Diagenesis in Deeply Buried Arkosic Sandstones, Central San Joaquin Basin, California

Patrick T. McCullough and Robert A. Horton Jr.

Deeply buried Eocene through Lower Miocene arkosic sandstones, including reservoir sands at Kettleman City and Tulare Lake, were deposited as laterally continuous sand bodies separated by shales in shallow marine environments. They have undergone nearly continuous subsidence since deposition. Pore fluids presently residing in these rocks are derived from sea water modified by diagenetic reactions. These reactions include extensive albitization of both plagioclase and potassium feldspars, alteration of volcanic lithic fragments to clays, precipitation of authigenic cements, and dissolution of framework grains (feldspars, quartz, and lithics) and cements. Cements are minor and include both clays and carbonates. Clays (mainly kaolinite) occur as pore-occluding cement and gra n coatings. Carbonates (mainly calcite with lesser dolomite, ferroan dolomite, siderite, and magnesian siderite) precipitated over a wide range of temperatures throughout the burial history of these rocks. In some places thin zones of calcite cement form seals for petroleum accumulation. The dominant controls on reservoir quality are compaction and dissolution of framework grains. Generally these sands are tightly packed with abundant long and/or sutured contacts, but zones of extensive secondary porosity are present. These zones occur in coarser-grain sands and adjacent to fractures which served as channel ways of enhanced permeability and fluid movement. Secondary porosity generally accounts for more than 50% of total thin-section porosity in these rocks; the amount of material removed through dissolution of aluminosilicates greatly exceeds that precipitated as authigenic clays in most of the samples studied, indicating significant removal of material, including aluminum, from the system.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California