SHEW, ROGER D., Shell Exploration and Production Technology Company, Houston, TX

Abstract: Descriptions, Properties, and Importance of Non-Reservoir Sediments in the Gulf of Mexico

Non-reservoir sediments that occur within and as bounding lithologies to Miocene to Pleistocene age reservoirs in the Gulf of Mexico are dominated by mudstones and claystones but there are also marls, volcanic ashes, carbonates, and cemented silts and sands present. The properties of these sediments, particularly the mudstones and claystones, are critical to understand as they are important to interpretations of 1) reservoir quality and producibility (baffles and barriers), 2) seal capacity, and 3) seismic data.

The classification of these "mudrocks" is mudstones (1/3 to 2/3 silt), claystones (<1/3 silt), silts (>2/3 silt but these are mostly of reservoir quality), and marls (>30% calcareous micro-fossils). Mudstones are part of the waning flow and suspension fallout, claystones are the final suspension fallout and hemi-pelagic "rain", and marls form during highstands of sea level as pelagic sediment and are often good regional markers. Another nonreservoir type is debris flows. These are admixtures of sand and clay that show mass flow and slump features; they may be mappable and separate reservoir bodies.

The properties of these sediments are quite variable depending on the silt, clay, and carbonate content as well as on the age, pressure, and temperature. Clay diagenesis is very important but it is not the only factor controlling the quality of the seals, baffles/barriers, and geophysical properties. Continuity, areal changes, and thickness are also important. If continuous, these sediments have been found to be capable of holding significant hydrocarbon columns: >900 m. Local and field scale properties and geometries must be known to determine trap, seal, and geophysical interpretations of the reservoir.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England