ABSTRACT: Geothermometry from Smectite and Silica Diagenesis in the Diatomaceous Monterey and Sisquoc Formations, Santa Maria Basin, California

Richard M. Pollastro

Indigenous, immature, low-gravity oils are produced from organic-rich diatomaceous rocks of the Monterey and Sisquoc formations of Miocene-Pliocene age in tectonically active basins along the coast of California. Original depositional components of these rocks are primarily biogenic silica (opal-A) and clay. Diagenetic changes in silica and interstratified illite/smectite (I/S), studied in the Union 51 Newlove well, Orcutt field, Santa Maria basin, using x-ray diffraction (XRD), constrain the thermal and burial histories of the diatomaceous source rocks.

The dominant clay mineral in these rocks is I/S, which composes about 60 wt. % of the clay minerals in the <2-µm fraction. There is a progressive decrease in expandability of I/S with increased burial depth from about 85% at 150 m to about 35% at 1100 m in the Union 51 Newlove well. The change from random I/S (R = O) to ordered I/S (R = 1) occurs at about 800 m in the well. Because of considerable uplift and erosion in the Orcutt area, no opal-A zone is present in the Union 51 Newlove well and the opal-CT zone occurs near the surface. The progressive diagenesis of opal-CT to quartz in the well is documented on XRD profiles. The transformation reaction of opal-CT to quartz begins at about 600 m and is complete at about 650 m; quartz is the only silica phase present below 650 m. /P>

The utility of I/S- and silica-phase geothermometry can be tested in the Union 51 Newlove well from the established profiles. After burial-history reconstructions, calculated temperatures from geothermal gradient profiles for changes in I/S (R = O to R = 1) and silica phase (opal-CT to quartz) in the well are 100-105°C and 85-90°C, respectively. These calculated temperatures are in good agreement with established diagenesis models; therefore, I/S- and silica-phase geothermometry is a useful tool for understanding the thermal and burial histories of these economically important rocks throughout the basin.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990