Changes

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, mid-fan, and 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. Gamma ray, SP, and 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, mid- 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 and mudflow deposits. Finer grained but better sorted distal fan deposits are highly permeable and porous. Because of increased sorting, mid- 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.