Print this pageHydrocarbon Fluid Inclusion Evidence for the P-T Conditions of Oil Migration and Accumulation in Folded Belts and Foreland Basins
BODNAR J., ROBERT, and VITYK O., MAXIM
Fluid inclusions have been used to constrain the P-T history of hydrocarbon generation and accumulation in folded belts and foreland basins. This was accomplished using oil and gas trapped in fluid inclusions in quartz crystals from nappes of the Ukrainian Folded Carpathians. Hydrocarbon generation within the Carpathian orogen occurred in Late Oligocene-Early Miocene during compressional deformation of the accreted sedimentary cover. During this process, underthrusted Oligocene source rocks were buried to different depths, resulting in variable levels of maturity. Fluid inclusions in quartz crystals from the nappes record the P-T conditions during burial and were used to determine whether or not the host source rocks passed through hydrocarbon generation window and, if so, at what depth generation was initiated, peaked and terminated. The fluid inclusions were thus used to placed absolute constraints on the depths and temperatures of the source rocks as a function of geologic time. The inclusions also identified the most likely zones in which petroleum could have migrated within the belt. For example, the Dukla nappe represents the least buried (1-2 km) nappe in the region. During the Miocene, quartz was precipitated in this nappe at P-T conditions within the oil window. Quartz from this nappe contains inclusions of light paraffinic oil (approximately 40-45 degrees API). Conversely, the Krosno zone was one of the most deeply buried (10 km) parts of the Ukrainian Folded Carpathians during Miocene compression. Quartz from the Krosno zone contains only dry gas (methane) inclusions. These gas inclusions show the highest densities that have been recorded for the region, and are consistent with burial beyond the oil window.