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Title: Silica diagenesis in Santa Cruz mudstone, Late Miocene, California

The silica-rich upper Miocene Santa Cruz Mudstone is similar to the Miocene Monterey Formation. Previous studies have suggested the Santa Cruz Mudstone was not buried deeply nor had it undergone extensive diagenesis. Because opaline diagenesis is temperature dependent, the author examined the silica diagenesis of the Santa Cruz Mudstone using scanning electron microscopy and x-ray diffraction methods to study its burial history. In a series of samples from Santa Cruz to Davenport, California (over 16 km), opal-CT is the dominant silica phase present and clay minerals are notably absent. The d(101)-spacing values of opal-CT range from 4.11 A (Santa Cruz area) to 4.06 A (north of Santa Cruz), exhibiting the complete range of d(101)-spacing values found in opal-CT zones. Scanning electron micrographs of crystalline microtextures show rosettes of opal-CT (lepispheres) in cavities of samples with medium to high d(101)-spacing values. The morphology of lepisphere crystallites grades from bladed to spiny with decreasing d(101)-spacing values, reflecting an internal crystal ordering with increased diagenesis. Further diagenetic changes occurred in a sample with 4.06 A d(101)-spacing where incipient quartz crystals signal the initial conversion of opal-CT to microcrystalline quartz. Silica diagenesis demonstrates that burial temperatures surpassed the range of opal-A to opal-CT conversionmore » and approached conversion temperatures (55/sup 0/C to 110/sup 0/C) of opal-CT to microcrystalline quartz. The conversion occurred when the Santa Cruz Mudstone was buried over 1900 m (depth calculated from a geohistory diagram). This burial temperature brings the Santa Cruz Mudstone within the oil generation window, and could account for the presence of hydrocarbons in the unit.« less
Authors:
Publication Date:
OSTI Identifier:
6255419
Report Number(s):
CONF-870606-
Journal ID: CODEN: AABUD
Resource Type:
Conference
Resource Relation:
Journal Name: AAPG (Am. Assoc. Pet. Geol.) Bull.; (United States); Journal Volume: 71:5; Conference: American Association of Petroleum Geologists annual meeting, Los Angeles, CA, USA, 7 Jun 1987
Research Org:
Univ. of California, Santa Cruz
Country of Publication:
United States
Language:
English
Subject:
02 PETROLEUM; CALIFORNIA; PETROLEUM DEPOSITS; SILTSTONES; RESERVOIR ROCK; DIAGENESIS; GEOLOGIC FORMATIONS; TERTIARY PERIOD; CENOZOIC ERA; FEDERAL REGION IX; GEOLOGIC AGES; GEOLOGIC DEPOSITS; MINERAL RESOURCES; NORTH AMERICA; RESOURCES; ROCKS; SEDIMENTARY ROCKS; USA 020200* -- Petroleum-- Reserves, Geology, & Exploration