Thermal Maturity Indicators Research Articles

0.25 Ga Salt Deposits Preserve Signatures of Habitable Conditions and Ancient Lipids.

Polygonal features in a ∼250 million-year-old Permian evaporitic deposit were investigated for their geological and organic content to test the hypothesis that they could preserve the signature of ancient habitable conditions and biological activity. Investigations on evaporitic rock were carried out as part of the MIne Analog Research (MINAR) project at Boulby Mine, the United Kingdom. The edges of the polygons have a higher clay content and contain higher abundances of minerals such as quartz and microcline, and clays such as illite and chlorite, compared with the interior of polygons, suggesting that the edges were preferred locations for the accumulation of weathering products during their formation. The mineral content and its strontium isotope ratio suggest that the material is from continental weathering at the borders of the Permian Zechstein Sea. The edges of the polygons contain material with mean δ13C and δ15N values of -20.8 and 5.3, respectively. Lipids, including alkanes and hopanes, were extracted from the interior and edges of the polygons, which are inferred to represent organic material entrained in the evaporites when they were formed. The presence of long-chain alkanes (C20-C35) that lack a carbon preference, low abundances of C23-C29 hopanes, and lack of marine, evaporitic, or thermal maturity indicators show that lipid biomarkers were, at least in part, potentially derived from a continental source and have not undergone significant thermal maturation since deposition. Lipid extractions using weak acids revealed significantly more lipids than those without acid, potentially indicating that encapsulation was not the only type of preservation mechanism occurring in Boulby salts. These data demonstrate the potential for ancient evaporites and their polygons to preserve information on local geological conditions, ancient habitability, and evidence of life. The data show that analogous martian evaporitic deposits are good targets for future life detection missions and the investigation of ancient martian habitability.

Application of organic facies and biomarkers in characterization of paleoenvironmental conditions and maturity of sediments from the Codó Formation in the west-central part of the São Luís Basin, Brazil

Paleoenvironmental conditions of the sediments of the Cretaceous Codó Formation in Brazil are still being debated. The main point of controversy is whether they were deposited in a strictly marine environment or under lacustrine conditions of varying salinity. In this study, organic facies and biomarker analyses were performed on samples from the Codó Formation in the São Luís Basin, northwest Brazil, to reconstruct the paleoenvironmental conditions, the origin, and the thermal maturity of the organic matter. Four types of palynofacies associations (PI – amorphous organic matter, PII – amorphous organic matter+sporomorphs, PIII – phytoclasts, and IV – marine palynomorphs) were identified. The total organic carbon (TOC) content exhibits a strong correlation (ρ = 0.75, p < .05) with amorphous organic matter (AOM) content suggesting that AOM is the main factor controlling TOC in the stratigraphic section, and that increases in the AOM contribution were associated with an increase in the water column and expansion of reducing conditions that favored organic matter preservation. The TOC and total sulfur (TS) plots suggest that there were salinity variations in the depositional environment. The identification of marine palynomorphs (Subtilisphaera genus) are indicative of several local marine incursions. The n-alkane distribution (unimodal and bimodal) confirmed the presence of strong input of higher plant components (phytoclast-dominated Palynofacies III) and the bacterially-reworked organic material (Palynofacies I/II). The aquatic terrestrial (ATR), terrigenous/aquatic (TAR), and proxy for aquatic macrophytes (Paq) ratios suggest autochthonous (Palynofacies I and II) and allochthonous (Palynofacies III) organic matter, and eutrophic and submerged-floating plants. Based on palynofacies associations and biomarkers we conclude that the depositional environment ranged from an anoxic/reducing to dysoxic/oxic within a closed-lacustrine system having a stratified water column. High evaporation rates favored a decrease in the water column and an increase in salinity, allowing the precipitation of salt by evaporation (CaCO3 > 80%). The thermal maturity indicators including vitrinite reflectance (%Ro), spore color index (SCI), Ts/(Ts + Tm) ratios, and C29 20S/(20S + 20R) and C29 ββ/(αα + ββ) indicate that the organic matter is immature to marginally mature.

Light hydrocarbons and diamondoids of light oils in deep reservoirs of Shuntuoguole Low Uplift, Tarim Basin: Implication for the evaluation on thermal maturity, secondary alteration and source characteristics

The Shuntuoguole Low Uplift in Tarim Basin is particular because it provides insights into geochemistry of light oils at ultra-deep Ordovician oil reservoirs (7200–8200 m). The light hydrocarbons and diamondoids of 16 oil samples from Shunbei No. I Fault Zone (F1) and V Fault Zone (F5) were analyzed to investigate their thermal maturity, source characteristics and secondary alteration. The oils show high similarity in the characteristics of light hydrocarbons and diamondoids. The light hydrocarbons are characterized by high contents of alkanes and low contents of cycloalkanes and aromatics, and the diamondoids show a domination of adamantanes and their alkylated isomers compared diamantanes. Thermal maturity indicators derived from diamondoids and C 5 –C 7 light hydrocarbons indicate that the oils are highly mature, and the corresponding equivalent vitrinite reflectance ( R c) determined by light hydrocarbons and methyldiadamantane index are in the ranges of 1.2%–1.5%. Signatures of biodegradation and thermochemical sulfate reduction were not observed in the oils. The geochemical information from the light hydrocarbons and distribution patterns of molar concentration of n -alkanes indicate that the oils from F1 experienced noticeable evaporative fractionation, and the oils from F5 did not. This was supposed to be the primarily reason for the physical difference between oils from the both fault zones. The parameters derived from the light hydrocarbons and diamondoids indicate the oils are from same source rocks, which are associated with marine materials with type II kerogen, such as algae and bacteria, and both of Cambrian–Lower Ordovician (Є-O 1 ) and Middle–Upper Ordovician (O 2–3 ) source rocks in the deep Lower Palaeozoic strata contributed to the oils. • Light hydrocarbons and diamondoids of the deep Ordovician oils were revealed. • The oil maturity was calibrated at equivalent vitrinite reflectance values of 1.2%–1.5%. • Both contributions of the Є-O 1 and O 2+3 source rocks to the oils were suggested. • No or negligible biodegradation and TSR alteration were suggested. • Noticeable evaporative fractionation was observed in the oils from No. 1 Fault.

Compound specific sulfur isotopes of saline lacustrine oils from the Dongpu Depression, Bohai Bay Basin, NE China

Lacustrine oils from type I-II kerogen from the Dongpu Depression of Bohai Bay Basin have been characterised by compound specific sulfur isotope and hydrocarbon biomarker analyses. The sample suite included sixteen saline lacustrine oils from the northern part of the Dongpu Depression, most of which (i.e., all but two interbedded samples) could be separated into four tectonically aligned oil families (OF1-4) on the basis of differences in their hydrocarbon composition (e.g. high MW n-alkanes, isoprenoids, polycyclic terpenoids). The other two oils studied were freshwater lacustrine oils from the south of the Dongpu Depression, the hydrocarbon composition of which indicated a fifth oil family (OF5). The δ34S values of organic sulfur compounds (δ34SOSCs) in the oils showed several general differences consistent with the five hydrocarbon biomarker defined oil families, including the relative magnitude of their δ34S values: Ave δ34SOSCs values: OF1 = 19‰ (relatively depleted δ34S); OF2 = 28‰ (enriched δ34S); OF3 = 24‰ (intermediate δ34S); OF4 = 25‰ (intermediate δ34S); and OF5 = 18‰ (depleted δ34S); and the respective δ34SOSCs profiles of the oils, e.g., the intermediate δ34S values of the LT oils (OF3) showed little variance with alkylation (<4‰), whereas the intermediate δ34S values of the higher maturity Wenliu oils (OF4) showed 34S enriched alkylated DBTs compared to base DBT (Δδ34SalkylDBT-DBT = +7‰). This demonstrates the potential of δ34SOSCs data assisting the genetic correlation of S-rich saline lacustrine oils. However, separate correlations between some of the δ34SOSCs data and hydrocarbon indicators of paleo-environmental (C35-/C34-hopanes), thermal maturity (C29 sterane and MDBT parameters) and TSR (Δδ34SBTs–DBTs = 6–12‰) impacts suggest each of these factors also influenced the δ34S character of the Dongpu oils. Therefore, δ34SOSCs data should be used cautiously in correlation of oils from complex petroleum systems and where possible integrated with other complimentary analytical information.

Preliminary Investigation of the Effects of Thermal Maturity on Redox-Sensitive Trace Metal Concentration in the Bakken Source Rock, North Dakota, USA.

Samples were taken at different levels of thermal maturity in the unconventional Bakken source rock. Programmed pyrolysis derived Tmax, solid bitumen reflectance, liptinite group maceral UV fluorescence, and nuclear magnetic resonance spectroscopy as different thermal maturity indicators were utilized in order to compare redox-sensitive trace metal (TM) concentration to maturity variations and disclose any probable relationship. Comparing redox-sensitive TMs with total organic carbon revealed the presence of anoxic/euxinic conditions in the depositional environment of the Bakken Shale. Although some of the TMs (V and Mo) exhibit slightly positive correlations with some of the thermal maturity indices used in this study, the correlations between other redox-sensitive TMs with maturity were neutral. Collectively, this study demonstrates that thermal maturity may have an impact on some redox-sensitive TMs such as Mo and V concentrations in marine sediments. Additional samples spanning higher maturities will need to be included because there is a possibility that an increase in thermal maturity may lead to the release and liberation of some redox-sensitive TMs from the organic matter (OM) directly. Remineralization and decomposition of OM with thermal maturity advance could release sulfur as a source of thermogenic H2S, which could accelerate pore water/rock interaction and authigenic Fe-sulfides. This could enhance the capability of uptaking of most of the redox-sensitive TMs and increase their concentration in pore water.

Depositional environment and maturity of Devonian Pimenteira Formation in the São Luís Basin, Brazil

We carried out a detailed organic facies study on shale cutting samples from the Devonian Pimenteira Formation in the São Luís Basin, northwest Brazil, to reconstruct the depositional paleoenvironment and origin of the organic matter. Samples were collected from the stratigraphic well 2-ANP-5-MA and were analyzed for total organic carbon (TOC), total sulfur (%S), insoluble residue (RI), spore coloration index (SCI), vitrinite reflectance (%Ro), palynofacies, and biomarkers. The TOC values of the analyzed sediments range from 0.19 to 0.51%, %S from 0.17 to 0.85%, and the total carbonate (CaCo3) is low (<15%); SCI ranges from 4.0 to 5.0, %Ro from 0.45 to 0.47%, and chitinozoan reflectance from 0.42 to 0.51%. The prasinophyte algae content exhibits a strong correlation (r2 = 0.73) with the TOC content; this suggests that this organic component is the main factor controlling TOC and that variations in prasinophyte content could reflect changes in the sea level associated with marine transgressions and regressions. Two types of palynofacies (PI—phytoclasts, amorphous organic matter [AOM], and spongiophyton-dominated—and PII—palynomorphs and zoomorphs-dominated) were identified. PI is characterized by a palynomorph association dominantly composed of marine palynomorphs like acritarchs and prasinophytes, whereas PII is characterized by phytoclasts and an association with AOM. Chitinozoan, conodonts and spongiophyton are common throughout this sedimentary sequence. The TOC (wt%) vs %S relationship indicates that the organic matter is dominantly marine but has some freshwater influence. The ternary graph including AOM, phytoclasts, and palynomorphs+zoomorphs suggests a depositional environment ranging from oxic proximal shelf to dysoxic proximal-distal shelf with deltaic influence. The presence of marine microplankton and land plants was corroborated by the distribution of the steranes components (C27, C28, and C29). The thermal maturity indicators, including %Ro, chitinozoan reflectance, SCI, Ts/(Ts + Tm) ratios, and C29 20S/(20S + 20R) and C29 ββ/(αα + ββ), indicate that the organic matter is immature.

Assessing the thermal maturity of black shales using vitrinite reflectance: Insights from Devonian black shales in the eastern United States

Thermal maturity of source rocks indicated by vitrinite reflectance (Ro) is an important and reliable parameter to determine the petroleum potential of sedimentary basins. Originally, Ro was used to characterize the degree of coalification of coals and thus works well for coal seams, but it has limitations when used to determine the thermal maturity of black shale successions. A suite of six coal-shale pairs from the Upper Devonian black shale formations in the eastern United States (New Albany Shale, Ohio Shale, Dunkirk Shale, and Rhinestreet Shale) was selected to study the applicability of Ro in assessing the thermal maturity of black shale successions. The results show that vitrinite in the studied coal samples is dominated by collotelinite, whereas vitrinite in black shales occurs as small dispersed particles (~5 μm) in the mineral matrix. When comparing the size and morphology of dispersed vitrinite particles in shales and collotelinite fragments in coals, vitrinite in shales and adjacent coals should have the same origin. The measured mean random Ro of vitrinite in coals ranges from 0.51–0.68%, and is 0.04–0.11% (average 0.07%) lower than that of dispersed vitrinite particles in enclosing shales. This observation contrasts with previously reported Ro suppression in liptinite-rich black shales. A high standard deviation of Ro measurements indicates a highly heterogeneous nature of vitrinite in black shales.Three mechanisms can be envisioned to contribute to the higher Ro of dispersed vitrinite particles in black shales relative to enclosed coals. First, small vitrinite particles may become more oxidized because small particles are more frequently suspended during transport to the site of deposition and reworking of bottom sediments after deposition than large pieces of driftwood that will turn into enclosed coal lenses during burial diagenesis. Second, misidentification of zooclast (e.g., chitinozoan) fragments as vitrinite due to loss of diagnostic morphology. Third, various macerals in the vitrinite group (e.g., corpogelinite and collotelinite) may have had different original reflectance values. Based on the petrographic characteristics of vitrinite and vitrinite-like particles, caution should be applied when using the Ro values of dispersed vitrinite and vitrinite-like particles in black shales as indicators of thermal maturity.

Earthquake slip surfaces identified by biomarker thermal maturity within the 2011 Tohoku-Oki earthquake fault zone

Extreme slip at shallow depths on subduction zone faults is a primary contributor to tsunami generation by earthquakes. Improving earthquake and tsunami risk assessment requires understanding the material and structural conditions that favor earthquake propagation to the trench. We use new biomarker thermal maturity indicators to identify seismic faults in drill core recovered from the Japan Trench subduction zone, which hosted 50 m of shallow slip during the Mw9.1 2011 Tohoku-Oki earthquake. Our results show that multiple faults have hosted earthquakes with displacement ≥ 10 m, and each could have hosted many great earthquakes, illustrating an extensive history of great earthquake seismicity that caused large shallow slip. We find that lithologic contrasts in frictional properties do not necessarily determine the likelihood of large shallow slip or seismic hazard.

Organic geochemical, palynofacies, and petrographic analyses examining the hydrocarbon potential of the Kharita Formation (Albian) in the Matruh Basin, northwestern Egypt

A recent study of selected samples from the Cretaceous (Albian) Kharita Formation of Egypt revealed very good to excellent source rock (SR) potential for six intraformational, organic-rich intervals. This work investigates the SR potential of the entire Kharita Formation across the Matruh Basin, using samples from two wells: the Abu Tunis 1X well from the central part of the basin, and the Siqeifa 1X well drilled on the eastern margin of the basin. More strongly reducing conditions were developed in the centre of the basin, and resulted in the deposition of more organic-rich shales by comparison to the less reducing conditions that prevailed on the eastern basin margin, where the shales contain less organic matter. Deltaic intraformational shales and carbonates in the Kharita Formation of Abu Tunis 1X constitute a significant 120 m net of the potential SR. The lower Kharita Formation contains 34 m net shale SR of good to very good/excellent organic richness, yielding values of 1.14–11.59 wt % total organic carbon (TOC). The organic matter has low Hydrogen Index (HI) values (184–389 mg HC/g TOC) and amorphous organic matter (AOM) and relatively high non-opaque phytoclast frequencies indicating mainly gas/oil-prone organofacies (kerogen Types III/II). The upper Kharita is rather important, containing 86 m net shale/carbonate SR that has fair to good organic richness (0.8–1.8 wt % TOC), and lower HI (126–250 mg HC/g TOC), a dominance of non-opaque phytoclasts, and subordinate AOM frequencies, which together indicate gas/oil-prone organofacies (kerogen III/II). In the Siqeifa 1X well, Kharita deltaic intraformational shales and shaley dolostones comprise 80 m net SR, which has mainly fair to good to less very good organic richness (0.8–2.1 wt % TOC), whilst low HI (93–220 mg HC/g TOC), dominance of non-opaque phytoclasts and subordinate AOM indicate gas-prone organofacies (kerogen Type III). A relative upward increase in deposition of lignite and coaly carbonaceous material supports a gas-prone organofacies. Whilst thermal maturity indices only point to immature to early mature (pre- to early oil-window) SRs in both the Abu Tunis 1X and Siqeifa 1X wells, hydrocarbon exploration focussing on this potential source rock may be justified in areas to the southeast of the Matruh Basin, where modelling indicates this unit may have reached the late mature oil- to main gas-generation windows.

Natural and experimental structural evolution of dispersed organic matter in mudstones: The Shimanto accretionary complex, southwest Japan

AbstractStructural changes induced by thermal maturation of dispersed organic matter (OM) in the Shimanto accretionary complex, southwest Japan, were investigated using micro‐Fourier‐transform infrared spectroscopy and micro‐Raman spectroscopy. Natural dispersed OM exhibits systematic structural changes inferred from D1‐ and G‐band FWHM values, Raman band separation (RBS), and intensity ratios of the D1‐ and G‐bands (ID1/IG ratio) from diagenetic zone to anchizone (IC values: 0.75–0.30). Infrared spectra indicate a loss of aliphatic CH x, aromatic CH x, and oxygen‐containing structures as temperature increases. These changes are consistent with discontinuities in thermal structures bounded by out‐of‐sequence thrusts. Kinetic pyrolysis experiments indicate that the ID1/IG ratio of synthesized OM has a power law relationship with heat treatment time. Kinetic models of temperature dependence were fitted using the ID1/IG ratio, and an effective activation energy of 106 ±17 kJ/mol was estimated using an Arrhenius equation. The activation energies estimated by power law rate and Avrami models have a least‐square correlation coefficient of 0.93, indicating the temperature dependence of carbonization. The estimated effective activation energy is consistent with that of coal, lignin, cellulose, and hemicellulose during thermal degradation. On the other hand, RBS, and D1‐ and G‐band FWHM values of OM display more complex changes with increasing heating temperature and time, and it is difficult to constrain rate parameters during pyrolysis experiments. Our data indicate that the ID1/IG ratio is controlled by a simple thermally activated process, whereas RBS and D1‐ and G‐band FWHM values can be affected by lithostatic pressure, fluid activity, hydrogen index, and host lithology, as well as temperature. Structural evolution of dispersed OM in mudstones differs between natural and anhydrous closed experimental systems. Natural carbonization based on micro‐Raman spectroscopy should be applied for a limited indicator of thermal maturation, especially for dispersed OM in diagenetic zone.

Palynofacies, paleoenvironment and thermal maturity of early Silurian shales in Saudi Arabia (Qusaiba Member of Qalibah Formation)

The early Silurian Qusaiba Shales are proven source rocks of the many Paleozoic petroleum systems of Saudi Arabia. In this study, a new, simple and cost-effective method for semi-quantitative analysis and graphical representation of palynological assemblage composition (PZC Phytoplankton–Zooplankton–Cryptospore ternary diagram) and a new visual thermal alteration index (IPA–TAI Integrated Palynomorph–Amorphous organic matter Thermal Alteration Index) integrating amorphous organic matter and palynomorph color, were developed based on the palynological/palynofacies analysis of some selected wells penetrating the early Silurian Qusaiba Shales throughout Saudi Arabia. The methods were tested against independent paleoenvironmental and thermal maturity indicators mainly based on geochemical parameters, confirming their validity as effective alternative methods for optical kerogen analysis and interpretation. The thermal maturity values estimated for core samples in the Qusaiba Shales from 13 wells in Saudi Arabia showed a strong correlation with Vitrinite Reflectance Equivalent (VRE%) maturity values determined from graptolite reflectance and pyrolysis Tmax measurements in the entire range of maturities (immature to overmature stages), proving that the present visual thermal alteration index can be successfully employed as a time efficient and reliable method for estimating thermal maturity in the Qusaiba Shales of Saudi Arabia. The present results show that the Qusaiba Shales were deposited in mostly mid- to outer shelf, and occasionally nearshore marine environments under prevailing dysoxic and anoxic water conditions, resulting in accumulation and preservation of organic matter.

Suppression of thermal maturity indicators in lacustrine source rocks: A case study of Dongying Depression, eastern China

Abstract The Paleogene Shahejie formation of the Dongying Depression in eastern China is a unique region and can offer insights into the suppression of thermal maturity in a large set of successive lacustrine shales. This study focuses on the Paleogene Shahejie formation of N-38 Well in the Dongying Depression. The unusual distribution of thermal maturity indicators, such as vitrinite reflectance (VRO) and C29 sterane thermal parameters was investigated through geochemical analysis. The research interval (2770–3376 m) was divided into three parts: the upper part of Es32 (2770–2990 m), the lower part of Es32 (2990–3190 m), and the Es31 (3190–3376 m). The Es31 mainly contained organic matter type I and some type II, which included mostly good and excellent source rocks. Meanwhile, the upper Es32 mainly contained organic matter type III, which included mostly poor and moderate source rocks. The parameters G/C30H, C35H/C31-35H, C31M/C31H, C29-20S/(20S + 20R), C31-22S/(22S + 22R), and C32-22S/(22S + 22R) showed a different distribution corresponding to the different lake sedimentary environments. Various organic facies were developed at the different evolution stages of the lake basin, which showed a noticeable influence on the parameters. The thermal maturity indicator VRO and molecular thermal parameters, such as C29-20S/(20S + 20R), C31-22S/(22S + 22R), C32-22S/(22S + 22R), and C31M/C31H, presented abnormal distributions in the large set of lacustrine shales, especially in the Es31. This observation suggested suppression of several thermal maturity indicators. VRO suppression positively correlated well with the corrected hydrogen index and alginite content in maceral composition. The high content of alginite group was considered to be the primary reason behind the evident negative VRO anomaly.

Organic matter variations and links to climate across the early Toarcian oceanic anoxic event (T-OAE) in Toyora area, southwest Japan

Marked climate warming during the early Toarcian oceanic anoxic event (T-OAE, ~182 Ma) has been shown to have had a significant effect on the global carbon cycle and biosphere. There is also emerging evidence that a primary short-term consequence of this warming was a marked increase in hydrological cycling. In this study, we have investigated local environmental responses to T-OAE climate change by conducting a palynofacies analysis through an expanded Panthalassic margin record of the event exposed in southwestern Japan (Nishinakayama Formation, Toyora area). Palynomorphs are poorly preserved and rare in the studied interval. A pronounced increase in terrestrial phytoclast abundance coeval with the peak of the T-OAE can be linked to increased fluvial supply driven by enhanced hydrological cycling that occurred in response to early Toarcian warming. Broader scale trends in phytoclast abundance through the studied interval may be linked to longer-term sea-level changes. Nitrogen-isotopes, commonly used to elucidate basin redox changes, are found to correlate with changes in the relative abundance of phytoclasts. In contrast, carbon-isotope data are probably not significantly influenced by changes in organic matter type through the succession. However, thermal maturity indicators demonstrate that the succession is overmature, and this likely suppresses the magnitude of the well-known T-OAE carbon-isotope excursion in this succession.

Estimation of thermal maturity in the Bakken source rock from a combination of well logs, North Dakota, USA

Numerous attempts have been conducted to construct a reliable model to relate petrophysical and geomechanical characteristics of shale to geochemical properties. In this study, continuous logs were built to predict the thermal maturity of the Bakken Formation from a set of petrophysical logs. First, dynamic modulus (Ed) was created from shear and compressional sonic and, density logs and then converted to Ei (nanoindentation based Young's modulus) using the existing experimental relationship between Ed and Ei. Next, continuous TOC logs were generated for the Bakken Shales based on an empirical relationship between well log derived Ei and TOC contents that were measured in the lab. There was found an acceptable difference between estimated log-based TOC and measured values in three separate wells. Solid bitumen reflectance as a reliable indicator of thermal maturity was measured on all samples from the same wells in the scarcity/absence of vitrinite maceral. The relationships between GR/NPHI/RHOZ wireline logs versus solid bitumen reflectance (BRO%) as the maturity index was then investigated in twelve wells. Based on the strength or weight of the relationship between each log and BRO%, a new parameter “Ʈ” was defined to represent thermal maturity from well logs. A good agreement between parameter Ʈ and BRO% values was observed which provided an empirical equation to make the estimation of BRO% values for the whole length of the shale members possible. Ultimately, the continuity of BRO% values, would enable us to establish a relationship between BRO% and measured Tmax values to generate continuous Tmax logs for the Bakken Shale members. Using these newly developed well logs, it would become possible to make a 3D property models for BRO% and Tmax (the thermal maturity property cube) for an accurate petroleum system evaluation based on GR/NPHI/RHOZ logs.

Comparing optical and Raman spectroscopic investigations of phytoclasts and sporomorphs for thermal maturity assessment: the case study of Hettangian continental facies in the Holy cross Mts. (central Poland)

Abstract Thermal maturity of six organic rich samples from Jurassic continental successions cropping out in the Holy Cross Mountains in Central Poland, has been characterized by classic thermal maturity indicators, micro-Raman spectroscopy and Palynomorph Darkness Index, in order to create a multi-method workflow for complex palynofacies thermal maturity assessment. Transmitted light observations on dispersed organic matter define a Hettangian lacustrine depositional environment, characterized by periods of reducing/oxidizing conditions and variable sedimentation rates. Thermal maturity detected by classical maturity indicators and PDI indicates an early maturation stage of hydrocarbon generation and is in agreement with spectroscopic analyses performed on phytoclast groups. Moreover, Raman parameters in the sporomorph group indicate a systematic shift toward a lower degree of aromatization compared to the phytoclast group. Finally, the multivariate statistical analysis performed on Raman spectra is found to be a promising tool to define and predict the heterogeneity of dispersed organic matter in sediments.

Organic petrography and geochemical characterization of the Upper Cretaceous Second White Specks and Upper Belle Fourche alloformations, west-central Alberta: Analysis of local maturity anomalies

The Second White Specks is an enigmatic unconventional shale play in the Western Canada Foreland Basin (WCFB). This play has an inconsistent production performance that has impacted exploration and production risk, preventing extensive development. In this contribution, geochemical properties are studied with the purpose to identify possible geochemical factors that could contribute to the production variability of this unconventional petroleum system. The studied succession consists of the Second White Specks (SWS) and Upper Belle Fourche (UBF) alloformations, which are part of the SWS shale play. Organic petrology and Rock-Eval pyrolysis were combined to characterize the organic matter (OM), and assess OM richness, thermal maturity and hydrocarbon generation potential in the foredeep section of the basin. The results show that the OM richness and hydrocarbon generation potential in this area is good, and comparison with other studies show that they do not exhibit major lateral variations. The kerogen is marine Type II and is dominated by liptinite, vitrinite, inertinite macerals, and solid bitumen. Alginite and bituminite are the most abundant macerals. Altered bituminite provides evidence of anaerobic bacteria degradation. There is a modest contribution of inertinite that may be responsible for a dilution effect of the pyrolyzable carbon. The thermal maturity indicators in this study such as vitrinite reflectance, Tmax, and alginite UV fluorescence color indicate that the controls on thermal maturity and hydrocarbon generation are not well understood. There are lateral short scale variations or maturity anomalies that require further investigation, to reduce the exploration risk.

Biomarker signatures of Upper Cretaceous hydrocarbon source rocks from the Latrobe Group, Gippsland Basin

Gas chromatography-mass spectrometry analyses have been carried out to investigate the geochemical characteristics of the Latrobe Group shales and coaly shales from the Gippsland Basin, Australia. The depositional environment, source of organic matter and thermal maturity of hydrocarbon source rocks in the study area were evaluated using molecular biomarker analyses. The distribution of isoprenoid alkanes and pentacyclic triterpanes reveals an oxic environment with fresh water (pristane/phytane > 3.0, gammacerane index < 0.3). The carbon preference indices (CPI) and odd-to-even predominance ratios of the n-alkanes are higher than 1.0, suggesting terrigenous higher plant-derived organic matter in the sediments. The high predominance of C29 sterane over C27 sterane, as well as the occurrence of conifer and angiosperm biomarkers (e.g., labdane, isopimarane, phyllocladane, rimuane, oleanane, retene, anthracene, and cadalene), corroborates input from higher vascular land plants. Biomarker and aromatic thermal maturity indices, such as the methylphenanthrene index, the methylnaphthalene ratio, C31 22S/(22S+22R) hopanes, C30 αβ/(αβ+βα) hopanes and C29 ααα 20S/(20S+20R) steranes, indicate rather thermally immature hydrocarbon source rocks, in agreement with the above CPI data. This maturity trend is also supported by the triaromatic sterane index [TA(I)/TA(I+II)], which is generally lower than 0.2.

Comments on empirical conversion of solid bitumen reflectance for thermal maturity evaluation

Recent studies raised doubts about the reliability of the correlation function between solid bitumen and vitrinite reflectance in diagenesis to catagenesis stages. Addressing this issue, data was surveyed from recent and classical studies following strict criteria. This compilation comprised an expressive sampling in the vitrinite reflectance range of 0.19 to 5.79% and was used to generate a new correlation function. This function was compared to previous ones and against data from recent studies. The results confirm that the reflectance of solid bitumen with homogeneous texture can be used as a thermal maturity index. Furthermore, the results also bring new insights on the identification of vitrinite-like solid bitumen when the petrographic criteria are ineffective.

Organic matter network in post-mature Marcellus Shale: Effects on petrophysical properties

Shale samples of the Marcellus Shale from a well drilled in northeastern Pennsylvania were used to study diagenetic effects on the mineral and organic matter and their impact on petrophysical response. We analyzed an interval of high gamma ray and anomalously low electrical resistivity from a high thermal maturity (mean maximum vitrinite reflectance > 4%) part of the shalehgas play. A suite of microanalytical techniques was used to study features of the shale down to the nanoscale and assess the level of thermal alteration of the mineral and organic phases. The samples are organic rich, with total organic carbon contents of 3–7 wt. %; the vast majority of the organic matter was identified as highly porous pyrobitumen. Matrix porosity is also present, especially within the clay aggregates and at the interface between rigid clasts and clay minerals. Mineral- and organic-based thermal maturity indices suggest that during burial the sediment had been exposed to temperatures as high as 285°C (545°F). Under these conditions, the residual, migrated organic matter assumed a partially crystalline habit as confirmed by the identification of turbostratic structures via electron microscopy imaging. Experimental dielectric measurements on organic matter–rich samples confirm that the anomalous electrical properties observed in the wire-line logs can be ascribed to the presence of an electrically conductive interconnected network of partially graphitized organic matter. The preservation of porosity suggests that this organic network can contribute not only to the electrical properties but also to the gas flow properties within the Marcellus Shale.

Biomarker signatures of the iconic Glossopteris plant

Glossopteris is the quintessential marker plant that dominated the forests of the Southern Hemisphere continents during the Permian before their abrupt extinction at the close of the Palaeozoic. Fossilized leaf and stem remains of Glossopteris plants, collected from the Permian succession of eastern India, were analysed to recognize the molecular signatures of solvent-extractable and non-extractable organic matter. Lipid-derived aliphatic and aromatic biomarkers were studied using GC–MS and GC×GC-TOFMS, while tetramethylammonium hydroxide (TMAH) thermochemolysis with GC–MS and GC×GC-TOFMS was used to reveal lignin precursors. The thermal maturity indices indicate low thermal rank for both samples. The presence of aromatic diterpanes in the leaf extract suggests that the Glossopteris plant was able to biosynthesize abietic acid and related plant terpenoids. The thermochemolysis products (thermochemolysates) of the samples contain monomethoxy-, and dimethoxybenzene derivatives produced from p‑hydroxyphenyl and guaiacyl units of lignin, respectively, which are the main constituents of extant gymnosperm lignin. The thermochemolysates indicate that lignin biosynthesis had evolved in Glossopteris.