Thermal Maturity Of Organic Matter Research Articles

Source rock assessment and organic matter characterization of the Oligocene upper Shahejie and Dongying formations in the Nanpu Depression of the saline lacustrine Bohai Bay Basin, China: Geochemical, palynological, and mineralogical perspectives

The Bohai Bay Basin, located in eastern China, is a significant petroliferous, non-marine rift basin, underwent intricate tectonic activities throughout the Cenozoic. The Nanpu Depression in the Huanghua sub-basin in the northern part of the basin was controlled by Paleogene rifting and increased subsidence. These geological processes resulted in the deposition of the Shahejie and Dongying formations, which exhibited varying quantities and qualities of organic matter. Eighty drill cuttings samples of the Paleogene First Member (Es1) and the overlying Third (Ed3) and Second (Ed2) members of the Shahejie and Dongying formations, respectively, were investigated. An integrated approach including organic and inorganic geochemistry, palynology, and mineralogy was employed to assess the paleoredox and marine incursion conditions, source rock quality and levels of thermal maturity, and factors that governed organic matter accumulation. Results indicated that the Es1 Member was deposited under anoxic conditions, resulting in the accumulation of organic matter-rich mudstones in the Nanpu Depression. Abundant pollen grains and acritarchs were found in the Es1 Member yielded along with minor occurrences of dinoflagellate cysts, freshwater algae, and spores. Terrestrial palynomorph composition provided evidence of increased terrestrial runoff, while the high abundance of acritarchs, accompanied by some dinoflagellate cysts, confirmed episodes of marine incursion and deposition under hypersaline conditions in the lacustrine Bohai Bay Basin. The Ed2 Member exhibited poor to fair amounts of organic matter and a limited potential of hydrocarbon generation with a variety of kerogen Types, including IV, III, II/III, and II. The Ed3 Member shows fair to good organic matter richness and hydrocarbon potential of kerogen Types III, II/III, and II. The Es1 Member emerged as the most promising source rock within the studied interval, boasting good to excellent organic matter richness, and fair to good hydrocarbon generation potential of kerogen Types III and II/III. Evaluating the thermal maturity of organic matter, based on Tmax and VRo-Eq% and related cross-plots, revealed that the Shahejie and Dongying formations are still in the immature to the early stage of the oil window. The mineralogical composition of the Es1 Member primarily consists of clay and quartz with some carbonate and accessory minerals, such as epigenetic halite, dolomite, and plagioclase. Weak correlations observed between the total organic carbon (TOC) and mineral components indicated that the availability of organic matter was primarily controlled by enhanced production and preservation rather than limited dilution under saline lacustrine, oxygen-deprived, and well-stratified water column conditions. Regional investigations of the Es1 Member confirmed the widespread anoxia and deposition of organic-rich source rocks in the Bohai Bay Basin during the Oligocene.

Palynostratigraphy of the Mississippian-Pennsylvanian boundary in the Tyler Formation, Williston Basin, USA: Implications for organic matter-rich source rocks and paleoenvironmental reconstruction

The Carboniferous was a period of intense environmental perturbations, climate changes between greenhouse and icehouse, eustatic sea level change, and accumulation of organic carbon-rich sediments. At this time, the Tyler Formation was deposited in the midcontinent USA. A detailed palynological analysis of the Tyler Formation revealed a highly diverse assemblage of spores with minor pollen content, represented by 100 species belonging to 51 genera. Stratigraphically constrained spores and pollen grains were used to construct three interval zones of middle-late Chesterian (late Viséan-Serpukhovian) to early Morrowan (middle Bashkirian) ages. The stratigraphic position of the Mississippian-Pennsylvanian boundary was determined in the lower Tyler Formation based on the last appearance of typical forms of the late Chesterian, including Tripartites vetustus, Knoxisporites triradiatus, Knoxisporites stephanephorus, Densosporites diatretus, and Schopfipollenites acadiensis, compared to the first appearance of early Morrowan Crassispora kosankei, Cirratriradites saturnii, Radiizonates aligerens, and Raistrickia saetosa. Palynofacies analysis and statistical clustering of the Tyler Formation showed three palynofacies assemblages. PFA-1 showed moderate relative abundances of phytoclasts and AOM, suggesting deposition close to fluvio-deltaic and shallow marine environments, while PFA-2 exhibited high abundances of phytoclasts, mostly of opaque wood, reflecting deposition in active river-dominated delta plains. PFA-3 showed the highest abundances of AOM, suggesting deposition in a shallow marine environment. Organic petrography and geochemistry data indicate that the Tyler Formation is one of the best source rock intervals throughout the midcontinent USA. Based on organic matter richness, the Tyler Formation is subdivided into three groups. The first group has TOC contents higher than 10 wt% of kerogen Types III, mixed II/III, and II with excellent hydrocarbon generation potential. The second group has TOC content in the range of 2–10 wt% of kerogen Types III, mixed II/III, and II, and fair to excellent hydrocarbon generation potential. The third group shows organic matter richness with TOC content below 2 wt% with good organic matter richness and kerogen Types III to IV, and poor to fair hydrocarbon generation potential. The organic matter thermal maturity is evaluated based on Tmax and VRo% values, suggesting that all samples of Tyler Formation are in the early to late stages of the oil window. However, care should be considered when assessing a mature source rock because the kerogen typing and generation is based on present-day TOC, S2, and HI rather than their original values.

Biomarker Hydrocarbons and Carbon Isotopes of Individual n-Alkanes in Paraffin Oils and Bitumens of the Middle Devonian–Lower Frasnian Terrigenous Complex of the Timan–Pechora Basin

Abstract ––The objects of study were bitumens of sedimentary rocks and paraffin oils of the Middle Devonian–lower Frasnian terrigenous complex of the Timan–Pechora basin. We studied the composition of biomarker hydrocarbons and the carbon isotope composition of individual n-alkanes of bitumens from the cores of the wells of the Omra–Lyzha saddle. The results were compared with similar data on the composition of paraffin oils from the south of the Pechora–Kozhva megaswell. The generation potential and thermal maturity of organic matter were studied by the Rock-Eval pyrolysis method. The maturity of organic matter in the rocks of the studied complex in the Omra–Lyzha saddle and the Pechora–Kozhva megaswell corresponds to the oil window. Data on the carbon isotopy of n-alkanes and on the composition of biomarker hydrocarbons do not contradict the possible genetic relationship between paraffin oils and the organic matter of the host sedimentary complex. Both the studied paraffin oils and the rock bitumens contain early eluting rearranged hopanes atypical of oils of other petroliferous complexes and show high contents of n-alkanes relative to iso-alkanes and polycyclic biomarkers. The content of 13C tends to decrease with an increase in the number of carbon atoms in the n-alkane molecule in both oils and bitumen rocks.

Biomarker signatures and depositional environment of ore-bearing black shale in the Luishia Cu–Co deposit, Democratic Republic of Congo: Implications for regional copper mineralization

The Central African Copperbelt is a globally important sediment-hosted stratiform copper cluster, with regional Cu–Co deposits controlled by Neoproterozoic rift basin. Ore formation in the Copperbelt involves multiple stages of mineralization during diagenesis, basin inversion, and collisional orogeny. Understanding the depositional environment of ore-hosting rocks is crucial for revealing the geological settings and pre-orogenic ore genesis. This paper presents a biomarker study focusing on the ore-bearing black shale in the Luishia Cu–Co deposit, Democratic Republic of Congo. The research investigates thermal maturity, source, and depositional environment of organic matter through rock organic carbon and pyrolysis analysis, chloroform bitumen “A” group component analysis, as well as saturated hydrocarbon gas chromatography-mass spectrometry (GC-MS) analysis. The results indicate significant thermal maturation of organic matter in Luishia black shale. Bacteria are identified as the primary source of organic matter followed by red algae and green algae. The sedimentary environment is characterized by a restricted rift basin under shallow marine conditions with a stratified water column. The processes of bacterial sulphate reduction (BSR) and thermochemical sulphate reduction (TSR) significantly influence the mineralization during diagenesis and basin inversion, respectively. Enhanced thermal maturity promotes methane generation which plays a role in reducing anhydrite and subsequently precipitating ore minerals. The findings have important implications for regional exploration strategies.

Thermal Maturity Constraint Effect and Development Model of Shale Pore Structure: A Case Study of Longmaxi Formation Shale in Southern Sichuan Basin, China

When the thermal maturity of the Longmaxi Formation in the southern Sichuan Basin is too high, the pore structure of shale becomes poor. Therefore, to investigate the effect of organic matter thermal maturity on shale pore structure, a study was conducted. Using the Longmaxi Formation shale in the southern Sichuan Basin as an example, the intrinsic relationship between shale porosity, pore structure parameters, organic matter laser Raman maturity, and organic matter graphitization degree was examined using X-ray photoelectron spectroscopy, particle helium porosity measurement, organic matter micro-laser Raman spectroscopy, and gas adsorption experiments. The results indicate that thermal maturity is the macroscopic manifestation of the graphitization degree of organic matter, and the correlation coefficient between the two is 0.85. A thermal maturity of 3.5% (with a corresponding organic matter graphitization degree of 17%) aligns with the highest values of shale porosity, pore volume, and pore-specific surface area across all pore size conditions. The evolution model of shale pore structure can be divided into two stages. The first stage is characterized by a thermal maturity between 2.0% and 3.5% (with a corresponding degree of graphitization of organic matter between 0% and 17%). During this stage, the number and connectivity of micro-macropores increase with increasing thermal maturity. The second stage is marked by a thermal maturity between 3.5% and 4.3% (with a corresponding degree of graphitization of organic matter between 17% and 47.32%). Basement faults are present, leading to abnormally high thermal maturity, poor preservation conditions, continuous generation of micropores, better connectivity, and a reduced number of pores. Medium macropores with good connectivity suffer from gas loss in the fracture network, leading to the collapse and disappearance of pores. The results mentioned in the statement have an important guiding role in the efficient exploration of shale gas in the Longmaxi Formation in the southern Sichuan Basin.

Characterization of a structural trap associated with an intrusive complex: the El Trapial oilfield, Neuquén Basin, Argentina

Abstract Volcanic plumbing systems emplaced in sedimentary basins may exert significant mechanical and thermal effects on petroleum systems. The last decade of research has evidenced that igneous intrusions may enhance thermal maturation of organic matter in source rocks and lead to both small- and large-scale structures that can deeply impact fluid migration or trapping. This contribution describes how the emplacement of a whole intrusive complex generated a dome structure of the overburden, which is the main trapping structure of a large producing oilfield. Our case study is the lower Miocene Cerro Bayo de la Sierra Negra (CBSN) intrusive complex, Neuquén Basin, Argentina, associated with the El Trapial oilfield where the main trapping structure is a large domal antiform centred on the CBSN complex. This study integrates the large subsurface dataset produced during the development of the El Trapial oilfield. More than 1200 vertical wells (producers and injectors) have been drilled in the flanks of CBSN complex. In addition, five 3D seismic cubes have been acquired over the years that have been merged and reprocessed into a single volume. Such a dataset allows a detailed characterization of both the structure affecting the Mesozoic strata and the geometry of the intrusive complex. Igneous rocks have been recognized along the entire stratigraphic section. Sill intrusions appear to concentrate in the shale units and the stacking of them has a direct impact on the doming structure generation. Our study allowed us to establish a direct correlation between the distribution of the intrusions and the extent, amplitude and style of doming, showing that the dome structure results from the emplacement of the intrusive complex. We also show that part of the doming is related to intrusions emplaced in the Mesozoic formations of the Neuquén Basin, whereas the other part of the doming is related to deeper structures not imaged on the geophysical data. We estimate that the amplitude of the doming reaches up to c. 500 m. The voluminous subsurface data, combined with exposed outcrops, makes the CBSN complex a world-class case study for showing how the shallow plumbing system of a volcanic complex may control the growth of large-scale trapping structures for various fluids, such as drinkable water, geothermal fluids and hydrocarbons.

Thermal maturity evaluation on mildly artificially oxidised sporomorphs: A comprehensive calibration of palynomorph darkness index (PDI) with vitrinite reflectance

The assessment of thermal maturity of organic matter dispersed in sedimentary rocks is a fundamental task in geological studies, including the modelling of burial history of sedimentary basins, palaeotemperature evolution and the hydrocarbon generation assessment. Palynomorph Darkness Index (PDI) is a quantitative method for evaluating thermal maturity, based on the RGB image-analysis of palynomorphs. The growing use of this method has demonstrated the need for calibration against other maturity indicators such as vitrinite reflectance (Avg. VRo), Tmax, SCI (Spore Color Index) and TAI (Thermal Alteration Index). In the present study, PDI was determined on continental palynomorphs and compared with a large dataset of vitrinite reflectance measurements from the same samples, covering a thermal maturity range between the immature and overmature stages of hydrocarbon generation. The selected samples, belonging to twenty-five different wells, were previously oxidised with nitric acid (60%HNO3) for approximately 20 min to facilitate routine palynostratigraphic investigation. Calibration against vitrinite reflectance resulted in two linear correlation equations, one for Avg. VRo below 0.7% and another for values above 0.7%. Additionally, six samples from the Selli level of the Aptian Gorgo a Cerbara section (Umbria-Marche Apennine, Central Italy) were analysed to understand the differences in PDI values between artificially oxidised and unoxidised organic matter. The analysis showed a discrepancy of approximately 19% in PDI values for the same thermal maturity between unoxidised and oxidised sporomorphs. Our results confirm that PDI is a reliable and valid method for evaluating thermal maturity of organic matter, particularly when standard methods cannot be applied. Moreover, comparison with published data concerning other palynomorph groups (i.e., acritarchs) further supports the usefulness of this technique in geological investigations.

Diagenetic characteristics and microscopic pore evolution of deep shale gas reservoirs in Longmaxi Formation, Southeastern Sichuan basin, China

The Lower Silurian Longmaxi Formation is the favorable target area for deep shale gas exploration and development in southeastern Sichuan Basin. Based on whole-rock X-ray diffraction analysis, scanning electron microscope, reservoir evolution thermal simulation experiment and nitrogen adsorption experiment, the diagenetic characteristics of deep shale reservoir in Longmaxi Formation were analyzed, and the reservoir pore evolution law was clarified. The results show that: ①The diagenetic minerals of the deep shale in the Longmaxi Formation are mainly quartz and clay minerals, with a small amount of carbonate minerals and feldspar. The primary inorganic pores are mainly controlled by mechanical compaction and cementation (quartz, carbonate, clay, pyrite). The organic pores are mainly controlled by the thermal maturity of organic matter, dissolution and later compaction. ②In the process of thermal simulation experiment, the organic pores of shale show a process of change from scratch, from small to large and then from large to small. Later, the organic matter is affected by compaction and graphitization, and the volume of micropores and mesopores begins to decrease. ③The shale pores of Longmaxi Formation have undergone several evolutionary stages. In the early stage of diagenesis, compaction caused a large number of inorganic pores to disappear. In the middle stage of diagenesis, kerogen hydrocarbon generation occupied pores, dissolution and cementation transformed pores. In the late diagenetic period, liquid hydrocarbon cracking gas and pressurization promote the development of organic pores.

A Comparison Study on the δD Value of Polycyclic Aromatic Hydrocarbons in Hydrous Pyrolysate of Herbaceous Peat with Different Water Medium

AbstractTo investigate the influence of diagenetic water media on the hydrogen isotopes of individual sedimentary aromatic compounds, a series of hydrous pyrolyses were conducted on herbaceous peat. Polycyclic aromatic hydrocarbons (PAHs) in hydrous pyrolysed samples and their hydrogen isotopic composition characteristics were studied. The aqueous medium demonstrated a significant influence on the hydrogen isotopic composition of the individual PAHs generated during pyrolysis. The results showed that the PAHs formed after pyrolysis in the presence of a saltwater medium with high δD value from a salt lake had a heavy hydrogen isotopic composition. The PAHs formed after pyrolysis in the presence of a fresh water medium with low δD value from a swamp had a light hydrogen isotopic composition. The difference in the average PAH δD value between the two hydrous experiments varied from –174‰ to –109‰, suggesting that the hydrogen isotopic composition of individual sedimentary PAHs can reflect the source of the diagenetic water medium. In addition, a comparative study found that the hydrogen isotopes of PAHs were superior to those of n‐alkanes in the same sample for diagenetic water indications. The results indicated that the exchange of water‐derived inorganic hydrogen and organic hydrogen was more intensive in freshwater experiments than in saltwater experiments. With an increase in the simulation temperature, the average δD value of PAHs generated in the hydrous simulation experiments showed an increasing trend, reflecting that the δD value of sedimentary PAHs formed with the participation of diagenetic water media was still closely related to the thermal maturity of organic matter. Comparative studies showed that the δD values of different types of organic compounds produced by hydrous pyrolysis of peat were in the order, PAHs > n‐alkanes > methane.

Organic richness and source rock potential of the La Peña Formation in the Sabinas and Piedras Negras basins, NE of Mexico

This paper investigates the petroleum generation potential of the Lower Cretaceous organic-rich rocks of the La Peña Formation in the Sabinas and Piedras Negras Basin (SB-PNB) of northeastern Mexico. In order to provide insights into the source rock potential of this unit, a total of 25 cutting samples collected from 15 wells at different stratigraphic intervals along the study area were analysed using Rock-Eval Pyrolysis technique. The analytical results revealed that all the analysed samples can be described as hydrogen-poor organic matter with HI <200 mg HC/g TOC. The organic richness indicates a poor to excellent Total Organic Carbon content suggesting that existed different conditions during the deposition and burial of this unit, causing variable organic matter production and preservation. Also, the results revealed a variation in the kerogen types that may be attributed to the relative stratigraphic positions of the selected samples in the wells.
 With respect to the organic matter thermal maturity, given by the geographical distribution of the Tmax values, the mature and overmature zone (435–508 °C) correspond to the Sabinas basin (SB).
 Overall, these results show a variability in the organic richness, thermal maturity and petroleum potential of the La Peña Formation, differentiating the values of the TOC content, petroleum potential and HI content for the samples from the Sabinas Basin and Piedras Negras Basin, respectively.

Diagenetic Evolution Sequence and Pore Evolution Characteristics: Study on Marine-Continental Transitional Facies Shale in Southeastern Sichuan Basin

Diagenesis and pore structure are essential factors for reservoir evaluation. marine-continental transitional facies shale is a new shale gas reservoir of concern in the Sichuan Basin. The research on its diagenesis pore evolution model has important guiding significance in its later exploration and development. However, the current research on pore structure changes, diagenesis, and the evolution of marine-continental transitional facies shale is not sufficient and systematic. In order to reveal the internal relationship between pore structure changes and diagenesis, the evolution of marine-continental transitional facies shale was tested by X-ray diffraction, field emission scanning electron microscopy, low-pressure gas adsorption, nuclear magnetic resonance, and the diagenetic evolution sequence and nanopore system evolution of Longtan Formation shale was systematically studied. The results show that the Longtan Formation shale underwent short-term shallow after sedimentation, followed by long-term deep burial. The main diagenetic mechanisms of the Longtan Formation shale include compaction, dissolution, cementation, thermal maturation of organic matter, and transformation of clay minerals, which are generally in the middle-late diagenetic stage. The pore structure undergoes significant changes with increasing maturity, with the pore volumes of both micropores and mesopores reaching their minimum values at Ro = 1.43% and subsequently increasing. The change process of a specific surface area is similar to that of pore volumes. Finally, the diagenetic pore evolution model of Longtan Formation MCFS in Southeastern Sichuan was established.

Intelligent geochemical interpretation of mass chromatograms: Based on convolution neural network

Gas chromatography-mass spectrometry (GC-MS) is an extremely important analytical technique that is widely used in organic geochemistry. It is the only approach to capture biomarker features of organic matter and provides the key evidence for oil-source correlation and thermal maturity determination. However, the conventional way of processing and interpreting the mass chromatogram is both time-consuming and labor-intensive, which increases the research cost and restrains extensive applications of this method. To overcome this limitation, a correlation model is developed based on the convolution neural network (CNN) to link the mass chromatogram and biomarker features of samples from the Triassic Yanchang Formation, Ordos Basin, China. In this way, the mass chromatogram can be automatically interpreted. This research first performs dimensionality reduction for 15 biomarker parameters via the factor analysis and then quantifies the biomarker features using two indexes (i.e. MI and PMI) that represent the organic matter thermal maturity and parent material type, respectively. Subsequently, training, interpretation, and validation are performed multiple times using different CNN models to optimize the model structure and hyper-parameter setting, with the mass chromatogram used as the input and the obtained MI and PMI values for supervision (label). The optimized model presents high accuracy in automatically interpreting the mass chromatogram, with R2 values typically above 0.85 and 0.80 for the thermal maturity and parent material interpretation results, respectively. The significance of this research is twofold: (i) developing an efficient technique for geochemical research; (ii) more importantly, demonstrating the potential of artificial intelligence in organic geochemistry and providing vital references for future related studies.

Silicified unicellular green microalga as a significant oil source in the Late Paleozoic alkaline lake-playa deposit, Junggar Basin, NW China

Siliceous mudstones and chert-shale couplets are primary source rocks for many large paleo-oil reservoirs over the world. It is well established that photosynthetic organisms can be exceptionally well preserved in silica precipitates, but the fate of the trapped organic matter (OM) during burial remains poorly studied. Here, dense unicellular microbial fossils have been recently discovered in the cherts of the Late Paleozoic alkaline saline lake deposit in NW Junggar Basin, NW China. We use petrographic observations and organic geochemical analysis to study the origin of unicellular microbes and how silicified organic matter generate and expel hydrocarbons. A 300-m-long oil shale core shows overall low levels of porosity, permeability and total organic carbon contents (generally <0.5 wt%), but very high extracts (on average 0.26 wt%), suggesting that the rich extracts were mostly indigenous and the original organic matter had a high hydrocarbon conversion. Analysis of biomarkers and silicified fossils both suggest that the bio-precursor for the organic matter is a special green alga. Reconstruction of silica diagenesis of cherts indicates that both the trapped lipids and hydrocarbons in silicified microbial cells and extracellular polymeric substances had been effectively expelled out during the transformations of Opal-A to microcrystalline quartz and during OM thermal maturation. Exceptional OM preservation and effective oil expelling ways render siliceous source rocks as excellent unconventional hydrocarbon reservoirs.

Formation and diagenesis of authigenic silicates in the Late Paleozoic alkaline lake deposits, Junggar Basin, NW China

Authigenic silicates are typical minerals of alkaline lake deposits. Their mineral composition and lateral distribution can be used for reconstructing paleoclimates, paleoenvironments, and diagenetic subsidence histories of hydrocarbon reservoirs. In particular the examples of pre-Mesozoic alkaline lake deposits were widely missing in the literature. To unveil this, a basin-wide high-resolution mineralogical investigation is conducted on the Late Paleozoic volcanic-related alkaline lake deposit of the Fengcheng Formation in the NW Junggar Basin, NW China. The investigated Late Paleozoic shales mainly contain stable silicates like K-feldspar, albite and reedmergnerite. Laterally, the authigenic silicates are mostly distributed in the lake-central and transitional zones and are especially abundant in organic-rich shales. Authigenic K-feldspar and albite were mainly formed by replacing detrital clay minerals and K-feldspar or directly precipitating from alkaline porewaters. Reedmergnerite was later formed by replacing acid-sensitive carbonate or feldspar minerals when organic matter was mature. Organic matter thermal maturation and hydrocarbon generation helped lower porewater pH and periodically open diagenetic systems. These processes facilitated fluid migration and element exchanges and promoted the transformation of thermodynamically unstable silicates and acid-sensitive carbonates to more stable silicates and quartz. A main result of this study is thus to emphasize the control of organic matter diagenesis and temperature on authigenic silicate evolution. This model can help revisit the genesis of the dominant felsic shales in alkaline lake deposits.

Evaluation of portable Raman spectroscopic analysis for source-rock thermal maturity assessments on bulk crushed rock

This study presents a simplified method and empirical relationships for determining organic matter thermal maturity using a portable Raman system equipped with a 785 nm laser, for analysis of crushed, whole-rock samples. Suites of rocks represented by shale and coal samples with various mineralogical composition, thermal maturity, and total organic carbon (TOC) were used to test the method and build correlations between Raman band separation (RBS) values and traditional thermal maturity indicators, organic matter reflectance (Ro), and programmed temperature pyrolysis (Tmax) values. A set of disparate shale samples, where both vitrinite and solid bitumen reflectance values were reported, have Ro values that range from 0.40 to 4.62%. Above 3.35% Ro, the corresponding RBS values plateau at ∼290 cm−1, thus correlations were evaluated with a linear regression (R2 = 0.96) between 0.40 and 3.35% Ro. Shale samples with Ro < 2% and Tmax < 551 were also used to correlate Tmax and RBS, yielding a linear correlation with an R2 of 0.94. For the coal data set, Ro values range from 1.21 to 4.08% and correlated RBS values plateau at ∼250 cm−1 above Ro = 3.0%, suggesting its correlative application below this maturity level. Several sample preparation methods were tested on cuttings material and standard deviation values for RBS were minimized by washing, drying, and hand crushing the material to pass through a 40-mesh sieve, although less preparation can still yield reliable results. The high degrees of correlation between whole-rock RBS data and two thermal maturity indicators demonstrate the utility of this approach for generating source rock thermal maturity data from minimally processed, whole-rock samples which could easily be applied in field or laboratory settings.

Diverse Origins of Gases From Mud Volcanoes and Seeps in Tectonically Fragmented Terrane

AbstractIdentification of methane origins remains a challenging work as current diagnostic signals are often not sufficient to resolve individual formation and post‐formation processes. To address such a knowledge gap in a tectonically active and fragmented terrain, samples from mud volcanoes, gas seeps, and springs distributed along structural features onshore and offshore of Taiwan were analyzed for their isotopic compositions of methane, nitrogen, helium, dissolved inorganic carbon, CO2, and water. Our analyses yielded Δ13CH3D and Δ12CH2D2 values ranging between +1.9‰ and +7.8‰ and between +3.0‰ and +19.9‰, respectively. A portion of the samples were characterized by values that represent the thermodynamic equilibrium at temperatures of 99°–260°C. These temperature estimates, together with the bulk isotopic compositions and local geothermal gradients (25°–30°C/km), suggest that methane was formed by thermal maturation of organic matter at depths of 2–9 km below the land surface and channeled upward along faults. Other samples were found to deviate from equilibrium by varying degrees. Considering the geological background, helium isotopic ratios, and nitrogen isotopologue compositions, and methanogens detected at some sites, these gases are either abiotic in origin or a mixture of microbial and thermogenic sources. Regardless of whether the equilibrium of methane isotopologues was reached, few sites hosted by sedimentary formations were characterized by mantle‐like helium signatures, indicating decoupled origins and potential degassing of helium from the relic igneous source. Overall, these results suggest the extraction of methane and other gases from multiple depths from strata fragmented by fault displacement in an active orogenic belt.

The environmental transformation mechanism of the coal- and oil shale-bearing interval in the Eocene Dalianhe Formation, Yilan Basin, Northeast China

Assemblages of multiple organic matter (OM)-related sedimentary energy systems are common in the Eocene nonmarine basins of Northeast China. During the Middle Eocene Climatic Optimum, oil shale developed in large quantities in the Yilan Basin. Numerous scientific papers have concentrated on the sedimentary cycle and industrial quality of the coal- and oil shale-bearing intervals, whereas few studies have studied the genetic environmental transformation mechanism of the coal and oil shale deposits. The Dalianhe Formation cover thick oil shale member and coal unit, which is a natural archive for studying the environmental evolution of lake swamp systems. In the present paper, the thermal maturity of organic matter (OM), oil shale industrial quality, coal rank, source of OM and paleoenvironmental conditions associated with OM accumulation in the Yilan Basin are investigated through bulk geochemical analyses, organic maceral analyses and biomarker composition analyses. The coal in the Yilan Basin is classified as subbituminous coal with high total organic carbon (TOC) values, high oil yields, low ash yields, and type II OM. The quality of the low-maturity oil shale in the Coal-Bearing Member is better than that in the Oil Shale Member. The oil shale in the Yilan Basin has type I-II OM and relatively high oil yields, which corresponds to the considerable content of liptinite with a predominantly alginite composition. Based on the higher Pr/Ph values, coal was deposited under oxic–dysoxic conditions, and the OM in the coal mainly originated from terrestrial plants, whereas the oil shale accumulated in a dysoxic environment with lower Pr/Ph values, and featured mixed OM sources, including terrestrial plants and low aquatic organisms. Affected by lake level fluctuations and wide peat swamping, the thickest and highest-quality coal and oil shale developed in swamps under a dysoxic–oxic water column in freshwater environments. Subsequently, the lake basin expanded rapidly, and the sedimentary environment changed to a dysoxic–oxic semideep–deep lake, in which extremely thick mudstone and a large amount of inferior oil shale with low oil yields accumulated under the influence of gravity flows. The basin continued to rapidly subside, the water column became anoxic–dysoxic, and the deposition of mudstone, siltstone and fine sandstone predominated.

Assessment of kerogen types and source rock potential of Lower Jurassic successions in the Mandawa Basin, SE Tanzania

Variability in organic matter types and quality of source rock intervals across evolving rift-basins has been the focus of previous studies, which have shown that due to dynamic sedimentary regimes and water depths such source intervals occur sporadically and restricted to specific stratigraphic horizons. Such heterogeneous settings require detailed geochemical analysis to document variations in source quality. This study presents a comprehensive geochemical analysis of source rock intervals in Lower Jurassic successions (Mbuo, Nondwa and Mihambia Formations) in the Mandawa Basin, on-shore Tanzania, in order to constrain organic matter types and quality. Organic matter richness was measured by Total Organic Carbon (TOC) analysis, while quality and thermal maturity of organic matter was assessed by programmed pyrolysis, complemented by organic carbon isotope analysis (δ13Corg) of sediment samples collected from outcrops, cores, and cuttings samples. TOC and programmed pyrolysis datasets from existing literature were synthesized in this study to better understand organic matter quality and thermal maturity evolution on regional scale. These data were examined in relation to paleogeography to gain insight into the factors regulating the distribution of organic matter quality within the Mandawa Basin.Average TOC contents in Lower Jurassic sediments fluctuate from 0.31 to 4.83 wt% with an average of 1.4 wt%, whereas, kerogen yield (S2) and hydrogen index (HI) remain generally low, with values ranging from 0.01 to 8.6 (mg HC/g rock) with an average (0.92 mg HC/g rock) as well as from 2 to 233 (mg HC/g TOC) with an average of 46 (mg HC/g TOC), respectively. Organic matter is composed mainly of type III (gas prone) to IV (inert) kerogens with lesser admixture of type II-III kerogen (oil-gas prone), which have been deeply buried and undergone a wider range of thermal diagenesis, from early to overmature stage. The δ13Corg isotopic composition of organic matter is consistent with kerogen typing, indicating terrestrial organic matter with a heavier signature during sea level low stands. Upon marine incursions δ13Corg shifts to isotopically more depleted marine organic matter. This implies that the investigated source intervals have poor to fair source potential. Only a few intervals of limited thickness in the Upper Mbuo, Nondwa, and Mihambia Formations of the Mita Gamma-1 and East Lika-1 wells, as well as in the Lower Mbuo Formation in the Mbuo-1 well, have been locally enriched. The enrichment is attributed to periodic marine ingressions that have persisted since the Pliensbachian period and suggests the presence of oil generation potential.A comparison of TOC and programmed pyrolysis borehole data from this study to those compiled from literature in a paleogeographical context reveals high spatial variability. The southeastern and northwestern parts of the study area have the highest source potential, indicating high organic matter productivity in the dysoxic to anoxic water column, which promoted the preservation of organic matter. In contrast, the eastern and western parts generally show low source potential, indicating degraded organic matter and significant terrestrial influxes from emergent land that were deposited in shallow, mixed oxygenated water with low to moderate organic matter productivity.

Relationship between fluorescence and IR parameters of oil inclusions and crude oils in the Dongpu Depression (Bohai Bay Basin, China)

The Dongpu Depression is an important petroliferous basin in China with abundance of hydrocarbons. The fluorescence characteristics of inclusion oils and their relationship with the present-day produced crude oils have never been systematically studied in the basin scale. This knowledge can further clarify ambiguities in petroleum system modeling and support exploration efforts in the area. To this end, core samples and crude oils were collected from the Shahejie Formation (Paleogene) in the Dongpu Depression to perform fluorescence and infrared spectroscopy from the source rock and the reservoir. Results show that the fluorescence characteristics of oil inclusions in the source rocks are consistent with oil inclusions in the reservoirs in the northern part of the depression, showing blue, green, yellow and orange fluorescence colors; whereas orange fluorescence is absent in the southern part in inclusions. Moreover, lack of a robust relationship between λmax and QF-535 suggests crude oils have gone through secondary alterations towards becoming heavy oil. Herein, comparison between the IR (CH2/CH3) and fluorescence parameters (λmax and QF-535) illustrates that CH2/CH3, although is suitable for organic matter thermal maturity assessment, cannot be directly applied to the expelled oils. Moreover, the occurrence of orange fluorescing oils in the horsts can be accounted for a higher hydrocarbon potential in these areas. Additionally, establishing a clear relationship between fluorescence and IR parameters failed in certain strata or structural unit, proving a complex tectonic setting of the depression. Overall, this study reveals the geologic nature of the Dongpu Depression is very complicated, which means additional analytical studies to resolve the oil-source correlation problem, in particular, would be necessary.

Organic matter accumulation and hydrothermal effect of the Barney Creek formation in the Glyde Sub-basin, northern Australia

The Glyde Sub-basin is a large petroliferous unit in the McArthur Basin, northern Australia, and it is situated ∼100 km southeast to the world-class McArthur River Zn–Pb hydrothermal deposit. The Paleoproterozoic Barney Creek Formation (BCF) is the hydrocarbon source rock in the sub-basin, and also the host rock for the McArthur River deposit. The controlling factors of organic matter accumulation and the hydrothermal effects on organic matter thermal maturity are not well-constrained for the BCF in the Glyde Sub-basin, hindering petroleum and mineral explorations. To address these issues, rock eval pyrolysis, reflectance measurement, trace element analysis, Nd–Hf and Re–Os isotopic analysis were conducted for the BCF samples taken from the drill-hole GRNT-79-5 in the Glyde Sub-basin. Analysis of trace elements and Nd–Hf isotopic ratios shows that cuttings of the BCF were derived from a mafic-dominant source (La/Ni = 0.89–2.27, La/Cr = 0.63–1.04, La/Sc = 3.16–4.95, La/V = 0.23–0.81, ϵNd = −5.01 to −2.98, ϵHf = −2.27 – 1.8), which may have contributed to a pronounced phosphorous delivery to the basin, resulting in enhanced primary productivity of seawater. Therefore, the high organic matter abundance in the BCF (TOC = 0.86%–5.04%) is most likely controlled by the primary productivity of seawater. The 187Re/188Os ratios (34.954–432.95) of a suite of samples negatively correlate with proportions of kerogen in organic matter (0%–94%), suggesting that burial temperature has reset the Re–Os isotope system. Hydrocarbon generation may have been promoted by hydrothermal fluid flows which relate to the McArthur River mineralization because the Re–Os isotope data yield an isochron age similar to the age of mineralization. The Re–Os isotope data provide direct evidence that the BCF in the Glyde Sub-basin was affected by hydrothermal fluids. However, the co-variation between thermal maturity indicators (Tmax = 436–444 °C; reflectance of solid bitumen = 0.34%–0.66%) and burial depth (97–512 m) suggests that organic matter thermal maturation primarily resulted from burial diagenesis rather than hydrothermal fluid flows. This indicates that temperatures of hydrothermal fluids was not high enough to significantly alter the thermal regime of the studied region.