Devonian samples

Bituminous Substances. The distribution of hydrocarbons and other bitumens in some of the Paleozoic rocks of the area is shown in Tables I and II, the latter summarized after Swain (7). The presumed saturated hydrocarbons of the Ordovician, Silurian and Lower Devonian samples are measurably less than in the Middle and Upper Devonian samples. The presumed aromatic hydrocarbons are not as well differentiated. The pyridine-plus-methanol-eluted chromatographic fractions, arbitrarily taken as asphaltic material, also appear, like the aromatic fractions, to be controlled more by local variations than by geologic age. 

Absorption spectra of the phenol-sulfuric acid solutions tested for total sugars show that 5-hydroxymethylfurfural from hexoses is more common in the uppermost Silurian and Devonian samples than in the earlier deposits. Furfurals from pentose sugars evidently form the bulk of the residual carbohydrates in these samples, however. There is no definite evidence as to the marine or terrestrial origin of the hexose products in the samples. 

The second important point is that not all samples contain vitrinite. Pre-Devonian samples do not contain vitrinite since higher plants did not evolve until around this time and vitrinite is derived from higher plant remains. However, there are a number of alternative maturity scales that can be used. For example, the thermal alteration index (Staplin, 1969) and spore coloration index (Burgess, 1974) have been used... 

Silurian, but not Devonian oil samples). The Silurian oil samples have smaller differences in S between pristane and the Cn n-alkane and between phytane and the Cig n-alkane than the Devonian samples. Different assemblages of organisms likely contributed to the acyclic isoprenoids versus n-alkanes in the source rocks for these two oil families. 

Higher dibenzothiophene/2-methylnaphthalene in the Hassi Messaoud and Zemlet oil samples (DBT/2MeN = 0.07-0.12, Table 3) compared to the Devonian samples (0.02-0.06) suggests that the Silurian source rock was deposited under conditions of lower redox potential (Eh) and contained less iron in clay minerals that could react with sulfides to form pyrite. Excess sulfide in anoxic sediments with little available iron can result in sulfur incorporation into the kerogen, which is then incorporated into the generated oil... 

Lower Eh at the time of deposition for the Zemlet source rock is supported by a higher C35/C32 hopane ratio for the Zemlet oil sample (0.20) compared to two of the Devonian samples (Assekaifaf and Zarzaitine = 0.12-0.13, Table 2). However, the C35/C32 hopane ratio is also higher for Oued Zenani and Dome oil samples (0.22). 

Stable carbon isotope ratios for the saturated and aromatic hydrocarbon fractions of the Algerian oil samples support a genetic relationship between the Silurian Hassi Messaoud and Zemlet oil samples, which differ from the Devonian Assekaifaf, Dome, Zarzaitine, and Oued Zenani samples (Table 4). However, these isotopic differences are small (< l%o) and cannot be generally applied because of larger differences that might be imposed by secondary processes, such as thermal maturity. The calculated canonical variables (Sofer, 1984) for the six oil samples indicate marine source-rock organic matter (CV < 0.47), but lack systematic differences between the Silurian and Devonian samples. 

Fig. 6. Diamondoid isomer concentrations are systematically lower in the Silurian-sourced Hassi Messaoud and Zemlet oil samples compared to the Devonian samples, despite higher thermal maturity. The jc-axis consists of compounds 1-7 in Table 5.

Formation of diamondoids requires acid catalysis by clay in the source rock (Dahl et al, 1999). Lower diamondoid concentrations in the Hassi Messaoud and Zemlet oil samples compared to the four Devonian samples, which have similar or lower thermal maturity, suggest lower clay content in the Silurian compared to the Devonian source rock. This is consistent with higher dibenzothiophene/2-methylnaphthalene ratios for the Hassi Messaoud and Zemlet oil samples, which also indicates lower Eh and possibly less clay in the source rock, as discussed above. 

The isotopic composition and the pattern of isotopic compositions among certain n-alkanes and isoprenoids (Table 6) and gasoline-range branched and cyclic hydrocarbons (Table 7) measured by CSIA differ from the Silurian-sourced Hassi Messaoud and Zemlet oil samples compared to the four Devonian samples. Except at mC and wCg, the Zemlet oil sample has more negative stable carbon isotope ratios for the Ce-Cig w-alkanes than the other Algerian oil samples (Fig. 7). The isotopic compositions of w-alkanes in Hassi Messaoud oil sample more closely resemble the other samples than the Zemlet sample, except at nCe and possibly nCf. Devonian source rock input may affect the heavier components in the Hassi Messaoud oil sample or their isotopic compositions may simply not be diagnostic of Silurian versus Devonian oil. The nCe in Hassi Messaoud and Zemlet oil samples is substantially depleted in (l.l-2.3%o) compared to the other samples. 

Fig. 7. Zemlet oil has generally more negative stable carbon isotope ratios for C6-C19 u-alkanes than the other Algerian oil samples (Table 6). The largest isotopic difference between Hassi Messaoud and Zemlet (Silurian) oil samples and the Devonian samples occurs at nC. ...

Fig. 9. Hassi Messaoud and Zemlet oil samples have smaller isotopic differences between pristane and nCi7 and between phytane and nCis than the Devonian samples. The Dome oil sample (Devonian) contained insufficient phytane for reliable measurement, but has pristane and nCu that differ by 2A2%o (Table 6).

In Kane JS (ed.) U.S. Geological Survey Bulletin 2046. The USGS Reference Sample Devonian Ohio Shale SDO-i pp D1-D5. 

The Guadiana Basin occupies a total area of 66,850 km2 in Spain and Portugal, 8,350 km2 of which comprises the Iberian Pyrite Belt (IPB) one of the world s largest concentrations of base metal sulphide deposits (Leistel et al., 1997 Ribeiro et al., 1990). Three plant species (Cistus ladanifer, Lavandula luisieri and Thymus vulgaris) were utilized in this study. The soils in which these plants were sampled are developed over varied geological formations. The oldest formation (PQ Group, Upper Devonian) consists of a thick sequence of arenites and shales... 

As seen in Figure 6, mean 8 Mo in these ancient sediments is significantly offset from that of modem seawater and from mean 8 Mo of recent euxinic sediments. The offset is in the direction of less fractionation of Mo isotopes in the oceans (closer to the likely 5 MOjj ), as would be expected if removal of Mo to euxinic sediments expanded at the expense of removal to oxic (and suboxic) sediments. The result is consistent with the hypothesis of expanded ocean anoxia, and consequent increase in the areal extent of euxinic depositional settings, during this time. A similar, albeit smaller, offset is seen in repeated measurements of a Devonian black shale sample (USGS SDO-1 Fig. 6). This shift could he interpreted as consistent with somewhat expanded euxinic deposition during this time—an interpretation consistent with other evidence of expanded Paleozoic ocean anoxia in inland hasins. While further work is needed, these initial results are promising. 

In general these substances are widely distributed in the acid distillates but appear to increase in number at two stratigraphic levels (1) in the middle Silurian McKenzie Limestone and (2) in the middle Devonian shales. Furthermore, possible two- and three-ring aromatic or heteroaromatic structures show similar increases. There is also a rough correlation with the Kjeldahl nitrogen content of the samples (Table VI). 

The Devonian shale sample generally had a darker appearance with fewer and less distinct bands than the Green River shale sample, and the organic matter appeared to be more evenly distributed over the 2.5-cm sample. Several areas of this sample were examined using the slow rastering microprobe techniques to determine the elemental distribution in this material. The observations made in two of the areas are presented below. 

This study has shown that the sample of Devonian shale is composed primarily of silicates with much lower amounts of carbonate minerals. Here, as with the Green River shale sample, the silicate minerals were associated primarily with the organic-rich areas of the shale, and when present, the carbonate minerals were found mainly in the organic-poor areas. The siliceous minerals of this Devonian shale sample were found to be quartz, illite, and muscovite, with trace amounts of kaolinite. Calcite, dolomite, Fe-rich dolomite or magnesite, and siderite were observed in the carbonate regions of the Devonian shale. However, the... 

Devonian shale had regions with much higher concentrations of siderite than were found in the Green River shale sample. As in the Green River shale, the FeS2 particles were less than 50ju.m in diameter and in most instances were observed only in the organic-rich areas of the samples. 

Bitumens, were separated by chromatography, urea clathration and 5A molecular sieve occlusion before and after analyses of many of the aliphatic sub-fractions by GC and gas chromatography-mass spectrometry (GC-MS). Experimental details are noted in a previous publication (16) in which the distribution of cyclic alkanes in two lacustrine deposits of Devonian (N.E. Scotland) and Permian (Autun, France) age, (the D and C series samples) were discussed, Chromatographic separation into aliphatic, aromatic and polar compounds of the bitumens extracted from the shales gave the results shown in Table VI. Carbon Preference Indices and pristane/phytane ratios were measured in this work space limitations precluded... 

Nearly all of the samples were immature as evidenced by rank determinations using vitrinite reflectance and spore colouration they may be considered to have a rank equivalent to that of late diagenesis-early catagenesis (5). Because of lack of spore fluorescence, the Devonian and Cambrian samples have been classified as having a vitrinite reflectance > 0.7%. The microscopical data is shown in Table II where the samples have been subdivided into five groups based on these rank parameters. 

---