarbons, New Delhi. Unpubl. Tech Rep. No. NGRI-2002-Exp-400.

Merh, S.S., 1995, Geology of Gujarat, Geological Society of India, Bangalore, 222 pp.

Pixler, B.O., 1969, Formation evaluation by analysis of hydrocarbon ratios. Journal Petroleum Technology, v. 21, pp. 665-670.

Schumacher, D., and Abrams, M.A., 1996, Hydrocarbon Migration and its Near Surface Expression, AAPG Memoir 66, 446 pp.

Sreenivasan, S., and D.M. Khar, 1995, Frontier basin exploration in India- Perspectives and challenges. Proceedings of the First International Conference & Exhibition Petrotech 1995, pp. 1-20.

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