Depositional and Diagenetic History of Bodcaw Sand, Cotton Valley Group (Upper Jurassic), Longwood Field, Caddo Parish, Louisiana: ABSTRACT

Abstract

ABSTRACT The Bodcaw Sand contains fine-grained sandstones and siltstones deposited within a barrier bar sequence. Based on vertical changes in sedimentary structures, texture, and mineralogical composition, three distinct lithofacies (upper, middle, and lower shoreface) within the Bodcaw Sand and two associated lagoonal lithofacies were identified. Cross-stratification and low angle laminations, rarely disrupted by biogenic structures, characterize the fine-grained upper shoreface sandstones. Middle shoreface sandstones have undergone extensive reworking by biotic and abiotic factors. Few primary sedimentary structures or early generation trace fossils are preserved in middle shoreface sandstones. Lower shoreface siltstones and very fine-grained sandstones contain lenticular and wavy bedding features that are often disrupted by bioturbation. The Bodcaw Sand has low porosity and permeability values. Vertical and lateral variation in porosity and permeability values are related to original deposition and subsequent diagenesis of Cotton Valley sediments. The Bodcaw Sand has experienced a complicated diagenetic history; compaction, cementation, replacement, and dissolution have modified primary rock properties following deposition of barrier bar sediments. Authigenic cementation plays an important role in modification of reservoir properties. Important authigenic minerals identified in the Bodcaw Sand include silica, carbonates, and phyllosilicates. Two major diagenetic sequence in the Bodcaw Sand are recognized based upon textural relationships between allogenic grains and authigenic constituents.