Reservoir Quality in the Deep to Ultradeep Lower Miocene Sandstone Play, Western Gulf of Mexico

The deep shelf gas play in the western Gulf of Mexico is focusing exploration attention on deep reservoirs (>4.5 km) below the shallow waters of the continental shelf. At these depths the greatest unknown and most critical risk factor is reservoir quality. This paper examines controls on reservoir quality of lower Miocene sandstones. We focus on the influence of regional variation in detrital mineral composition on diagenesis and porosity of lower Miocene reservoirs in five areas across the western Gulf: (1) Louisiana, (2) upper Texas coast, (3) lower Texas coast, (4) Burgos Basin, Mexico, and (5) Veracruz Basin, Mexico. Lower Miocene sandstones in the western Gulf of Mexico were derived from a wide geographic area with varied provenance, which is reflected in detrital mineralogy. Lower Miocene sandstones from offshore Louisiana are quartz-rich subarkoses; feldspar and rock-fragment content increase southward as source areas shifted to include volcanic and carbonate rocks. Abundance of chemically and mechanically unstable detrital grains influenced diagenesis, pore types, and porosity loss. Porosity is higher in quartz-rich sandstones in Louisiana and upper Texas than in lithic-rich sandstones of the lower Texas coast and Mexico at the same temperature.