Permeability patterns in point bar deposits: Tertiary Loranca Basin, central Spain

A composite point bar sediment body in the Tertiary Loranca Basin of central Spain was the subject of a detailed analysis of the relation between sedimentary facies heterogeneities and permeability contrasts.

Comparison of field-measured permeability data with grain size, sorting and diagenetic features shows that a strong decrease in permeability (from 10D to 0.1D) over the contact zone of two stacked point bar units is related to the occurrence of a cemented, rootletted mud layer which formed during a channel abandonment stage. This type of low-permeability horizon is observed to extend from the adjacent floodplain downslope onto the abandonment layer. In the topographically lowest part of the abandonment slope, however, the mud layers usually are eroded during a subsequent channel reactivation stage, and stacking of point bar units is characterized by sand-on-sand contacts with normal, high-permeable transitions.

Statistical processing of permeability data from cross-bedded sets within a point bar unit reveals that the average foreset permeability is significantly higher than the average bottomset value (total permeability range from 0.5 to 17 D), and that the foreset values are highly variable. Both observations are directly related to grain size and sorting differences.

In reservoir equivalents of the composite point bar sediment body, the effect of the permeability contrasts on fluid flow is two-fold:

1 channel abandonment layers, when preserved after channel reactivation, cause partial compartmentalization of the reservoir unit;

2 in water-flooded oil-reservoirs, permeability contrast between adjacent foreset laminae and bottomsets may cause a capillary pressure-determined water fingering effect.