Organic Matter and Associated Minerals on the Dwarf Planet Ceres

Ceres is the largest object in the main belt and it is also the most water-rich body in the inner solar system besides the Earth. The discoveries made by the Dawn Mission revealed that the composition of Ceres includes organic material, with a component of carbon globally present and also a high quantity of localized aliphatic organics in specific areas. The inferred mineralogy of Ceres indicates the long-term activity of a large body of liquid water that produced the alteration minerals discovered on its surface, including ammonia-bearing minerals. To explain the presence of ammonium in the phyllosilicates, Ceres must have accreted organic matter, ammonia, water and carbon present in the protoplanetary formation region. It is conceivable that Ceres may have also processed and transformed its own original organic matter that could have been modified by the pervasive hydrothermal alteration. The coexistence of phyllosilicates, magnetite, carbonates, salts, organics and a high carbon content point to rock–water alteration playing an important role in promoting widespread carbon occurrence.

Geochemical Criteria for Possible Identification of Hydrocarbons Generated by Upper Proterozoic Source Rock

The article summarizes the literature data of domestic and foreign researchers on the geological and geochemical conditions of the formation and transformation of the original organic matter of ancient Upper Proterozoic strata, as well as key points in understanding the processes of their accumulation at the R-V boundary.

Influence of geological and geochemical conditions of deposits formation on the composition and properties of hydrocarbon fluids (on the example of Bovanenkovo oil and gas condensate field)

The article provides an analysis of the variability of the composition of hydrocarbon fluids from the Bovanenkovo field (Yamal, Western Siberia) in order to better understand the geological processes that led to the formation of this field and the reformation of the composition of fluids in the layers of this field. It is shown that geochemical information on the composition and properties of oil, condensate and gas allows to “reconstruct” the geological stages of the formation of deposits in their modern form. The data on the properties of fluids, group and molecular composition of the light and middle parts of oils and condensates of various layers are given. Differences in the composition of hydrocarbons from different reservoirs of the field, the relationship of the composition of fluids with oil deposits and superimposed geological reorganizations in the Yamal area already formed were demonstrated. In terms of composition, three groups of oils were identified: residual, whose composition was formed as a result of the migration of relatively low-molecular components, light oils with a high content of paraffins and light (low-paraffinic) oils and condensates. It is shown that differences in the composition of oils are due both to the composition of the original organic matter for them and to migration processes after the formation of accumulations of hydrocarbon fluids.

Ideas and perspectives: Hydrothermally driven redistribution and sequestration of early Archaean biomass – the hydrothermal pump hypothesis

Archaean hydrothermal chert veins commonly contain abundant organic carbon of uncertain origin (abiotic vs. biotic). In this study, we analysed kerogen contained in a hydrothermal chert vein from the ca. 3.5 Ga old Dresser Formation (Pilbara Craton, Western Australia). Catalytic hydropyrolysis (HyPy) of this kerogen yielded n-alkanes up to n-C22, with a sharp decrease in abundance beyond n-C18. A very similar distribution (≤ n-C18) was observed in HyPy products of pre-extracted recent bacterial biomass, while abiotic compounds synthesised via Fischer-Tropsch-type synthesis exhibited a modal distribution. We therefore propose that the original organic matter in the Archaean chert veins has a primarily microbial origin. We hypothesise that the microbially-derived organic matter accumulated in different aquatic and subsurface Dresser environments, and was then assimilated, redistributed and sequestered by hydrothermal fluids (hydrothermal pump hypothesis).

Organic petrology of peak oil maturity Triassic Yanchang Formation lacustrine mudrocks, Ordos Basin, China

An organic petrology evaluation and a determination of solid bitumen reflectance [Formula: see text] were completed for organic-rich Triassic Yanchang Formation mudrocks ([Formula: see text]) from the Ordos Basin, north-central China, as part of a larger investigation of “shale gas” resources. These data were integrated with information from Rock-Eval programmed pyrolysis to show that the samples are in the peak oil window of thermal maturity and that organic matter is dominated by solid bitumen with minor amounts of type III kerogen (vitrinite and inertinite) from vascular land plants. Describing a “kerogen type” for these rocks based strictly on parameters determined from programmed pyrolysis is misleading because the original organic matter has converted to hydrocarbons (present as solid bitumen), a large proportion of which may have been expelled into adjacent reservoir facies. However, based on the comparison with immature-early mature lacustrine mudrock (Garden Gulch Member of Green River Formation) and marine shale (Boquillas Formation), we suggest that the original organic matter in the organic-rich samples examined for our study may have been type I/II kerogen with hydrogen index values of [Formula: see text] TOC.

Density and loss on ignition as indicators of the fossilization of silicified wood

Measuring density of silicified wood and determining weight loss after 450°C heating provides useful data for interpreting the process of permineralization. These simple gravimetric methods do not replace X-ray diffraction, electron microscopy, polarized light microscopy, Raman spectroscopy, and other specialized techniques for studying fossil wood, but they can be performed rapidly, and require minimal laboratory facilities. Woods mineralized with opal have densities of 1.9–2.1 g/cm3, compared to 2.3–2.6 g/cm3 for wood mineralized with chalcedony or quartz. Weight loss after 450°C heating, commonly described as “loss on ignition” can be used to roughly estimate the % of original organic matter that remains in chalcedony or quartz-mineralized wood, using the density of extant taxa for comparison. For opalized wood, 450°C weight loss mostly represents dehydration of the hydrous silica. Data from specimens from 20 localities reveal two characteristics: 1) silicified woods typically consist either of opal or chalcedony/quartz, not an intermediate mixture of the two silica polymorphs; 2) the percentage of organic matter that remains after petrifaction is usually very small.

Programmed pyrolysis (Rock-Eval) data and shale paleoenvironmental analyses: A review

We use public-domain data from programmed pyrolysis, collected using Rock-Eval or similar apparatuses, to illustrate the utility and challenges of using this method for paleoenvironmental analyses. These data are widely available and commonly collected and analyzed for a variety of purposes in the petroleum industry in the fields of paleo-oceanography and paleoclimate research and elsewhere. We show how factors such as sample condition, mineralogy, organic content, and others can influence the results, leading to potential interpretational complications. We also evaluate ways that the data can be plotted to maximize their utility for interpretation purposes. Four types of organic matter are commonly identified based on programmed pyrolysis results. Type I organic matter has a high (e.g., [Formula: see text]) hydrogen index (HI), is oil-prone and is commonly assumed to represent lacustrine depositional settings. Type II organic matter is also oil-prone (i.e., having an HI between 350 and 700) but is commonly assumed to represent marine organic matter. Type III organic matter has a lower HI ([Formula: see text]), will tend to generate gas, and is commonly assumed to represent terrigenous (e.g., land plants) organic matter. Type IV organic matter has very low HI values and is associated with nongenerative (i.e., inert) organic matter. We show that these simple associations between organic matter type and paleoenvironment were not always valid. Preburial degradation of the organic matter and mixing of different organic matter types are two possibilities that can lead to erroneous paleoenvironmental interpretations. Furthermore, the programmed pyrolysis results characterize the present composition of the organic matter which, especially at high thermal maturities (e.g., dry gas window), can be significantly different to that of the original organic matter. Other types of advanced geochemical and sedimentologic analyses should be integrated with the programmed pyrolysis results during their interpretation.

Even and Odd Carbon Predominance Coexisting N-Alkanes in Paleogene Dark Shale in Jinhu Sag of Subei Basin

Gas chromatograms of saturated hydrocarbons of shale rock extracts show three patterns of n-alkanets in the second member of Funning formation (E1f2) in Jinhu Sag of Subei basin, eastern China. Three patterns of n-alkanets are the odd-to-even carbon predominance, the even-to-odd carbon predominance, and the even and odd carbon predominance coexisting, respectively. In this paper, the characteristics of n-alkanet coexisting even and odd carbon predominance and its origin and sedimentary environment are investigated only. The short-chain n-alkanets and long-chain n-alkanets are characterized by pronounced odd carbon predominance, with the dominant carbon of nC15, nC17 and of nC25, nC27, nC29 and nC31. The mid-chain n-alkanets are dominated by even carbon predominance, with the dominant carbon of nC20 and nC22. The composition of biomarkers are characterized by phytane preference, high gamma cerate abundance, the presence of β-carotene and C24-tetracyclic terrapin, which indicate that the organic matter in source rocks deposited in an anoxic, hyper saline lacus trine environment. The biomarkers are no homogeneous, which shows that salinity and redo is not constant in pale-lacus trine. The n-alkanets are mainly derived from algae in the ancient lake and macro pyres on land or around the edges of the ancient lake. The n-alkanets distribution patterns of the dark shales studied are believed not to be dependent on the redo in the depositional environments or the salinity of the water body, but to be associated with original organic matter composition.

Monosaccharide status of pre-Mazama Ahb horizons in Alberta, Canada

Palaeosols, formed prior to deposition of Mazama tephra (6600 yr B.P.), are widespread in Alberta. Palaeo-Ahb horizons are readily identifiable, with colors ranging from very dark gray to gray and brown. Establishment of fundamental and stable properties of the original organic matter would facilitate the classification of these buried soils. Use of a suite of eight monosaccharides divided the 15 palaeosols examined in this study into one group of three, possibly formed under Brown Chernozemic soil-forming conditions or drier, and the remainder, possibly formed under Black Chernozemic soil-forming conditions. The former had C/N ratios approaching those of microorganisms and low xylose to mannose ratios. It is concluded that, because of the long time-window of surface stability between deglaciation and the Mazama tephra fall, pre-Mazama palaeosols can only be studied on a site-specific basis. Grouping these palaeosols, based on their pre-Mazama tephra deposition only, is not realistic. Synthesis of a unified hypothesis of palaeoenvironmental conditions is, therefore, not possible using only their monosaccharide status. Key words: Holocene soils, palaeosols, buried soils, monosaccharides, landscape

Mineralogy and geochemistry of Mesozoic black shales and interbedded carbonates, southeastern Sicily: evaluation of diagenetic processes

Upper Triassic/Lower Jurassic organic-rich shales and interbedded carbonates (Rhaetian → Sinemurian) are widespread in the subsurface of southeastern Sicily where important oil fields have been found hosted in Triassic reservoirs. Core samples from wells drilled offshore and onshore were studied from petrographie and geochemical viewpoints.In the Hettangian/Sinemurian shale-carbonate sequences, which accumulated in a rapidly subsiding basin, the micritic aragonitic mud is still largely preserved. Mixed-layer I/S has remained randomly interstratified to a depth > 4000 m. Diagenetic carbonates are non-stoichiometric finely crystalline, pore-filling dolomite and/or calcite. The carbonate component exhibits a high Sr content and fair amounts of Fe and Mn. Carbon and oxygen isotopic values suggest a subsurface interstitial formation for the digenetic carbonates in an essentially closed system. Based on all accumulated data it is suggested that anoxic marine waters were retained in the sediment pores for a long time after deposition, thus enhancing the preservation of significant amounts of the original organic matter.In contrast, Rhaetian tidal-flat deposits hosting black shales display a clay component characterized by ordered illite-rich I/S and a carbonate mineralogy dominated by low-Mg calcite in the uppermost beds and by near-stoichiometric dolomite in the lowermost ones. Petrographie, chemical and isotopic data indicate early cementation in an oxidizing phreatic environment and lower down in the sequence pervasive dolomitization in a sabkha-type environment.