Geochemical Analyses of Samples from Hole 515B, Vema Channel, and Hole 516F, Rio Grande Rise

Abstract

The petroleum-generating potential of five samples from Hole 515B, Vema Channel, and of 23 samples from Hole 516F, Rio Grande Rise, was analyzed. Organic carbon and pyrolysis data indicated that source rocks of good quality are not present. Microscopic examination showed predominance of woody organic matter, which is more favorable for the generation of gas in a mature stage; all samples, however, are still thermally immature. INTRODUCTION Samples from Holes 515B and 516F, Leg 72 (Fig. 1), were studied in detail in order to characterize and evaluate the petroleum-generating potential of the drilled section. In this investigation, routine geochemical parameters were measured and considered in conjunction with more detailed analyses (character of the Cf5 = saturated hydrocarbons and types of organic matter, qualitatively determined by microscopic examination). Lithologies and ages of the samples are summarized in Table 1. ANALYTICAL METHODS The organic carbon content was determined by combustion in a Leco carbon analyzer. Standard samples were analyzed frequently in order to check precision of the instrument. A pyrolysis apparatus developed in the PETROBRAS Research Center was used to quickly determine source rock characteristics. Light hydrocarbon gases (Cj-C4) were analyzed isothermally at 100 °C from canned samples by gas chromatography, using a Perkin Elmer Model 990 equipped with a 1.2-m Porapak column. Kerogen (insoluble organic matter) was separated according to the standard procedure used in palynology (Staplin, 1969), but an oxidation step was omitted. The separated kerogen was mounted on regular glass slides and studied under the microscope (400 × magnification, plain transmitted light). Bitumen was extracted in a Soxhlet apparatus by refluxing with pure choloroform at its boiling point. The extracts were fractionated via solid-liquid chromatography (after Cummings and Robinson, 1964). The total alkanes from rock extracts were analyzed in a Perkin Elmer Model 990 gas chromatograph equipped with hydrogen flame ionization detectors and two identical eutectic analytical columns. RESULTS AND INTERPRETATION Organic carbon. On the basis of our experience and published studies of organic carbon content of petroliferous and nonpetroliferous sedimentary basins, we have considered 1.0% organic carbon to be a minimum requirement for a potential source rock of petroleum. In general, the sediments contain very small amounts of organic matter (Table 2); the average is 0.22% (very poor source rock). Light hydrocarbon gases. Methane was the dominant gaseous hydrocarbon in the air space of the cans, so that Barker, P. F., Carlson, R. L., Johnson, D. A., et al., Intl. Repts. DSDP, 72: Washington (U.S. Govt. Printing Office). the wetness is very low; this indicates that the sediments are still immature (Table 2). Pyrolysis. In all samples the hydrocarbon source potential was very low, less than 450 ppm (Table 2). Organic extracts. The extracts (Table 3) range from 230-892 ppm. They are composed predominantly of heteroatomic compounds (NSO), indicating immature material. Chromatography. The «-alkane distribution from rock extracts revealed a distribution with predominance in the C2\-C21 range (Fig. 2). The high proportion of naphthenic compounds indicates that the organic matter of all samples is immature. Visual kerogen. The types of organic matter, as determined by microscopic examination of the kerogen slides, were reported as percentage of amorphous, herbaceous, and woody material (Table 2). Amorphous organic matter represents lipid-rich unstructured material, which can yield many petroleum hydrocarbons when subjected to appropriate diagenetic conditions. Herbaceous types include structured material, such as cuticles, spores, and pollens; they can yield oil and gas. Woody represents hydrogen-lean organic matter typically derived from land plants and includes coaly material; woody matter can yield only gas. Figure 3 shows the relationship of each of these organic matter types to petroleum generation and destruction zones. CONCLUSIONS Potential petroleum source rocks were not identified in Holes 515B and 516F, DSDP Leg 72, in the studied intervals. Analyses of light hydrocarbon gases, organic extracts, and /i-alkane distribution suggests that the sediments are thermally immature. REFERENCES Cummings, J. J., and Robinson, W. E., 1964. Normal and isoprenoid hydrocarbons isolated from oil shale bitumen. J. Chem. Eng. Data, 9:304-307. Dow, W. G., 1977. Kerogen studies and geological interpretations. / . Geochem. Explor., 7(2):77-79. Staplin, F. L., 1969. Sedimentary organic matter, organic metamorphism, and oil and gas occurrence. Bull. Can. Pet. Geol., 17: 47-66. Date of Initial Receipt: December 1, 1981