Fan-Delta Reservoirs in Lower Cotton Valley Group (Jurassic), Kildare Field, Northeast Texas

Cynthia E. Black, Robert R. Berg

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) thick. 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 sandstones, and (3) an offshore facies of very fine-grained sandstones and interbedded black shales.

Facies 1 is a fine-grained (0.21 mm), nearly structureless unit that is 15 ft (5 m) thick, lacks bedsets, and has a quartz content that approaches 95%. Facies 2 is conglomeratic, and is in beds that fine upward, and are 0.5-3 ft (0.15-1 m) thick. 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 at the top by wave action. The dip-trending channel facies (facies 2) is overlain by the strike-trending beach facies (facies 1). Higher porosities of 12% and permeabilities of 7 md are found in the beach facies.

AAPG Search and Discovery Article #91042©1987 GCAGS and GC-SEPM Section Meeting, San Antonio, Texas, October 28-31, 1987.