Thermal Maturity Of Organic Matter Research Articles

Aspects of palyno-organic facies analysis: Comprehensive evaluation of the source rocks and age of some subsurface Lower Cretaceous core materials in the north Western Desert, Egypt

Abstract The palyno-organic facies aspects of the subsurface Lower Cretaceous core samples are extensively investigated in three wells in the northern part of the Western Desert of Egypt. These wells are Burg El Arab-1 and Mamura-1, Mersa Matruh-1 (BAX-1, MAX-1 and MMX-1 respectively). The study is based on the palynofacies and organic geochemical analyses. The palynofacies examination includes a statistical analysis of the different classes of organic particles by cluster analysis and principal component analysis (PCA). This analysis leads to identify 4 palynofacies types (A, B, C1 and C2) which are dominated by various levels of terrestrial floral materials. The paleoenvironmental settings that are attributed to these palynofacies types are the distal suboxic-oxic, proximal oxic, transitional oxic and highly distal oxic. The principal component analysis (PCA) differentiates between these palynofacies types mathematically by the biplot of components 1 and 2. The component 1 corresponds to distance from the terrestrial floral sources. The identification of quantity, type, quality and thermal maturity of organic matter is discussed depending on the palynofacies and organic geochemical analyses. The thermal maturity is assessed digitally by measuring the RGB color channels in the spore images and their corresponding thermal alteration index (TAI incorporated with values of Tmax). These measures are integrated in burial history models with geothermal gradient and depositional history. The study reveals that most of the core samples represent generally fair to good enriched sources of gas prone type III kerogen and less mixed gas-oil type II/III kerogen. Additionally, some occasional high levels of organic conversions are recorded. The burial history models indicate that oil expulsion window initiated in the Cretaceous. The examinations indicate mature oil sources in the Alam El Bueib Formation in MMX-1 well. The organic geochemical parameters are correlated versus the component 1 and sum of all components of PCA. The correlation coefficients (R2) pointed to good correlation between the oxygen index (OI), TOC and S2 and lower correlation with hydrogen index (HI).

High microscale variability in Raman thermal maturity estimates from shale organic matter

Raman spectroscopy has recently received attention as a means to estimate thermal maturity of organic matter in petroleum generating source rocks to complement more traditional approaches such as vitrinite reflectance and programmed pyrolysis. While many studies have observed positive correlations between source rock thermal maturity and Raman spectral parameters, little attention has been given to the degree of variation in the Raman response across individual organic grains, especially for shales or mudrocks with highly dispersed organic matter. Here the spatial variation in Raman estimates of thermal maturity within individual organic grains is assessed from shales from the Boquillas, Marcellus, Niobrara, and Woodford Formations. The thermal maturity parameters extracted from Raman spectra can vary widely across distances of ≤5 μm within the same organic grain. These results illustrate the high degree of chemical heterogeneity inherent to the organic matter within these source rocks. Additionally, the spatial pattern of the Raman parameters, as revealed by 2D Raman mapping, imply that organic matter structure is influenced by associations with mineral surfaces within the surrounding rock matrix. Chemical heterogeneity and matrix effects directly impact the Raman response from these types of materials and thus the extracted thermal maturity estimate. These findings highlight the care which must be adopted when making Raman measurements of organic matter within source rock matrices, especially for samples which feature highly dispersed, heterogeneous organic matter as found in petroliferous mudrocks.

Biomarkers, Tmax and vitrinite reflectance as organic matter thermal maturity indices in sedimentary rocks – correlations and scope of applicability

Biomarkers, Tmax and vitrinite reflectance as organic matter thermal maturity indices in sedimentary rocks – correlations and scope of applicability

Geochemical impacts of hydrothermal activity on surface deposits at the Southwest Indian Ridge

Submarine hydrothermal circulation has attracted much scientific interest since seafloor hydrothermal activity was first observed in the 1970s; an area of particular interest is the impact of exported inorganic and organic materials from hydrothermal vent systems into the open ocean. In 2007, the first active hydrothermal vent field, with vent fluid temperatures up to 379 °C, was discovered at the ultraslow spreading Southwest Indian Ridge (SWIR), where active vents are much less abundant than fast spreading ridges, and the effect of hydrothermal extrusion on surface sediments is not fully understood. To explore how geochemical proxy signatures respond to hydrothermal activity, we investigated the distributions of elements, minerals and lipids in surficial normal marine sediments, metalliferous sediments and low-temperature hydrothermal deposits collected from the SWIR. The results showed different effects of hydrothermal activity on the surface deposits. The normal marine sediments were predominantly calcium carbonate characterized by > 42% CaO and > 90% calcite, with a significant autochthonous marine contribution to organic matter (OM) and a predominance of lower molecular weight alkanols and fatty acids; they were uninfluenced by hydrothermal activity but received some terrigenous input represented by abundant high molecular weight n-alkanes with an odd-over-even predominance. The near-field metalliferous sediments and hydrothermal deposits were very different. Some near-field metalliferous sediments were influenced by low-temperature hydrothermal activity, and their distributions of elements and minerals were similar to those of hydrothermal deposits, which were characterized by abundant Fe/Si and opal/nontronite. Other near-field metalliferous sediments were evidently influenced by mixing of high-temperature hydrothermal sulfides typically containing abundant Cu/Zn. With respect to the organic matter assemblages, near-field deposits contained little evidence for thermal maturation of organic matter and all were characterized by a strong microbial signature, including hopanoids, isoprenoidal and non-isoprenoidal dialkyl glycerol ether lipids, and low molecular weight n-alkanes with an even carbon number predominance. The far-field metalliferous sediments, despite the influence of non-buoyant plumes and slightly higher concentrations of hydrothermal-derived metals (e.g., Fe, Cu and Zn), had the same distribution of organic lipids and major mineral composition (> 90% calcite) as did normal marine sediments. Thus, the influence of non-buoyant plume inputs appears to have been minimal possibly due to the dilution of in situ microorganisms by normal marine organisms in sediment and seawater. Furthermore, these characteristics indicate inorganic indices based on abundant metal elements derived from the hydrothermal systems (such as Fe/Cu/Zn content, ∑REE/Fe, the ternary diagram of Fe, Cu×100 and Ca) are more sensitive, serving as better proxies than organic matter assemblages to differentiate the effects of diverse hydrothermal activity on surface deposits.

Assessing low-maturity organic matter in shales using Raman spectroscopy: Effects of sample preparation and operating procedure

Laser Raman spectroscopy is used to assess the thermal maturity of organic matter in sedimentary rocks, particularly organic-rich mudstones. However, discrepancies exist between quantified Raman spectral parameters and maturity values obtained by vitrinite reflectance. This has prevented the adoption of a standard protocol for the determination of thermal maturity of organic matter (OM) by Raman spectroscopy. We have examined the factors influencing the Raman spectra obtained from low-maturity OM in potential shale gas reservoir rocks. The inconsistencies in Raman results obtained are due to three main factors that are critically evaluated: (1) different operational procedures, including experiment setup and spectral processing methods; (2) different methods of sample preparation; (3) the analysis of diverse types of OM. These factors are scrutinized to determine the sources of inconsistency and potential bias in Raman results, and guidance is offered on the development of robust and reproducible analytical protocols. We present two new Raman parameters for un-deconvolved spectra named the DA1/GA ratio (area ratio of 1100–1400 cm−1/1550–1650 cm−1) and SSA (scaled spectrum area: sum of total area between 1100 and 1700 cm−1) that offer potential maturity proxies. An automated spreadsheet procedure is presented that processes raw Raman spectra and calculates several of the most commonly used Raman parameters, including the two new variables.

Stratiform siderites from the Mesoproterozoic Xiamaling Formation in North China: Genesis and environmental implications

Stratiform or layered siderites are much less common than siderite concretions in the stratigraphic record. In the middle part of the Mesoproterozoic Xiamaling Formation (ca. 1.40–1.35Ga), layered siderites are widespread across the North China Platform. Facies analysis indicates that the Xiamaling siderites were formed in shallow-water environments above the storm wave base. High Fe/Al and V/Al ratios and the lack of pyrites in siderite-bearing strata indicate ferruginous bottom-water conditions during the formation of siderites. The low thermal maturity of organic matter and clay mineral structures suggest that the layered siderites were formed at burial temperatures ≤90°C. The negative δ13C values (−24‰ to −12‰) of siderites and the presence of μm-scale hematite inclusions in siderite grains suggest that dissimilatory iron reduction (DIR) during early diagenesis has played critical roles in siderite formation. The source of Fe-oxyhydroxides required for DIR could be from Fe(II) oxidation in shallow seawater above the redoxcline in response to enhanced hydrothermal activities or, alternatively, directly from continental Fe-oxide input. The positive cerium (Ce) and europium (Eu) anomalies of clay minerals in siderite-bearing strata and their positive correlation with total Fe contents (TFe2O3) seem to favor a hydrothermal origin of iron. Syndepositional hyalophane concretions, intermittent tuff layers, and remarkable enrichment of Cu and Zn in some of the siderite-hosting rocks provide further support to hydrothermally sourced iron. The enhanced hydrothermal activity for the development of stratiform siderites in the Xiamaling Formation probably related to the final breakup of the North China Craton from the Columbia supercontinent, during which active hydrothermal systems provided sufficient Fe(II) for Fe-redox recycling and formation of layered siderites.

Thermal maturity assessment study of the late Pliensbachian-early Toarcian organic-rich sediments in southern France: Grands Causses, Quercy and Pyrenean basins

Thermal maturity of late Pliensbachian-early Toarcian organic-rich sediments in the Grands Causses (Suèges section), Quercy (Caylus section) and Pyrenean (Pont de Suert section) basins was determined through multiple parameters, including Spore Coloration Index (SCI), hydroid random reflectance (HRr) and spectral fluorescence of Tasmanites algae (λmax). The main objective of this study is to test the effectiveness and make comparisons of organic matter thermal maturity in these three sections by different techniques and particularly hydroid reflectance.For the Suèges section SCI presented a value of 3.5–4.0. HRr ranges between 0.36% and 0.47%, which corresponds to 0.45%–0.52% VReq, and shows a good correlation with SCI values. Spectral fluorescence analysis presents a λmax of 560 nm for most samples. The fluorescence spectral maximum seems to be redshifted in comparison to other thermal maturity parameters.The Caylus section exhibited SCI of 3.5–4.0. HRr ranges between 0.32% and 0.50% corresponding to 0.42%–0.54% VReq. Spectral fluorescence analysis points out a λmax of 560 nm, displaying a good correlation with SCI. When Req values determined through SCI are compared with equivalent vitrinite reflectance values determined through λmax, a redshift of the fluorescence spectrum maximum is detected.For the Pont de Suert section SCI ranges between 8.0-8.5 and 8.5–9.0. HRr varies between 0.85% and 1.18% which corresponds to 0.76%–0.98% VReq. Even though there are lower values than those obtained by SCI, VReq is reliable and acceptable to determine the thermal evolution stage. Furthermore, HRr values also present a good correlation with SCI.These data indicate that the majority of the samples from the Suèges and Caylus sections are in the immature to early mature evolution stages and Pont de Suert section samples are in late mature to early overmature evolution stages for liquid hydrocarbons generation, supporting a very similar thermal evolution for the Grands Causses and Quercy basins and an entirely different thermal history for the Pyrenean Basin. This is in accordance with the similar tectonic and sedimentary context of the Quercy and Grands Causses basins. The Pont de Suert section, located near the collision front of the Pyrenees mountains, records a more complex sedimentary and tectonic history. In addition, this study presents the first reflectance data for Jurassic hydroids, which show a good correlation with other rank parameters.

Geochemical and geological characteristics of Permian Lucaogou Formation shale of the well Ji174, Jimusar Sag, Junggar Basin, China: Implications for shale oil exploration

Abundant shale oil resources were recently found in the organic‐rich Permian Lucaogou (P2l) Formation shale, Jimusar Sag, Junggar Basin. Although some scholars have conducted some researches on the basic geochemical characteristics of the Lucaogou Formation shale, it is not enough to effectively guide the shale oil exploration in the area. This is because during the exploration process of shale oil, besides the basic geochemical characteristics of shale, its mineral composition, physical properties, and oil‐bearing characteristics are all very important evaluation parameters. To systematically evaluate the favourable shale oil exploration sections in the Lucaogou Formation, 265 pieces of shale cores were sampled from well Ji174 in this study. Besides the basic geochemical characteristics of the 265 pieces of cores, the mineralogical, petrophysical, and oil‐bearing characteristics were also analysed. Results show that the shale has abundant organic matter (average total organic carbon 3.51%, average petroleum generation potential 15.67 mg/g), dominated by Type II1 and Type II2 kerogen, and has entered the early‐mature to mature stage. The Upper Member of the Lucaogou Formation has relatively higher organic matter and organic matter type is relatively more oil prone compared to the Lower Member; however, organic matter thermal maturity is lower. The P2l Formation shale is characterized by high carbonate minerals and low clay minerals, and when considering mineralogical brittleness, both the Upper and Lower members are ideally fracturable in shale oil development. The P2l Formation shale has a wide porosity range (1.1–13.9%), and the Lower Member's porosity is slightly greater. The inversion of shale porosity in the P2l Formation is controlled by the differentiated mineral composition. The oil saturation index values of the Lower Member shale are significantly greater than the Upper Member shale. From the above, the Lower Member of the P2l Formation shale is ideally suited and has greater shale oil potential in the Jimusar Sag, Junggar Basin, especially in the depth intervals of 3,255–3,272, 3,282–3,298, and 3,317–3,334 m in the well Ji174.

Shale oil potential of lacustrine black shale in the Eocene Dongying depression: Implications for geochemistry and reservoir characteristics

The geochemistry and reservoir characteristics of the lacustrine shale in the Eocene Dongying depression are described in detail based on thin-section and field-emission–scanning electron microscope observations of well cores combined with x-ray diffraction, physical property testing, and geochemical indicators. The Eocene Shahejie (Es) Formation Es4s–Es3x shale member is predominantly carbonate, clay minerals, and quartz. Six lithofacies were identified: (1) laminated limestone (organic-rich laminated limestone and organic-poor laminated limestone), (2) laminated marl, (3) laminated calcareous mudstone, (4) laminated dolomite mudstone, (5) laminated gypsum mudstone, and (6) massive mudstone. The Es4s–Es3x shale samples from three cored wells had total organic carbon (TOC) contents in the range of 0.58 to 11.4 wt. %, with an average of 3.17 wt. %. The hydrocarbon generation potential (free hydrocarbons [S1] + the hydrocarbons cracked from kerogen [S2]) values range from 2.53 to 87.68 mg/g, with an average of 24.19 mg/g. The Es4s–Es3x shale of the Dongying depression has a high organic-matter content with very good or excellent hydrocarbon generation potential. The organic maceral composition is predominantly sapropelinite (up to 95%). The hydrogen index (being S2/TOC) versus the maximum yield temperature of pyrolysate (Tmax) indicates that the organic matter is predominantly type I kerogen, which contains a high proportion of convertible organic carbon. The Es4s–Es3x shale is thermally mature and within the oil window, with the vitrinite reflectance values ranging from 0.46% to 0.74% and the Tmax value ranging from 413°C to 450°C, with the average being 442°C. The shale contains interparticle pores, organic-matter pores, dissolution pores, intracrystalline pores, interlaminar fractures, tectonic fractures, and abnormal-pressure fractures. The primary matrix pore storage is secondary recrystallized intercrystal pores and dissolution pores that formed during thermal maturation of organic matter. The TOC content and effective thickness of the organic-rich shales are the primary factors for hydrocarbon generation. The reservoir capacity is related to the scale, abundance, and connectivity of pore spaces, which are controlled by the characteristics of the lithofacies, mineral composition, TOC content, and microfractures.

Fingerprint of hydrocarbon generation in the southern Georgina Basin, Australia, revealed by small angle neutron scattering

Small angle neutron scattering (SANS) and ultra-small angle neutron scattering (USANS) are used to directly detect the processes of hydrocarbon generation in the 10nm to 10μm size pores in Middle Cambrian carbonate and siliciclastic rocks which contain no land-plant material suitable for conventional maturity determination by vitrinite reflectance. The method takes advantage of the pore-size-specific variation of neutron scattering contrast between the solid rock matrix and pore-space content with depth, which is caused by thermal maturation of organic matter through the oil and gas generation windows. SANS and USANS measurements were performed on bedding plane-orientated core slices, extracted from a series of 10 to 12 depth intervals for three wells CKAD0001, MacIntyre 1 and Baldwin 1, in the southern Georgina Basin, central Australia. The depth intervals, intersecting the organic-rich basal ‘hot’ shales of the middle Cambrian Arthur Creek Formation, were selected based on Rock-Eval pyrolysis data. SANS and USANS results indicate that oil generation has occurred in the past in nanometer-sized pores in rocks that are now at depths of around 538.4m in CKAD0001 and 799.3m in MacIntyre 1. Furthermore, in the CKAD0001 well, the oil-wet pores extend into the larger pore-size range (at least up to 10μm) due to the efficient expulsion of oil. At around 880m in Baldwin 1, the influence of pyrobitumen reverts pore space from gas wet to oil wet. These hydrocarbons have remained in situ since the Devonian when the Neoproterozoic to Paleozoic section was exhumed in the Alice Springs Orogeny and subsequently eroded, preserving only remnants of the once extensive basin sediments.

Modified RGB-based kerogen maturation index (KMI): Correlation and calibration with classical thermal maturity indices

Thirty-eight cutting samples from the Lower Jurassic to the Upper Cretaceous succession of the Abu Tunis-1X well, north Western Desert of Egypt, were processed palynologically to extract their psilate sporomorphs. This designated practice was made to assess the digitally-quantified progressive changes in the triple Red-Green-Blue (RGB) coloration of sporomorph exine with depth. Stabilization and calibration of the illumination system was applied. Certain precautions were also implemented prior to such digital RGB measurements; for example, some exceptionally lighter (caved) or darker (reworked) specimens were excluded from RGB coloration population samples to avoid misleading results. A minimum of three and up to ten spore grains were studied according to the sample richness and two RGB readings were taken for each selected specimen. The average, standard deviation, and confidence level values for each sample were calculated due to the variable palynological productivity of the samples. In the present study, an attempt was made to monitor and distinguish which of the color components and/or parameters is the most sensitive detector of the progressive maturation changes with depth. By investigating the trends of different parameters of the RGB (i.e. total RGB, R, G, B, R/G) with depth, we were able to distinguish and deduce a linear maturation index from the triple RGB readings. Such a linear maturation index was based on the R component and was successfully correlated and calibrated with other conventional, linear maturity indices (Tmax°C, vitrinite reflectance-VRo%, and thermal alteration index-TAI). The newly modified, RGB-based maturation index called the Kerogen Maturity Index (KMI) can confidently and effectively detect the subtle and progressive changes in kerogen maturation with depth. Such correlation and calibration practices presented herein enabled us to present a modern, reliable, easy to use, and inexpensive measuring technique to assess the thermal maturity of organic matter. The KMI covers the immature and most of the mature phases (early-upper mature) of kerogen and it has its own vitrinite-calibrated cut-offs, which can be correlated with both the Tmax and TAI parameters. This undoubtedly allows the KMI to be easily correlated and complement other classical, expensive maturity indices. This opens the door widely for the palynologists and organic petrologists to use the KMI with good confidence and high reliability for its proven good maturity appraisal.

Effect of ion milling on the perceived maturity of shale samples: Implications for organic petrography and SEM analysis

Sample polishing by argon ion beam is a widely used method for examining shale samples for inherent porosity characteristics; the high quality of these surfaces suggests that this technique may also be used for optical reflectance measurements to provide information about the thermal maturity of samples. Yet, the inevitable surface heating that this polishing method engenders has raised concerns that the measured reflectance properties are no longer those of the original sample. To explore the impact of ion milling on the maturity of shale samples as measured by vitrinite and solid bitumen reflectance, five different ion milling configurations were applied to a set of organic-rich New Albany Shale (Late Devonian-Early Mississippian) samples that range in maturity from immature to post-mature. Using two ion mill designs, edge milling vs planar milling, single and dual ion beams, variable acceleration voltages, and milling at room temperature vs samples cooled by liquid nitrogen, provided a wide range of beam heating scenarios. Reflectance of macerals was measured before and after ion milling to investigate whether and to what extent various ion-milling approaches change the reflectance values, and by extension the perceived thermal maturity of organic matter in these samples. Our results demonstrate that more aggressive milling methods, such as the use of multiple beams and higher acceleration voltages elevate reflectance values, and that this effect is most pronounced in immature samples and diminishes for samples of increasing original maturity. Specifically, for the two least mature samples, the most aggressive milling method (configuration D) increased reflectance of vitrinite from 0.48% to 0.58%, and from 0.58% to 0.74%. Increase of reflectance (perceived maturity) can be counteracted by reducing beam intensity (e.g., fewer beams, lower voltage) and cooling of samples with liquid nitrogen. The severity of heating artifacts depends partially on the ion mill design, and non-damaging settings must be determined experimentally for a given ion mill model. Because thermal alteration of organic matter typically involves the expulsion of volatiles, there is also a danger that ion beam heating of immature and oil window samples can skew the porosity characteristics of shale samples. Thus, determining non-damaging ion mill settings has the dual benefit of avoiding measuring false maturity levels and misleading porosity characteristics.

Organic petrology and micro-spectroscopy of Tasmanites microfossils: Applications to kerogen transformations in the early oil window

The transformation of kerogen to hydrocarbons in the early stages of oil generation is critical for understanding the resource potential of liquid-rich shale plays. Organic petrology commonly is used for visual evaluation of type, quality, and thermal maturity of organic matter, but the relationship of visual petrographic changes to chemical transformations is not well characterized. To improve understanding of these processes, organic-walled microfossils of the unicellular green alga Tasmanites (composed of algaenan) in Upper Devonian Ohio Shale (Huron Member, Appalachian Basin) were analyzed by micro-spectroscopy techniques including micro-Fourier transform infrared (micro-FTIR), X-ray photoelectron (XPS), electron probe microanalysis (EPMA), and fluorescence. Immature to mid-oil window maturation sequences of core and outcrop samples with solid bitumen reflectance (BR) and vitrinite reflectance (VR) values ranging from 0.45 to 0.80%Ro were used. Hydrous pyrolysis was applied to low-maturity (BR: 0.25–0.39%Ro) Huron and time-correlative New Albany shale samples to create similar artificial maturation sequences for comparison. Micro-FTIR spectroscopy revealed a decrease in the CH2/CH3 ratio with increasing maturity, indicating Tasmanites aliphatic chains become shorter and more branched. Oxygenated functional groups decreased relative to aliphatic stretching bands and increased aromaticity was noted at the highest maturities. In samples that were pyrolyzed for 72h at temperatures of 300–320°C (BR: 0.56–0.68%Ro), Tasmanites showed similar trends, whereas at pyrolysis temperatures of 340°C and higher (BR>1.0%Ro), Tasmanites was pseudomorphed by accumulations of solid bitumen, carbonate and sulfide. Replacement of Tasmanites by these phases in hydrous pyrolysis experiments ≥340°C and its absence at higher maturities (peak oil, VR and BR≥0.9%Ro) in naturally matured samples, as documented in a previous study, implies that a large fraction of the algaenan component of original organic carbon is converted to petroleum during thermal maturation. XPS analysis indicated the molar proportion of aliphatic carbon increases with increasing thermal maturity, accompanied by decreases in oxygenated functional groups and olefinic carbon. EPMA of Tasmanites showed highest concentrations of S, with concentrations of redox-sensitive trace elements U, Mo, Ni and V generally at or below detection limits. Decrease in organic S with increasing thermal maturity may be related to cleavage of Tasmanites at CS linkages; however, this relationship was inconsistent and presence of adjacent or entrained nanoscale silicate or sulfide phases may impact measured trace element concentrations. Fluorescence microscopy and spectroscopy showed a red shift in spectral maxima and decreased emission intensities with increasing maturity, interpreted as due to non-radiative energy loss possibly because of increased aromaticity. Collectively, these results provide new insights into the in situ chemical transformations that accompany petrographic changes as oil-prone kerogen converts to petroleum with thermal advance from immature conditions into the mid-oil window.

Isotopically zoned carbonate cements in Early Paleozoic sandstones of the Illinois Basin: δ18O and δ13C records of burial and fluid flow

SEM/SIMS imaging and analysis of δ18O and δ13C in sandstones from a transect through the Illinois Basin (USA) show systematic μm-scale isotopic zonation of up to 10‰ in both carbonate and quartz cements of the middle-Ordovician St. Peter and Cambrian Mt. Simon formations. Quartz δ18O values are broadly consistent with the model of Hyodo et al. (2014), wherein burial and heating in the Illinois Basin is recorded in systematically zoned quartz overgrowths. Observations of zoned dolomite/ankerite cements indicate that they preserve a more extended record of temperature and fluid compositions than quartz, including early diagenesis before or during shallow burial, and late carbonates formed after quartz overgrowths. Many carbonate cements show innermost dolomite with δ18O values (21–25‰ VSMOW) that are too low to have formed by deposition at low temperatures from ancient seawater (δ18O>−3‰) and most likely reflect mixing with meteoric water. A sharp increase in Fe content is commonly observed in zoned carbonate cements to be associated with a drop in δ18O and an abrupt shift in δ13C to higher or lower values. These changes are interpreted to record the passage of hot metal-rich brines through sandstone aquifers, that was associated with Mississippi-Valley Type (MVT) Pb-Zn deposits (ca. 270Ma) of the Upper Mississippi Valley. Local variability and individual trends in δ13C are likely controlled by the sources of carbon and the degree to which carbon is sourced from adjacent carbonate units or thermal maturation of organic matter. Quartz overgrowths in sandstones provide an excellent record of conditions during burial, heating, and pressure-solution, whereas carbonate cements in sandstones preserve a more-extended record including initial pre-burial conditions and punctuated fluid flow events.

Nanoscale Pore Characteristics and Influential Factors of Niutitang Formation Shale Reservoir in Guizhou Province

Nanoscale pore characteristics are crucial in assessing the resource potential of gas shales. Although the Niutitang formation was widely deposited in the upper Yantze Platform, South China and has been recognized as a promising shale gas reservoir, there lacks substantial breakthrough in the exploitation of shale gas from the Niutitang formation. Aiming at better understanding the reservoir properties and corresponding influential factors, 14 core samples from the Lower Cambrian Niutitang formation locating in the central Guizhou province were investigated in the current study to characterize the nanoscale pore system in the shale. Organic geochemical analyses (i.e., total organic carbon content and thermal maturity), X-ray diffraction, low pressure nitrogen adsorption, and field emission scanning electron microscopy were employed to obtain complementary information of the pore system. Measured TOC in this study is generally > 1.50% and averages 3.35%. All of the samples are in the over-maturity stage with R-o ranging from 2.39% to 3.29%. X-ray diffraction shows that quartz, clay minerals and plagioclase are the dominant minerals. Nitrogen adsorption results indicate that all of samples show type IIb nitrogen adsorption isotherms with type H3 hysteresis loops, which imply the coexistence of micropores, mesopores and macropores in the shale. The mesopores account for 60-70% of total pore volume, and are likely contributed by clay minerals and quartz. Organic matter appears to be the major contributor of the micropores and specific surface area, and is closely linked to the rapid decrease of average pore size with increasing burial depth. The field emission scanning electron microscopy reveals abundant organic matter pores in the middle-upper Niutitang formation, but lesser or smaller in the bottom of Niutitang formation. The lower Niutitang formation seems to develop substantial amounts of organic-clay aggregates, which preferentially lie parallel to the shale bedding and contain lots of nanoscale pores. The perpendicular variation of pore structure features is explained with multiple mechanisms, including thermal maturation of organic matter, compaction by strata pressure, dissipation of shale gas, etc. The results of our study have emphasized the interesting and complex features of the nanoscale pore structures in the gas shales, which may facilitate future assessment and exploitation of shale gas resources.

Nanoscale analysis of preservation of ca. 2.1 Ga old Francevillian microfossils, Gabon

The FC Formation of the Francevillian of Gabon displays the oldest Gunflint-type assemblage of microfossils that is hosted in shallow-water stromatolites. The FC Formation was deposited between 2.14 and 2.08 billion years ago (Ga), after the Great Oxygenation Event (∼2.4–2.3Ga) and near the end of the Lomagundi event (∼2.3–2.06Ga). Although they have been used asa benchmark for the search for older microfossils, the nature of Gunflint-type microfossils has remained elusive due to their simple shapes, their small sizes, and their alteration. Here, we report the first nanoscale study of Francevillian Gunflint-type microfossils. We used a combination of Raman spectroscopy, palynology, in situ focused ion beam sectioning, and analytical electron microscopy. In spite of the relatively high thermal maturity of organic matter (inferred peak burial temperature ∼296±30°C), spherical cell walls (Huroniospora) and filamentous sheaths (Gunflintia minuta: ≤3µm in diameter, and broader filaments) are preserved. Organic matter in/on cell walls and sheaths, is associated with nanocrystalline quartz, whereas coarser quartz crystals fill and surround the microfossils. This pattern, likely inherited from recrystallization of texturally heterogenous opal generations, could have allowed the observed preservation of organic structures and limited migrations of organic matter. Moreover, we demonstrate the preservation of thick-sheathed broad (>3µm) filaments for the first time in a stromatolitic Gunflint-type assemblage; such thick sheaths are common in cyanobacteria, but not in other filamentous bacteria. We distinguished two types of star-shaped organic microstructures (Eoastrion) but found no diagnostic evidence for/against a microfossil nature at the nanoscale. Furthermore, we show that titanium is commonly associated with organic structures of microfossils, likely asa result of diagenetic mineralization. In contrast, iron-rich nanocrystals associated with microfossils occur in quartz, not in organic matter, and could possibly represent recrystallized biominerals.

Geochemistry of cretaceous rocks, Pakistan: I. Biomarker approach to assess thermal maturity of OM

ABSTRACTThe sediment samples from cretaceous sequences, Lower Indus basin, were analyzed using diagnostic biomarkers to predict the thermal maturity of organic matter. The sediments were tested for aliphatic biomarkers ratios, i.e. homohopanes and steranes isomerization ratios and carbon preference indices. These biomarker maturity parameters reveal that the Lower Goru (including its members Upper shale, Lower shale, and Talhar shale) and Sembar Formations in the Lower Indus Basin of Cretaceous age have reached the maturity level equivalent to the main zone of hydrocarbons generation, whereas Parh and Upper Goru Formations are immature and far from oil window. Geochemical analyses carried out stresses that hydrocarbons derived from the Sembar and Lower Goru Formations (particularly deeper Members) are within the zone of peak oil generation.

Origin and characterization of Eagle Ford pore networks in the south Texas Upper Cretaceous shelf

ABSTRACT Recent studies have shown that the loss of primary pores and the development of secondary pores in mudrocks are primarily controlled by burial diagenesis of the mineral matrix and thermal maturation of organic matter (OM). However, the lack of quantitative data on nanometer- to micrometer-scale rock properties has limited the ability to define and predict petrophysical properties and fluid flow in these fine-grained rocks. To upscale these rock properties, quantitative data are needed at multiple scales. Representative Eagle Ford Group samples were collected from continuous cores taken from two adjacent oil-producing wells in Karnes County, Texas, to investigate small-scale variations in mineralogy, diagenesis, and pore type. Point-count and pore-tracing methods were used to systematically quantify pore types and determine the size and shape of the identified pores. The two cores from the Eagle Ford are dominated by modified mineral pores, although secondary OM pores in migrated petroleum (bitumen) are also important. The mineral-pore network includes (1) primary mineral pores originally saturated with formation water and (2) modified mineral pores containing migrated petroleum (bitumen and/or residual oil). The OM-pore network includes (1) primary OM pores and (2) secondary OM pores including relatively large, less abundant OM bubble pores and relatively small, more abundant OM spongy pores. The abundance of OM spongy pores correlates positively with total-organic-carbon (TOC) content, and that of mineral pores weakly correlates with the volume of quartz plus feldspar. Studied samples have similar thermal maturities, although samples from one deeper core are slightly more mature than the other. Except for thermal maturation, the strong, micrometer-scale heterogeneity of rock components and properties (texture, fabric, mineralogy, and TOC) impacts the abundance, distribution, and type of pores. This micrometer-scale heterogeneity in porosity and pore networks would, in turn, significantly impact matrix permeability.

Quantity, thermal maturity of organic matter and relation to prospective source rock horizons in Tut-1x well, North Western Desert, Egypt

ABSTRACTThe quantity and thermal maturity of organic matter in the present study are discussed throughout the Rock-Eval pyrolysis for 27 rock samples of Tut -1x well used to describe the quantity, type, maturity, and petroleum generative potentials of prospective source rock horizons. The results showed that Alam El Bueib Formation has type-III kerogen. Masajid, Khatatba, and Ras Qattara Formations display mixed kerogen type II-III, which has the ability to generate mixed oil and gas accumulations under thermal maturation. Meanwhile, Alam El Bueib and Masajid Formations entered the early mature stage of oil generation at vitrinite reflectance values of 0.60 Ro%. The source rock of the Khatatba and Ras Qattara Formations reached to the peak of oil generation at vitrinite reflectance values of 1.36 Ro%. This indicates that Khatatba and Ras Qattara Formations can be consider to have generative potentials of prospective source rock horizons in the Tut-1x well.

Vibrational spectroscopy assessment of kerogen maturity in organic-rich source rocks

Source rocks are sedimentary rocks containing organic matter. In order to evaluate if a source rock is suitable for economical exploitation, the evaluation of many factors is required, among which the most important are concentration of the organic matter and its quality (e.g. kerogen type and thermal maturity) that evolves following temperature and pressure over burial history. The most commonly used techniques to characterize the thermal maturity of organic matter are the maximum pyrolysis temperature of generated hydrocarbons and the reflectance of vitrinite macerals (%Ro). The limitations to the applicability of these analyses is the driving force to develop new methods and in particular spectroscopic ones. In this work the maturity of different kerogens (i.e. solid organic matter entrapped into rocks), both naturally and artificially aged, is determined by Raman and Attenuated Total Reflection-InfraRed (ATR-IR) spectroscopies, applied directly to source rocks. The parameterization of spectra by suitable variables proved to allow evaluating kerogen maturity with the best reliability at lower rank for ATR-IR and at higher rank for Raman analysis. Good correlations were obtained between Raman and ATR-IR spectroscopic parameters and the above mentioned vitrinite reflectance (%Ro).