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ABSTRACT: Stratigraphic Sequence of the Monterey and Sisquoc Formations in the Hondo Oil Field, Santa Barbara Channel, California

Caroline M. Isaacs, Robert G. Arends, Mary Lou Thornton

The stratigraphic sequence of the Monterey and Sisquoc formations in the Hondo oil field was evaluated by analyzing a 5000-ft sequence of well cuttings. In this sequence, 10-ft intervals from the Sisquoc Formation generally average (by weight) 55-70% detritus, 15-30% biogenic/diagenetic silica, 5-15% carbonate minerals (principally dolomite), less than 0.3% apatite, and 2-3% organic matter. Within the Sisquoc Formation (about 2200 ft thick), the average abundance of carbonate minerals decreases downward from 15 to 5% and the average silica abundance increases from 15 to 30%, whereas detritus and apatite abundances remain essentially constant. Compared to the overlying Sisquoc Formation, the 1500-ft thick siliceous zone comprising the upper part of the Monterey Formation c ntains markedly less abundant detritus (range of 10-ft averages, 15-40%), but more abundant silica (30-65%), carbonate minerals (10-35%), apatite (0.3-4.4%), and organic matter (3.8-9.9%). In comparison with the upper part of the Monterey, within the 1500-ft-thick sand-shale zone comprising the lower part of the Monterey, detritus is more abundant and increases downward (from 25 to 75%), silica is less abundant (range, 10-25%), carbonate minerals are more abundant at the top of the zone and decrease downward (from 40 to 5%), apatite is sparser (average <0.05%), and organic matter is also less abundant (average 2.5%). Within the entire sequence of the Monterey and Sisquoc formations, average apatite exceeds 1% in only two parts: (1) a 400-ft interval comprising most of the lower calcar ous zone (distinctly apparent on the gamma-ray log) and (2) a 200-ft interval in the middle shale zone and upper part of the massive chert zone (not distinct on the gamma-ray log).

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