A three-dimensional, quantitative depositional model of gravelly braided river sediments with special reference to the spatial distribution of porosity and permeability.

摘要

A quantitative, three-dimensional depositional model of gravelly, braided fluvial deposits is developed based on the deposits of the Sagavanirktok River in Northern Alaska. These deposits were described using cores, wireline logs, trenches, and ground-penetrating radar profiles. The nature and origin of the deposits are inferred from observations of channel and bar formation and migration, and channel filling, interpreted from aerial photographs, and observations of water flow during floods and of the river-bed at low-flow stage. This depositional model quantitatively represents the geometry of the different scales of strataset (related to geometry and migration of bedforms and channels), the spatial relationships between the different scales of strataset, and the distribution of sediment texture within these stratasets. Porosity and permeability in the model are related to sediment texture. The spatial distribution and connectivity of open-framework gravel, sand and silt strongly influences fluid flow and contaminant transport over tens to hundreds of meters horizontally.; The geometry of the different scales of strataset is related to the geometry and migration of their formative bedforms. In particular, the wavelength-to-height ratio of bedforms is similar to the length-to-thickness ratio of their associated deposits. Furthermore, the wavelength and height of bedforms like dunes and bars are related to channel depth and width. Therefore, the thickness of a particular scale of strata set (i.e., medium-scale cross sets and large-scale sets of inclined strata) will vary with the scale of the paleoriver. These relationships between the dimensions of stratasets, bedforms and channels mean that the depositional model developed from the Sagavanirktok River can be applied to other gravelly fluvial deposits.; The depositional model can be used to guide geostatistical simulation of gravelly fluvial aquifers, given sparse site-specific data on sediment textures (or porosity and permeability) and the thickness of different scales of strataset. Characterization of gravelly fluvial aquifers and hydrocarbon reservoirs using this model will result in improved borehole placements, hydrocarbon recovery, and simulations of groundwater flow and contaminant transport.