Analysis of wax hydrocarbons in petroleum source rocks from the Damintun depression, eastern China, using high temperature gas chromatography

A detailed organic geochemical study; utilising petrography, biomarker hydrocarbon analysis and high temperature GC analysis of extractable wax hydrocarbon constituents was performed on four marginally oil window-mature source rocks from the Shahejie Formation (Eocene), Damintun depression in eastern China. The main maceral components in the source rocks were vitrinite, liptinite and exinite, with vitrinite being more abundant (>50 vol.%) in organic-lean samples whose TOC contents were between 1 and 2 wt.%. Large differences in pristane/phytane ratios suggested that the organic-rich samples were deposited in a less oxic depositional environment than that for the organic-lean rocks. The distribution of extractable wax hydrocarbons, determined by high temperature GC, showed a marked difference between these two sample types. The organic-rich samples contained high molecular weight hydrocarbons (HMWHCs) dominated by macrocrystalline n-alkanes (n-C₂₃–n-C₃₇, typically maximising at n-C₂₉), while the organic-lean samples contained lower amounts of extractable wax hydrocarbons but were relatively rich in microcrystalline components (> n-C₃₅). In all source rocks (Es3 and Es4), a noticeable odd-over-even predominance (OEP) of n-alkane chain lengths (up to n-C₆₅) was evident, consistent with a direct biological origin for the long n-alkyl chains. They were most probably formed during diagenesis from decarboxylation of predominantly even-carbon-numbered aliphatic acids originating from higher plant or lacustrine algal sources and/or were directly biosynthesised in hydrocarbon form. At least two other homologous series of branched/cyclic HMWHCs were observed, one of which was confirmed as a series of branched alkanes (probably methyl-branched). The carbon number distribution patterns of HMWHCs may be primarily controlled by thermal maturity and biogenic source input as well as being influenced by diagenetic reactions governed by depositional environmental conditions, as shown previously [Carlson, R.M.K., Teerman, S.C., Moldowan, J.M., Jacobson, S.R., Chan, E.I., Dorrough, K.S., Seetoo, W.C., Mertani, B., 1993. High temperature gas chromatography of high wax oils. In: Indonesian Petroleum Association, 22nd Annual Convention Proceedings. Jakarta, Indonesian, pp. 483–507. Carlson, R.M.K., Jacobsen, S.R., Moldowan, J.M., Chan E.I., 1994. Potential application of high temperature gas chromatography to Middle Eastern petroleum exploration and production. In: Al-Husseini, M.I. (Ed.), Geo'94, Vol 1., Selected Middle East Papers from The Middle East Petroleum Geoscience Conference, 1994; Gulf PetroLink. Manama, Bahrain, pp. 258–267]. Our study indicates for the first time that Es3 source rocks as well as Es4 facies contain HMWHCs. The distributions of extractable wax hydrocarbons suggest that both Es4 and Es3 members may potentially serve as important parent source rocks for generating waxy petroleum in this region.     

Orientation of framboidal pyrite in shale

Ordered framboids from Devonian Chattanooga Shale (U.S.A.) and Rammelsberg Banderz (Germany) show a common orientation in any one sample. This observation was made using a stereographic method of reconstruction in which the lineations within the framboids were measured on each of three planes cut mutually at right angles from the rock samples. The explanation for this preferred orientation is not yet known, but it may originate from crystal-growth during compaction of the shales.

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Zusammenfassung Pyrit-Himbeeren aus dem devonischen Chattanooga Shale (U.S.A.) und vom Rammelsberg Banderz zeigen bevorzugte Orientierungen ihrer inneren Struktur (Figure I) gegenüber der Sedimentschichtung. Diese Beobachtungen sind in stereographischen Projektionen aufgezeichnet. Der Vorgang der Orientierung mag auf die Kristallisation der Pyrit-Netzwerke während der Diagenese zurückzuführen sein.

     

Use of radiometric (Cs-137, Pb-210), geomorphic,and stratigraphic techniques to date recent oxbow sediments in the rio puerco drainage Grants uranium region,New Mexico

In the absence of historic geochemical baseline data for the Grants uranium region, environmental changes resulting from uranium mine-mill activities can be determined only by indirect methods. A methodology for determining the age of recent sediments in streams draining the region has been established based on combined geomorphic, stratigraphic, and radiometric dating techniques. Because clayrich sediments retain possible radionuclides and heavy metals derived from mineralization and mined sources, sample sites which contain fine-grained deposits that both predate and postdate mine-mill activity were located in abandoned-channel segments (oxbows) of major streams draining the eastern Grants uranium region. Aerial photographs (and derivative maps) taken between 1935 and 1971 provided the historical and geomorphic documentation of approximate dates of oxbow formation and ages of alluvial fills in the abandoned-channel segments. Pits were dug at these oxbow sites to determine stratigraphy and composition of the deposits. Samples collected from pit walls and auger holes below the pits were subjected to radiometric analysis by gamma ray spectrometry for the artificial radionuclide Cs-137 and the natural radionuclide Pb-210 as well as other U-238 and Th-232 daughters. Because of the dynamic nature of the system, absolute dating with Cs-137 was not possible but samples could be dated as either pre-or post-1950. The 1950 date is important because it marked the beginning of the uranium exploitation in the region. The Pb-210 dating was not possible because background Pb-210 was very high relative to fallout Pb-210.     

Origin of petroleum in the Neoproterozoic–Cambrian South Oman Salt Basin

The South Oman Salt Basin (SOSB) is host to the world’s oldest known commercial deposits. Most of the South Oman oils have been proven to be associated with the source rocks of the Neoproterozoic to Cambrian Huqf Supergroup, but the assignment of oils to specific Huqf intervals or facies has been hampered by the geochemical similarity of the organic matter across the entire Huqf sequence, possibly as a consequence of limited change in the local palaeoenvironment and biota over the time of its deposition. This study was conducted to establish improved correlations between organic-rich rock units and reservoir fluids in the SOSB through detailed molecular and isotopic analysis of the Huqf Supergroup, with special emphasis directed towards understanding the Ara carbonate stringer play.Unusual biomarkers, tentatively identified as A-norsteranes, show distinctive patterns among carbonate stringer oils and rocks different from those observed in Nafun sediments and Ara rocks from the Athel basin. These putative A-norsteranes form the basis for new oil-source correlations in the SOSB and provide for the first time geochemical evidence of a self-charging mechanism for the carbonate stringer play. The paucity of markers specific to the Nafun Group (Shuram, Buah and Masirah Bay formations) confounds attempts to quantify their respective contributions to Huqf oil accumulations. Nafun inputs can only be determined on the basis of subtle differences between Nafun and Ara biomarker ratios. The most useful geochemical characteristics delineating Nafun Group organic matter from Ara Group intra-salt source rocks included: low relative abundance of mid-chain monomethyl alkanes (X-compounds); low relative abundance of gammacerane, 28,30-dinorhopane, 25,28,30-trinorhopane and 2-methylhopanes; low C₂₂T/C₂₁T and high C₂₃T/C₂₄T cheilanthanes ratio values. Based on these parameters, molecular evidence for major contributions of liquid hydrocarbons from Nafun Group sediments (Shuram, Buah and Masirah Bay formations) is lacking. Our results suggest that the majority of SOSB hydrocarbon accumulations originate from within the Ara group, either from the carbonate stringers or from the package of sediments that comprises the Thuleilat, Athel Silicilyte and U shale formations. Subtle aspects of the composition of some carbonate stringer and post-salt Huqf oils could suggest some degree of sourcing from the Nafun rocks but stronger evidence is needed to confirm this.     

Orographic controls on rain water isotope distribution in the Mount Lofty Ranges of South Australia

Hydrogen and oxygen isotopes of water are common environmental tracers used to investigate hydrological processes, such as evaporation, vegetation water use, surface water–groundwater interaction, and groundwater recharge. The water isotope signature in surface water and groundwater evolves from the initial rain signature. In mountain terrain, rain water stable isotope composition spatially varies due to complex orographic precipitation processes. Many studies have examined the isotope–elevation relationships, while few have quantitatively investigate the terrain aspect and slope effect on rain isotope distribution. In this paper, we examine the orographic effects more completely, including elevation, terrain slope and aspect, on stable isotope distribution in the Mount Lofty Ranges (MLR) of South Australia, using a multivariate regression model. The regression of precipitation isotope composition suggests that orographic effects are the dominant controls on isotope spatial variability. About 75% of spatial variability in δ¹⁸O and deuterium excess is represented by the regression using solely orography-related variables (elevation, terrain aspect and slope), with about 25% of δ¹⁸O spatial variability attributed to the terrain aspect and slope effect. The lapse rate is about −0.25‰ for every 100 m at both windward and leeward slopes. However, at the same elevation, δ¹⁸O at the leeward slope (eastern MLR) is 0.5‰ larger than that at the windward slope. The difference can be explained by different mechanisms – continuous rain-out processes on the windward side and sub-cloud evaporation on the leeward side. Both δ¹⁸O and deuterium excess maps (1 km resolution) are constructed based on the regression results for the MLR. Both maps are consistent with groundwater of local precipitation origin, and useful to examine groundwater recharge.     

The age and petrology of the Chimbadzi Hill Intrusion, NW Zimbabwe: first evidence for early Paleoproterozoic magmatism in Zimbabwe

The mafic-ultramafic Chimbadzi Hill intrusion in the NW of the Zimbabwe craton is a dyke with inward-dipping margins comprising magnetite peridotite, troctolite and magnetite melatroctolite. The magnetite peridotite is composed of about equal amounts of V- and Ti-bearing magnetite and olivine (Fo₆₀). The troctolite is composed of about 50% olivine (Fo_50-54), 40% plagioclase (An_53-58), 7% clinopyroxene and minor apatite and magnetite with ilmenite lamellae. Geochemical trends suggest that the Chimbadzi Hill Intrusion formed by fractional crystallisation from a single initial magma. However, the more primitive magnetite peridotite overlies the more evolved troctolite in the intrusion. This ‘apparent’ inverted stratigraphy may be due to emptying of a fractionated magma chamber from the top, or to floor subsidence during intrusion.U–Pb dating on baddeleyite reveals that the age of the Chimbadzi Hill Intrusion is 2262 ± 2 Ma. This age does not correspond to any known tectono-thermal event in the Zimbabwe Craton or adjacent metamorphic belts. It is 300 Ma younger than the late Archean Great Dyke, and 230 Ma older than other Paleoproterozoic events in and around the craton. Therefore, it may represent a so far undocumented very early Proterozoic igneous event in the Zimbabwe Craton. The intrusion represents a vanadium resource for Zimbabwe, with titanium potentially being mined as by-product.