Use of Well Cuttings for Determining Compositional Trends in the Monterey Formation, California
Caroline M. Isaacs
In the Santa Maria and Santa Barbara-Ventura basins of California, the Monterey Formation is extremely variable in composition, hand samples showing values widely scattered over the ranges 2-95% biogenically derived silica, 0-95% carbonate minerals, 5-80% terrigenous detritus, 1-35% organic matter, and 0-25% apatite. Histograms of compositional abundances show markedly non-Gaussian distributions. Silica abundance, for example, is almost uniformly distributed over its range (i.e., silica is as likely to be 10-15% as 80-85%). Such abundance patterns make reliable determination of mean values extremely difficult; statistical analysis indicates that for sequences only 5 m (20 ft) thick, several hundred hand samples are needed for confident determination of a mean within 5% of its true value. Variation thus poses a major problem in accurately evaluating and comparing compositional trends, both lateral and stratigraphic.
The proposed practical solution is analysis of bulk well cuttings, which are composite samples. The reliability and reproducibility of the method were tested by analyses (major oxides, total carbon, organic carbon) of (1) cuttings from one well and cores from an adjacent well in the Orcutt oil field, onshore Santa Maria basin, and (2) cuttings from two adjacent wells in the Hondo offshore oil field, Santa Barbara Channel. Comparison of cores with cuttings from the two Orcutt wells generally confirms the accuracy of cuttings analysis for representing the compositional sequence. Comparison between cuttings from the two Hondo wells shows that cuttings analysis yields a distinctive reproducible sequence that provides an excellent basis for detailed correlation.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.