James G. Palacas

Determination of Organic Carbon in Modern Carbonate Sediments

ABSTRACT In the routine analysis for organic carbon in modern unconsolidated sediments the initial step commonly is to remove the carbonate carbon by acid treatment and to analyze directly the organic carbon in the residue. We find, however, that as much as 44% of the organic carbon in modern carbonate sediments from Florida Bay, Florida, is solubilized and lost during the acid treatment. Therefore, the amount of carbon in the acid solution in these, and in similar modern sediments, must also be included in the analysis for an accurate determination of the percentage of total organic carbon in the sample.

Source Rock Potential of Middle Cretaceous Rocks in Southwestern Montana

The middle Cretaceous in southwestern Montana is composed of a marine and nonmarine succession of predominantly clastic rocks that were deposited along the western margin of the Western Interior Seaway. In places, middle Cretaceous rocks contain appreciable total organic carbon (TOC), such as 5.59% for the Mowry Shale and 8.11% for the Frontier Formation in the Madison Range. Most samples, however, exhibit less than 1.0% TOC. The genetic or hydrocarbon potential (S1+S2) of all the samples analyzed, except one, yield less than 1 mg HC/g rock, strongly indicating poor potential for generating commercial amounts of hydrocarbons. Out of 51 samples analyzed, only one (a Thermopolis Shale sample from the Snowcrest Range) showed a moderate petroleum potenti l of 3.1 mg HC/g rock. Most of the middle Cretaceous samples are thermally immature to marginally mature, with vitrinite reflectance ranging from about 0.4 to 0.6% R0. Maturity is high in the Pioneer Mountains, where vitrinite reflectance averages 3.4% R0, and at Big Sky, Montana, where vitrinite reflectance averages 2.5% R0. At both localities, high R0 values are due to local heat sources, such as the Pioneer batholith in the Pioneer Mountains.

Baseline concentrations of hydrocarbons in barrier-island quartz sand, northeastern Gulf of Mexico

The amount and approximate molecular distribution of indigenous hydrocarbons in coastal sand provide a baseline above which petroleum pollution is measurable. Organic carbon, alkanes, total hydrocarbons, alkane/organic carbon and hydrocarbon/organic carbon ratios, and gas chromatographic analyses were used to characterize marine, lagoon, and pond facies of the barrier-island sand. Results from six localities provide no evidence for petroleum pollution. Such pollution would be indicated by complex molecular distributions resembling petroleum and by alkane/organic carbon ratios greater than 0.003 and hydrocarbon/organic carbon ratios greater than 0.01.

Thermal Maturation and Organic Richness of Potential Petroleum Source Rocks in Proterozoic Rice Formation, North American Mid-Continent Rift System, Northeastern Kansas

A recent well in northeastern Kansas penetrated 296 ft (90.2 m) of dark gray siltstone in the Precambrian Mid-Continent rift (Proterozoic Rice Formation). Correlations indicate this unit may be as thick as 600 ft (183 m) and is possibly time-equivalent to the Nonesuch Shale (Middle Proterozoic) in the Lake Superior region. The upper half of this unit qualifies as a lean source rock (averaging 0.66 wt. % TOC), and organic matter in it is in the transition stage between oil and wet gas generation. The presence of the gray siltstone in this well and similar lithologies in other wells is encouraging because it indicates that source rock deposition may be common along the Mid-Continent rift, and that parts of the rift may remain thermally within the oil and gas window. Microscopic examination of calcite veins penetrating the dark gray siltstone reveals numerous oil-filled and subordinate aqueous fluid inclusions. Homogenization temperatures indicate these rocks have been subjected to temperature of at least 110-115°C (230-239°F). Burial during the Phanerozoic is inadequate to account for the homogenization temperatures and thermal maturity of the Precambrian rocks. With the present geothermal gradient, at least 8250 ft (2.5 km) of burial is necessary, but lesser burial may be likely with probably higher geothermal gradients during rifting. Fluorescence colors and gas chromatograms indicate compositions of oils in the fluid inclusions vary. However, oils in the fluid inclusions are markedly dissimilar to the nearest oils produced from Paleozoic rocks.

Hydrocarbons in Estuarine Sediments of Choctawhatchee Bay, Florida, and Their Implications for Genesis of Petroleum

Analyses were made on 159 sediment samples from Choctawhatchee Bay, Florida, to determine the distribution and significance of bitumen (benzene-soluble organic substances), particularly the hydrocarbons. Column and gas chromatography was used to characterize the bitumen. In the upper 10-16 cm of estuarine mud, which contains an average of 3.6 percent organic carbon, the bitumen content ranges from 60 to 470 ppm and averages 170 ppm, or about 0.5 percent of the organic carbon. In the relatively clean quartz estuarine sand the bitumen content averages only 20 ppm, or 1.3 percent of the average organic carbon (0.15 percent). The hydrocarbons make up an average of about 50 percent of the bitumen in the mud and an average of about 22 percent of the bitumen in the sand. The distribution in the mud of normal paraffins C19 to C33 is remarkably uniform among the samples, and shows a marked predominance of odd- over even-carbon-number molecules (average odd over even ratio is about 8), characteristic of a land-plant source of hydrocarbons. In contrast, the n-paraffin distribution in the sand is quite variable, and in many samples the odd over even ratio is less than 2, suggesting brackish-water and marine organisms as a major source of hydrocarbons. Results of this study and related studies show that recent sands contain bituminous substances, including hydrocarbons, in geologically significant amounts. The inference is drawn that, if the bitumen disseminated in large volumes of interconnected sands is converted, even in part, to crude oil and concentrated in pools, the sands themselves may contribute substantial amounts of the petroleum crude oil in sandstone reservoirs.