Modern Analogs for Paleocave Sediment Fills and their Importance in Identifying Paleocave Reservoirs

Abstract

ABSTRACT Coalesced, collapsed-paleocave reservoirs occur in Lower Ordovician Ellenburger, Arbuckle, and Knox carbonates and less commonly in other Paleozoic strata. One of the most distinctive features of these paleocave systems, on wireline logs and in core data, is sediment fill. Modern analogs of paleocave-sediment fills from several Gulf of Mexico cave systems characterize the texture, fabric, and associated sedimentary structures of these fills. The sediment-fill facies can consist of carbonate and/or siliciclastic debris. Clastic cave sediment, ranging from clay- to cobble-sized material, commonly contains variable amounts of breakdown-derived breccia. Sections of passages can be completely plugged by sediment. Depositional processes include suspension, traction, and mass-flow mechanisms. Sedimentary structures such as parallel laminae, chaotic bedding, cross bedding, graded bedding, and scour are common. Reservoir quality of the sediment-fill facies depends on the texture and mineralogy of the material. Sediment fills are distinctive in core and can be found more than 100 m beneath the unconformity that produced the cave system. On wireline logs, they are characterized by high positive spontaneous-potential and high gamma-ray responses that reflect the terrigenous content in an otherwise carbonate system. Sediment fills, distinctive eatures of paleocave reservoir systems, are important in substantiating the origin of these reservoirs.