GEOLOGICAL INDIA P.JOURNAL LAKSHMI SRINIVASA SOCIETY RAO ANDOF OTHERS Vol.82, December 2013, pp.666-674
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Correlation of Trace Elements with Hydrocarbon Microseepage P. LAKSHMI SRINIVASA RAO, D. SRINU, M. A. RASHEEDD, M. S. KALPANA, D. J. PATIL and A. M. DAYAL Stable Isotope & Surface geochemical prospecting of Hydrocarbon, CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad – 500 606 Email:
[email protected] Abstract: Direct correlation have been observed between certain trace element and hydrocarbon anomalies in the near subsurface soils of Vindhyan basin, India. This relationship with hydrocarbon is very useful in hydrocarbon exploration. 52 soil samples from Vindhyan basin were collected from a depth of 2.5m. All the soil samples were analyzed for light hydrocarbon, isotope and trace element concentrations. The adsorbed light gaseous hydrocarbon analyses show the presence of methane (8 - 328 ppb), ethane (0 - 27 ppb) and propane (0 -11 ppb) respectively and these values indicate the presence of hydrocarbon micro-seepage in the study area. The carbon isotopic values determined for methane and ethane for these soil samples are (-26.41 to -47.70 ‰ PDB) and (-20.07 to -35.30 ‰ PDB) respectively and they are thermogenic in nature. The trace element concentrations of nickel (33-220 ppm), vanadium (72-226 ppm), copper (20131 ppm), chromium (94-205 ppm), zinc (66-561 ppm) and cobalt (9-39 ppm) have higher than the normal concentrations in soils. Trace element concentrations are used to plot with the data obtained from light gaseous hydrocarbon concentrations and carbon isotopic values of soil samples of the Vindhyan basin. Trace element anomalies have been observed around the hydrocarbon anomalies in the study area. Keywords: Trace elements, Hydrocarbons, Adsorbed soil gas, Vindhyan basin. INTRODUCTION
The trace metal in oil and gas reservoir are nano particles, whereas methane is the important constituent of the ascending air current into the earth’s crust from the oil and gas reservoir through the faults and fractures. The light gaseous hydrocarbons in deep oil and gas reservoirs are ascending with trace element particles to the earth surface (Wang et al. 2008; Wang, D.Y. et al. 2006; Yang, F.G. et al. 2000; Zaho, B.L. et al. 2005). Long term hydrocarbon seepages develop arrays of chemical and mineralogical changes in surface soils and set up near surface oxidation and reduction zones (Shuhab Khan and Jacobson, 2008). The increase in trace metal concentrations near oil/gas producing areas, suggests a changes in soil chemistry under reducing environment, presumably due to the influence of hydrocarbon microseepage (Madhavi et al. 2011). Continuous leakage of hydrocarbons and associated fluids like hydrogen sulphide produce a reducing environment and initiate digenetic Eh/pH controlled reactions in the rocks and soils overlying hydrocarbon accumulations. These chemical reactions cause dissolution mobilization of various compounds as well as precipitation of minerals in the stratigraphic column above the leaking reservoir (Petrovic
et al. 2009). Trace metals in the soil are able to form organometallic complexes under reducing conditions that can be found above petroleum reservoir (Schumacher and Abrams, 1996). The organic matter interaction with trace element is due to their adsorption properties because of the modification of mineral surface properties with organo mineral complexes (Staunton, 2002). Trace element associations forming organometallic compounds have been found “haloed” or concentrated over or around underlying hydrocarbon reservoirs (Duchscherer, 1983). Geochemical halos for vanadium (V), chromium (Cr), manganese (Mn), nickel (Ni), cobalt (Co), copper (Cu), molybdenum (Mo), uranium (U), iron (Fe) and zinc (Zn) have been reported by Duchscherer (1984) over oil fields in Texas and Montana, United States. Manganese (Mn), vanadium (V), nickel (Ni), and copper (Cu) trace elements, as well as the radioactive elements like uranium (U) and radium (Ra), were found to be adsorbed to a greater extent around the periphery of Kyurov-Dag Oil Field in the Azerbaijan than actually over the field (Alekseev et al. 1961). The main objective of this study is to appraise the usefulness of trace elements data by comparing with the data obtained using adsorbed soil gas analyses in
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hydrocarbon exploration. Trace elements may be path finder or otherwise important in search for hydrocarbon anomalies. GEOLOGY
The Vindhyan succession of central India is twittered, about 4500m thick and consists of mildly deformed and unmetamorphosed siliciclastics and carbonates exposed over wide areas in central India (Fig. 1). Vertical section of Vindhyan supergroup is given in Table 1 (Bose et al. 2001). The Semri Group comprises of siliciclastics, carbonates and volcanic clastics. The Kajrahat limestone and the Rohtas limestone are the two important carbonate units of the Semri Group. A thin, discontinuous dolomite unit occurs at the mid-level of the predominantly siliciclastic Kheinjua Formation and is known as the Fawn dolomite. The upper Vindhyan Group is dominated by clastics and the sole carbonate unit, the Bhander limestone occurs near the top of the succession. Preservation of delicate sedimentary features is a distinct characteristic of the entire Vindhyan succession. Detailed facieses analysis is thus possible on all the carbonate units of the Vindhyan Supergroup. Excellent preservation of sedimentary features, dominantly micritic nature of the carbonate sediments and lack of compaction features rule out significant burial diagenesis of the
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carbonates (Bose et al. 2001; Chakraborty, 2004; Arun K. Shandilya, 2008). Vindhyan sedimentation took place largely in a shallow marine setting (Chanda and Bhattacharya, 1982; Bose et al. 2001), although minor paleo geographic shifts have been reported in a few recent studies (Chakraborty, 2004; Banerjee and Jeevankumar, 2005). It is believed that the initial sedimentation took place in an intracratonic rift basin and then a transition to intracratonic sag basin took place during the upper Vindhyan. MATERIALS AND METHODS
A total of 52 soil samples (Fig. 2) were collected from a depth of 2.5 m using a hollow metal pipe by manual hammering to the required depth. The cores collected were wrapped in aluminium foil and sealed in poly-metal packs. The light gaseous hydrocarbons were desorbed from soil samples by treating with orthophosphoric acid. The desorbed gas was injected into Varian CP-3380 Gas Chromatograph (GC). The gas concentrations are reported in ppb on dry weight basis. The GC accuracy of measurement of C1 to C4 components is ±1 ppb with a precision of 5%. Carbon isotopic composition of light hydrocarbons in soil samples is determined using GC-CIRMS. The carbon isotopic composition is reported in
Table 1. Lithostratigraphic table of Vindhyan basin (after Bose et al., 2001)
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Fig.1. Geological Map of Vindhyan basin (modified after Rajendra Prasad et al. 2008).
Fig.2. Sample location map of study area.
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per mil (‰) relative to the Pee Dee Belemnite (PDB). The precision of the isotopic analysis is ± 0.5 ‰. Analysis of Trace Elements
52 soil samples of Vindhyan basin were analyzed for trace elements by ICP-MS (Inductively Coupled Plasma Mass Spectrometer) technique at CSIR-National Geophysical Research Institute (NGRI), Hyderabad. Using this method precession of 1-7% relative to standard deviation and comparable accuracy was obtained. The analytical procedures and protocols followed at CSIR-NGRI are the same as given in Naqvi et al. (2006), Balram et al. (1996) and Ganeshwar and Govil (1995). RESULTS AND DISCUSSION
Soil samples were analyzed for trace elements nickel (Ni), vanadium (V), copper (Cu), chromium (Cr), zinc (Zn) and cobalt (Co) concentrations. Trace elements have been used as indirect surface indicator for petroleum accumulations. The objective of this paper is to illustrate the relationship between hydrocarbon anomalies and trace element anomalies. The trace elements concentration ranges and useful statistical values are summarized in Table 2. It was observed that the trace elements concentrations were tremendously increased when compared with actual concentration in the soils (Vinogradov, 1959). A comparison of the contour maps of hydrocarbons with contour maps of trace elements (Figs. 3 to 8), clearly indicate that trace elements are accumulated around the hydrocarbon anomalies. Particularly nickel (Fig. 3), vanadium (Fig. 4), zinc (Fig. 7) and cobalt (Fig. 8) which are clearly located in the region of hydrocarbon anomalies. The halo pattern is readily apparent. Good correlation has not been observed in copper (Fig. 5) and chromium (Fig. 6) with hydrocarbon anomalies. Trace elements occur as a near surface ‘halos’ which will be pathfinder or otherwise important in the search for hydrocarbon anomalies (Siegel, 1974). The main retention processes of metal ions at soil surfaces include adsorption, surface precipitation and Table 2. Statistical data of trace elements Nickel (ppm)
Vanadium (ppm)
Copper (ppm)
Chromium (ppm)
Zinc Cobalt
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fixation. Adsorption is a major process responsible for their accumulation. Adsorption process of metals in soils depends on the soil pH, competing ions and soil composition. Surface functional groups are vital for adsorption process. Oxygen contained functional groups are almost universally implicated in metal bonding. Due to interaction between organic matter and metals, soil organic matter influences the immobilization of trace elements in soils. Most of the organic matter surfaces are negatively charged. These are called exchange sites. Since most metals are positively charged, they are attracted and held in these exchange sites. Increased amounts of soluble Ni, V, Cu, Cr, Zn and Co have been observed in the reducing environment caused by the seepage of hydrocarbons. In the reducing environment solubility of metals increases and transport occurs. Due to hydrodynamic flow, soluble elements move around the soil, although they do not leave the reducing area (Tedesco, 1995). The boundary is formed between the reducing and oxidizing zone by the deposition of carbonate, oxide and sulphide minerals. Several metals including Ni, V, Cu, Cr, Zn and Co are mobilized in soils and accumulated around the hydrocarbon anomaly when impeded hydrocarbon accumulation leads to reducing condition (Arie Nissenbaum and Swaine, 1976). This feature in combination with other geochemical, geophysical and geological techniques may help to understand the characteristics of the petroleum reservoir. The number of sample sites, ranges of values and useful statistical values are summarized in Table-3. Good correlation observed among C1-C2, C1-C3, C1-σC2+, C2-C3, C2-σC2+, C3-SC2+. The geochemical signature (gas, gas/oil) is determined by using ratios of hydrocarbon constituents detected in the soil-gas samples. Based on Pixler (1969) diagram (Fig. 9), the majority of the soil samples fall in oil and gas condensate zone. Bernard et al., (1978) suggested a genetic diagram by correlating [C1/(C2+C3)] ratio with the δ13C concentration of methane. In the present study, based on the Bernard diagram (Fig. 10) all the samples fall with in the thermogenic field and might be derived from Type-III Table 3. Summary statistics of adsorbed soil hydrocarbon gas concentration in the study area Methane (ppb)
(ppm) (ppm)
Ethane (ppb)
Propane (ppb)
ΣC2+ (ppb)
Minimum
33
72
20
94
66
9
Minimum
8
0
0
0
Maximum
220
226
131
205
561
39
Maximum
328
27
11
37
Average
68
131
59
127
113
19
Average
94
8
3
10
Actual concen-
40
100
20
100
50
8
Std. Deviation
68.7
6.5
2.6
9.16
Soil gas sample (%)
100
94.3
79.2
94.3
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Fig.3. C2+ (Ethane + Propane) vs Nickel anomaly.
Fig.4. C2+ (Ethane + Propane) vs Vanadium anomaly
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Fig.5. C2+ (Ethane + Propane) vs Copper anomaly map.
Fig.6. C2+ (Ethane + Propane) vs Chromium anomaly map.
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Fig.7. C2+ (Ethane + Propane) vs Zinc anomaly.
Fig.8. C2+ (Ethane + Propane) vs Cobalt anomaly
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Fig.9. Pixler diagram.
kerogen. Carbon isotope variations in ethane related to carbon isotope variations in methane (Schoell, 1983) in this study shows that all samples (Fig. 11) fall in deep migration of thermogenic source (arrow of Md). Arrow Md indicates that mixing a deep dry gas (predominantly methane) with an associated gas change the ä 13C1 / 13C 2 relationship accordingly.
Fig.11. Schoell diagram. CONCLUSIONS
Trace element concentrations have been used as indirect indicator for hydrocarbon microseepage. This study compares the trace elements concentrations with obtained adsorbed soil gas results and explains how the trace elements concentrations vary near hydrocarbon anomalies. The compositional correlation is fundamental for understanding the link between trace element concentrations and hydrocarbon anomaly. This type of analogy is needed to be confirmed by analyzing the samples from other productive and prospective zones. In this present prospective zone the microseepage of hydrocarbon adsorbed on the soil is petroliferous in nature. Hydrocarbon constituents of the microseeps are cogenetic and have not been influenced significantly by the secondary effects during their migration. Molecular analyses of gases indicate that the gases are associated with oil and gas condensate zone. δ13C1 and δ13C2values indicate thermogenic source for these light gaseous hydrocarbons present at the near surface soil samples. Enrichment of trace element concentrations around the hydrocarbon anomaly suggests the correlation between microseepage and trace elements concentrations. Enrichment of trace element concentrations in one or more prospective zones around hydrocarbon anomalies can judge the trace elements may
Fig.10. Bernard diagram JOUR.GEOL.SOC.INDIA, VOL.82, DEC. 2013
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be a pathfinder or other wise important search tool for hydrocarbon anomaly. Acknowledgement: The project coordinators and coworkers are thankful to Director, NGRI for providing
necessary facilities. Dr. V. Balram is highly acknowledged for ICP-MS analysis. Shri Arun Kumar, Secretary, OIDB is gratefully acknowledged for encouragement and funding towards setting up of “National Facility for Surface Geochemical Prospecting of Hydrocarbons” at NGRI.
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(Received: 7 April 2010; Revised form accepted: 19 December 2012)
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