Colin P. North

Ephemeral-Fluvial Deposits: Integrated Outcrop and Simulation Studies Reveal Complexity

Ephemeral-fluvial sandstones form the main reservoirs in many oil and gas fields. Production histories and development difficulties suggest these deposits are more complex than is commonly supposed in the petroleum industry. There is inadequate published information to account for this observation. Relevant sedimentological literature on such deposits is qualitative and unsuitable for detailed reservoir characterization studies; therefore, integrated sedimentological, petrophysical, and fluid-flow simulation studies have been conducted on outcrop analogs in the southwestern United States. Our key objectives were to define flow units and to find areas of sensitivity inherent both in the sediments and in the analytical and statistical procedures used to describe and model s ch systems. We report results of integrated studies on the Lower Jurassic Kayenta Formation in southeastern Utah. The Kayenta Formation is a relatively proximal, sand-rich succession dominated by channel-fill sandstones. Sedimentological analysis identified eight main genetic units. In contrast, reservoir simulations, based on probe-permeameter data collected at outcrop, show that six of the main genetic units have uniform flow behavior. Intraformational mudclast conglomerates are the dominant permeability barriers, both because of the clasts themselves and because they act as sites of preferential cementation. Stochastic permeability models conditioned to the outcrop data produce results little different from models based on averaged values for each facies. The most sensitive feature of the mode s concerns the assumptions made about the mudclast conglomerates. The model results show that only by integrating analytical approaches can we understand the full character of an outcrop analog in such a way that we can use it predictively.

Spectral Gamma-Ray Logging for Facies Discrimination in Mixed Fluvial-Eolian Successions: A Cautionary Tale

Accurate assessment of downhole natural gamma-ray logs requires a reference database built up from well-exposed outcrops where the recognition and interpretation of lithofacies is unequivocal. Although only a few cases have been published for spectral gamma-ray logs, some of these show there is a strong correlation to depositional environment. We present detailed spectral gamma-ray data, within an interpretative framework of the sedimentology and petrography, for a mixed fluvial-eolian succession in the Cutler Group of southeastern Utah, United States. In this case, these data did not help identify depositional environment. Discrimination of eolian from fluvial deposits is not reliable because there is much overlap between the gamma-ray emissions of each lithofacies, although the eolian sediments tend to have lower values than the fluvial sediments. Interdune deposits cannot be discriminated from flood-plain sediments. The thorium/potassium ratio is approximately constant for all lithofacies, and is of no use as a facies discriminator. The thorium/uranium ratio is less than 7, much lower than might be expected for continental lithofacies from a review of the literature. These results have general application to mixed fluvial-eolian successions because they reflect the typical interactions between eolian and fluvial sedimentary processes, and do not depend especially on the character of the original sediment.

Dominance of lateral over axial sedimentary fill in dryland rift basins

ABSTRACT Facies models for continental rift basins suggest longitudinal (axis-parallel) fluvial activity dominates sediment transport and deposition. In contrast, modern basins in the arid to semi-arid northern Basin and Range Province, USA, show axial drainage development to be characterized by short endorheic systems that contribute little to the basin fill. Mapping and calculation of the proportionate distribution of surficial facies of three representative basins at different stages of rift evolution show that basin fill is dominated by the deposits of transverse catchments, and axial fluvial deposits are restricted to a narrow corridor by the progradation of lateral systems. Drainage integration in these dryland rifts is limited by the moisture-stressed climate, with a resultant reduction in stream power, and the complex, tectonically-induced physiography, which limits potential drainage pathways. River systems that flow through multiple structural basins are rare, restricted to those systems with catchment headwaters lying outside the dryland climatic regime. These data imply that long-range, axial fluvial deposits should not automatically be included as a significant part of dryland rift sedimentary facies models. Sediment routing pathways in ancient dryland rift systems may be much shorter than commonly predicted, so affecting the spatial distribution of lithofacies. Climate, therefore, has a much stronger control on drainage and lithofacies at all stages of rift development than is generally stated.