ABSTRACT In recent years, outcrop analogue studies have become a powerful tool in sedimentology for the assessment of reservoirs, both in hydrocarbon and aquifer studies. Data from exploratory drilling campaigns can be augmented significantly by observations on the outcrop of the corresponding stratigraphical interval with the objective to validate the borehole information through direct observation. In addition, through the physical separation of the outcrop area and the subsurface, the increased spatial coverage of a reservoir and its equivalents provides additional information about facies and their changes and thus on reservoir properties. This chapter presents results of a study on the Cretaceous sedimentary aquifers in Saudi Arabia (Wasia–Biyadh–Aruma) in order to better assess the storage volume of fossil ground-water, which is of fundamental importance for the hyper-arid kingdom. Besides the regional 3-D stratigraphic framework, the focus was on measurements of porosity and permeability of approximately 150 samples and the interpretation of reservoir quality in terms of sedimentary facies and its diagenetic overprint. In general, both porosity and permeability are varying on a high level (Biyadh: 1–36% / 2100–6500 mD; Wasia: 3–42% / 2100–6500 mD; Aruma: 1–38% / 10 –6 –0.15 Darcy). Apparently, the storage volume and hydraulics of these regional aquifers are controlled not only by their fracturing but also by their matrix porosity. Permeability varies by about an order of magnitude among samples or between vertical and horizontal permeability within some samples. This variation can be well explained by heterogeneity due to sedimentary facies, for example, cross-bedding and bioturbation. In some areas, the kind of cementation and its intensity have a large effect on the permeability. The data obtained enhance the quality of the hydraulic interpretations of this aquifer system. Spectral gamma-ray logs proved to be useful for a regional correlation and the correlation of aquifers and aquicludes. This is based on the recognition of the major unconformities in the logs but also on the identification of various paleosol horizons, which regularly show high emissions of U and Th radionuclides. Intensive weathering during the Cretaceous is responsible for dominantly kaolinitic clay mineralogy and consequently negligible K emissions.

Abstract Lacustrine deltaic systems differ in several ways from marine ones, and classic sequence stratigraphical concepts need to be modified. This study uses a quantitative outcrop analogue approach with an exceptionally high-resolution record of a lacustrine delta complex (centimeter to decimeter) of the Upper Triassic at the southern margin of the Junggar Basin (Xinjiang, northwestern China). Sedimentological logging of lithofacies and architectural elements were combined with gamma-ray measurements and two-dimensional maps of outcrop faces. The data are analyzed and interpreted in terms of depositional dynamics, cyclicity, stacking pattern, accommodation vs. sediment supply, and preservation potential. The sedimentary inventory comprises various types of gravelly channel bodies, sheetlike sandy and clayey units, as well as ferrocrete horizons and coal seams organized in four depositional environments: Delta slope, delta front, delta top and distal alluvial plain. In contrast to existing fan-delta models, weakly incised, coarse-grained channel fills reaching far beyond the delta front are intercalated into fine-grained delta-slope deposits. Thus, we propose a new type of a mixed-load fan delta, which might be common in other very large lake systems. A fourfold cycle hierarchy with systematic superposition of cycles was identified. It shows a closely linked control of sediment transport, depositional processes, and accommodation space typical for lake systems. According to preservation of cycles and regional geodynamic data, tectonic rates did not change markedly in the Late Triassic. However, a decrease of bisaccate pollen and biodiversity but an increase of spores suggests an increase in humidity, but also in climate “extremes” towards the Jurassic-Triassic boundary. Preservation potential and cycle symmetry are related to the accommodation space by the sediment supply (A/S) ratio, which shows an overall decrease during Norian times with a subsequent increase during the Rhaetian. Gamma-ray measurements (GR) also show a distinct cyclicity, however frequently independent of grain sizes. Channelized coarse-grained sediments show increased natural gamma-ray emission (total counts) due to 40 K in claystone clasts. The U/Th ratio is often higher in the delta-front environment due to interaction of fresh river water and lake water.