Abstract This article deals with the stratigraphic record of a climatic or tectonic perturbation of an experimental coupled catchment‐fan system. Following Bonnet & Crave's results (2003), which suggest that it is possible to differentiate between climatic or tectonic causes of surface uplift of an erosional topography from the record of sediment flux output, we design a new experimental device to test this proposition in the sedimentary signal. This device allows the study of a coupled erosion–sedimentation system at the laboratory scale for given and changing uplift and rainfall rates. On the basis of experimental results, we propose a methodology to study alluvial fan architecture from large‐scale geometries to stacking pattern and sequence analysis. In these experiments, the erosional perturbation resulting from climate or tectonic forcing induces a typical dynamic in terms of both sediment supply and the ratio between the sediment and water supply, which controls the transport capacity. The four possible forcings (rainfall rate and uplift rate increase or decrease, respectively) then result in unique dynamics of the combined parameters such as the fan slope, apex aggradation, mean sedimentation rate, grain size distribution, bed thickness and frequency and facies stacking. We first analyse large‐scale geometries (onlap, toplap, downlap or truncation) and then fine‐scale sedimentological features (fining, thinning, coarsening, thickening) in order to discriminate the nature of the forcing. This conceptual model could be adapted to real world alluvial fans in order to recognize and separate the driving mechanisms from each other.
Abstract In the Middle Jurassic series of the Cleveland basin (Northeastern England), the Scalby Formation comprises two main members, interpreted as a valley-fill complex overlain by a deltaic aggradational series. The valley was incised during a relative sea-level drop and infilled during a stepped relative sea-level rise by a retrogradational fluvial to estuarine complex. The valley fill is composed of three superposed sandsheet prisms showing, respectively, 1. fluvially dominated straight channels, 2. tidally influenced meander belts, and 3. a mixed fluvial-tidal meander belt The deltaic aggradational member corresponds to highstand deposits. Small ribbons or meandering channels are isolated in shaly floodplain and lacustrine facies. A precise three-dimensional reservoir reconstruction has been carried out on a 1-km2 area using cliff sections, drill cores, wireline logs and petrophysical measurements. Accurate reservoir reconstructions are presented here as large cross sections in terms of lithology, porosity and permeability. At Cloughton, the valley-fill member consists of a lower and upper prism. Each corresponds to a channel-fill stage with a basal aggradational fluvial unit overlain by an estuarine or paralic transgressive unit The basal prism displays a fluvial stacked-chaanel unit with side bars and sand or clay plugs, the tops of which are truncated by a transgressive surface. The unit is overlain by an estuarine sand-flat complex. The upper prism corresponds to tidally-influenced meander belts with hectometric point bars composed of argillaceous sandstone and siltstone. It is capped by shaly marsh deposits, corresponding to the period of maximum flooding of the valley fill. The aggradational deltaic member consists mainly of delta-plain shales with multistory ribbon channels and wide meander belts. The best potential reservoirs are located in the basal fluvial sucked channels of the valley fill. Reservoir qualities decrease in the estuarine sand-flat complex and are very poor in the mixed fluvial-tidal meander belts. The small ribbon channels generally constitute good but small disconnected reservoirs. The sequence stratigraphy of the Scalby Formation shows that reservoir geometry and architecture could be controlled by minor relative sea-level variations.
2SONATRACH Triassic reservoirs are major exploration and production targets in Algeria, and contain around 60% of the proved oil reserves of the country. In the late 1950’s and early 1960’s, giant oil fields such as El-Borma at the Algero-Tunisian border and Hassi-R’Mel in the western province were discovered in the Trias. Three wells have been selected to illustrate the Triassic sedimentologic environments and their sequential organization: − ALD-1 illustrating the Oued Mya Trough area, is characterized by a thick reservoir of clean sandstone interpreted as a braided plain sedimentary environment. − ASL-2 is located in the southern part of the Berkine Trough and shows some proximal braided facies associations of the TAGS. − ELB-1, is located in the El borma area, close to the Libyan border, and presents more distal facies associations. For each well, a collection of the most characteristic facies associations are illustrated with core photographs, core descriptions, wireline log signatures and associated paleosedimentary environments. The facies associations and sedimentary environment will be replaced in their dynamic evolution in term of sequence stratigraphy and a correlation panel will be proposed.
Abstract The facies associations of the overflow deposits associated with turbidite channels were studied in the outcrops of the Maastrichtian Pab Sandstone, in SW Pakistan. In this area, a basin-floor fan was preserved from the platform to the deep basin setting. In the mid-fan setting, a channel complex crops out in three dimensions, and consists of a dozen channels and their overflow deposits, including levees, crevasse lobes and spill-over lobes. The overbank deposits can be directly in contact with the channel-fill, but, in many cases, matrix-supported debris-flow deposits made lenses close to the channel base. Heterolithic drapes of thin-bedded turbidite deposits were also preserved along the channel margins. A three-dimensional (3D) static model describing the heterogeneity distribution within the turbiditic channel complex was compiled using both stochastic and deterministic approaches in a site where two channels were laterally connected by overbank deposits. Petro-acoustic properties derived from a subsurface database were then assigned to the facies to perform seismic simulations. The synthetic seismic simulations showed that the channel base can easily be misinterpreted compared to the geological model. Dynamic modelling, such as well test and streamline simulations, was also performed using the model to assess the transition between the channel and overbank deposits from a dynamic point of view. As a result of the streamline simulations, the overbank deposits connecting the channel homogenized the pressure regime in the reservoir. However, the sweeping efficiency of water injection can be affected by the heterogeneity distribution along the channel margins. A significant volume of oil could also be by-passed because of the occurrence of early water breakthrough through the spill-over lobes, or because the flow slowed down when it reached the heterogeneity along the channel margin.
Abstract The truncated pluri-Gaussian approach is a powerful method for modeling geology. Its main strength is its flexibility in representing complex lateral and vertical facies transitions with different anisotropies. In addition, it is easy to condition simulations to data points. This makes it an excellent method for modeling reservoirs with a complex architecture, such as carbonate bioconstructions or reservoirs affected by diagenesis. This chapter presents a method for obtaining a tractable and mathematically consistent model for lithotype semivariograms and cross-semivariograms in complex cases. The method is illustrated using an example involving algal bioconstructions in the outcrops of the Paradox basin (Utah). Complex facies transitions, both vertically and laterally between the mound and intermound facies, together with the complex geometry of the algal mounds, make it virtually impossible to simulate these sorts of deposits using object-based models or classical pixel-based methods. The truncated pluri-Gaussian model is introduced to handle these complex facies transitions using the concept of a lithotype rule. Such a rule, when expressed diagrammatically, can be a valuable tool for synthesizing geological information, and it can serve as one of the key inputs into the stochastic model. As illustrated here, combining the rule with proportion curves is a very effective way for analyzing and modeling geology in terms of facies sequences, even in complex depositional environments.
The Eocene Annot Sandstone in the French Alps corresponds to a turbiditic system up to 1000m thick, which has filled large morphologic gutters induced by tectonic activity. Detailed field studies have recently been undertaken on well-exposed 3D outcrops at a reservoir scale (kilometric), in order to analyse and reconstruct the architecture of deep-marine gravity deposits. These selected sites belong to different sedirnentary settings (inner fan valley, channel-levee system, depositional lobes) and different morphological configurations (narrow trough controlled by faulting, unconfined basin), and they exhibit a large variety of facies (slumps, debris-flows, high and low density turbidites).
