Gray Sandstones (Jurassic) in Terryville Field, Louisiana--Basinal Deposition and Exploration Model: ABSTRACT

Deep (~ 13,000 ft, 4,000 m) hydrocarbon production at Terryville field is from various zones within Upper Jurassic siliciclastics that are referred to informally as the "gray sandstone" section. This sequence of interbedded sandstones and shales occurs in the Smackover section or within the lower Bossier shale seaward of the Upper Jurassic shelf edge, and is correlative to and coeval with inner shelf facies deposited north of the field area. The "Gray" section at Terryville field consists of at least four sandstone bodies separated by black shales and silty to sandy shales. The shales are thinly bedded and harbor a locally abundant ichnocommunity of Chondrites, Planolites, and Scalarituba; small ammonoids and bivalves are present locally in these beds. Thin layers and lenses of sandstones (lenticular and flaser bedding, partial Bouma sequences) are intercalated with the shales locally, and commonly are heavily bioturbated by Teichichnus and Arenicolites. The sandstones are fine grained, feldspathic sublitharenites, locally conglomeratic (shale clasts), with rare ooids and comminuted skeletal fragments. The four sandstones in the field area are of stacked, lobate geometry. The lobes consist internally of anastomosing lense of sandstones and conglomeratic sandstones interbedded with and replaced laterally by shales and sandy shales. The long axes of these lobes and lenses are oriented normal to regional upper Smackover shelf-edge trends. Stacked "megasedimentation packages" are recognized within each sandstone lobe. These packages consist internally of repetitive second-order sedimentation units, including partial Bouma sequences, locally conglomeratic graded beds (normal and reverse), massive textureless beds, and coarse rhythmites. The thickness and internal grain size of these component units have a tendency to decrease systematically upward from the base of and laterally within each megasedimentation package. Stacked packages within and immediately surrounding the depoaxes of each lobe coarsen upward f om repetitive units of sandstones to conglomeratic sandstones. The areal distribution, vertical stratigraphy, geometry, bedform characteristics, and texture of the "Gray" sandstones, and their regional relation to upper Smackover carbonate facies to the north, suggest their formation as progradational submarine-fan complexes deposited in a basinal environment. The sandstones and conglomeratic sandstones are interpreted as braided distributaries and associated facies deposited in upper and midfan environments. At distances from these distributaries, the thinner sandstone packages and the interbedded shales and sandstones represent proximal overbank to midfan deposits. The intervening shales are interpreted as basin plain and distal overbank deposits. Although trapping at Terryville field is mainly structural, sandstone trends and geometries control reservoir occurrence, and this aspect of stratigraphic entrapment should be expected in future "Gray" sandstone fields in this area. Reservoir permeabilities in the "Gray" sandstones are limited because of the presence of pore-filling chlorite, illite-smectite, and dolomite. End_Page 1432------------------------------ The pore system is almost entirely of secondary origin, having resulted from the partial dissolution of labile framework grains and carbonate cements. The most effective stimulation of these reservoirs appears to be an acid-enhanced hydraulic fracture. End_of_Article - Last_Page 1433------------