Hydrocarbon seeps cause chemical and mineralogical changes at the surface, which can be detected by remote sensing. This paper aims at the detection of mineral alteration induced by gas seeps in a marly limestone formation, SW Iran. For this purpose, the multispectral Advance Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and the high spatial resolution WorldView-2 (WV-2) data were utilized for mapping surficial rock alteration. In addition, the potential of Visible Near Infrared (VNIR) bands of the WV-2 and its high spatial resolution for mapping alterations was determined. Band ratioing, principal component analysis (PCA), data fusion and the boosted regression trees (BRT) were applied to enhance and classify the altered and unaltered marly limestone formation. The alteration zones were identified and mapped by remote sensing analyses. Integrating the WV-2 into the ASTER data improved the spatial accuracy of the BRT classifications. The results showed that the BRT classification of the multiple band imagery (created from ASTER and WV-2) using regions of interest (ROIs) around field data provides the best discrimination between altered and unaltered areas. It is suggested that the WV-2 dataset can provide a potential tool along higher spectral resolution data for mapping alteration minerals related to hydrocarbon seeps in arid and semi-arid areas.

The presence of hydrocarbon seeps at the surface is indirect evidence of the presence of mature source rocks within a geological system at depth. Chemical changes in the environment of surface rocks caused by hydrocarbon seeps cause mineralogical alterations. To determine the nature of the alterations and the influences of lithology and type of seep, rock samples were collected from altered and unaltered evaporite and marly limestone formations in the Dezful embayment, southwest Iran. Reflectance spectroscopy, bulk rock/wet chemical analyses, and sulfur, carbon, and oxygen isotopic analyses were used to delineate surficial alterations and relate alterations to hydrocarbons seeping from underlying reservoirs. In addition, the boosted regression trees (BRT) method was used to predict the presence of alterations from spectral indices. Comparisons of geochemical data and spectral data of altered evaporites and altered marly limestones showed that the minerals within alteration facies have distinctive spectral, chemical, and isotopic signatures. Gas-induced alterations were characterized by the formation of gypsum and native sulfur and depletion in S34. The released H2S in natural gas reacted with gypsum in the evaporite sediments and calcite in the marly limestone formations, which led to precipitation of secondary gypsum and native sulfur. Oil-induced alterations were characterized by formation of secondary calcite and depletion in C13. The oxidation of seeping oil and reactions between this oil and host rocks caused precipitation of secondary calcite within both formations. The combination of fieldwork data and spectral-geochemical data showed a connection exists between surficial alterations and underlying petroleum reservoirs, which can be used in exploration campaigns.

The persistent natural hydrocarbon seepage in onshore basins challenges observation and exploration technologies, which are required to document and assess these valuable indications of the presence of oil and gas in the subsurface. This paper aims at demonstrating the relationship between the compositional variation of an evaporite cap rock and the types of seeps occurring at the surface. For this purpose, the multispectral Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data was utilized for mapping mineral variations of a petroleum system in the Zagros oil fields. Relative absorption-band depth (RBD), band rationing and the boosted regression trees (BRT) were applied to enhance and classify the mineral composition of evaporite, sandstone, and marly limestone formations. The gas seeps were associated with the areas of gypsum-bearing evaporite cap rock while oil seeps were mostly associated with calcite and clay zones within the cap rock, which was more prone to fracturing during the tectonic activities of the basin. It is suggested that the application of remote sensing in the oil and gas industry could be widened by detection of sleep-induced alteration to assess the efficiency of cap rock and to evaluate the productivity of reservoirs at a regional scale.