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

R. G. Loucks
Bureau of Economic Geology, University of Texas at Austin, Austin, TX

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 wells.

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. The shape and size of the original passage control the dimensions of these deposits. Most sediment fills originate outside of the cave system and are transported into cave passages by surface runoff and back flooding. Depositional processes include suspension, traction, and mass-flow mechanisms. Sedimentary structures such as parallel lamina, chaotic bedding, cross bedding, graded bedding, and scour are common. Reservoir quality depends on the texture and mineralogy of the material.

Sediment fills are distinctive in core and can be found over 100 m beneath the unconformity that produced the cave system. On wireline logs, they are characterized by high positive SP and high gamma-ray responses that reflect the terrigenous content in an otherwise carbonate system. Sediment fills, distinctive features of paleocave reservoir systems, are important in substantiating the origin of these reservoirs.

AAPG Search and Discovery Article #90901©2001 GCAGS, Annual Meeting, Shreveport, Louisiana