Characterization of microseepage-induced diagenetic changes in the Upper-Red Formation, Qom region, Iran. Part I: Outcrop, geochemical, and remote sensing studies

Abstract

This article explores the footprints of hydrocarbon microseepage in the Upper-Red Formation covering the Alborz petroleum reservoir in the Saveh-Qom basin, Iran. The adopted multidisciplinary approach combines field observation and sampling, detailed petrographical and geochemical analysis, and remote sensing mapping using Sentinel-2 and ASTER satellite data. The study revealed significant changes in the microseepage-affected zones when contrasted to unaffected lithofacies and led to the establishment of a new array of mineralogical and geochemical indicators for the phenomenon. These comprise partial bleaching of the red-beds, secondary goethite coatings, diagenetic clay minerals (e.g. illite-smectite, chlorite), and pervasive calcite, silica, and gypsum cementations. In terms of geochemistry, the major oxides, particularly the ratio of CaO + K 2 O/Na 2 O + MgO + CaO + K 2 O proved to be a reliable index for distinguishing microseepage-prone areas. Based on isotopic studies, a combination of three sources of carbon, comprising oxidized hydrocarbons, was recognized in the carbonate cements; yet the strata were shown to be moderately flushed by percolating fluids. Satellite remote sensing was successful in mapping reservoir-scale footprints of the microseepage systems. It indicated that the extent of the induced alterations, and thereby the outline of the underlying reservoir, is much larger than previously known. Overall, the secondary changes were shown to be a function of the geochemistry of the host-rock, the local environmental variables (i.e. climate and hydrogeology), and the 3D architecture of the trap. The substantiated indicators carry direct implications for onshore microseepage targeting and hold high potential to help reduce the exploration and development risks in frontier and mature basins. • The footprints of petroleum microseepage were shown to be traceable at multiple scales. • The structural geology has governed the extent, locality, and intensity of the anomalies. • The extent of the reservoir was indicated to be larger than what was previously envisaged.