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Organic and inorganic geochemical analyses were completed on 44 samples from the Sisquoc and Monterey Formations, including 37 samples from 6 wells in a north-south transect in the onshore Santa Maria basin and 7 samples from surface outcrops along the Santa Barbara coast. Determinations included major oxides, total carbon, carbonate carbon, vitrinite reflectance, thermal alteration index, elemental analysis of kerogen (C, H, N, 0, S), pyrolysis assay by thermal evolution analysis-flame ionization detector, and gasoline-range hydrocarbon analysis. Results show considerable variation in both inorganic sedimentary components and organic carbon (range, 0.6-14.7%). The kerogens in the sample set are immature and marginally mature type I and type II kerogens, and the set averages 65% amorphous material, and 35% herbaceous, humic, and inertinite particles. In terms of petroleum-source potential, the sample set is on average good to excellent. Most kerogens in the sample set exceed 1% total organic carbon (TOG), about half exceed 2%, and about a quarter exceed 5%. Nearly all samples have total hydrocarbon yield from pyrolysis assay exceeding 0.2%, and all samples have volatile hydrocarbon yield exceeding 200 ppm. Some thermal maturity parameters have distinct trends related to maximum thermal exposure as estimated from silica diagenetic grade. With increasing maximum thermal exposure, TAI increases, Tmax increases, atomic 0/C decreases, and total C^-Cy gasoline range hydrocarbon abundance increases. Trends related to maximum thermal exposure are indistinct or slight in values for the volatile proportion of total hydrocarbon from pyrolysis assay, atomic H/C, and vitrinite reflectance. INTRODUCTION The Monterey Formation in the Santa Maria and Santa Barbara-Ventura basins has received considerable attention in recent years both as an oil source and as an oil reservoir. As an oil-source, the Monterey Formation has some unusual characteristics. For example, increasing evidence suggests that most parameters widely used to judge thermal maturity of kerogen are either misleading or difficult to interpret in the Monterey Formation, partly because of compositional characteristics and partly because much Monterey oil is probably generated at levels of organic metamorphism mormally considered as immature (McCulloh, 1979; Orr, 1983; Walker and others, 1983; Kruge, 1983; Petersen and Hickey, 1984, 1987). Because of its economic importance, Monterey organic geochemistry has been studied mainly by the petroleum industry, and very few data on Monterey kerogens and oils have been made available in the public domain. An area where data are especially sparse is the Santa Maria and Santa Barbara-Ventura basins. The purpose of this report is to make accessible a set of kerogen data for the area. The report provides organic and inorganic analytical data on 44 rock samples from the Monterey Formation and overlying Sisquoc Formation of the Santa Maria and Santa Barbara-Ventura basins (Figure 1). Included are 37 samples from 6 wells comprising a north-south transect within the onshore Santa Maria basin from the Santa Maria Valley oil field to the Lompoc oil field (Figure 2; Tables 1 and 2). This transect was designed as a reconnaissance study of kerogen thermal maturity parameters in the Monterey Formation and lower part of the overlying Sisquoc Formation. Because the Monterey is relatively thin «1500 ft or 450 in) in this area, the total range of thermal exposure within the Monterey in an individual well is relatively small about 1 19-31°C, based on geothennal gradients of 40-70°C/km (French, 1940; Pisciotto, 1981). In order to cover a fuller range of thermal exposure, the study was thus based on laterally equivalent sets of samples from 6 different wells at various levels of maturity. Part of the analytical data reported here for the 37 subsurface samples was previously summarized in an abstract by Isaacs and Magoon (1984) and cited by Isaacs and Petersen (1987). The remaining 7 samples are outcrop samples representing a small reconnaissance set of laterally equivalent strata from the transitional marlsiliceous member of the Monterey Formation in the coastal area between Point Conception and Rincon Point (Figure 3). Based on well-studied silica diagenetic patterns (Isaacs, 1982; Keller, 1984), samples represent increasing diagenetic grade as follows: Goleta Slough (opal-A), Rincon Point (opal-A + opal-CT), Gaviota East Beach (opal-CT), and Black Canyon (quartz) (Figure 3). METHODS Sample Selection Subsurface samples were selected from core material previously collected from Union Oil Company and Arco Oil and Gas Company. Within each well, samples were chosen to represent the siliceous shale/mudstone of the Sisquoc Formation (where available) and a broad range of the lithotypes in the Monterey Formation; included with the Monterey are 2 fine-grained samples from the underlying Point Sal Formation. The Monterey samples should not be interpreted as lithotypes especially representative or typical of the interval from which they were sampled. Nor should average values of the samples be interpreted as average values of the sequence as a whole. Heterogeneity of composition is so marked in the Monterey Formation in this area that several hundred samples would be needed for confident determination of mean values (Isaacs, 1987). Outcrop samples from the Santa Barbara-Ventura coastal area were selected from the transitional-marl siliceous member of the Monterey Formation, a member which has ubiquitous carbonate and on average more organic matter than the formation as a whole (Isaacs, 1983). Samples were intended to represent pairs of calcareous silica-rich rocks and calcareous silica-poor rocks. However, the two samples from Rincon Point are similar in composition (see Tables 7 and 8), and only one sample was processed from Black Canyon due to limited availability of fresh outcrops. This small set of samples is highly select and was not designed to represent average values for the Monterey Formation in the Santa Barbara-Ventura coastal area as a whole. Sample Preparation Samples were prepared for three different types of analysis: (1) organic analysis; (2) inorganic analysis; and (3) (not reported here) determination of bulk density and grain density. Samples were cut on a free-flowing water saw or (where friable) on a dry saw. Amounts submitted for organic analysis ranged from 138 to 330 g, and amounts for inorganic analysis from 6 to 15 g. In comparing results from the organic and inorganic analysies, note that the two sets of analyses were not performed on powder splits of the same piece of sample, but on separate pieces of sample. Where a single piece of bulk sample was large enough, all analytical pieces (including density plugs) were cut from the same chunk and represent matching or nearly matching pieces. However, a uniform chunk of each sample was not in all cases large enough or properly shaped to allow matching pieces for both sets of analyses. In some cases, moreover, separate chunks of sample had to be aggregated in order to obtain sufficient material (av 200 g) for the organic analyses. The level of correspondence between the material used for inorganic analysis and the material used for organic analysis is listed in Table 2. In this table, "exact match" means that a single chunk of sample was trimmed to represent a single stratigraphic horizon and was cut vertically into pieces for the two sets of analyses so that the two pieces were as exactly matching as possible (except for any lateral differences). "Near match" means that a single chunk of sample was trimmed and cut vertically into two pieces that were almost matching, but one of the pieces included 5-10% material not included in the other piece (for example, where the bulk chunk was slightly tapered). "Part match" means that a single chunk of sample was cut into two pieces, but the bulk sample was quite irregular in shape so that the two pieces did not exactly correspond (for example, the inorganic piece may have included only half the thickness represented by the organic piece). "Similar" means that the two pieces were not cut from the same bulk chunk but represented separate pieces of rock that looked lithologically similar. Organic Analytical Techniques Samples in this study were processed for organic geochemical parameters along with samples in the U.S. Geological Survey's oil and gas source rock study of the National Petroleum Reserve in Alaska (NPRA) (Claypool and Magoon, 1988). Parameters determined were: total carbon (of whole rock); organic carbon (of whole rock); vitrinite reflectance; thermal alteration index (TAI); elemental composition including C, H, N, 0, S, ash (of kerogen concentrate); results from pyrolysis assay by thermal evolution analysis-flame ionization detector (TEA-FID); and abundance of C^-Cy gasoline-range hydrocarbon compounds (of whole rock). The TEA-FID method (including results for total hydrocarbon, volatile hydrocarbon, and Tmax) is similar to Rock-Eval pyrolysis, but results are not exactly comparable.* All organic-matter analyses were performed by Geochem Research, Inc., of Houston, Texas. The techniques used in the analysis were detailed by Claypool and Magoon (1988) except for 0 and S determinations. Oxygen abundance was determined by the Unterzaucher technique. Sulfur abundance was determined by the conversion of sulfur to sulfate (decomposition of organic matter by combustion in a Schoeniger flask in oxygen, with the 802-803 mixture absorbed in aqueous hydrogen peroxide), and the sulfate was measured in 80% isopropanol by titration with Ba(ClO^) solution using Thorin indicator. Inorganic Analytical Techniques A matching or similar piece of each sample was analyzed for major element abundances by X-ray fluorescence spectroscopy (XRF). These analyses were performed on undried powders by the U.S. Geological Survey's Branch of Geochemistry as de

u.s. geological survey open file report/open-file report

Reconnaissance study of petroleum source-rock characteristics of core samples from the Sisquoc and Monterey formations in a north-south subsurface transect across the onshore Santa Maria basin and in surface sections along the Santa Barbara-Ventura coast, Southern California