Reflectance spectroscopy
Three groups of minerals predominantly present in the hydrocarbon microseepage environments were attempted to identify by reflectance spectroscopy. These were: ferric iron oxides, clay minerals, and carbonate minerals. The most common ferric iron oxides in sediments are hematite and goethite. Hematite shows three distinct absorption bands at 520, 650, and 880 nm (Clark 1999; Viscarra Rossel et al. 2010). Goethite exhibits weaker absorptions at 420, 480, 600, and 1700 nm and a strong absorption near 920 nm (Zheng et al. 2016). Among the clay minerals, kaolinite, montmorillonite, illite, vermiculite, and chlorite are the common clay minerals in the hydrocarbon microseepage environments. Clay minerals exhibit diagnostic absorption features near 1400 nm (caused by OH overtones), 1900 nm (overtones caused by water molecules), 2200 nm (due to Al-OH combination tones) and some weaker absorptions in the 2300–2500 nm range due to presence of Fe- or Mg-OH (Clark 1999; Zhao et al. 2018; Fang et al. 2018). Carbonate minerals show spectral characteristics absorption near 2350 and 2500 nm (Hunt and Salisbury, 1971).
The spectra of the sediments from both the hydrocarbon microseepage affected as well as unaffected areas show characteristics absorptions near 480 nm, 920 nm, 1400 nm, 1900 nm, 2200 nm, 2340 nm, and 2440 nm (Fig. 3). The absorptions near 480 and 920 nm are characteristics of ferric iron minerals. It is observed that the reflectance spectra of fine-grained goethite (ID: MPCMA2-B of USGS Spectral Library) perfectly matches with the spectra of the samples in the ferric iron absorption wavelengths (Fig. 3). The shallow and asymmetric shape of the absorption feature in the interval 800–1000 nm indicates that the goethite is fine-grained non-crystalline in nature and mostly occur as grain coatings (Sheldon and Tabor 2009).
The characteristics absorption features at 1400 nm, 1900 nm, 2200 nm wavelength indicate the presence of clay minerals. The presence of kaolinite is ruled out by the absence of its characteristics spectral doublets near 2160 and 2210 nm (Fang et al. 2018) in the samples. Also, the absorption features characteristics of montmorillonite, vermiculite, and chlorite do not match with any of the spectra of the samples implying their absence in the samples. The characteristics prominent absorption at 1400 nm, 1900 nm, 2205 nm, and the two weaker absorptions near 2340 nm and 2440 nm indicate that the clay minerals are predominantly composed of illite. The reflectance spectrum of illite with sample ID: IL 101 of USGS Spectral Library perfectly matches with the clay absorption features of the samples (Fig. 3). In the spectra of the sediments, absorption features are observed near 2350 nm, indicating the presence of carbonate minerals, but the absorption feature also coincides with the 2340 nm absorption of illite. Also, the absorption at 2350 nm is not very deep, indicating that the carbonates in the sediments are of very low abundance.
The abundance of minerals was attempted by analyzing the continuum removed (CR) spectral curves of the samples. Continuum removal technique is one of the most efficient methods of determining the abundance of minerals and is considered as a feasible substitute for chemical statistical methods in mineralogy studies (Gomez et al. 2008; Viscarra Rossel et al. 2009). The technique generates a hull of boundary points using the local maxima (Gomez et al. 2008). It removes the background noise and highlights the particular absorption feature (Clark and Roush 1984). The strength and depth of the characteristic absorption feature of a mineral on the whole rock reflectance curve show the abundance of the mineral: the more profound the absorption feature, the more mineral is present in the rock sample (Clark 1999). The band depth (BD) at the characteristic absorption wavelength of a mineral was calculated by subtracting the continuum removed reflectance value from 1 (Viscarra Rossel et al. 2009) and was used as a measure of the mineral abundance. The characteristics absorption wavelength for goethite was taken as 941 nm, and for illite at 2205 nm in the continuum removal curves (Fig. 4). As seen in Fig. 4, the continuum removed spectra of fine-grained goethite standard (MPCMA2-B) of the USGS spectral library, also exhibit absorption at the 2205 nm band wavelength. Thus, it is evident that the presence of goethite influences the band depth at 2205 nm for illite. Assuming a linear mixing model in the whole rock, the actual band depth of illite at 2205 nm was calculated by subtracting the band depth contribution of goethite from the gross band depth at 2205 nm. The CR band depth calculation shows that the average CR band depth of goethite at 941 nm is lower (0.0227) in the hydrocarbon microseepage bearing sediments than that of the hydrocarbon non-anomalous sediments (0.0280). Again, it is also observed that the average CR band depth of illite at 2205 nm is higher (0.03128) for samples from microseepage bearing areas than that of the hydrocarbon unaffected sediments (0.0265). Thus, it is evident that the average content of ferric iron mineral is lower, and the average clay content is higher in the hydrocarbon microseepage bearing sediments in comparison to the hydrocarbon unaffected sediments.
XRD Studies
The X-ray diffractograms of the sediment samples from both the hydrocarbon affected and unaffected areas show prominent peaks for quartz, illite, muscovite, and feldspar. Illite is identified from the prominent peaks near 100A, 4.480A, and 3.330A d-spacings (Fig. 5). The peaks at 4.480A and 3.330A of illite are, however, observed to be coincided with peaks of muscovite and quartz. It is, therefore, the XRD studies of the samples strongly support the spectroscopic observations. However, It is to be noted that though the sediments contain goethite as major ferric iron oxide as revealed by the spectroscopic studies, the prominent peak of goethite at 4.18A is absent in the X-Ray diffractograms. This is due to the fact that the goethites occur as a fine-grained coating around the constituent grains, which are opaque to XRD (Swayze et al. 2000). The X-ray diffractograms of the samples also lacked prominent peaks characteristics of carbonate minerals, indicating very low abundances of these minerals supporting the spectroscopic observations.
XRF Studies
Selective specimens were analyzed for XRF studies from hydrocarbon affected and unaffected areas. The average and median concentrations of the major element oxides are shown in Table.2. The table shows that the average concentrations of Al2O3, Fe2O3 (total), CaO, MnO, K2O, and P2O5 are slightly higher, and SiO2 and Na2O content are slightly lower in the microseepage bearing sediments. It is observed that, though, the average Al2O3 content is slightly higher in the microseepage affected samples, the median value of Al2O3 is slightly higher in the microseepage unaffected sediments. Similarly, though the average P2O5 content is marginally higher in the hydrocarbon affected samples, the median values of P2O5 are the same in both the samples. The following geochemical indices were determined on the major element oxide data of the samples to get a better understanding of the degree of alterations:
Chemical index of alteration (CIA) is the most commonly used index to quantify the degree of chemical weathering the sediments have undergone and is expressed as (Nesbitt and Young 1982):
CIA = {Al2O3/ (Al2O3 + CaO + Na2O + K2O)} *100
High CIA values indicate washing out of the more mobile elements like Ca, Na, and K with respect to relatively immobile Al and thus represent high chemical weathering. The greatest CIA values (close to 100) correspond to kaolinite weathering, values between 75–90 represent illite, and for feldspars, it is 50 (Nesbitt and Young 1982; Fedo et al. 1995; Nadłonek and Bojakowska 2018). The average CIA value for the hydrocarbon affected sediments (85.89) is nearly the same as that of the hydrocarbon unaffected sediments (85.80). These CIA values fall in the advanced stage of weathering in the illite zone, reflecting that the major constituent of the sediments/sediments is illite. This is also supported by the spectroscopic studies, which indicated illite as major constituent clay mineral in the sediment samples.
The silicification index (SI) is a measure of silica content and is defined as:
SI = [ SiO2 / (SiO2 + Al2O3)] * 100 (after Pirajno, 2009)
The higher values of SI indicate a higher amount of silica in the samples. It is observed that the average SI value is lower (72.76) for the microseepage affected samples in comparison to the microseepage unaffected samples (74.51). Thus, it is indirectly evident that some amount of silica has been depleted in the hydrocarbon microseepage induced sediments.
Alteration index (AI) is a measure of the degree of the alteration as suggested by Ishikawa et al. (1976) and is measured by
AI = { (K2O + MgO) / (K2O + MgO + Na2O + CaO)} *100
The alteration index calculation reveals that the average AI for the hydrocarbon induced sediments is higher (83.14) than that of the hydrocarbon unaffected sediments (81.29). Therefore, although the average bulk compositions of the sediments are similar, the different alteration indices indicate a higher degree of alteration in the hydrocarbon microseepage bearing sediments.
A new geochemical index to quantify the hydrocarbon microseepage related alterations termed as Microseepage-Induced Alteration Index (MIAI) has been introduced by Asadzadeh et al. (2020). The MIAI is defined as under:
MIAI = { (CaO + K2O) / (Na2O + K2O + CaO + MgO)}*100
The MIAI calculation shows that average MIAI values are higher (54.95) in the microseepage affected sediments with respect to the hydrocarbon microseepage unaffected sediments (52.54). This indicates that the hydrocarbon microseepage affected sediments have undergone more alteration than that of the hydrocarbon unaffected ones. The results, therefore, prove that the MIAI is an efficient index in differentiating hydrocarbon microseepage induced alterations in sediments.
Trace element (excluding REEs) studies
Limited numbers of specimens were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS). The result of the trace elements (excluding REEs) is shown in Table 3. The distribution of the average trace element concentrations reflects that the hydrocarbon-bearing sediments are enriched in average Be, V, Cu, Zn, Ga, Zr, and Mo and are depleted in Li, Cr, Co, Ni, Rb, Sr, Sc, and Y.
REE studies
The REE values of each specimen were normalized with respect to Post-Archaean Average Australian Sedimentary rock (PAAS) (McLennan l989) values. The average normalized value of the REEs for the specimens are shown in Table 4. The table indicates that the average REE values are higher in the hydrocarbon unaffected sediments. The PAAS normalized REE plots for the samples are shown in Fig. 6. The REE patterns show that all the sediment samples exhibit positive Eu positive anomalies. These results, in general, also support the observations by Asadzadeh et al. (2020).