Vitrinite recycling: diagnostic criteria and reflectance changes during weathering and reburial

The aim of this study was first to review or even identify reliable diagnostic criteria to distinguish recycled and autochthonous vitrinite particles and, second, to examine and try to explain the impact of weathering and reburial on optical changes (reflectance) of recycled material. The work was based on indigenous and recycled particles of two sample sets from two wells drilled in the Senegalese margin basin and the Ardèche paleomargin (SE France), respectively. In addition to reflectance measurements, the studied samples have also been analysed by Rock-Eval pyrolysis. For both sample sets, hydrogen index (HI) and oxygen index (OI) values suggest a type III organic matter of dominant terrestrial origin.Two main diagnostic criteria appear particularly useful to identify indigenous and recycled vitrinite particles: the first is the clear genetic relationship between telinite and collotelinite; the second is the presence of pyrite framboid inclusions in indigenous vitrinite. The pyrite–vitrinite association illustrates the relationship between sulphate reduction, pyritisation, and in situ organic matter transformation through gelification process. However, application of both diagnostic criteria requires some additional information about depositional conditions (i.e., anoxicity) and thermal maturity of the studied samples. Weathering has no or only very limited effect on vitrinite reflectance. During their new burial history, recycled vitrinites follow a reflectance evolution path close to that of inertinites and thus appear as transitional materials between vitrinite and inertinite macerals. Consistently, this behaviour implies that recycled vitrinite reflectance changes during burial diagenesis do not solely depend on their chemical composition but, most probably too, on combined chemical and structural changes inherited from their past burial history and from the impact of weathering during their recycling stage.     

Statistical analysis of vitrinite reflectance data—a new approach

The reflectance of vitrinite (collotelinite) particles is a widely used parameter as a geothermometer for the estimation of the thermal maturity of organic matter enclosed in rocks. However, several problems have occurred during the last decades, which can be traced back to basically three causes: human mistakes, technical problems, and problems associated with the structural and compositional inhomogeneity of organic matter. Whilst in most cases the first two types of uncertainties can be handled by standardization, the third can cause significant problems during interpretation due to its generally inestimable character. The suppression of vitrinite reflectance and statistical problems originated from small sample size, and outliers belong to this latter type.International standards, such as the ASTM and the ISO, define the vitrinite reflectance parameter as a statistical average of measured data, disregarding the fact that the average is an unresisting and unrobust statistical parameter. In other words, the average is very sensitive to outliers and distribution.The aim of this research was to find and test a better, more resistant, and robust statistical parameter used by traditional parametric and nonparametric statistics, which can be applied in practice instead of the average. Three categories of statistical problems were studied on coal and disperse organic matter (DOM) samples: the distribution of measured values, the effect of data number, and the effect of outliers on statistical parameters. The statistical experiments carried out on numerous original and generated sample sets show that the median (med) and the most frequent value (Mn), a special weighted average, are better parameters to estimate the thermal maturity of organic matter especially above 1% reflectance value.     

镜质体反射率热演化规律研究

镜质体反射率的演化并不是人们传统意义上认为的随着温度（深度）的增加成指数增加,而是分阶段演化的。通过对有机质热模拟实验数据和自然演化数据进行分析后发现镜质体反射率的演化可以分为明显的3段：1）缓慢变化阶段,随着温度的增加镜质体反射率缓慢地变化,2）中等快速阶段,随着温度的增加镜质体反射率相比较前一阶段增加速率有明显的加大,3）快速增加阶段,随着温度的增加镜质体反射率呈指数快速增加。这种分阶段的演化是由镜质体本身的化学组成和化学结构所决定的。镜质体反射率演化阶段的划分对于热历史的恢复以及认识镜质体的热演化机制和规律具有重要的意义。     

Using reflectance crossplots and rotational polarization for determining first-cycle vitrinite for maturation studies

Vitrinite reflectance on dispersed sedimentary organic matter is traditionally determined from histograms of vitrinite and inertinite reflectance measured on either whole rock samples or kerogen concentrates, or from particles usually in whole rock preparations chosen by morphology and relative reflectance to be first-cycle vitrinite. This paper discusses an alternative method: discrimination of first-cycle vitrinite using apparent maximum and minimum reflectance data plotted on reflectance crossplots. The use of crossplots increases the confidence that the macerals identified as first-cycle vitrinite are optically similar. The maximum–minimum data, in this study, were collected on a quantitative reflectance microscope outfitted with rotational polarization, a technique that eliminates the usual difficulty in collecting such data on very small macerals by stage rotation.     

Secretinite—reflectance and chemical data from two high volatile bituminous coals (Upper Carboniferous) of North America

Secretinite—a maceral of the inertinite group as recognized by the ICCP in 1996—is a noncellular maceral of seed fern origin. New reflectance data indicate that this maceral has primary anisotropy with bireflectances of 0.4% to 0.9% in high-volatile B bituminous (Ro=0.6%) Carboniferous coal of North America. The highest reflectance is in cross-section as opposed to longitudinal section. Characteristic feature of secretinite is the virtual absence of Si and Al, unlike that in associated vitrinite. This indicates the absence of submicron aluminosilicates in secretinite and their presence in vitrinites. Secretinite is highly aromatic as indicated by low O/C ratios and high contribution of aromatic hydrogen bands detected by FTIR analysis.     

华北太原组镜质组中的超微类脂体及其煤性质异常原因

利用超薄切片透射电镜分析技术，对华北太原组和山西组煤中镜质组的超微结构和组分进行了分析，确认太原组镜质组中含有较丰富的超微类脂体，且超微类脂体主要来自菌藻类，山西组和太原组镜质组中超微类脂体数量存在明显差异。证实镜质体中超微类脂体数量和其化学性质紧密相关。从超微组分层次解释了山西组和太原组镜质组性质差异的原因，并分析讨论了太原组煤性质异常性的原因和煤还原程度问题，提出成煤过程中明显菌藻类低等生物贡献是太原组煤性质异常的一个重要原因。     

From Middle Jurassic heating to Neogene cooling: The thermochronological evolution of the southern Alps

The Dolomite region is located in the Southern Alps, which were affected by Mesozoic extensional tectonics and by consequent thermal perturbations. In this work, vitrinite reflectance and apatite fission-track analysis are used to estimate the thermal evolution. These methodologies have been applied to the Permo-Mesozoic succession, which crops out along the TRANSALP seismic profile. The regional distribution of the organic matter maturity seems to be mainly controlled by different burials reached during the Norian-Liassic extensional phase, in connection with high heat flow values. The best solutions obtained from thermal modelling of both vitrinite and fission-track data suggest that peak of high heat flow occurred during Bajocian–Bathonian ages, when western Tethys was characterized by intrusions of gabbros and plagiogranites and extrusion of tholeiite basalts. This time coincides with the onset of the drifting phase and related thermal subsidence. The following thermal relaxation occurred during continuous sedimentation and the maximum burial does not coincide with peak temperatures. Cooling history has been carefully analysed through apatite fission-track data on samples collected close to the Valsugana overthrust, which document an important exhumation event at about 10 Ma. The related erosion has been analysed through the combined use of arenite petrography and fission-track analysis on detrital samples of the Veneto foredeep succession, which represents the storage of detritus during Tertiary. These data confirm that after Serravalian the Southalpine domain and related covers were affected by subaerial erosion.     

Molecular characterisation of vitrinite in relation to natural hydrogen enrichment and depositional environment

The Py-GC/MS results of the study carried out on two groups of vitrinites (perhydrous and non-perhydrous) of different age and properties and on a trimaceralic coal associated with one of the perhydrous group are discussed. Such a study provides information about the effect of natural hydrogen enrichment on vitrinite structure at the molecular level. Moreover, the influence of the different conditions in the sedimentary environment on the chemical structure of the vitrinite is also discussed. This influence is inferred through differences in the distribution and relative amount of phenolic compounds found in the pyrolysates from two samples of two different coal-beds in the same basin but formed under different paleoenvironmental conditions. For vitrinites with a high H/C atomic ratio, despite having a strong perhydrous character, their pyrolysates exhibit the highly phenolic signature typical of lignin-derived material with only minor aliphatic compounds. Thus, the major chemical structural elements in these vitrinites are simple phenols with a high contribution of para alkyl-substituted derivatives. However, there is no parallel relationship between the evolution of the oxygenated functionalities and the reflectance values. From the results obtained a coalification pathway where hydrogenation processes predominate over thermal ones is proposed. The presence of resin-like substances and/or oils (which are two of the causes of natural hydrogen enrichment) in the molecular structure of vitrinites have, therefore, affected the normal evolution of the lignin and contributed to the special properties of this type of materials.     

Physical and chemical environments of abnormal vitrinite reflectance evolution in the sedimentary basins

Based on the tested data of pressure and vitrinite reflectance of some wells in sedimentary basins, abnormal high pressure is regarded as not the only factor to retard the increase of vitrinite reflectance (R _o). Apart from the types of the organic matter, the physical environment (temperature and pressure) and chemical environment (fluid composition and inorganic elements) will result in the abnormal vitrinite reflectance values in the sedimentary basins. This paper tested trace elements and vitrinite reflectance data from the the abnormal high pressure and normal pressure strata profiles, respectively, and found that the acidic and lower salinity starta are favorable for the increase of R _o. By discussing the corresponding relationship between the contents of some trace elements in the mudstone and the vitrinite reflectance values, the typical trace elements were found to suppress and/or catalyze the vitrinite reflectance of organic matter, while the elements of Ca, Mn, Sr, B, Ba and P may result in the retardation of R _o. However, elements of Fe, Co, Zn, Ni and Rb may catalyze the organic matter maturation. This study is conductive to the organic maturation correction, oil and gas assessment and thermal history reconstruction by the paleothermometry. Translated from Acta Geologica Sinica, 2006, 80(11): 1760–1769 [译自: 地质学报]     

The application of FAMM (Fluorescence Alteration of Multiple Macerals) analyses for evaluating rank of Paraná Basin coals, Brazil

Combining vitrinite reflectance (VR) and fluorescence alteration of multiple macerals (FAMM) analyses provide insights into the chemical nature of vitrinites (i.e., perhydrous vs. orthohydrous vs. subhydrous compositions) in Permian Gondwana coals of the Paraná Basin, Brazil. The FAMM-derived equivalent VR (EqVR) values and relationships with VR can be determined according to calibration curves based largely on Permian Gondwana coals of eastern Australia.The analytical results indicate that vitrinites in the Paraná Basin coals studied generally range from orthohydrous to perhydrous, with interpreted VR suppression ranging up to 0.2% absolute for the most perhydrous case. The EqVR values of the Santa Catarina coals, which range from about 0.85% to 0.95% differ from VR values by about 0.10–0.15% absolute, potentially having significant implications on coal utilization.The causes of vitrinite reflectance suppression in the Paraná Basin coals are as yet poorly understood, but are likely to be related to a combination of factors.     

Quantitative chemical characterization of vitrinite concentrates using pyrolysis-gas chromatography. Rank variation of pyrolysis products

A series of hand-picked vitrinite samples from the Lower Kittanning Seam, Pennsylvania have been examined using quantitative pyrolysis-gas chromatography. These vitrinites ranged in rank from 0.59 to 1.71% reflectance, a rank range from high volatile C bituminous to low volatile bituminous. High molecular weight pyrolysis products included alkyl aromatic and phenolic compounds. Attempts have been made to correlate the pyrolysis product composition to rank parameters including vitrinite reflectance, volatile matter yield, carbon content, atomic H/C ratio and Rock-Eval determined T_max. Total yield of phenols was found to be strongly and inversely rank related. A clear relationship between C₈ alkyl-benzene yield and rank was not found for the sample set.     

Potential source rocks, organic geochemistry and thermal maturation in the southern depocenter (Kipourio–Grevena) of the Mesohellenic Basin, central Greece

The study area is the southern depocenter (depth > 4200 m) of the Mesohellenic Basin which extends between Kipourio and Grevena, central Greece. The Mesohellenic Basin is a Middle-Tertiary intramontane basin developed within the Hellenide orogen. Previous studies have focused on the depositional environments, configuration and hydrocarbon potential of the basin. In this paper we present additional geochemical and petrographic data from outcrop samples of the basin's southern depocenter, which is considered the most promising area, in terms of hydrocarbon prospectivity. A total number of thirty six samples were analysed: Rock-Eval pyrolysis, maceral analysis, vitrinite reflectance and thermal alteration index, bitumens extraction, liquid chromatography, and GC-MS. The samples were collected from deltaic deposits and submarine fan sediments of Late Eocene to Late Oligocene age. The TOC values of the analysed samples range between rich and very rich and the organic matter consists mainly of type III kerogen and the organic matter consider to be predominately gas prone. The thermal maturity assessed from T_max and vitrinite reflectance shows an immature stage of the organic matter along with the presence of layers having reached the very early mature stage. Vitrinite reflectance measurements and maturity calculations (applying the Lopatin modeling), reveal that the lower part of the depocenter sediments falls within the ‘oil window’. The extractable organic matter (EOM) (mg bitumens/g TOC) indicate the existence of samples (from deltaic deposits) with high ratio of transformation (EOM) (> 100 mg bitumen/g TOC). The GC and GC-MS analyses of the biomarkers indicate mainly the occurrence of terrestrial organic matter reflecting oxidizing conditions and both immature and very early mature stages. The results of the Rock-Eval pyrolysis and the distribution of the isoprenoids support the assumption of the input of an organic matter mixture.     

Sedimentary organic matter and source rock potential of the Paleocene Aaliji Formation in Qumar Oil Field,NE Iraq

The organic matter content of the Paleocene Aaliji Formation has been studied from the Qm-1 well in the Qumar Oil Field, NE Iraq. A palynofacies analysis revealed the obvious domination of amorphous organic matter (AOM) in the samples studied in addition to the different ratios of palynomorphs, phytoclasts and opaque organic material. The deposition of the various percentages of organic matter components and types of palynomorphs appear to have occurred in a proximal suboxic–anoxic basinal environment. The samples analysed showed relatively low percentages of total organic carbon, indicating a generally poor source rock. The thermal alteration index for the palynomorphs (dinoflagellates) observed and identified ranged between 2, 2+ and 3?, indicating an immature to early stage of maturity for the section studied. No clear differentiation between the stages of maturity within the section was identified. The reflectance measured for a few vitrinite particles at a depth of 2,900 m showed uncertainly of 0.46 % Ro, indicating a still thermally immature stage. The values obtained from pyrolysis analysis also supported the formation being in the early stages of maturity, i.e. an immature condition, with T _max values between 416 and 435 °C. The quality of the organic matter examined and analysed appeared to be mostly type III gas-prone kerogen, as discerned from the hydrogen index, oxygen index and other pyrolysis parameters. The parameters obtained and calculated from gas chromatography analysis performed on a sample at a depth of 2,900 m found marginally mature marine source organic matter.     

窑街矿区海石湾井田油A层腐泥煤有机质成熟度辨析

在石油、天然气、页岩气和煤层气资源勘探开发方面,镜质体反射率指标被公认是唯一可对比的烃源岩有机质成熟度评价指标。为准确测定窑街矿区海石湾井田油A层腐泥煤的有机质成熟度,采集井田钻孔煤岩样品,开展显微镜下显微成分反射光、荧光光学特征观测。测试发现,油A层腐泥煤中存在2种不同特征镜质组,一种为发荧光的镜质组,反射色低、反射率测值低;另一种为不发荧光镜质体,反射色高、反射率测值相对偏高。油A层沉积环境分层明显,上部为腐泥煤,显微成分以藻类体为主,含有大量的黄铁矿,反映深部沼泽强还原环境;下部显微成分以腐泥基质、基质镜质体、角质体为主,相较于上部,藻类体及黄铁矿含量明显降低,反映出其沉积物源发生变化,藻类等低等生物输入比例降低,高等植物物源比例增高,反映为相对较弱的还原环境。根据井田地层沉积环境、油A层腐泥煤显微组成及其与下部煤二层腐植煤的镜质组成熟度对比分析,认为油A层腐泥煤中不发荧光镜质体的反射率能够反映其真实的成熟度,与下部煤二层处于同一变质阶段。研究认识为开展腐泥煤有机质成熟度分析提供了新的方法、思路,当煤岩层中高等植物含量较少,或者与其相邻地层成熟度差异较大时,建议将局部测试结果与整个煤系各煤/岩层进行比对,同时结合其镜质体荧光特征和沉积环境综合判断分析。      

四川盆地五峰-龙马溪组页岩成熟度研究

上奥陶统五峰组-下志留统龙马溪组海相页岩是四川盆地下古生界主要的烃源岩和页岩气勘探目标,有机质成熟度不仅是油气生成评价的关键,也是页岩品质评价的重要指标之一.下古生界页岩有机质成熟度一直以来是有机岩石学研究的难点与热点问题.由于下古生界缺乏镜质体,先前的研究多是采用沥青反射率转换为等效镜质体反射率的方法,并且由于沥青的局限性和不确定性,使得五峰-龙马溪组页岩的成熟度缺乏统一的认识和系统研究.通过采集四川盆地及其周缘的岩心和露头样品,系统分析了页岩有机显微组分光学反射率特征.结果表明笔石和固体沥青是最主要的两类有机显微组分.根据固体沥青的显微结构形态和光性特征,将固体沥青大体上分为两类:（1）颗粒状-棱角状的充填在孔隙和微裂缝中高反射率焦沥青;（2）以细小不规则表面的有机质颗粒大量分散于粘土矿物基质中的低反射率基质固体沥青.焦沥青与笔石随机反射率均可以表征下古生界页岩有机质成熟度.但焦沥青反射率略低于笔石反射率,并且随着成熟度的增高,笔石反射率的增速大于焦沥青,各向异性也显著增强.相对于固体沥青反射率,笔石随机反射率分布更为集中,更适合作为含笔石页岩有机质成熟度指标.但是笔石反射率与等效镜质体反射率在过成熟阶段的换算关系需要进一步研究.      

沉积盆地中镜质组反射率异常的物理化学环境探讨

根据实测的压力和镜质组反射率数据讨论了沉积盆地中镜质组反射率异常的物理化学环境.认为异常压力并不是导致镜质组反射率受到抑制的唯一原因;沉积盆地中有机质镜质组反射率异常的原因除与有机质本身的性质有关外,还与其所处的温度、压力等物理环境及其周围的流体性质、无机元素(矿物)的组成等化学因素密切相关.通过对钻井异常高压带和正常压力带剖面密集取样进行的微量元素与镜质组反射率分析表明,偏酸性、低盐度的流体介质有利于有机质镜质组反射率的增加;通过分析泥岩中的某些元素丰度与镜质组反射率的对应关系,初步确定钙、锰、锶、硼、钡、磷等元素对镜质组反射率的演化有抑制作用,而铁、钴、锌、镍、铷等元素对有机质的热演化具有催化作用.本研究对于有机质成熟校正、油气资源评价和利用成熟度古温标恢复盆地热历史都具有意义.     

Geochemical study on a well in the Western Canada Basin: relation of the aromatic distribution pattern to maturity of organic matter

Thirty-one rock samples from a Western Canada Basin well have been analysed for aromatics, using glass capillary gas chromatography with simultaneous flame ionization and sulfur-selective flame photometric detection. A uniform aromatic distribution pattern with a marked predominance of di- and tricyclic aromatic hydrocarbons was observed throughout a depth interval of 3000 meters comprising Cretaceous and Jurassic sediments. The very high relative abundance of sulfur aromatics at two narrow intervals in the Triassic and Mississippian is attributed to a major change in facies. Gradual changes with increasing depth have been observed for a series of compound ratios, which had been calculated from the isomers of methyl- and dimethylphenanthrene, and their parent compounds. These changes reflect the increasing thermal maturation of the sedimentary organic matter. A Methylphenanthrene Index (MPI) has been introduced, which exhibited a very good correlation with vitrinite reflectance data. The MPI is the first maturity parameter which is based on a series of aromatic hydrocarbons. This hydrocarbon internal maturity parameter permits the recognition and definition of maturity levels in extracts which can be compared to the maturity (e.g. vitrinite reflectance) of source rocks.     

煤及有机岩研究的新进展：第44届国际煤及有机岩会议

通过对44届国际煤及有机岩会议及会后访问所讨论和研究的主要问题的分析,介绍了煤的分类、新的煤显微组分、煤岩学与有机岩石学、镜质组反射率抑制、成熟度、盆地分析与评价、以及工艺转化中的活性惰性组组分等方面研究的新进展。     

Mineralogy and organic petrology of oil shales in the Sangkarewang Formation, Ombilin Basin, West Sumatra, Indonesia

The Ombilin Basin is filled by late Eocene to early Oligocene marginal fan deposits (Brani Formation) and lacustrine shales (Sangkarewang Formation), unconformably overlain by a late Oligocene to early Miocene fluvial sequence (Sawahlunto and Sawahtambang Formations) and capped by an early to mid-Miocene marine sequence (Ombilin Formation). Significant oil shale deposits occur in the Sangkarewang Formation, intercalated with thin laminated greenish-grey calcareous sandstones. X-ray diffraction shows that the sediments consist mainly of quartz, feldspar, carbonates and a range of clay minerals, together in some cases with minor proportions of sulphides, evaporites and zeolites. Feldspar and non-kaolinite clay minerals decrease up the sequence, relative to kaolinite, suggesting a changing sediment source as the basin was filled. Calcite, thought to be mainly of authigenic origin, is also more abundant in the middle and upper parts of the sequence.The organic matter in the oil shales of the sequence is dominated by liptinite macerals, particularly alginite (mainly lamalginite) and sporinite. Cutinite also occurs in some samples, along with resinite and traces of bituminite. The dominance of lamalginite in the liptinite components suggests that the material can be described as a lamosite. Samples from the Sangkarewang Formation have vitrinite reflectance values ranging between 0.37% and 0.55%. These are markedly lower than the vitrinite reflectance for coal from the overlying Sawahlunto Formation (0.68%), possibly due to suppression associated with the abundant liptinite in the oil shales.Fischer assay data on outcrop samples indicate that the oil yield is related to the organic carbon content. Correlations with XRD data show that, with one exception, the oil yield and organic carbon can also be correlated directly to the abundance of carbonate (calcite) and inversely to the abundance of quartz plus feldspar. This suggests that the abundance of algal material in the lake sediments was preferentially associated with carbonate deposition. High yields of oil are noted in some samples, as a percentage of the organic carbon content. This may indicate that partial generation of hydrocarbons from the material has already taken place, in association with thermal maturation of the Sangkarewang succession.     

Chitinozoan reflectance in the Lower Palaeozoic of the Welsh Basin

Vitrinite (woody matter) and spores are generally uncommon or absent in pre-Devonian sediments. To determine organic maturity in the Lower Palaeozoic alternative methods, such as chitinozoan reflectance, must be employed. Chitinozoa are marine, vase-shaped organic-walled microfossils of unknown affinity. The reflectance of incident light from polished chitinozoa walls can provide reliable and accurate maturity data and, due to their relative facies-independency, a high degree of resolution to regional maturity studies. Calibration of the chitinozoan reflectance scale with vitrinite reflectance enables palaeotemperature estimates to be made from pre-Devonian sediments.
The range of chitinozoan reflectance for the Welsh Basin is from under 1% to 8%. This corresponds to estimated palaeotemperatures of under 100°C to over 300°C. The regional spatial distribution of organic maturation recorded by chitinozoan reflectance in the Welsh Basin is attributed to palaeobasin form. The increase in reflectance with age in the rocks of the Myddfai Steep Belt and associated areas allows an estimation of a palaeogeothermal gradient. In the Silurian sequences around Llandovery the gradient is calculated to be in the region of 50°C km⁻¹. Similar gradients are derived from other studies (clay minerals, metabasite mineral assemblages, conodont colours). The results from Myddfai Steep Belt suggest the thermal peak occurred during the Acadian event.
Chitinozoan reflectivity demonstrates that, with respect to hydrocarbon generation, mature conditions occur on the palaeoplatform, where the sedimentary sequences are relatively thin. Overmature conditions are prevalent in the palaeobasin where there is a thick sedimentary pile which has been subsequently deformed.