Aerospace detection of hydrocarbon-induced alteration

This chapter provides an overview of the background to the direct detection of onshore hydrocarbon microseepage by remote sensing techniques and the results achieved to date. The occurrence at surface of hydrocarbon seeps suggests that an oil or gas reservoir leaks even though it acts as a trap for hydrocarbons. Macroseepage is the visible presence of oil and gas seeping to the surface. Microseeps are invisible trace quantities of hydrocarbons seeping to the surface. The most persuasive evidence for microseepage is the measurement, sometimes over many years, of statistically-significant anomalous amounts of light hydrocarbons in soil gases and soils directly over oil and gas reservoirs. The occurrence of hydrocarbon microseepage directly above reservoirs points to vertical migration of hydrocarbons, despite the fact that groundwater movement might be expected to militate against this. The cross-sectional shape of the hydrocarbon leakage pattern is termed a “chimney,” and most chimneys are nearly vertical. Vertical migration through the strata has been attributed to at least four mechanisms: effusion, diffusion, solution, and gas bubbles.