Geobody architecture, genesis and petrophysical characteristics of the Budakalász travertines, Buda Hills (Hungary)

Abstract Subsurface Pre-salt Lower Cretaceous reservoir systems encountered offshore Brazil and Angola that consist of porous continental carbonates are difficult to access because of the limited core material that is under concession. Therefore, it is important to study comparable continental carbonate analogues, such as travertines and lacustrine carbonates, that potentially possess similar porosity and permeability characteristics. The Budakalász Pleistocene travertines (Buda Hills, Hungary) have been studied from a sedimentological, geochemical and reservoir properties point of view in order to understand their formation, petrophysical properties and 3D reservoir architecture. The three-dimensional geobody architecture of the Budakalász travertines illustrates the potential complexity of kilometre scale continental carbonate systems. Heterogeneous low angle sloping travertine lobes consisting of terrace slope, smooth slope and reed facies, are covered by a lime mudstone dominated, gastropod- and charophyte-rich marsh-pool facies. The observed facies represent a change from a subaerial travertine to a palustrine to lacustrine system, reflecting the ceasing of the system due to tectonic uplift. The dominant paleo-flow direction of the sloping lobes was towards the Danube depression, with several subordinate flow systems identified. The SrCO isotope geochemical signatures reflect travertine precipitation at ambient to superambient temperatures from meteoric derived fluids with a mixed CO2 origin of the marine carbonate substrate rocks, a potential atmospheric and a soil-derived component. Although the uniform signatures of the stable oxygen and carbon isotopes, in addition to petrographic observations suggest partial diagenetic isotopic resetting, the primary isotope trends seem to have been preserved. A reservoir-oriented facies evaluation in the three-dimensional geobody frame of the Budakalász carbonates based on porosity and permeability core plug analyses reveals their heterogeneous nature. Terrace, smooth slope and marsh-pool facies have considerable porosities of 5–15%, while the porosity of the reed facies may reach up to 27%. Permeability of all facies is strongly anisotropic, mainly controlled by a better connectivity along layer-parallel primary pores. The reconstruction of the 3D geobody architecture of the Budakalász lobe complex coupled to its geochemical and petrophysical properties provides a unique dataset that contributes in understanding fossil travertine systems