Vitrinite Reflectance Measurements Research Articles

Impact of thermal maturity on the concomitant evolution of the ultrafine structure and porosity of marine mudstones organic matter; contributions of electronic imaging and new spectroscopic investigations

This study aims to better understand the evolution of the organic matter (OM) ultrafine structure (designating the nanoscopic structure of the OM/macerals) and porosity with increasing thermal maturity of mudstones source-rocks. To this end, the particulate fractions of kerogen from organic-rich Kimmeridge clay mudstones were isolated by acidic treatment before and after laboratory thermal maturation. The evolution of the composition, the chemical structure and the porosity of kerogen from the immature stage (Ro = 0.42%) to the dry gas zone (Ro = 2.12%) were documented using a combination of elemental analysis, vitrinite reflectance measurements, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM), Raman spectroscopy and Small Angle X-Ray Scattering (SAXS). The evolution of the porosity of OM was then compared to the evolution of the pore volume of the total rock measured by nitrogen adsorption. The results show that the progressive densification, reorganization and aromatization of the amorphous kerogen particles into a more ordered, but heterogeneous carbon-rich residue is responsible for significant variations in the kerogen porosity with increasing maturity. Contrary to the previous observations on these Kimmeridge clay mudstones, the variations that occur during the peak of oil generation (Ro of ca. 0.90%) mark the onset of the OM-hosted pores development. There appears to be a natural close relationship between the evolution of OM porosity and total pore volume of organic-rich mudstones during gas generation. Indeed, a similar alternation of pore collapse and pore development is observed in response to gas generation. The development and the evolution of pores in these organic-rich mudstones seems thus mainly driven by the evolution of the chemical structure and the composition of OM during thermal maturation. In the dry gas zone, the porosity and the specific surface area of the kerogen are significant (19% and 57.1 m2.g−2 respectively). This highlights the importance of the OM content, type and composition in the porosity and gas storage capacities of mudstone reservoirs, increasingly discussed in the available literature in recent years.

Regional hydrocarbon potential and thermal reconstruction of the Lower Jurassic to lowermost Cretaceous source rocks in the Danish Central Graben

The Danish part of the Central Graben (DCG) is one of the petroliferous basins in the offshore region of north-western Europe. The source rock quality and maturity is here reviewed, based on 5556 Rock-Eval analyses and total organic carbon (TOC) measurements from 78 wells and 1175 vitrinite reflectance (VR) measurement from 55 wells, which makes this study the most comprehensive to date. The thermal maturity is evaluated through 1-D basin modelling of 46 wells. Statistical parameters describ-ing the distribution of TOC, hydrocarbon index (HI) and Tmax are presented for the Lower Jurassic marine Fjerritslev Formation, the Middle Jurassic terrestrial-paralic Bryne, Lulu, and Middle Graben Formations and the Upper Jurassic to lowermost Cretaceous marine Lola and Farsund Formations in six areas in the DCG. For the Farsund Formation the source-rock richness is presented for selected stratigraphic sequences. The upper part of the Farsund Formation is immature in the southern part of the Salt Dome Province, and late oil mature in and near the Tail End Graben and in the Søgne Basin. The lower part of the Farsund Formation is immature in local areas, yet post-mature in the Tail End Graben and in the Salt Dome Province. The Lower and Middle Jurassic shales are gas-prone in most of the DCG. The depth of the oil window, as defined by a VR of 0.6% Ro, ranges between 2200 and 4500 m. The variations are ascribed to heat flow differences in the DCG and can be modelled by a simple depth model, which includes the thickness of the Cretaceous to Palaeo-gene Chalk and Cromer Knoll Groups. According to the model, a thick Chalk Group offsets the oil window to deeper levels, which likely can be attributed to the thermal properties of the highly thermally conductive chalk compared to the underlying less thermally conductive clays. The DCG is an overpressured basin, and high-pressure, high-temperature conditions are expected to occur deeper than 3.8 km except for the Feda and Gertrud Grabens where such conditions, due to generally lower tem-peratures, are expected to occur deeper than around 4.7 km.

Organic metamorphism as a key for reconstructing tectonic processes: a case study from the Austroalpine unit (Eastern Alps)

At the northwestern margin of the Gurktal Alps (Eastern Alps), Eoalpine (Cretaceous) thrusting of carbonaceous material (CM) bearing metasediments formed a very low- to low-grade metamorphic nappe stack above higher-grade metamorphic basement nappes. Sedimentary burial as well as progressive metamorphism transformed the enclosed CM to anthracite, metaanthracite and semigraphite. In a kinematically well-constrained section at the northwestern frontal margin of the nappe stack, this transformation has been investigated by vitrinite reflectance measurements and Raman spectroscopy of carbonaceous materials (RSCM). Automated, interactive fitting of Raman spectra estimates the metamorphic peak temperatures in a complete section through the upper part of the Upper Austroalpine unit. A RSCM trend indicates a temperature profile of ca. 250–600 °C. The top part of the gradient is reconstructed by one-dimensional thermal modeling. The certainty of ca. ± 25 °C at a confidence level of 0.9 resembles the data variability within a sample location. Due to the large calibration range, the method is able to reconstruct a thermal crustal profile in space and time. The study highlights the versatility of RSCM, which characterizes almost 250 Ma of a complex and polyphase tectonic history. RSCM data characterize the Variscan metamorphic grade in nappes now imbricated in the Eoalpine nappe stack. They additionally constrain a numerical model which emphasizes the significance of an increased thermal gradient in a continental margin towards the western Neotethyan ocean during Permo-Triassic lithospheric extension. It finally characterizes the Eoalpine metamorphic gradient during nappe stacking and a significant metamorphic jump related to exhumation and normal faulting.

Thermal history and extension of the lithosphere in the Mannar basin and realization its hydrocarbon potential, offshore Sri Lanka

Data on the structure of the sedimentary blanket and the composition of its rocks, measurements of deep temperatures and vitrinite reflectance published in the open press were used for the numerical 1-D reconstructions of the thermal evolution of the Mannar Basin and the history of its tectonic subsidence near wells Pearl-1, Dorado North, Pseudo Mannar Deep and Barracuda. Analysis of variations in the basin's tectonic subsidence and published geophysical data on the modern depth of the MOHO boundary made it possible to numerically estimate the degree of stretching of the basin's lithosphere at different stages of its evolution. Our analysis shows that a significant part of the lithosphere extension occurred before the start of the formation of the basin's sedimentary cover. The moderate lithosphere extension with amplitude β = 1.15–1.25 accompanied the thermal activation of the basin lithosphere during the second stage of the basin rifting in the Late Cretaceous. Our modeling suggests a more moderate history of changes in the thermal regime of the basin's lithosphere and lower degree of maturity and realization of hydrocarbon potential in sedimentary sections of the Pearl-1, Dorado North, Pseudo Mannar Deep and Barracuda wells than those reconstructed numerically in recent studies of the Mannar Basin.

Polycyclic aromatic hydrocarbons in Middle Jurassic sediments of the Polish Basin provide evidence for high-temperature palaeo-wildfires

The distributions and concentrations of polycyclic aromatic hydrocarbons (PAHs) and plant-derived aromatic biomarkers were analysed in Middle Jurassic strata from central Poland. These compounds were abundant in all samples indicating the occurrence of specific palaeoflora families and the intensity of palaeo-wildfires. Moreover, calculated fusinite reflectance values revealed that the temperature of Middle Jurassic wildfires may have exceeded 950 °C, consistent with high oxygen concentrations in the atmosphere during this period. The high content of charred organic matter has an influence on certain biomarker maturity indicators based on terrestrial organic matter such as C29 20S/(S + R). A new indicator was developed, benzo[ghi]perylene/(perylene + benzo[ghi]perylene), abbreviated as BgP/(Per + BgP, high values of which are characteristic of samples in which terrestrial organic matter is mostly charred. The use of this indicator is limited to immature organic matter, since perylene disappears when vitrinite reflectance is higher than 0.7 %Ro. Measurements of vitrinite reflectance indicated that Middle Jurassic organic matter with a reflectance value < 0.51 %Ro was immature or early-mature. Juxtaposition of the modified higher plant index (mHPI) calculated from mHPI = (retene + cadalene + ip-iHMN)/(retene + cadalene + ip-iHMN + 1,3,6,7-TeMN) and BgP/(Per + BgP) showed that samples from the central part of the Polish Basin contained more charred than unburnt terrestrial organic matter, as values of BgP/(Per + BgP) > 0.5 are correlated with lower mHPI values. This is evidence for the transport of wildfire products by rivers, wind, and/or sea currents over large distances. Tracers for conifers, non-vascular plants, and fungi were identified.

Vitrinite reflectance and consolidation characteristics of the post‐middle Miocene Forearc Basin in central and eastern Boso Peninsula, central Japan: Implications for basin subsidence

AbstractTo understand the sedimentary development of the Boso Forearc Basin, central Japan, since ~ 3 Ma, we investigated paleothermal structure and consolidation trends in the central and eastern parts of the forearc basin through vitrinite reflectance measurements and consolidation tests. Vitrinite reflectance (Rm) was in the range 0.33 % to 0.61 % for the Miura Group in the central part of the forearc basin and 0.34 % to 0.41 % for the Miura and Kazusa Groups in the eastern. These values suggest a roughly uniform vitrinite reflectance for the Miura Group from the central to eastern parts. No significant vitrinite reflectance difference is observed across the ~ 3 Ma Kurotaki Unconformity in the eastern part of the basin. The consolidation yield stress (pc) was calculated as 27.5 MPa and 32.2 MPa for the Kiyosumi and Amatsu Formations of the Miura Group in the eastern part, respectively. Both the pc values are consistent quantitatively with represent the trend of the maximum overburden pressure estimated from the thickness and density of overlying sediments, and the difference in pc is expected by the maximum burial depths of the strata at the sampling localities. Values of pc in the eastern part of the basin increase with thickness of overlying sediment, showing no break across the Kurotaki Unconformity. Considering the eroded thickness of the Miura Group, the continuous trends in vitrinite reflectance and consolidation between the Miura and Kazusa Groups in the eastern part reflect the greater deposition of the eastern part of the Boso Forearc Basin since ~ 2.3 Ma.

Burial and thermal history of the Upper Silesian Coal Basin (Poland) constrained by maturity modelling – implications for coalification and natural gas generation

Maturity modelling was carried out using basin and petroleum system modelling (BPSM) software in the lithologic sections of 17 wells of the Upper Silesian Coal Basin (Poland). The best fit between calculated and measured vitrinite reflectance (VR), porosity and density data was obtained by applying a thickness of eroded sedimentary overburden from 1700 m in the east to 4500 m in the west and relatively low to moderate heat flow values during the maximum late Carboniferous burial. These heat flow values were in the range 50–71 mW/m 2 , most likely owing to rapid deposition of molasse sediment that led to a downward deflection of the isotherms. The upper Carboniferous strata were heated to temperatures in the range c. 90–170 °C, which is in accordance with the moderate to high levels of thermal maturity of these rocks (c. 0.6–1.7%VR). The coal rank pattern was reached before the Variscan tectonic inversion at the transition between the Carboniferous and the Permian (c. 300 Ma). This coalification level was not overprinted by any later thermal processes. The coalification resulted in the generation of natural gases. The upper Carboniferous strata are characterized by early to late phases of hydrocarbon generation. The kerogen transformation ratio (c. 5–75%TR) values vary across the basin. Most coals generated significant amounts of hydrocarbons, which reached over 80 mg of methane per gram of total organic carbon. Although most hydrocarbons generated most likely were lost during the intense post-Variscan exhumation, the hydrocarbon potential of the basin is still substantial.

Correlation of Maturity Parameters Derived from Methylphenanthrenes and Methyldibenzothiophenes in the Carboniferous Source Rocks from Qaidam Basin, NW China

Twenty-one core samples from the Carboniferous Keluke Formation in the Qaidam basin, NW China, have been geochemically characterized to investigate thermal maturation influence on the evolution behaviors of aromatic hydrocarbons and the validity of commonly used maturity parameters. The Keluke Formation was deposited in marine to continental transitional facies and dominated by type III kerogen. Rock-Eval Tmax and vitrinite reflectance (%Ro) measurement suggested that the studied samples are highly matured at peak oil to gas condensate generation stages. Most biomarkers lost their sensitivity to indicate maturity level due to either approaching the equilibrium point or too low concentrations, while isomer distributions in alkylnaphthalenes, alkylphenanthrenes, and alkyldibenzothiophenes still show systematic variations with increasing maturity. The present study focused on the maturity parameters derived from methylphenanthrenes (MP) and methyldibenzothiophenes (MDBT). The most widely used methylphenanthrene index 1 (MPI‐1=1.5×2‐MP+3‐MP/P+9‐MP+1‐MP) shows no correlation with known maturity indicators, but the methylphenanthrene ratio (MPR=2‐MP/1‐MP) and methyldibenzothiophenes ratio (MDR=4‐MDBT/1‐MDBT) increase steadily with increasing maturity levels and are proved to be valid maturity parameters. However, empirical vitrinite reflectance estimations derived from MPR and MDR have dramatically overestimated the maturity levels. Our quantitative data illustrated that concentrations of thermally stable isomers (3-MP, 2-MP, and 4-MDBT) increase continuously with increasing maturity while thermally unstable isomers (9-MP, 1-MP, and 1-MDBT) are almost invariable in the studied maturity range. The invalidity of MPI-1 is caused by the involvement of phenanthrene in the maturity parameter formula possibly due to a variable degree of alkylation. Dealkylation of methylphenanthrenes to form parent phenanthrene occurs much earlier than Ro of 1.35% reported in the literature. The increment of MPR and MDR values with maturity levels is mainly caused by different generation rates with a higher generation rate of thermally stable isomer than thermally unstable counterpart rather than isomerization between them. Caution should be taken when empirical formula published in literature based on commonly used maturity parameters is directly applied for maturity estimation as no universal applicable correlation is likely available.

Testing reproducibility of vitrinite and solid bitumen reflectance measurements in North American unconventional source-rock reservoir petroleum systems

Abstract An interlaboratory study (ILS) was conducted to test reproducibility of vitrinite and solid bitumen reflectance measurements in six mudrock samples from United States unconventional source-rock reservoir petroleum systems. Samples selected from the Marcellus, Haynesville, Eagle Ford, Barnett, Bakken and Woodford are representative of resource plays currently under exploitation in North America. All samples are from marine depositional environments, are thermally mature (Tmax >445 °C) and have moderate to high organic matter content (2.9–11.6 wt% TOC). Their organic matter is dominated by solid bitumen, which contains intraparticle nano-porosity. Visual evaluation of organic nano-porosity (pore sizes 1.0 produced lowest R values, generally ≤0.5% (absolute reflectance), similar to a prior ILS for similar samples. Other traditional approaches to outlier removal (outside mean ± 1.5*interquartile range and outside F10 to F90 percentile range) also produced similar R values. Standard deviation values

Quantitative characterization of low-rank coal reservoirs in the southern Junggar Basin, NW China: Implications for pore structure evolution around the first coalification jump

Abstract The assessment of coalbed methane (CBM) reservoirs can provide insights into the evolution of the pore structure systems around the first coalification jump (FCJ). The southern Junggar Basin, a typical low-rank coal-bearing basin in northwestern China, has been of interest to scientists and policy-makers due to its abundance of CBM resources and favourable conditions for CBM enrichment. A total of 28 coal samples from the Middle Jurassic Xishanyao Formation collected from 11 sampling sites in the central segment of the southern Junggar Basin were analyzed in this study. These samples were quantitatively characterized and comprehensively evaluated using macroscopic to nanometer-scale methods including macroscopic coal description, proximate analysis, vitrinite reflectance measurements, maceral analysis, low pressure nitrogen adsorption (LPNA), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP) and nuclear magnetic resonance (NMR). The results show that the Xishanyao low-rank coals are characterized by low moisture contents, low or low-medium ash yields (eastern and western parts), high volatile matter contents, moderate vitrinite contents, and high inertinite contents. The physical property of the Xishanyao coals generally displays heterogeneity, with the specific surface area, the total pore volume, the air permeability and the transverse relaxation time (T2) ranging from 0.116 to 15.983 m2/g, 0.461 × 10−3 to 13.37 × 10−3 cm3/g, 0.001 to 151.66 mD (0.987 × 10−6 to 0.149 μm2) and 1.44–174.22 ms, respectively. A new parameter, namely micropore volume proportion per unit nano-meter, is proposed. Micropore and transition pore volume per nm, specific surface area, total pore volume, DN1 and DN2 (LPNA fractal dimension) all first increase, then decrease and finally increase with rising coal rank, while the maximum and minimum values of the maximum vitrinite reflectance (Ro, max) occur at 0.58–0.59% and 0.65–0.66%. The boundary of Ro, max corresponding to the FCJ can be further defined from 0.5-0.6% to 0.58–0.59% based on the evolution of adsorption pore structures. Thus, pore structure parameters related to adsorption capacity are controlled predominantly by coal rank. In addition, tectonic deformation has a strong impact on seepage pore structure but only a slight impact on that of adsorption pore, which leads to an ambiguous correlation or a complex trend between mesopore, macropore content, DMIP (fractal dimension of MIP) and coal rank around the FCJ. This study reveals the complex variation of pore structure evolution of low-rank coals and the significant influence of the FCJ on physical properties of coalbed methane reservoirs.

Burial and thermal history modelling of the Upper Carboniferous strata based on vitrinite refl ectance data from Bzie-Dębina-60 borehole (Upper Silesian Coal Basin, southern Poland)

Integrated modeling of the coal rank in the Upper Carboniferous sediments of the BzieDebina-60 borehole from SW of the Upper Silesian Coal Basin was performed based on a new vitrinite reflectance measurements from the Westphalian A. Microscopic investigations of Westphalian samples in the Bzie-Debina-60 well have shown that organic matter is dominated by vitrinite maceral group. The coal rank in the well shows an increase with depth. The mean vitrinite reflectance values in these samples range from 0.76 to 1.07 %. One-dimensional forward modelling of thermal maturity in the Bzie-Debina-60 borehole section has been performed in order to explain the burial and thermal history. The maximum temperature (c. 145 °C) causing the coalification occurred in the Latest Carboniferous (c. 300 Ma). These processes were likely related to signifi cant late Variscan burial heating in this area as indicated by previous authors.

Evaluation of the Gas Content in Archived Shale Samples: A Carbon Isotope Study

We examined 14 archived samples of shale for the chemical and 13C isotopic composition of residual gases produced as part of rock-crushing operations at a hammer mill. Results were compared with data on maturity from Rock-Eval pyrolysis and vitrinite reflectance measurements. The samples originated from three different formations (Mikulov Marls, Ostrava Formation, and Liteň Formation) located in the Czech Republic. For comparison, we examined a gas-prone shale sample from the Polish Silurian. We used changes in the chemical and isotopic composition of released gases to evaluate the isotope fractionation during gas loss and retroactively calculated the initial content of gas in the shale samples. The gas content estimates (in L of gas per ton of rock) correspond with the maturity parameters of the shales. Calculated isotope fractionation for the gas release was −3‰ for both methane and ethane. The archived samples primarily lost methane (up to 90%), with subsequent changes in the content of ethane and higher hydrocarbon levels.

Elastic anisotropy and its influencing factors in organic-rich marine shale of southern China

Shale is observed to have strong anisotropy due to its unique mineralogy and microstructure, and this anisotropy property has significant impact on seismic and well-log data. The organic-rich marine shale in the southern and eastern Sichuan Basin is one of the most important shale-gas reservoir formations in China. To investigate the elastic anisotropy of this shale and its influencing factors, we performed ultrasonic velocity measurements, X-ray diffraction analysis, rock-eval pyrolysis and vitrinite reflectance measurement on the samples from the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation. The experimental results show the that: (1) the velocity anisotropy of the Wufeng-Longmaxi (WL) shale varies from 10% to 50%, and most samples have strong anisotropy; (2) the P- and S-wave anisotropy parameters (Thomsen’s e and γ) increase with clay contents, but this relationship can be greatly affected by the clay orientation index; (3) organic matter content (OMC) is found to have little influence in seismic anisotropy for the over mature WL shale, whereas the OMC determines the magnitude of anisotropy of immature/mature shales (e.g. the Bakken shale or the Bazhenov shale) according to the published literatures, because organic matters in shales of different maturity have different morphologies and distributions; (4) the OMC of WL shale has positive correlation with quartz content, and this weakens the correlation between OMC and the magnitude of anisotropy to a certain extent. The results of this study provide an important rock-physics basis and data support for seismic anisotropy exploration, quantitative interpretation and resource evaluation of the organic-rich marine shales in southern China.

Gas-generation potential of shales in small and medium-sized basins: a case study from the Xuanhua Basin, north China

Small- and medium-sized basins are widely distributed, and some contain commercial gas reservoirs demonstrating their gas-generation potential. The Xuanhua Basin, which is a small-sized coal-bearing basin in north China, includes a promising target for shale-gas exploration in the Xiahuayuan Formation. In this study, we used this basin as a case study to assess the critical geochemical features for small or medium-sized basins to form commercial gas reservoirs. Total organic carbon (TOC) analysis, Rock-Eval pyrolysis, microscopic observation of macerals, vitrinite reflectance measurement and kerogen stable carbon isotope analysis were performed to characterise the organic geochemistry of the Xiahuayuan shales. The original total organic carbon (TOCo) content and hydrocarbon-generative potential (S2o) were reconstructed to further evaluate the gas-generation potential of these shales. In addition, geochemical data of shales from other similar-sized basins with gas discoveries were compared. The results showed that the kerogen from the Xiahuayuan Formation is Type III (gas-prone), and macerals are dominated by vitrinite. TOC values showed a strong heterogeneity in the vertical profiles, with most higher than 1.5 wt%. The measured Ro values ranged from 1.4 to 2.0%. However, thermal maturity was not correlated with the present-day burial depth with higher maturity in the wells closest to the diabase intrusion centre. The remaining generation potential (S2) averaged 0.91 mg HC/g rock, equal to 1.4 cm3 CH4/g rock, and the average amount of hydrocarbon generated was 4.33 cm3 CH4/g rock. In small and medium-sized basins, the TOC content of commercially developed gas shales ranged from 0.5 to 2.5 wt%, organic matter was mainly humic (gas-prone), and the burial depth was generally shallow. Biogenic gas reservoirs for commercial exploitation tend to have larger shale thicknesses (120–800 m) than thermogenic gas reservoirs (60–90 m).The Xiahuayuan Formation is a good gas-source rock with gas-prone kerogen type, relatively high TOC values and moderate thermal maturity.The average amount of hydrocarbon generated from the Xiahuayuan shales is about 4.33 cm3 CH4/g rock, indicating a potential to form a shale gas reservoir.Owing to the influence of diabase intrusions, the Xiahuayuan shales have entered the dry gas window at relatively shallow-buried depths.Small- and medium-sized basins have the potential to generate commercial gas reservoirs with the generated volume mainly a product of the thickness and maturity of black shales.

THE OLIGOCENE–MIOCENE MENILITE FORMATION IN THE UKRAINIAN CARPATHIANS: A WORLD‐CLASS SOURCE ROCK

This study investigates the hydrocarbon potential of Oligocene–Miocene shales in the Menilite Formation, the main source rock in the Ukrainian Carpathians. The study is based on the analysis of 233 samples collected from outcrops along the Chechva River in western Ukraine in order to analyse bulk parameters (TOC, Rock‐Eval), biomarkers and maceral composition.In Ukraine, the Menilite Formation is conventionally divided into Lower (Lower Oligocene), Middle (Upper Oligocene) and Upper (Lower Miocene) Members. The Early Oligocene and Early Miocene ages of the lower and upper members are confirmed by new nannoplankton data. The Lower Menilite Member is approximately 330 m thick in the study area and contains numerous chert beds and turbidite sandstones in its lower part together with organic‐rich black shales. The shales have a high content of silica which was probably derived from siliceous micro‐organisms. The TOC content of the shales frequently exceeds 20 wt.% and averages 9.76 wt.%. HI values range between 600 and 300 mgHC/gTOC (max. 800 mgHC/gTOC). The Middle Member contains thin black shale intervals but was not studied in detail. The Upper Member is about 1300 m thick in the study area and is composed mainly of organic‐rich shales. Chert layers are present near the base of the Member, and a prominent tuff horizon in the upper part represents a volcanic phase during shale deposition. The member grades into overlying molasse sediments. The average TOC content of the Upper Menilite succession is 5.17 wt.% but exceeds 20 wt.% near its base.Low Tmax and vitrinite reflectance measurements for the Lower (419°C and 0.24–0.34 %Rr, respectively) and Upper (425°C and 0.26–0.32 %Rr, respectively) Menilite Member successions indicate thermal immaturity. Biomarker and maceral data suggest a dominantly marine (Type II) organic matter input mixed with varying amounts of land‐plant derived material, and indicate varying redox and salinity conditions during deposition.Determination of the Source Potential Index (SPI) shows that the Menilite Formation in the study area has the potential to generate up to 74.5 tons of hydrocarbons per m2. The Chechva River outcrops therefore appear to have a significantly higher generation potential than other source rocks in the Paratethys realm. These very high SPI values for the Menilite Formation may explain why a relatively small area in Ukraine hosts about 70% of the known hydrocarbon reserves in the northern and eastern Carpathian fold‐thrust belt.

Upper Jurassic-Lower Cretaceous Source-Rock Evaluation and Oil—Source Rock Correlation in the Abadan Plain, Southwest Iran

Eight oil samples from Lower Cretaceous Fahliyan reservoirs and 51 core and cutting samples from Upper Jurassic-Lower Cretaceous Garau Formation in Abadan Plain, Southern Iran, were analyzed using bulk property and molecular methods in order to identify their geochemical characteristics such as maturity and source depositional environment, as well as to correlate them to respective source rock(s). Rock-Eval pyrolysis and vitrinite reflectance measurements indicated that the Garau Formation, classified as a good-to-excellent source rock, the kerogen is mainly of Type II and the maturity is in peak oil generation. The crude oils of the Fahliyan reservoir are classified as light oil (API more than 35), non-biodegraded, saturate fraction, slight even/odd predominance, and front-end biased distribution in gas chromatogram. According to molecular and isotopic data, the oils were generated by a marine carbonate source rock which was deposited under the anoxic condition. Furthermore, all the studied samples reveal compositional similarity and hence can be assigned to one oil family, originating from a common source rock. Based on compositional similarities of biomarkers and isotope data in the studied crude oils and source rock, the Upper Jurassic-Lower Cretaceous Garau Formation can be regarded as the main source rock of the oils.

Shale gas content evolution in the Ordos Basin

Gas content is a crucial parameter for shale reservoir evaluation and shale gas accumulation. Taking Shanxi and Taiyuan Formation shale samples from the Ordos Basin of China as an example, this research used thermal simulation, Rock-Eval pyrolysis, total organic carbon content (TOC) measurement, vitrinite reflectance (Ro) measurement, X-ray diffraction analysis, and methane isothermal adsorption based on formation conditions to study the evolution of shale gas content. A thermogenic gas content model was established through thermal simulation, Rock-Eval pyrolysis, and Ro measurement, while the influential factors of adsorbed gas content were analysed and a model was established through TOC and Ro measurements, X-ray diffraction analysis, methane isothermal adsorption, and reservoir conditions. Then, the research characterized the evolution of thermogenic gas, gas-in-place (GIP), adsorbed gas, and free gas through geologic time using a combination of thermogenic gas content, hydrocarbon expulsion and adsorbed gas content models.

Vitrinite Maturation in Confined Pyrolysis Experiments

Laboratory simulation (pyrolysis) is an important approach for the study of petroleum formation and evolution. It is useful to determine the maturity levels under experimental temperature and time conditions. Mean random vitrinite reflectance (%Ro) is a parameter widely used to demonstrate source rock maturity. Confined pyrolysis experiments were conducted for simulating vitrinite maturation on six coal samples with initial hydrogen indices (HI) ranging from 155 to 259 mg HC/g TOC at 2 and 20 °C/h heating rates. Coal JC24 having higher initial HI (259 mg HC/g TOC) has relatively lower measured vitrinite reflectance (%Ro) compared with that of coals JC23 and JC25 having lower initial HI (i.e., 171 and 155 mg HC/g TOC, respectively), at the same thermal stress levels. For all six coals, the measured %Ro is lower compared to the predicted EASY%Ro at >1.5 EASY%Ro. The differences between the measured %Ro and calculated EASY%Ro are substantially greater at 20 °C/h compared to 2 °C/h at the same thermal stress ...

Palaeomagnetic dating of hydrothermal alteration in the Woodford Shale, Oklahoma, USA

AbstractAn oriented Woodford Shale core from the eastern Ardmore Basin was sampled to test if the shale was an open or closed system to hydrothermal fluids, and to determine the timing of alteration. Mineralized fractures are ubiquitous in the core, and the shale exhibits a complex paragenesis with multiple hydrothermal minerals, including biotite, magnesite, norsethite, gorceixite and potassium feldspar present in and around the fractures. These minerals suggest that the Woodford Shale was an open system during part of its diagenetic history. Vitrinite reflectance (Ro) measurements indicate values of ∼1.81 % (∼230 °C). Palaeomagnetic analysis reveals a characteristic remanent magnetization (ChRM) with south-southeasterly declinations and shallow inclinations that resides in magnetite. This ChRM is interpreted to be either a chemical remanent magnetization (CRM) or a thermochemical remanent magnetization (TCRM) that was acquired at 245 ± 10 Ma during Late Permian time based on the pole position (51.0° N, 115.6° E). Because the palaeomagnetic specimens show evidence of extensive hydrothermal alteration, the CRM/TCRM is interpreted to date the migration of hydrothermal fluids through the shale. The agreement in timing with other studies that report hydrothermal alteration in southern Oklahoma and the Ouachita Mountains in Late Permian time, suggest that there were post-collisional fluid-flow events which accessed reservoirs of warm fluids.

Properties of solid bitumen and other organic matter from Oligocene shales of the Fore-Magura Unit in Polish Outer Carpathians: Microscopic and geochemical approach

In this study, we apply microscopic and geochemical (Rock-Eval) methods to evaluation of both the origin and maturity of solid bitumen and other organic matter dispersed in Oligocene shales from the Skrzydlna Tectonic Bay and the Mszana Dolna Tectonic Window of the Fore Magura Unit, Outer Carpathians, Poland. Homogenous solid bitumen occurs in early mature and mature samples of rocks from the Skrzydlna Tectonic Bay (vitrinite reflectance VR from 0.60 to 0.80%). It reaches reflectance values (BR) from 0.31 to 0.61% and is interpreted as the intermediate product between kerogen and oil originated during the oil generation process. In the late mature and post-mature samples from the Mszana Dolna Tectonic Window (VR from 1.10 to 1.49%), homogenous solid bitumen reflectance varies from 1.45 to 1.76%. This is an indigenous, thermally altered solid bitumen or residue after hydrocarbon generation, hence, it should be called the pyrobitumen. In samples from both locations, granular solid bitumen was found, as well, of reflectance values (GBR) from 0.25 to 0.92%.A very strong correlation of homogenous solid bitumen, pyrobitumen and granular solid bitumen reflectances with vitrinite reflectance was found in studied samples. When measured in the same sample, the solid bitumen from the Skrzydlna Tectonic Bay always showed lower reflectance values than the coexisting vitrinite. In samples from the Mszana Dolna Tectonic Window, in which measured vitrinite reflectance VR was always above 1,0%, the, observed pyrobitumen invariably showed the higher reflectance values whereas the granular solid bitumen consistently revealed lower reflectance than vitrinite, homogenous solid bitumen or pyrobitumen coexisting in the same sample. The Rock-Eval analyses confirmed that bitumen is indigenous in all studied samples. The degree of thermal maturation of organic matter based on vitrinite reflectance measurements was equivalent to organic matter maturity determined by the Rock-Eval Tmax values, which could be confirmation that our microscopic observations and selection of vitrinite fragments were correct.We were able to perform more individual measurements of homogenous solid bitumen particles and we obtained lower average standard deviation for BR values than for VR values. Due to the dark vitrinite abundant in samples taken in the Skrzydlna Tectonic Bay and the small-size vitrinite particles seen in samples from the Mszana Dolna Tectonic Window we found it difficult to select proper vitrinite particles for microscopic examinations. By contrast, both the solid bitumen and the pyrobitumen particles were often easier to recognize due to their characteristic shape and larger size than those of vitrinite fragments. For these set of samples we conclude that reflectance measurements run on homogenous solid bitumen are more reliable in comparison with those performed on vitrinite.