Potencjał węglowodorowy skał macierzystych i geneza gazu ziemnego akumulowanego w utworach miocenu zapadliska przedkarpackiego w strefie Rzeszowa

Abstract

Hydrocarbon potential of source rocks and origin of natural gases accumulated in Miocene strata of the Carpathian Foredeep in Rzeszów area.S u m m a r y. In the autochthonous Upper Badenian and Lower Sarmatian strata of the Carpathian Foredeep in the Rzeszów area the total organic carbon (TOC) contents vary from 0.02 to 1.26 wt.% (average 0.64 wt. %). Geochemical studies on the dispersed organic matter demonstrated the presence of gas-prone type III (humic) kerogen with small admixtures of algal type II kerogen. At depths less than 2,500 metres the organic matter is immature, thus microbial processes predominated. Methane concentration in natural gases accumulated within Miocene strata usually exceeded 94 vol.% and was generated by microbial reduction of carbon dioxide. Microbial methane was generated mainly during deposition of autochthonous Miocene sediments, but it is possible that this process continues today on a small scale. Higher gaseous hydrocarbons (mainly ethane and propane), which are usually minor constituents (concentrations less than 0.4 vol. %), were generated during diagenetic processes and at the initial stage of the low-temperature thermogenic processes. The slight changes in the geochemical indices of dispersed organic matter and isotope ratios of natural gases with depth are evidence of the homogeneity of deposition of humic organic matter in shallow marine basin during the Late Badenian and the Early Sarmatian and the fact that similar gas generation conditions can be found in the Miocene sequence. Generation and accumulation of microbial methane and the formation and charging multiple stacked reservoirs within the autochthonous Miocene strata, e.g., Palikówka, Jasionka, Stobierna and Terliczka deposits, was facilitated by rhythmic and cyclic deposition of clays and sands and their very high sedimentation rate.