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===Alluvial fan deposits===

===Alluvial fan deposits===

An ''alluvial fan'' is a wedge of clastic detritus that forms at the base of a mountain front as sediments eroding from the mountains are transported downslope by streams or debris flows and deposited at the base (Figure 3e). The fan-shaped body is generally characterized by a gradation from coarser sediments at the apex to finer sediments at the toe. Alluvial fans are commonly divided into proximal, mid-fan, and distal fan subenvironments.

An ''alluvial fan'' is a wedge of clastic detritus that forms at the base of a mountain front as sediments eroding from the mountains are transported downslope by streams or debris flows and deposited at the base (Figure 3e). The fan-shaped body is generally characterized by a gradation from coarser sediments at the apex to finer sediments at the toe. Alluvial fans are commonly divided into [[proximal alluvial fan|proximal]], [[middle alluvial fan|mid-fan]], and [[distal alluvial fan|distal]] fan subenvironments.

Vertical sequences through the proximal fan are generally dominated by gravelly deposits with subordinate sandy deposits. Sequences through the mid- and distal fan are increasingly sand dominated. [[Basic open hole tools#Gamma ray|Gamma ray]], [[Basic open hole tools#Spontaneous potential|SP]], and [[Basic open hole tools#Resistivity|resistivity]] log responses throughout a fan can generally be expected to be blocky to irregular, depending on the amount of clay.

Vertical sequences through the proximal fan are generally dominated by gravelly deposits with subordinate sandy deposits. Sequences through the mid- and distal fan are increasingly sand dominated. [[Basic open hole tools#Gamma ray|Gamma ray]], [[Basic open hole tools#Spontaneous potential|SP]], and [[Basic open hole tools#Resistivity|resistivity]] log responses throughout a fan can generally be expected to be blocky to irregular, depending on the amount of clay.

[[Permeability]] and [[porosity]] of alluvial fan deposits vary greatly as a function of depositional process and differential response to [[diagenesis]]. In general, streamflow deposits have greater permeability and porosity than debris and mudflow deposits. Finer grained but better sorted distal fan deposits are highly permeable and porous. Because of increased sorting, middle and distal parts of the fan probably have better and more predictable reservoir quality than proximal parts. Little is known of directional permeability within alluvial fan reservoirs, but paleochannels can be expected to act as preferred pathways of flow.

[[Permeability]] and [[porosity]] of alluvial fan deposits vary greatly as a function of depositional process and differential response to [[diagenesis]]. In general, [[streamflow deposits]] have greater permeability and porosity than [[debris flow deposits|debris]] and [[mudflow deposits]]. Finer grained but better sorted distal fan deposits are highly permeable and porous. Because of increased sorting, middle and distal parts of the fan probably have better and more predictable reservoir quality than proximal parts. Little is known of directional permeability within alluvial fan reservoirs, but paleochannels can be expected to act as preferred pathways of flow.

Where alluvial fans prograde into standing bodies of water (that is, oceans or lakes), they are called ''fan deltas''. The distal parts of these fans are generally much better sorted and cleaner as a result of reworking by wave and/or tidal processes. Proximal and mid-fan log responses are the same as alluvial fans. Log response in the distal part depends upon the intensity of wave and tidal processes and on whether the fan is actively prograding or being transgressed. Typically, distal parts will have an upward-coarsening gamma ray, SP, and resistivity log character. Barring adverse diagenetic effects, permeability can be expected to be much greater in marine than in more proximal parts of the fan delta because of increased sorting, destruction of compositionally immature grains, and winnowing of fines. [[Directional permeability]] trends in distal parts may be different from more proximal locations because of different sand body trends between these different parts of the fan delta.

Where alluvial fans prograde into standing bodies of water (that is, oceans or lakes), they are called ''[[fan deltas]]''. The distal parts of these fans are generally much better sorted and cleaner as a result of reworking by [[wave processes|wave]] and/or [[tidal processes]]. Proximal and mid-fan log responses are the same as alluvial fans. Log response in the distal part depends upon the intensity of wave and tidal processes and on whether the fan is actively prograding or being transgressed. Typically, distal parts will have an upward-coarsening gamma ray, SP, and resistivity log character. Barring adverse diagenetic effects, permeability can be expected to be much greater in marine than in more proximal parts of the fan delta because of increased sorting, destruction of [[grain maturity|compositionally immature grains]], and winnowing of fines. [[Directional permeability]] trends in distal parts may be different from more proximal locations because of different sand body trends between these different parts of the fan delta.

===Braided and meandering fluvial deposits===

===Braided and meandering fluvial deposits===