Subsurface hydrocarbon traps are not correctly sealed, and hydrocarbons move vertically from the reservoir as invisible traces in the form of microseepages. Long-term hydrocarbon microseepages cause surface or near-surface alterations such as bleaching of red beds, enrichment of ferrous iron minerals and higher concentrations of clay and carbonate minerals in soils/ rocks. Multi-and hyperspectral remote sensing data have successfully been used to detect such alterations in many parts of the world. In India, such studies have not been carried out till now. In this study, Landsat 7 ETM+ images have been used to find out hydrocarbon microseepage-bearing areas in part of Assam-Arakan Fold Belt in the northeastern part of India. Based on the spectral characteristics of the hydrocarbon microseepage-induced altered minerals, two spectral enhancement techniques, viz. principal component analysis (PCA) and band ratio analysis, have been carried out on the Landsat 7 ETM+ images. PCA reveals that three principal component images-1457PC3, 1345PC2 and 3457PC4-show relatively better enhancement for the hydrocarbon-bearing alteration areas. Again, band ratio analysis of the images indicates that ratio images-3/1, (2 + 5)/(3 + 4) and 7/5-show excellent spectral enhancement for the hydrocarbon-induced mineral alterations. The three PC images have been combined with the three band ratio images to find out probable hydrocarbon microseepage areas. The remote sensing-derived prospect areas have been validated with surface geochemical, seismic/geologic and gravity data available in the area.

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Long term hydrocarbon microseepages create a reducing environment on the surface rocks and sediments, which induces an array of mineralogical alterations. Among these alterations, the reduction of ferric iron minerals to ferrous iron minerals and precipitation of clay and carbonates are significant. Several studies have been carried out to characterize these hydrocarbon induced rocks/sediments. Almost all these studies have been carried out for arid to semi-arid climatic regions. The present study attempts to characterize the geochemical properties of the hydrocarbon induced sediments in part of Assam-Arakan Fold Belt (AAFB), NE India characterized by heavy rainfall. Reflectance spectroscopy, X-ray diffraction (XRD), X-ray fluorescence (XRF) and inductively coupled plasma emission - mass spectrometry (ICP-MS) studies have been carried out on the sediments. The reflectance spectroscopy reveals that microseepage-induced sediments have higher clay content and lesser ferric iron mineral content. Geochemical indices also suggest that the hydrocarbon-affected sediments are relatively more altered than the unaffected ones. Studies of trace element patterns indicate that the hydrocarbon-induced 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. The normalized rare earth element (REE) distribution patterns are the same for both the microseepage affected and unaffected sediments though the microseepage-induced sediments are slightly depleted in the REEs. The present study, thus, points out that the hydrocarbon microseepage-induced alterations are also evident in the high precipitation terrains though the alteration levels are less pronounced than that of the arid to semi-arid climatic regions due to abundant surface and groundwater which mobilize the minerals/elements from the microseepage system and tries to homogenize the compositions.

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