This outcrop analogue study investigates Triassic fluvial sandstones (Stubensandstein Formation) deposited on a terminal alluvial plain under semi-arid to sub-humid climatic conditions in the land-locked South German Keuper Basin. The Stubensandstein may serve as an analogue for reservoir units in comparable continental basins. Data came from studies of (1) 13 large sandpits: in total, these have wall faces that cover over 80,500 sq.m, while single wall faces are to 480m wide and 50m high; (ii) three subsurface well-log clusters (comprising up to 20 boreholes), and four single wells. In this paper, two representative outcrops in proximal and distal palaeogeogrqphic positions are discussed in detail. This study combines the results of the following methodologies: 1. Sedimentological analysis: Fourteen lithofacies types and nine types of architectural element were identified. Exposed sandpit walls allowed distinct styles of alluvial architecture to be mapped, ranging from proximal to distal end members. Channel geometries change from ribbon-like to sheet-like, and lateral accretions change their lithofacies make up from sandy to muddy. 2. Ground penetrating radar: 2km of radar lines were recorded, and buried architectural elements were traced behind sandpit faces revealing their 3D geometries. 3. Petrophysics: A total of 1,860 permeability, 1,360 porosity and 4,500 gamma-ray measurements were made in order to characterize fluid-flow units. Porosity and permeability data show characteristic trends and distributions within the architectural elements. Lithofacies types can be grouped into four petrophysical rock types which show differences in poro-perm values and frequency distributions which are related to depositional energy. The average permeability of sandbodies decreases by two orders of magnitudes from about 700md to 1md along palaeogeographic proximal to distal trends. Based on these data five types of fluid flow units with characteristic features were distinguished. Data derived from the outcrop studies concerning sandbody dimensions and geometry, reservoir architecture and interconnectedness, poro-perm trends and fluid-flow units may be useful for subsurface reservoir characterization and modelling in similar settings. Part 2 of this paper will focus on the larger-scale cyclicity, and provide depositional, palaeoclimatic and reservoir models.