Diagenetic Mineral Assemblage Prediction of Reservoir Burial History

Lori A. Hathon and David W. Houseknecht

Mineralogy of Oligocene Sespe Formation sandstones, siltstones and shales was used to establish a relationship between depth of burial and diagenetic process within the Oxnard Shelf tectonic subprovince of the Ventura Basin, California. Shallow (700-2000m), low temperature diagenetic products/processes observed at the eastern end of the shelf include: smectite- kaolinite-iron oxide pigmentation-labile grain dissolution. Plagioclase feldspar framework grains exhibit intermediate compositions (Ab₈₀). Deep (3700- 5300m), high temperature diagenetic products/processes observed at the west end of the shelf include: illite/smectite-illite-chlorite-quartz-laumontite- intergranular pressure solution. Plagioclase feldspar framework grains are nearly completely albitized (Ab₉₅).

Apatite fission track analyses confirm depth of burial/temperature exposure estimates made from diagenetic mineral assemblages observed along the sample traverse. The eastern end of the Oxnard shelf has not been significantly more deeply buried than at present day; the South Mountain area experienced approximately ~2000 m of deeper burial prior to uplift. Diagenetic mineral assemblage was used to infer that Sespe sandstones from the Topa Topa Uplift area (present depth 312m) have experienced significantly deeper paleoburial (on the order of approximately ~4,000-5,000m/13,000-16,000ft) and exposure to temperatures on the order of 130°C (270°F). Apatite fission track analysis supports the interpretation made from diagenetic assemblage, suggesting paleotemperature >110° .

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