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Fan deltas have been defined as progradation of alluvial fans into a standing body of water from a proximal highland area. Sedimentary environments associated with fan delta complexes have been described in detail in Holocene examples. The subaerial fan is composed of braided channels, gravelly beaches, flood plains, and marshes. The subaqueous fan includes tidal lagoons, channel-fill complexes, marginal islands, breaker bars, and is also characterized by steep slopes and submarine channels where mass-gravity processes may dominate.
Few fan delta complexes have been recognized in the subsurface. The Cotton Valley Taylor "B" Sandstone is interpreted as the distal part of a prograding fan delta based on the vertical sequence in cores from Kildare field, Cass County, Texas. Three
major sandstone facies are distinguished on the basis of sedimentary structures, composition and texture. They are, in descending order, (1) a massive to laminated, well-sorted sandstone, (2) a laminated, pebbly sandstone, and (3) a very fine-grained sandstone interbedded with black shale.
Facies 1 is fine-grained (0.21 mm), lacks bedsets, and is mostly massive or indistinctly laminated. Monocrystalline quartz content is high, reaching 95%. Other minerals are minor at 4%, and rock and shell fragments range from 5 to 15%. Facies 1 is only 10 ft (3.1 m) thick and is interpreted as bar or beach deposits formed by reworking of sand by waves at the margins of the delta plain. Facies 2 is composed of conglomeratic bedsets which fine upward from a pebbly erosive base to a parallel or cross-laminated top. Bedsets range from 0.5 to 3 ft (15 cm to 1 m) thick. There is an increase in rock and shell fragments and a decline in monocrystalline quartz from Facies 1, averaging 34% and 63% respectively. The facies remains fine-grained (0.19 mm); however, average grain size and bed thick ess increase upward. Facies 2 is interpreted as a complex of shallow, braided, distributary channels. Thickness is variable but ranges from 10 to 20 ft (3.1 to 6.2 m). Facies 3 bedsets are graded and the sequence of sedimentary structures reflects deposition under conditions of decreasing flow regime. The thin, 1 ft (0.3 m), graded sandstone beds are separated by black laminated shale. This facies is very fine-grained (0.09 mm), relatively poor in quartz, 70%, and rich in matrix which may exceed 30%. Facies 3 is interpreted as thin-bedded turbidites deposited in channels and channel margins downdip from the break in slope at the edge of the delta platform. The thin AE, ABE, BE and CDE bedsets alternate with intervals, 1 to 3 ft (0.3 to 1 m) thick, of bioturbated siltstone or rippled sand tone indicative of normal, shallow marine biotic and current activity. There is a sharp, erosive relationship between Facies 2 and 3.
Resistivities are variable and SPs depressed due to extensive carbonate cement (up to 50% total rock) which is also responsible for low porosity and permeability. Dissolution of authigenic carbonates results in secondary porosity up to 17%. Cross-plots of resistivity (Rt) versus porosity are useful in characterizing facies and enable identification of facies in non-cored wells.
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