Fan deltas are alluvial fans that prograde into a standing body of water from a proximal highland area. Few fan deltas have been recognized in the subsurface, but the Cotton Valley Taylor "B" sandstone can be interpreted as the distal part of a fan delta in Kildare field, Cass County, Texas. Three facies are distinguished in cores of two sandstones that are 40 ft (12 m) in thickness. They are, in descending order: (1) a beach facies of massive to laminated, well-sorted sandstone, (2) a channel facies of massive to laminated, pebbly sandstone and (3) an offshore facies of very fine-grained sandstones and interbedded black shales.

Facies 1 is a fine-grained (0.21 mm) and nearly structureless unit that is 15 ft (5 m) thick, lacks bedsets and has a high quartz content that approaches 95%. Facies 2 is conglomeratic in beds that fine upward and are 0.5 to 3 ft (0.15 to 1 m) in thickness. Facies 3 is thinly bedded in massive to laminated sets that are graded and separated by black shales. The total section represents a coarsening upward, rapidly prograding sequence that was partly reworked by wave action at the top.

The three facies were identified in noncored wells by plots of true resistivity as a function of porosity. Based on the identification in well logs, the beach facies (1) shows a narrow strike trend, the channel facies (2) shows a dip trend, and the offshore facies (3) thickens in a dip direction and replaces the beach and channel facies in a seaward direction. Higher porosities of 12% and permeabilities of 7 md are found in the beach facies.

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