Low-temperature thermochronology and diagenesis of the Amagá Basin: insights into hydrocarbon generation and its relationship with tectonothermal and hydrothermal processes

The mineralogical characterization of dark green cements of intraclasts and their host sandstones, developed using optical petrography, X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX) and organic petrography, provides new insights about the diagenetic evolution and depositional conditions of the late Oligocene-middle Miocene Amagá Formation, northwestern Colombia. Dark green intraclasts are found as medium-grained, slightly feldspathic sandstones, in which the main authigenic mineral is pyrite that formed during a mesogenetic (burial diagenesis) stage. The host sandstones of the dark green intraclasts were deposited in a meandering river environment and are related to point bar deposits over swampy areas. These strata are medium-grained quartzose sandstones cemented by ankerite as a product of replacement of early calcite through hydrothermal alteration. Solid hydrocarbons analysis together with new low-tempereture thermocronology apatite (U–Th)/He data, available apatite fission-track (AFT), zircon fission-track (ZFT) and vitrinite reflectance (%Ro) data, indicate maximum burial temperatures of ca., 100°C for the deeper parts of the basin. Solid hydrocarbons were emplaced in the late stages of the diagenetic history and occur in a porous system where fractures played an important role. They constitute the first evidence of migration and accumulation of hydrocarbons in the Amagá Basin. However, the latest event of fracturing during a major cooling event associated with basin inversion at ca., 4 Ma, caused complete hydrocarbon biodegradation and seepage, and increased the effect of meteoric water inflow during the telogenetic stage.