Thermogravimetry-differential thermal analysis coupled with chromatography as a thermal simulation experimental method and its application to gaseous hydrocarbons from different source rocks

In this paper a thermogravimetry-differential thermal analysis method coupled with chromatography （TG-DTA-GC） has been adopted to simulate the generation of gaseous hydrocarbons from different hydrocarbon source rocks such as coals, mudstones, and carbonate rocks with different maturities. The temperature programming for thermal simulation experiment is 20℃/min from ambient temperature to 700℃. As viewed from the quantities and composition of generated gaseous hydrocarbons at different temperatures, it is shown that low-mature coal has experienced the strongest exothermic reaction and the highest loss of weight in which the first exothermic peak is relatively low. Low-mature coal samples have stronger capability of generating gaseous hydrocarbons than high-mature samples. The amounts and composition of gaseous hydrocarbons generated are closely related not only to the abundance of organic carbon in source rocks, but also to the type of kerogen in the source rocks, and their thermal maturity. In the present highly mature and over-mature rock samples organic carbon, probably, has already been exhausted, so the production of gaseous hydrocarbons in large amounts is impossible. The contents of heavy components in gaseous hydrocarbons from the source rocks containing type- Ⅰ and - Ⅱ kerogens are generally high ; those of light components such as methane and ethane in gaseous hydrocarbons from the source rocks with Ⅲ-type kerogens are high as well. In the course of thermal simulation of carbonate rock samples, large amounts of gaseous hydrocarbons were produced in a high temperature range.     

Evaluation of the petroleum composition and quality with increasing thermal maturity as simulated by hydrous pyrolysis: A case study using a Brazilian source rock with Type I kerogen

Hydrous pyrolysis (HP) experiments were used to investigate the petroleum composition and quality of petroleum generated from a Brazilian lacustrine source rock containing Type I kerogen with increasing thermal maturity. The tested sample was of Aptian age from the Araripe Basin (NE-Brazil). The temperatures (280–360 °C) and times (12–132 h) employed in the experiments simulated petroleum generation and expulsion (i.e., oil window) prior to secondary gas generation from the cracking of oil. Results show that similar to other oil prone source rocks, kerogen initially decomposes in part to a polar rich bitumen, which decomposes in part to hydrocarbon rich oil. These two overall reactions overlap with one another and have been recognized in oil shale retorting and natural petroleum generation. During bitumen decomposition to oil, some of the bitumen is converted to pyrobitumen, which results in an increase in the apparent kerogen (i.e., insoluble carbon) content with increasing maturation.The petroleum composition and its quality (i.e., API gravity, gas/oil ratio, C₁₅₊ fractions, alkane distribution, and sulfur content) are affected by thermal maturation within the oil window. API gravity, C₁₅₊ fractions and gas/oil ratios generated by HP are similar to those of natural petroleum considered to be sourced from similar Brazilian lacustrine source rocks with Type I kerogen of Lower Cretaceous age. API gravity of the HP expelled oils shows a complex relationship with increasing thermal maturation that is most influenced by the expulsion of asphaltenes. C₁₅₊ fractions (i.e., saturates, aromatics, resins and asphaltenes) show that expelled oils and bitumen are compositionally separate organic phases with no overlap in composition. Gas/oil ratios (GOR) initially decrease from 508–131 m³/m³ during bitumen generation and remain essentially constant (81–84 m³/m³) to the end of oil generation. This constancy in GOR is different from the continuous increase through the oil window observed in anhydrous pyrolysis experiments. Alkane distributions of the HP expelled oils are similar to those of natural crude oils considered to be sourced from similar Brazilian lacustrine source rocks with Type I kerogen of Lower Cretaceous age. Isoprenoid and n-alkane ratios (i.e., pristane/n-C₁₇ and phytane/n-C₁₈) decrease with increasing thermal maturity as observed in natural crude oils. Pristane/phytane ratios remain constant with increasing thermal maturity through the oil window, with ratios being slightly higher in the expelled oils relative to those in the bitumen. Generated hydrocarbon gases are similar to natural gases associated with crude oils considered to be sourced from similar Brazilian lacustrine source rocks with Type I kerogen of Lower Cretaceous, with the exception of elevated ethane contents. The general overall agreement in composition of natural and hydrous pyrolysis petroleum of lacustrine source rocks observed in this study supports the utility of HP to better characterize petroleum systems and the effects of maturation and expulsion on petroleum composition and quality.     

Organic geochemistry and petroleum potential of Jurassic and Cretaceous deposits of the Yenisei–Khatanga regional trough

We present results of geochemical studies of organic matter of the Jurassic–Cretaceous deposits in the west of the Yenisei–Khatanga regional trough. The studies were carried out on a representative set of well cores by a complex of modern organic-geochemistry methods (determination of organic-carbon content in rocks, pyrolysis, estimation of the carbon isotope composition in the kerogen of rocks, extraction, liquid and gas–liquid chromatography, and chromato-mass spectrometry). Based on the distribution of biomarkers in the studied bitumens and pyrolysis of rocks, two groups of the samples were recognized: with terrigenous (type III) and marine (type II) organic matter. The terrigenous bitumens are characterized by a low hydrogen index (HI) and a predominance of hydrocarbons C₂₉ among steranes and C₁₉ and C₂₀ among tricyclanes. The marine bitumens, revealed in stratigraphic analogs of the Bazhenovo Formation and in the Malyshevka, Nizhnyaya Kheta, and Shuratovka Formations, show an even distribution of sterane homologues and a predominance of medium-molecular tricyclanes. The Pr/Ph and C₃₅/C₃₄ ratios and the presence of diahopanes testify to the burial of organic matter in suboxidizing sea coast environments. In the Yanov Stan (J₃–K₁), Gol’chikha (J₂–K₁), and, to a lesser extent, Malyshevka (J₂), Nizhnyaya Kheta, and Shuratovka (K₁) Formations, we have recognized widespread stratigraphic levels with marine organic matter of rocks. Its contents and degree of maturity permit these rocks to be considered oil-generating.     

Characteristics of light hydrocarbons (C1–C7) in mesozoic and paleozoic rocks in the Jurong Basin of Jiangsu Province,China

Characteristics have been studied of light hydrocarbons (C₁–C₇) from crude oils and source rocks ranging from Devonian to Triassic in age in the Jurong Basin where carbonate rocks are dominating. The results show that light hydrocarbon compositions (C₁–C₇) can be used to classify organic matter types and maturities as well as to make oil-source rock correlations. It is also an effective method in organic geochemical studies of oils, gases and source rocks in terrains of old carbonate rocks.     

The contribution of bacteria to organic matter in coal-measure source rocks

On the basis of GC–MS analysis, a suite of nine coal-measure source rocks (Ro 0.51%–0.63%) from the southern margin of Junggar basin was found to contain many biomarkers for bacterially-generated hydrocarbons: hopane, sesquiterpene, C₂₃+ monomethyl alkanes (even carbon predominance), and C₂₄+ alkyl cyclohexane. Rock–eval and microscope analysis indicate that vitrinite (especially desmocollinite and homocollinite) plays a significant role in the generation of hydrocarbons in coal-measure source rocks. Vitrinite performs this role by absorbing ultramicroscopic organic matter, generally in the form of resins or bacterial plastids. C₂₃+ monomethyl alkanes (even carbon predominance) and C₂₄+ alkyl cyclohexane series compounds are derived from bacterial metabolites of higher plants. The ultramicro organic matter adsorbed by vitrinite source rocks in the study area is probably ultramicro bacterial plastids. Because the organic matter of higher plants with low hydrogen content has been transformed into organic matter rich in hydrogen by bacteria, the hydrocarbon generation capacity of source rocks is greatly improved. In other words, in coal-measure source rocks, bacteria play an important role in hydrocarbon generation.

     

The influence of a salt dome on the diagenesis of organic matter in the Jeanne d'Arc Subbasin of the northeast Grand Banks of Newfoundland

This study elucidates the geothermal influence of a salt dome on the diagenesis of organic matter present in sedimentary sequences directly overlying the salt strata. Despite similarities in geological setting, geochemical characteristics in sedimentary column and proximity of location (～3 km), the exploratory wells Adolphus 2-k-41 and D-50 drilled on the crest and flank of a salt dome respectively, show considerable differences in the quantitative and qualitative distribution of hydrocarbons. The well drilled on the crest of the salt dome tested oil of 31°APl gravity. The concentrations of gaseous (C₁-C₄) and heavy (C₁₅⁺) hydrocarbons of this well were several times higher than those found in the corresponding sequences of a flank well. The coloration studies of isolated kerogen and gas chromatographic investigations of hydrocarbons suggest that the organic matter present in Adolphus 2-k-41 has undergone a greater degree of thermal alteration than that of D-50. The heat radiating from the salt strata is believed to be responsible for the early diagenesis of the organic matter.     

东营凹陷牛38井沙河街组烃源岩生排烃评价

东营凹陷牛38井沙河街组三段烃源岩从沙三下亚段向沙三中-上亚段为一从高水位深湖相向低水位前三角洲相发展的沉积旋回,该沉积环境控制从下向上发育不同类型的有机质。相应地,该段烃源岩品质从下向上变差,其中沙三中亚段上部及沙三中亚段中部烃源岩产烃能力小,沙三下亚段烃源岩整体品质好,局部存在优质烃源岩。其优劣性体现了在该段烃源岩中,Ⅰ型干酪根具有高的生产力,而Ⅲ型、Ⅱ₂型干酪根生烃潜力很低,基本为无效有机质。计算结果显示不同数据点的烃源岩单位质量原始生烃潜量可相差3个数量级,体现了烃源岩存在强烈的非均质性。有机碳的非均质性分布为其提供了物质基础,而湖盆的旋回式沉积及湖平面的波动性变化是其内在原因。同时,本文也提供一种计算生排烃量的新方法,结果表明该方法较为有效可行。     

Thermal alteration experiments on organic matter from recent marine sediments in relation to petroleum genesis

Three fractions of organic matter: lipid (benzene:methanol-extractable), humic acid (alkali-extractable) and kerogen (residue) were extracted from a young marine sediment (Tanner Basin, offshore southern California) and heated for different times (5–116 hr) and temperatures (150°–410°C). The volatile (gases) and liquid products, as well as residual material, were then analyzed. On a weight basis, the lipid fraction produced 58% of the total identified n-alkanes, the kerogen fraction 41%, and the humic acid <1%. Whereas n-alkanes produced from lipid show a CPI > 1.0, those produced by thermal alteration of kerogen display a CPI < 1.0. The volatiles produced by heating the lipid and humic acid fractions were largely CO₂ and water, whereas those produced from heated kerogen also included methane, hydrogen gas and small amounts of C₂–C₄ hydrocarbons. A mechanism for hydrocarbon production due to the thermal alteration of organic constituents of marine sediment is discussed.     

Evaluating the timing of oil expulsion: about the inverse behaviour of light hydrocarbons and oil asphaltene kinetics

This paper deals with natural temperature records in the heavy (asphaltenes) and the light fractions (C₇—light hydrocarbons) of petroleum. Two sets of marine oils formed from different source rocks and petroleum systems were studied using asphaltene kinetics and light hydrocarbon analysis. Both fractions have been reported to contain information about the temperature the respective oils have been exposed to in the subsurface. These indicated temperatures generally correspond to the conditions in the source rock when expulsion occurred. Bulk kinetic analysis of reservoir oil asphaltenes as well as light hydrocarbon (LH) analysis (of dimethylpentanes) were used here in order to evaluate the expulsion temperatures. Surprisingly, when considering information coming from both fractions, an inverse trend between LHs expulsion temperatures (Ctemp) and asphaltenes (Tasph.) can be observed—high T_asph (asphaltene temperatures) occur with low LH C_temp (light hydrocarbon expulsion temperatures) and low T_asph can be observed when Ctemp is high. These differences are of fundamental importance for the use of such geochemical data in calibrating numerical basin models. The reason for this inverse behaviour is possibly due to the different expulsion behaviour of light hydrocarbons and the heavy fraction of oils, especially when the source rocks contain only moderate amounts of organic matter. In addition it has to be considered that the temperature predictions obtained using asphaltene kinetic analysis are related to the onset temperature of petroleum expulsion, while light hydrocarbons provide, at best, average expulsion temperatures.     

Release of sulfur- and oxygen-bound components from a sulfur-rich kerogen during simulated maturation by hydrous pyrolysis

An immature sulfur-rich marl from the Gessosso-solfifera Formation of the Vena del Gesso Basin (Messinian, Italy) has been subjected to hydrous pyrolysis (160 to 330°C) to simulate maturation under natural conditions. The kerogen of the unheated and heated samples was isolated and the hydrocarbons released by selective chemical degradation (Li/EtNH₂ and HI/LiAlH₄) were analysed to allow a study of the fate of sulfur- and oxygen-bound species with increasing temperature. The residues from the chemical treatments were also subjected to pyrolysis–GC to follow structural changes in the kerogens. In general, with increasing hydrous pyrolysis temperature, the amounts of sulfide- and ether-bound components in the kerogen decreased significantly. At the temperature at which the generation of expelled oil began (260°C), almost all of the bound components initially present in the unheated sample were released from the kerogen. Comparison with an earlier study of the extractable organic matter using a similar approach and the same samples provides molecular evidence that, with increasing maturation, solvent-soluble macromolecular material was initially released from the kerogen, notably as a result of thermal cleavage of weak carbon–heteroatom bonds (sulfide, ester, ether) even at temperatures as low as 220°C. This solvent-soluble macromolecular material then underwent thermal cleavage to generate hydrocarbons at higher temperatures. This early generation of bitumen may explain the presence of unusually high amounts of extractable organic matter of macromolecular nature in very immature S-rich sediments.     

ESR assessment of kerogen maturation and its relation with petroleum genesis

Petroleum hydrocarbons are formed by breakdown of kerogen preserved in source rocks throughout the process of catagenesis. This process is accompanied by free radical generation in kerogen. The availability of measuring free radicals in kerogen using ESR to deduce a maturation estimation of potential source rocks has been hindered due to the presence of the solvent-extractable organic molecules (SEOM) trapped within the kerogen matrix. Spin concentration (Ns) of the kerogen treated with pyridine (K_Py) represents the paramagnetic centers of the kerogen matrix itself and provides a potential parameter to evaluate kerogen maturation.     

http://www.sciencedirect.com/science/article/pii/S1674987113001114

The Deccan Syneclise is considered to have significant hydrocarbon potential.However,significant hydrocarbon discoveries,particularly for Mesozoic sequences,have not been established through conventional exploration due to the thick basalt cover over Mesozoic sedimentary rocks.In this study,near-surface geochemical data are used to understand the petroleum system and also investigate type of source for hydrocarbons generation of the study area.Soil samples were collected from favorable areas identified by integrated geophysical studies.The compositional and isotopic signatures of adsorbed gaseous hydrocarbons(methane through butane) were used as surface indicators of petroleum micro-seepages.An analysis of 75 near-surface soil-gas samples was carried out for light hydrocarbons(C1-C4) and their carbon isotopes from the western part of Tapti graben,Deccan Syneclise,India.The geochemical results reveal sites or clusters of sites containing anomalously high concentrations of light hydrocarbon gases.High concentrations of adsorbed thermogenic methane(C_1 = 518 ppb) and ethane plus higher hydrocarbons(ΣC_(2+) = 977 ppb) were observed.Statistical analysis shows that samples from 13% of the samples contain anomalously high concentrations of light hydrocarbons in the soil-gas constituents.This seepage suggests largest magnitude of soil gas anomalies might be generated/source from Mesozoic sedimentary rocks,beneath Deccan Traps.The carbon isotopic composition of methane,ethane and propane ranges are from-22.5‰ to-30.2‰ PDB,-18.0‰to 27.1‰ PDB and 16.9‰-32.1‰ PDB respectively,which are in thermogenic source.Surface soil sample represents the intersection of a migration conduit from the deep subsurface to the surface connected to sub-trappean Mesozoic sedimentary rocks.Prominent hydrocarbon concentrations were associated with dykes,lineaments and presented on thinner basaltic cover in the study area,which probably acts as channel for the micro-seepage of hydrocarbons.     

The effect of oil expulsion or retention on further thermal degradation of kerogen at the high maturity stage: A pyrolysis study of type II kerogen from Pingliang shale,China

High maturity oil and gas are usually generated after primary oil expulsion from source rocks, especially from oil prone type I/II kerogen. However, the detailed impacts of oil expulsion, or retention in source rock on further thermal degradation of kerogen at the high maturity stage remain unknown. In the present study, we collected an Ordovician Pingliang shale sample containing type II kerogen. The kerogens, which had previously generated and expelled oil and those which had not, were prepared and pyrolyzed in a closed system, to observe oil expulsion or oil retention effects on later oil and gas generation from kerogen. The results show that oil expulsion and retention strongly impacts on further oil and gas generation in terms of both the amount and composition in the high maturity stage. Gas production will be reduced by 50% when the expulsion coefficient reaches 58%, and gas from oil-expelled kerogen (less oil retained) is much drier than that from fresh kerogen. The oil expulsion also causes n-alkanes and gas compounds to have heavier carbon isotopic compositions at high maturity stages. The enrichment of ¹³C in n-alkanes and gas hydrocarbons are 1‰ and 4–6‰ respectively, compared to fresh kerogen. Oil expulsion may act as open system opposite to the oil retention that influences the data pattern in crossplots of δ¹³C₂–δ¹³C₃ versus C₂/C₃, δ¹³C₂–δ¹³C₃ versus δ¹³C₁ and δ¹³C₁–δ¹³C₂ versus ln(C₁/C₂), which are widely used for identification of gas from kerogen cracking or oil cracking. These results suggest that the reserve estimation and gas/source correlation in deep burial basins should consider the proportion of oil retention to oil expulsion the source rocks have experienced.     

Comparison of source rock geochemistry of selected rocks from the Schei Point group and Ringnes formation, Sverdrup basin, arctic Canada

Organic-rich from the Schei Point group (middle to late Triassic in age) and the Ringnes formation (late Jurassic) from the Sverdrup basin of the Canadian arctic archipelago have been geochemically evaluated for source rock characterization. Most samples from the Schei Point group are organic-rich (> 2% TOC and are considered as immature to mature oil-prone source rocks [kerogen types I, I–II (IIA) and II (IIA)]. These kerogen types contain abundant AOM1, AOM2 and alginite (Tasmanales, Nostocopsis, Leiosphaeridia, acritarch and dinoflagellate) with variable amounts of vitrinite, inertinite and exinite. Samples from the Ringnes formation contain dominant vitrinite and inertinite with partially oxidized AOM2, alginite and exinite forming mostly immature to mature condensate- and gas-prone source rocks [kerogen type II–III (IIB), III and a few II (IIA)]. Schei Point samples contain higher bitumen extract, saturate hydrocarbons and saturate/ aromatic ratio than the Ringnes samples. Triterpane and sterane (dominant C₃₀) distribution patterns and stable carbon isotope of bitumen and kerogen suggest that the analyzed samples from the Schei Point group are at the onset of oil generation and contain a mixture of sapropelic (algal) and minor terrestrial humic organic matter. Sterane carbon number distributions in the Ringnes formation also suggest a mixed algal and terrestrial organic matter type. There are some variations in hopane carbon number distributions, but these are apparently a function of thermal maturity rather than significant genetic differences among samples. Pyrolysis-gas chromatography/mass spectrometry of the two samples with similar maturity shows that the Schei Point sample generates three times more pyrolyzate than the Ringnes sample. Both samples have a dominant aliphatic character, although the Ringnes sample contains phenol and an aromaticity that is higher than that of the Schei Point sample.     

Studying of liquid thermolysis products of various types of immature kerogen in sedimentary lacustrine rocks from the valjevo-Mionica basin,Serbia, and the effect of Pt<Superscript>4+</Superscript> and Ru<Superscript>3+</Superscript> ions on their yield and the hydrocarbon composition

Liquid thermolysis products of various types of immature kerogen from sedimentary lacustrine rocks from the Valjevo-Mionica basin in Serbia were studied to evaluate the generation potential of kerogen contained in the organic matter (OM) of the rocks, determine the composition of the biomarkers and alkylaromatics in the liquid thermolysis products, and elucidate the effect of Pt⁴⁺ and Ru³⁺ ions (which were added in the form of inorganic salts) on the yield and hydrocarbon composition of the liquid thermolysis products. For this purpose, representative bitumen-free samples A and B of the sedimentary rocks were subjected to thermolysis under various conditions. Rock A contains high amount of immature organic matter, which is dominated by kerogen type I/II and was generated under strongly reduced sedimentation conditions at a high salinity. Sample B is poorer in immature OM than sample A, and the OM of the former contains kerogen type II/III and was generated predominantly in a reduced environment. The content of the liquid products and the concentrations of hydrocarbons obtained in the course of thermolysis of bitumen-free sample A and the typical oil distribution of the biomarkers and alkylaromatics in the thermolysis products confirm a high generation potential of OM in this rock. In all of our experiments on the thermolysis of bitumen-free sample B, the yield of liquid products and hydrocarbons is low. According to the kerogen type, the thermolysis of this rock generates much gases. The Pt⁴⁺ and Ru³⁺ ions (added in the form of simple inorganic salts) increased the yield of liquid (kerogen type I/II) and gaseous (kerogen type II/III) products. During the thermolysis of various type of immature kerogen in the lacustrine sedimentary rocks at a temperature of 400°C, the OM attained maturation corresponding to the early catagenesis level. Saturated biomarkers and alkylaomatics in the thermolysis products of both samples display typical oil distributions. The type of the source OM most strongly affects the composition of n-alkanes and alkylnaphthalenes. The metal ions used in this research served as catalysts for the methylation process during the thermolysis of immature kerogen, regardless of its type. The effect of the Pt⁴⁺ and Ru³⁺ ions on other transformations of the hydrocarbons, for example, the destruction of high-molecular n-alkanes to low-molecular ones and on isomerization reactions in molecules of polycyclic biomarkers and alkylaromatics to thermodynamically more stable isomers in the thermolysis products is controlled, first of all, by the type of the source OM.     

Primary cracking of kerogens. Experimenting and modelling C1, C2–C5, C6–C15 and C15+ classes of hydrocarbons formed

Mathematical models of hydrocarbon formation can be used to simulate the natural evolution of different types of organic matter and to make an overall calculation of the amounts of oil and/or gas produced during this evolution. However, such models do not provide any information on the composition of the hydrocarbons formed or on how they evolve during catagenesis.From the kinetic standpoint, the composition of the hydrocarbons formed can be considered to result from the effect of “primary cracking” reactions having a direct effect on kerogen during its evolution as well as from the effect of “secondary cracking” acting on the hydrocarbons formed.This report gives experimental results concerning the “primary cracking” of Types II and III kerogens and their modelling. For this, the hydrocarbons produced have been grouped into four classes (C₁, C₂–C₅, C₆–C₁₅ and C₁₅₊). Experimental data corresponding to these different classes were obtained by the pyrolysis of kerogens with temperature programming of 4°C/min with continuous analysis, during heating, of the amount of hydrocarbons corresponding to each of these classes.The kinetic parameters of the model were optimized on the basis of the results obtained. This model represents the first step in the creation of a more sophisticated mathematical model to be capable of simulating the formation of different hydrocarbon classes during the thermal history of sediments. The second step being the adjustment of the kinetic parameters of “secondary cracking”.     

Distinguishing Cambrian from Upper Ordovician source rocks: Evidence from sulfur isotopes and biomarkers in the Tarim Basin

The reported source rocks for the abundant petroleum in the Tarim Basin, China range from Cambrian to Lower Ordovician and/or Upper Ordovician in age. However, the difference between the two groups of source rocks is not well characterized. In this study, pyrite was removed from eleven mature to over mature kerogen samples from source rocks using the method of CrCl₂ reduction and grinding. The kerogen and coexisting pyrite samples were then analyzed for δ³⁴S values. Results show that the kerogen samples from the Cambrian have δ³⁴S values between +10.4‰ and +19.4‰. The values are significantly higher than those from the Lower Ordovician kerogen (δ³⁴S of between +6.7‰ and +8.7‰), which in turn are generally higher than from the Upper Ordovician kerogen samples (δ³⁴S of between ?15.3 and +6.8‰). The associated pyrite shows a similar trend but with much lower δ³⁴S values. This stratigraphically controlled sulfur isotope variation parallels the evolving contemporary marine sulfate and dated oil δ³⁴S values from other basins, suggesting that seawater sulfate and source rock age have an important influence on kerogen and pyrite δ³⁴S values. The relatively high δ³⁴S values in the Cambrian to Lower Ordovician source rocks are associated with abundant aryl isoprenoids, gammacerane and C₃₅ homohopanes in the extractable organic matter, indicating that these source rocks were deposited in a bottom water euxinic environment with water stratification. Compared with the Upper Ordovician, the Cambrian to Lower Ordovician source rocks show abundance in C₂₈ 20R sterane, C₂₃ tricyclic terpanes, 4,23,24-trimethyl triaromatic dinosteroids and depletion in C₂₄ tetracyclic terpane, C₂₉ hopane. Thus, δ³⁴S values and biomarkers of source rock organic matter can be used for distinguishing the Cambrian and Upper Ordovician source rocks in the Tarim Basin.     

Investigation of the pyrolytic liberation of molecular nitrogen from Palaeozoic sedimentary rocks

Open-system non-isothermal pyrolysis up to 1,200°C in combination with elemental analysis was used to study the thermal liberation of molecular nitrogen (N₂) from sedimentary rocks and kerogen concentrates of Palaeozoic age from the Central European Basin system and an Eocene shale (Liaohe Basin, China) with a high content (36%) of ammonium feldspar (buddingtonite). The N/C_org (atomic) ratios of the kerogen concentrates ranged from 0.005 to 0.014, which represents the range commonly observed for coals. Bulk N/C_org ratios of the Palaeozoic shales extended from 0.035 to 0.108, indicating the presence of significant amounts of inorganic nitrogen. Namurian A and A-B (CnA; CnA-B) samples typically exhibited the earliest onset of N₂ generation with intense, characteristic peaks around 600°C. N₂ liberation from the buddingtonite-rich sample occurred at higher temperatures, with a broad peak around 700°C. Pyrograms of the kerogen concentrates showed no or strongly reduced N₂ generation in the 500–700°C range. On-line isotope-specific analysis of the pyrolytically liberated N₂ on one sample revealed a variability of ∼10‰ in the δ¹⁵N values and a steady increase in δ¹⁵N with temperature during the main phase of N₂ generation.     

Primary alteration—oxidation of marine algal organic matter from oil source rocks of the North Sea and Norwegian Arctic: new findings

The presence of partially oxidized algal organic matter in oil-prone marine source rocks, is the rule rather than the exception. Partially oxidized, algal kerogen can still act as a significant source of liquid hydrocarbons. However, the corresponding peak of C₁₂ + hydrocarbon generation is shifted to a considerably lower maturity level compared with that of the classical Type II kerogen. The extent of primary alteration-oxidation of marine algal kerogen is monitored by means of solid state microfluorescence spectroscopy. A new parameter, the Primary Alteration Factor (PAF) is established, and the relationships between PAF and H/C, O/C, HI, TOC and between PAF and %₀δ¹³C are determined. The present data show large variations in the bulk chemistry of immature marine algal kerogens, and reveal evidence for gradational dehydrogenation/oxidation of the source organic matter. This contrasts with the recently proposed mechanism for kerogen formation. SEM analysis reveals a relationship between the physical breakdown of algal organic matter and the formation of liptodetrinite. FTIR analysis shows that the incorporation of primary oxygen in the kerogen macromolecules is not in the form of carbonyl or carboxyl functionalities. The presence of highly unreactive, stable oxygen, associated with aromatic structures in partially oxidized algal kerogen, is suggested by resistance of the kerogen to graphitization. The FTIR data also suggest the presence of aryl ether oxygen. The present findings raise fundamental questions regarding the mechanisms of kerogen cracking and kerogen formation, and have important implications for petroleum exploration.     

Stepwise pyrolysis-gas chromatography of kerogen in sedimentary rocks

Stepwise pyrolysis-gas chromatography is used to examine and characterize the carbonaceous matter in sedimentary rocks. Low-temperature steps remove material normally volatile or extracted by benzene-methanol. Successively-higher temperature steps degrade the insoluble carbonaceous matter (kerogen) into smaller molecular pieces. The sequential pyrolysis steps have the advantage of breaking the kerogen at several temperatures which may be related to bond type or strength. The pyrolysis product chromatograms from each step can be compared. The molecular sizes (chain length) of kerogens fragments can be determined. The results presented here show the molecules in the range C₁₁ to C₂₃ because: (1) they can be compared to normal petroleum source rock extractables; and (2) these large molecules give a feeling for the molecular construction of the kerogen.Green River and Antrim shales show low-temperature material which is indigenous and not modified compared to the pyrolyzed kerogen fragments in the range C₁₁C₂₃. Kupferletten shows low-temperature material of a narrow molecular weight range of C₁₅C₁₉ which is probably derived from the kerogen. Monterey shale low-temperature material appears to be unrelated to the kerogen as represented by its pyrolysis products. The Pierre shale kerogen shows molecules over the range C₁₁C₂₃. Kerogen from the Romney shale has no molecules large than C₈ in its pyrolysis products and no petroleum potential due to thermal and tectonic diagenesis.