Hydrostratigraphic decomposition of fluvio-deltaic sediments inferred from seismic geomorphology and geophysical well logs in the Pannonian Basin, Hungary

Abstract

Heterogeneity and anisotropy of lithological units influences hydraulic conductivity on several scales. The purpose of this study is to develop a novel method supporting hydrostratigraphic classification by considering horizontal and vertical variations of sand content in relation with the paleo-depositional environment. The workflow was applied for the uppermost ca. 1800 m thick fluvio-deltaic deposits (Great Plain Aquifer) of late Neogene to Quaternary age basin-fill succession in the eastern part of the Pannonian basin in Hungary. Five combined 3D seismic volumes, seven master horizons, and 30 well logs were analyzed. First, RMS amplitude maps were extracted to interpret the seismic geomorphological features and depositional architectures. Their associated lithology was inferred from wireline logs (GR and SP) by calculating the shale volume and the net-to-gross sand ratio for 30 m thick intervals. These were used to calibrate the seismic facies as a proxy for the horizontal distribution of sand versus shale. This method allowed the identification of sand bodies, i.e.: deltaic lobes, complex channel belts, simple fluvial channels behaving as aquifers, and the dominantly muddy delta plain to flood plain suits as aquitard. The vertical pattern of sand distribution was also evaluated. Three major stratiform and some corridor-like minor hydrostratigraphic units were defined instead of the former regional (basin) scale aquifer unit. 1) Laterally extended interval of stacked deltaic lobes of high sand ratios and high rate of connectedness, at the bottom of the studied interval. 2) General presence of extended muddy floodplains with anastomosing river systems, characterized by 100–200 m wide channels, low sand ratio, and limited connectedness: 3) widespread meandering and/or braided river systems with high sand ratios and high connectedness in the Quaternary. Within unit 2) spatially and temporally variable appearance of major 500–3600 m wide meandering channel belts produce locally high sand-ratio corridors in the NE-SW direction. A workflow adapted from the oil-and-gas industry was successfully applied to distinguish units of varying sand content and hydraulic conductivity. This approach can be used on basin to local scale to build a spatially complex facies-based model of hydrostratigraphy. Thus, a robust heterogeneous geological model serves as a base for investigating fluid flow on the required scale.