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Organic geochemical investigation of recent marine ooids (457 ± 76 to 1,516 ± 86 years) from the Schooner Cays area, Bahamas, has yielded data that suggest a probable source bed function for their ancient equivalents. Chromatographic analysis of gas desorbed from the ooids reveals the presence of C1 to C5+ compounds believed to be authigenic. These include between 1.7 and 3.6 × 10-5 gm C1 to C4 saturated hydrocarbons per gram organic carbon. Total organic carbon (TOC) content varies between 1.23 and 4.13 wt. %, depending on the purity of the sample, with the lowest values reflecting an increased contribution of skeletal debris to the ooids. Total organic extract (TOE) values range from 550 to 650 ppm an show a slight transformation in the direction of oil formation. The organic matter isolated from ooids (termed protokerogen) is dominantly of algal facies. Elemental composition of this protokerogen showed mean atomic H/C, O/C, and N/C ratios of 1.76, 0.24, and 0.19, respectively. Following pyrolysis, CR/CT ratios were found to be very low, with a mean of 0.18. All the results, including a thermal alteration index (TAI) of 1 to 1.5 on Staplin's scale and a very strong green to blue-green fluorescence under ultraviolet light excitation, point toward an immature, high grade, kerogen-type material with enormous potential for generating hydrocarbons. Additional experiments using a high pressure cell to simulate diagenesis in the ooids showed profound changes in their organic geochemistry wi h the contained organic matter following the predicted evolution path for type II kerogen. There is strong evidence that with deeper burial and prolonged exposure to higher temperatures, and perhaps to catalytic influence of the clay minerals (0.05%) and traces of metals (e.g., Ti, Mn, Sr, V) found in ooids, the organic matter will generate significant amounts of hydrocarbons. In those oolites with a favorable history of porosity development, the hydrocarbons would migrate along the continuous groundmass of organic matter within the ooids and into the pore spaces to accumulate as petroleum.
The indigenous origin of petroleum in oolites, which obviates a long and wasteful primary migration process, affords an explanation for the exceptionally rich petroleum accumulations in such reservoirs. It is proposed that with the exception of leached oolites with oomoldic porosity, other petroleum-bearing oolites could be viewed as integrated source-reservoir beds. The prolific oolite reservoirs of the Arab Formation in Saudi Arabia were selected as natural case studies and the result found to be consistent with the hypothesis.
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