Thermal Maturity Research Articles

Fault characterization and its significance on hydrocarbon migration and accumulation of western Tazhong Uplift, Tarim Basin: Insight from seismic, geochemistry, fluid inclusion and in situ U-Pb dating

Understanding the mechanisms of deep hydrocarbon accumulation is of paramount importance in the field of petroleum geology. Studying deep hydrocarbon accumulation mechanisms contributes to the development of effective reservoir management strategies and efficient extraction techniques. In this study, fluid inclusion observation, homogenization temperatures, in situ calcite U-Pb dating, geochemical analyses and seismic data were applied to investigate the hydrocarbon charging history of the western Tazhong Uplift, and the migration pathways were further analyzed to establish the history of oil and gas accumulation in the study area. The results reveal three distinct episodes of oil and gas charging processes in the western Tazhong Uplift. The first episode, determined by in situ calcite U-Pb dating bearing primary oil inclusions, occurred during the late Caledonian to early Hercynian orogeny. This episode is characterized by yellowish fluoresce color oil inclusions. The second episode involved a complex variation of the thermal maturity of oil. Mature oil, with oil inclusions preferentially fluorescing yellow-green color, was charged into the reservoirs near the Tazhong No.1 fault. Subsequently, the high mature oil characterized by blue-fluorescing oil inclusions was charged. At the same time, the mature oil migrated vertically into the reservoir away from the Tazhong No.1 fault along the strike slip faults. The third charging process is dominant by high mature oil in the Southwest of the western Tazhong Uplift, whereas the gas intrusion occurred in the area near the Shuntuoguole Low Uplift. Furthermore, the strike slip faults and reverse faults were distinguished based on the seismic profiles and coherence diagram. The intersections of strike slip faults and Tazhong No.1 fault developing more fractures served as oil and gas charging points. The oil production revealed that the strike slip faults acted as important hydrocarbon migration pathways. This study provides valuable insights into the processes of hydrocarbon accumulation and offers further evidence for deep oil exploration in the region.

Thermal Maturity Map Based on Vitrinite Reflectance of Yamama Formation in Basrah Oil Fields, South of Iraq

This study aims to create a detailed thermal maturity map based on vitrinite reflectance in the Basrah oil fields in southern Iraq. The specific objective was to determine areas with the highest thermal maturity within the Yamama Formation. The selected fields for analysis were WQ-60, Lu-12, Zb-42, Nr-12, Mj-12, and Ru-72, which belong to the West Qurna, Luhas, Zubair, Nahr Umar, Majnoon, and Rumaila oilfields, respectively. The Petromod 1D software was used for basin modeling and thermal history reconstruction. By inputting geological and geophysical data, the software enabled the construction of a comprehensive thermal basin history. The primary focus was on evaluating vitrinite reflectance values, which serve as a proxy for thermal maturity. The results of the study revealed significant variations in thermal maturity within the Yamama Formation across the selected fields. The Nahr Umar field (Nr-12) exhibited the highest thermal maturity, characterized by elevated vitrinite reflectance values. This indicates that organic matter within the Yamama Formation has undergone significant heating and maturation processes in this particular field. On the other hand, the Zubair field (Zb-12) displayed the lowest vitrinite reflectance values, suggesting a lower level of thermal maturity. This could be attributed to different burial histories, variations in the thermal gradient, or other tectonic movements in the area.

Cenozoic tectono-sedimentary evolution of the external rif chain (Morocco) derived from mineralogical and geochemical analysis of mudrocks

A model of the Cenozoic tectono-sedimentary evolution of the External Rif Chain (Morocco) is provided by means of the study of the mineralogical and geochemical composition of mudrocks. To date there was a lack of homogeneous data and of a complete and extensive study of the whole External Rif Zone (ERZ). Therefore, this work shows the study of the whole ERZ where the most representative stratigraphic sections have been selected. This work provides important information about the geodynamic evolution and the variations in source-area provenance related to the growing of the Rif orogenic belt. Although there is still much work to be done, this study aims to improve the knowledge of the Cenozoic tectono-sedimentary evolution of the entire western ERZ with a homogeneous method, with a focus on the paleogeographic and paleotectonic evolution, the paleoweathering and the source areas deduced from mineralogical and geochemical data of the Cenozoic mudrocks. The bulk mineralogy is mainly characterized by the presence of calcite, quartz and dolomite plus ankerite. Feldspars have few percentages. The clay minerals are principally represented by mixed-layer illite/smectite (I/S). Illite and kaolinite are in little amount. Femic minerals, mixed-layer chlorite/smectite (C/S) and chlorite are the most abundant. The I/S features suggest a different thermal condition for the three domains. The chemical composition indicates that the mudrocks can be described as mixtures of carbonates with aluminosilicate components. The Al/Ti, Th/Cr, Th/Sc, La/Th and La/Sc ratios, the Cr/V vs. Y/Ni plot, the V-Ni-Th∗10 and La-Th-Sc ternary diagrams indicate a predominantly felsic source with a minor mafic input more evident in the Paleocene-Eocene samples of the External Intrarif and Mesorif. The External Rif Zones changed in the Cenozoic from a passive margin to a complex foreland system with the incoming of the Alpine tectonic phases. In general, the felsic contribution should be linked to the foreland area consisting in the Middle Atlas and Mesetas massifs made of a crystalline domain. This margin probably presented an intermediate narrow oceanic branch in the External Intrarif-Mesorif boundary that surprisingly should start to close during Paleogene times providing the mafic contribution. This Paleogene tectonic activity in these domains is corroborated by the thermal maturity indicating late diagenesis. The chemical weathering indices, such as the CIA (Chemical index of Alteration) and its modifications, show medium-high values and thus suggest generally moderate paleoweathering conditions in agreement with the predominant amount of I/S.

The unconventional petroleum system of a mixed siliciclastic-carbonate sequence: Irati Formation, Paraná Basin, Brazil

Widespread rhythmic intercalation of a millimeter- to meter-scale hydrocarbon source rock (organic-rich shale and marlstone) and a carbonate reservoir (micritic and peloidal dolostone) are unusual. Here, the case of the Irati Formation (Paraná Basin) is reported, in which source rock strata have high total organic carbon (averaging 10 to 20 wt%), with dominance of highly oil-prone kerogen type I and II. Multivariate statistical analyses were applied to find patterns in the geochemical multivariate data set. The data points to three main productive sectors in the basin: Paraná Sector, Santa Catarina Sector, and Rio Grande do Sul Sector. Quality, quantity, and petroleum potential vary both laterally and vertically. On the other hand, the reservoir carbonate beds are predominantly composed of low-primary pore space, in which the effective reservoir capacity is determined by the presence of postdepositional secondary porosity. In addition, the Paraná-Etendeka magmatism, which occurred during the break-up of the Atlantic Ocean, played an important role in thermal maturation and primary oil migration.

Effects of organic richness, mineral composition, diagenesis, and thermal maturity on the viscoelastic properties of organic-rich Cretaceous mudstones in the Middle Magdalena Valley basin, Colombia

A major geological risk factor for engineering and energy-related applications, such as CO2 storage and unconventional hydrocarbon production, is the geomechanical integrity of mudrocks. The goal of this work is to better constrain the macroscopic mechanical properties resulting from microstructural components by investigating the effects of thermal maturity, mineral composition, diagenesis and organic richness on the mechanical properties of Cretaceous mudstones in two surface sections (San Vicente de Chucurí and La Cristalina Creek) and one well (La Luna-1) from the Middle Magdalena Valley Basin (MMVB), Colombia. The thermal maturity of 22 rock samples was evaluated via organic source-rock screening and determination of illite crystallinity (Reichweite). Both geothermometers placed the La Luna Formation rock samples from San Vicente de Chucurí and the La Luna-1 well within the oil generation window (80°C–120 °C). In contrast, the rock samples of the Guaguaquí Group from La Cristalina Creek in the southern part of the basin are within the gas window (>180 °C). The mineralogy of organic-rich mudstones in the MMVB is dominated by quartz and calcite. Moreover, mudstones from the MMVB have a wide range of organic richness and clay contents, with organic abundances comparable to and clay contents lower than those found in other source-rock reservoirs. The Young's modulus (EIT), Martens hardness (HM), and creep (CIT), as determined through microindentation experiments, highly varied, but overall, the findings suggest that the Cretaceous rocks of the MMVB present conditions favorable for hydraulic fracturing. Furthermore, the viscoelastic properties of Cretaceous rocks in the MMVB are influenced primarily by the concentrations of strong minerals (quartz and calcite) and weak components (clay and TOC). Specifically, an increase in the concentration of weak components compared with that of strong minerals leads to an increase in CIT and a decrease in HM and EIT, and vice versa. Unlike other source-rock reservoirs, there are no indications of a strong correlation between thermal maturity and mechanical properties, suggesting a decoupling of these parameters in rocks of the MMVB. A close scanning electron microscopy examination of the rocks indicated that the observed weak to moderate correlation between thermal maturity and mechanical properties appears to result from rock silicification and calcite cementation caused by the precipitation of these minerals at early stages of diagenesis. This observation implies that the impact of diagenesis on rock geomechanics of silica-rich and calcareous-rich mudstones may override or be more important than the effects of thermal maturity, which, along with clay and organic matter contents, are the greatest controls in the geomechanics of argillaceous source-rock reservoirs.

Mudstone diagenesis with depth and thermal maturity in the Cenomanian–Turonian Eagle Ford group. PART II: Diagenetic processes and paragenetic sequence

The Cenomanian–Turonian (C–T) Eagle Ford Group (EFG) is composed of massive, laminated, and/or burrowed limestones, marls, and lime mudstones deposited on the drowned south Texas shelf below storm-wave base. A burial diagenesis investigation using seven cored wells from shallow to deep burial (depth ranges from 7684 to 13,670 ft [2342 to 4166.6 m]; BHT from 183 to 285 °F [83.9–140.6 °C]) and from east (San Marcos Arch) to west (Maverick Basin) delineated a complex diagenetic history as a result of changes in water chemistry, redox condition, and burial history. The second part of this comprehensive study shows the variety of diagenesis and paragenesis. This study focuses especially in the organic-rich Lower Eagle Ford (LEF) member deposited largely under anoxic/euxinic conditions. Processes such as near seafloor sulfate-reduction, Fe-reduction, and methanogenesis drive various chemical reactions during lithification. Feldspar diagenesis included albitization and transformation of K-feldspar to kaolinite and albite, and from kaolinite to Mg-chlorite with depth. Clay mineral diagenesis is complex with authigenic kaolinite, mixed kaolinite/Mg-chlorite, and Mg-chlorite form in biota tests and matrix. Additionally, calcite dissolution, burial dolomitization, late Fe-chlorite replacement, and oxidation of pyrite to marcasite are also observed. Detritus-derived smectite starts to transform to mixed layered clay, illite-smectite (I/S) in the oil window; however, illitization trends were not observed in depth. This is because most smectite forms from alteration of volcanic ashes and the volcanic origin of the disorientated smectite did not follow regular illitization kinetics. The increase in Mg content from illitization transforms not only the kaolinite to chlorite, but precipitates authigenic dolomites. The clay minerals precipitated in the original interparticle pores, further reducing porosity, and impacted permeability, fluid saturation, wettability, and capillary pressure. With thermal maturation, bitumen and oil conversion increases oil-wetness of the rock and the resistivity measurement from wireline logs.

Hydrocarbons, Hydrogen, and Organic Acids Generation by Ball Milling and Batch Incubation of Sedimentary Rocks

Pulverized rock samples are widely used in laboratory experiments, e.g., to assess microbial or abiotic processes in batch incubation tests. However, it is unclear if ball-milled samples accurately reflect in-situ conditions and if observed processes are affected by by-products artificially generated during the sample preparation procedure. As such, this study examined the effects of dry ball milling on the release of gases, which include C1-C4 hydrocarbons, carbon dioxide (CO2), hydrogen (H2), and low molecular weight organic acids (LMWOAs) from different sedimentary rocks. The experiments involved pulverization using a gas-tight zirconium oxide planetary ball mill followed by wet and dry batch incubation and thermal desorption up to 200 o C. During milling, all sedimentary rocks, except a low organic carbon sandstone sample, yielded methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), H2, CO2, and unsaturated hydrocarbons, e.g., ethene (C2H4). Sandstones only yielded H2, CH4, and CO2. Stable carbon isotope signatures of these products are similar to thermogenic gases. The gases were also detected in subsequent wet incubation experiments (after gases from milling had been removed). Additionally, formate, acetate, and citrate, were detected in all samples except for sandstone. Pyruvate, malate, and succinate were also detected in some samples. Thermal desorption products of powdered limestone, shale, and pyrite concretion samples included organic acids, such as acetate and formate, which were higher in milled samples than in crushed particles (∼1 mm). The original geological thermal maturity of the studied sedimentary rock samples was low (below “oil window”) and, thus, considerably below “gas window”, suggesting that most of the detected gases were generated during ball milling. H2 generated during ball milling may be derived from the reduction of water from fluid inclusion or phyllosilicates, involving the formation of reactive mineral surfaces or radicals. Notably, the concentrations of gaseous hydrocarbons derived from milling are relatively high and comparable to “wet” gases. At the same time, these data indicate that substantial amounts of gases and LWMOAs might be artificially generated during laboratory batch experiments with milled samples. Hence, ball milled rock samples must be used with caution if assessing (bio-)geochemical processes for natural environments.

Comparative organic geochemistry of shale deposits of northern Appalachian Basin

The organic-rich black shales in the Appalachian basin are a vital producer of natural gas. In this study, we present new multiproxy geochemical data from the Ordovician and Devonian black shales in New York (NY) and Pennsylvania (PA). The samples include outcrop samples collected in NY (Utica Group and Marcellus Formation) and core samples from PA (Marcellus Formation, Skaneateles Formation, and Genesee Group). We combined organic geochemical data (% total organic carbon or %TOC, δ13Corg, C/N ratio, and lipid n-alkane distribution) with trace element (TE) data to identify the organic matter (OM) sources and depositional conditions. The TE analysis data shows that water conditions were variable during the deposition of these black shales, fluctuating between oxic and dysoxic conditions with occasional anoxia. There was probably a change from an open water condition (Co∗Mn = 0.2) during the deposition of the Flat Creek Formation to a more restricted exchange later during the deposition of the Indian Castle Formation (Co∗Mn = 2.9) in the Ordovician. Basin circulation likely remained restricted during the deposition of the Devonian black shales (Co∗Mn ranges from 0.4 to 1.3). Based on δ13Corg values (−32.9‰ and −29.6‰) that are more depleted than marine OM δ13Corg, C/N ratios (11.2 and 9.2) higher than marine OM, and the presence of longer chain n-alkanes in the range of C25 to C33, we suggest that bryophytes were possibly a significant organic source to the Ordovician Utica Group in NY. The kerogen type in Utica Group samples is type III, mostly terrestrial OM. The Devonian NY and PA samples show mostly bimodal distributions. In some samples, a secondary, though sometimes dominant, shorter chain peak in the range of C14 to C20 is present, in addition to the long chain peak. We suggest the bimodal n-alkane distribution signifies a mixed OM source consisting of terrestrial and marine contributions with differing degrees of thermal maturation compared to the samples with a unimodal distribution. Our results suggest that samples from the NY Marcellus Group are composed of type III kerogen, while samples from the PA Marcellus Formation, Skaneateles Formation, and Genesee Group contain both type II and type III kerogens.

Source Rock Characteristics of Loa Janan Area, Kutai Kartanegara, East Kalimantan, Indonesia

The source rock analysis was involved examining the geochemical content of hydrocarbons in fine-grained sedimentary rocks. This study was conducted on claystone in the Loa Duri area to determine the source rock characterization, which includes the level of richness, the organic matter type, and the level of maturity. The methods used are field data collection and laboratory analysis. A field study is used to collect rock sampling, while laboratory analysis consists of the analysis of Total Organic Carbon content, Rock-Eval pyrolysis, and vitrinite reflectance (Ro). The results of the analysis of Total Organic Carbon on six claystone samples showed values ranging from 0.50-8.97%. Generally, they had the potential to form hydrocarbons in the moderate to special category. Rock-Eval pyrolysis shows Hydrogen Index values from 52–149 mg HC/g Total Organic Carbon has the potential to generate gas, with Tmax values ranging from 414–432, indicating an immature to early mature level of thermal maturity. Ro analysis on three claystone samples with values ranging from 0.40–0.48 %. The hydrocarbon potential of the research area indicates the category of organic material richness is quite excellent, with kerogen including type III. The quality of the source rock based on the value of the Hydrogen Index is included in the category of gas-prone. The low maturity value (both Tmax and Ro) is thought to be caused by the suppression phenomenon in coal-dominated areas, so the values displayed by the two parameters do not correspond to actual values.

New Advance in the Study of Shale Oil Generation Peak Determination and Diagenetic Pore Evolution

Shale formations globally are widely distributed with abundant resources and varied thermal maturation ranges. However, the understanding of shale’s oil generation peak, diagenetic stages, and pore evolution remains incomplete. This study investigates shale samples of varying maturities and organic matter content from representative oil and gas basins in China and the United States. Comprehensive characterization was conducted using thermal simulation, rock X-ray diffraction analysis, N2 and CO2 adsorption, and mercury injection analysis. The study delineates the hydrocarbon generation process in shale, identifies the oil generation threshold, determines the peak oil generation, and categorizes shale’s diagenetic stages based on clay minerals and pore evolution. The results indicate: (1) highly mature shale exhibits delayed hydrocarbon expulsion and peak oil generation, starting at Ro values greater than 0.75% and reaching peak oil generation at Ro levels surpassing 1.2%. In contrast, peak oil generation in less mature shale initiates at Ro values of 1.1%, providing a more precise depiction of the shale’s diagenetic evolution stages; (2) the higher the TOC content of shale, the greater its hydrocarbon generation capacity, showing a robust positive correlation between hydrocarbon generation and TOC; (3) the diagenesis and pore evolution of shale can be categorized into four distinct stages: the early diagenesis stage (Ro < 0.5%), dominated by mesopores, and with reduced pore volume and surface area; the middle diagenesis stage A (0.5%–1.1%), where shale pore volume has been enhanced while the surface area has been reduced; the middle diagenesis stage B (1.1%–2.0%), where an initial decrease followed by an increase in mesopore volume occurs, along with a modest increase in macropores; and the late diagenesis stage (Ro > 2.0%), with increased organic pores and microfractures, while both pore volume and surface area expand. The study suggests that a Ro of 1.1% marks the peak oil generation period for shale, occurring during the early stage of middle diagenesis, characterized by larger pore volume and surface area, crucial for shale oil and gas enrichment.

Petroleum source-rock characterization and the depositional environment of Kimmeridgian–Tithonian sequences, Jaisalmer Basin, western Rajasthan, India

In hydrocarbon exploration, total organic carbon (TOC) content and Rock-Eval pyrolysis are commonly employed geochemical techniques that offer concise insights into kerogen type, effective source-rock identification and thermal maturity. In the current study, the data obtained from Rock-Eval pyrolysis has been used to define the source-rock quality, generative potential, kerogen type, maturity of the source sediments and kerogen kinetics of the Baisakhi–Bhadesar Formation of Kimmeridgian–Tithonian (154.7–145.6 Ma) age. Basinal level hydrogen index (HI), TOC content, source-rock maturity, transformation ratio and heat-flow maps have been generated by integrating the data from pyrolysis with previously available data from wells drilled in the basin. The TOC content of the Kimmeridgian–Tithonian sequence ranges from 0.03 to 12.71% in the studied samples, with an average TOC content of 1.28%, indicating good source-rock quality. The HI, in collaboration with T max and vitrinite reflectance (VR o ) data, demonstrates that the Baisakhi–Bhadesar Formation is characterized by type II, a mixture of type II/III and type III kerogen facies and exhibits good source-rock quality and poor to good generative potential in the basin. The studied samples are marginally mature to mature in nature ( T max , 430–450°C; VR o , 0.52–0.72%). A maturity analysis of the basin suggests that during the Late Jurassic most areas were under the oil window zone, except for the Bhakhari Tibba and Miajlar areas. The transformation ratio overlay for the Kimmeridgian–Tithonian source sequences shows better transformations of the source rock in the area of the Shagarh Sub-basin. Kerogen kinetics of the studied Baisakhi–Bhadesar Formation demonstrate that the activation energy ranges between 46 and 74 kcal mol −1 with the significant distribution of activation energy being 54 kcal mol −1 (42.07%), representing a strong heterogeneous type of organic matter in the sediments. Based on lithological, palaeontological and electrolog studies, a shallow-marine to nearshore environment of deposition with a sediment-input direction from the SE has been inferred for the Kimmeridgian–Tithonian sequences. The results of this study quantitatively establish the role of the Kimmeridgian–Tithonian sequence as a source rock, ultimately contributing to the generation of hydrocarbons in the basin along with spatial changes in the quality of source sediments in different parts of the basin.

Comprehensive evaluation of the Paleogene lacustrine source rocks within the Bodong Sag, Bohai Bay Basin using kinetics-based petroleum system modeling

The hydrocarbon potential of the Bodong Sag remains unclear. Investigation and comprehensive evaluation of Paleogene lacustrine source rocks is essential to determine the exploration potential and direction. This study examined the Paleogene source rocks using organic geochemical methods, including total organic carbon (TOC), Rock-Eval pyrolysis, maceral composition, vitrinite reflectance (Ro), and kinetic analysis. For the first time in this study area, a three-dimensional (3D) kinetics-based petroleum system modeling approach was employed to reconstruct the burial history, thermal maturity, and hydrocarbon generation history of the source rocks. The resource mass was evaluated based on the calculated generation mass. Results indicate that the third member (E2s3) and first member (E3s1) of the Shahejie Formation are mainly Type II1 organic matter, exhibiting high generation potential. The third member of the Dongying Formation (E3d3) contains mixed Type II1 and II2 organic matter, with moderate potential. The E2s3 and E3s1 source rocks matured early, entering the oil window in the Early Oligocene (∼30.3 Ma) and Middle Oligocene (∼28 Ma), respectively, and are currently in the wet gas to dry gas stage. The E3d3 source rocks matured later, entering the oil window at the end of the Oligocene (∼24.6 Ma) and are currently in the late oil to the wet gas stage. Subsidence and burial have resulted in higher maturity in the southern subsag compared to the northern subsag, with the margins remaining in low maturity to immature stages. Resource estimates for the Bodong Sag source rocks are quantified at 8.33 × 108 t of oil and 1.68 × 1011 m3 of gas. E2s3, E3s1, and E3d3 contribute 41.62%, 33.95%, and 24.43% respectively, with E2s3 source rocks being the major contributor. The southern subsag, accounting for 76.92%, is the primary hydrocarbon generation kitchen. The significant increase in natural gas resources highlights the prospects for natural gas exploration in the Bodong area.

Source Rock Assessment of the Permian to Jurassic Strata in the Northern Highlands, Northwestern Jordan: Insights from Organic Geochemistry and 1D Basin Modeling

The present study focused on the Permian to Jurassic sequence in the Northern Highlands area, NW Jordan. The Permian to Jurassic sequence in this area is thick and deeply buried, consisting mainly of carbonate intercalated with clastic shale. This study integrated various datasets, including total organic carbon (TOC, wt%), Rock-Eval pyrolysis, visual kerogen examination, gross composition, lipid biomarkers, vitrinite reflectance (VRo%), and bottom-hole temperature measurements. The main aim was to investigate the source rock characteristics of these strata regarding organic richness, kerogen type, depositional setting, thermal maturity, and hydrocarbon generation timing. The Permian strata are poor to fair source rocks, primarily containing kerogen type (KT) III. They are immature in the AJ-1 well and over-mature in the NH-2 well. The Upper Triassic strata are poor source rocks in the NH-1 well and fair to marginally good source rocks in the NH-2 well, containing highly mature terrestrial KT III. These strata are immature to early mature in the AJ-1 well and at the peak oil window stage in the NH-2 well. The Jurassic strata are poor source rocks, dominated by KT III and KT II-III. They are immature to early mature in the AJ-1 well and have reached the oil window in the NH-2 well. Biomarker-related ratios indicate that the Upper Triassic oils and Jurassic samples are source rocks that received mainly terrestrial organic input accumulated in shallow marine environments under highly reducing conditions. These strata are composed mostly of clay-rich lithologies with evidence of deposition in hypersaline and/or stratified water columns. 1D basin models revealed that the Upper Triassic strata reached the peak oil window from the Early Cretaceous (~80 Ma) to the present day in the NH-1 well and from ~130 Ma (Early Cretaceous) to ~90 Ma (Late Cretaceous) in the NH-2 well, with the late stage of hydrocarbon generation continuing from ~90 Ma to the present time. The present-day transformation ratio equals 77% in the Upper Triassic source rocks, suggesting that these rocks have expelled substantial volumes of hydrocarbons in the NH-2 well. To achieve future successful hydrocarbon discoveries in NW Jordan, accurate seismic studies and further geochemical analyses are recommended to precisely define the migration pathways.

Depositional environments and thermal maturity of the hydrocarbon source rocks in the Devonian–Early carboniferous Ora Formation from palynological organic petrographic investigations in northern and western Iraq

The study investigates the palynofacies, organic matter character, and hydrocarbon generation potential of this rock unit, based on surface and subsurface samples from the Ora Formation type-section in extreme northern Iraq and the Akkas-3 well in western Iraq, respectively. Dark, mostly oxidized, gelified platy amorphous organic matter (AOM) dominates the palynological components in the surface section, in addition to a few rounded spores and structured phytoclasts. In contrast, palynomorphs, including Ambitisporites avitus, Aneurospora spp., Vallatisporites verrucosus, Acinosporites spp., Verrucosisporites nitidus and the algae Botryococcus, are predominant in studied samples from the Akkas-3 well. Statistical cluster analysis identified three palynofacies types in the surface section based on stratigraphic variations in the particulate organic matter. These vary from high amorphous organic matter (AOM), moderate phytoclast, and low palynomorph abundances that represent proximal suboxic–anoxic shelfal environments to moderate to good AOM and low to moderate palynomorph abundances that represent distal suboxic–anoxic or distal dysoxic–anoxic shelfal environments. The organic petrographic study of the outcrop section also revealed the strong effect of oxidation, where dispersed terrigenous and amorphous organic materials in the form of granular and gelified forms dominate and reflect a terrestrial origin of these components. In the subsurface section, a mixed terrestrial and less marine or lacustrine origin characterized the studied organic matter, where land plant spores (sporinite), in addition to vitrinite and inertinite, are dominant with a few scattered liptinite macerals. The difference in thermal maturity between the outcrop and subsurface samples is likely due to the higher tectonic burial of the outcrop samples that form part of the Northern Thrust Zone of Iraq. Nevertheless, higher abundances of AOM (oil-prone kerogen type II) accumulated in the northern outcrop type section. This might imply that the Ora Formation has a higher potential for hydrocarbon production north of the Akkas field.

A new non-destructive method to decipher the origin of organic matter in fossils using Raman spectroscopy.

Ancient biomolecules provide a unique perspective on the past but are underutilized in paleontology because of challenges in interpreting the chemistry of fossils. Most organically preserved soft tissues in fossils have been altered by thermal maturation during the fossilization process, obscuring original chemistry. Here, we use a comprehensive program of thermal maturation experiments on soft tissues from diverse extant organisms to systematically test whether thermally altered biosignatures can be discriminated using Raman spectroscopy. All experimentally matured samples show chemical signatures that are superficially similar. Comparative analysis of Raman spectra following peak deconvolution, however, reveals strong tissue-specific signals. Application of this approach to fossils from the Bolca (49 Ma) and Libros (10 Ma) Konservat-Lagerstätten successfully discriminates fossil vertebrate soft tissue from that of fossil plants. Critically, our data confirm that a robust interrogation of Raman spectra coupled with multivariate analysis is a powerful tool to shed light on the taxonomic origins of thermally matured fossil soft tissues.

Crystal Resorption as a Driver for Mush Maturation: an Experimental Investigation

Abstract The thermal state of a magma reservoir controls its physical and rheological properties: at storage temperatures close to the liquidus, magmas are dominated by melt and therefore mobile, while at lower temperatures, magmas are stored as a rheologically locked crystal network with interstitial melt (crystal mush). Throughout the lifetime of a magmatic system, temperature fluctuations drive transitions between mush-dominated and melt-dominated conditions. For example, magma underplating or magma recharge into a crystal mush supplies heat, leading to mush disaggregation and an increase in melt fraction via crystal resorption, before subsequent cooling reinstates a crystal mush via crystal accumulation and recrystallisation. Here, we examine the textural effects of such temperature-driven mush reprocessing cycles on the crystal cargo. We conducted high-P-T resorption experiments during which we nucleated, grew, resorbed, and recrystallised plagioclase crystals in a rhyolitic melt, imposing temperature fluctuations typical for plumbing systems in intermediate arc volcanoes (20–40 °C). The experiments reproduce common resorption textures and show that plagioclase dissolution irreversibly reduces 3D crystal aspect ratios, leading to more equant shapes. Comparison of our experimental results with morphologies of resorbed and unresorbed plagioclase crystals from Mount St. Helens (MSH) (USA) reveals a consistent trend in natural rocks: unresorbed plagioclase crystals (found in MSH dacite, basalt and quenched magmatic inclusions [QMIs]) have tabular shapes, while plagioclase crystals with one or more resorption horizons (found in MSH dacite, QMIs, and mush inclusions) show more equant shapes. Plagioclase crystals showing pervasive resorption (found in the dacite and mush inclusions) have even lower aspect ratios. We therefore suggest that crystal mush maturation results in progressively more equant crystal shapes: the shapes of plagioclase crystals in a magma reservoir will become less tabular every time they are remobilised and resorbed. This has implications for magma rheology and, ultimately, eruptibility, as crystal shape controls the maximum packing fraction and permeability of a crystal mush. We hypothesise that a mature mush with more equant crystals due to multiple resorption–recrystallisation events will be more readily remobilised than an immature mush comprising unresorbed, tabular crystals. This implies that volcanic behaviour and pre-eruptive magmatic timescales may vary systematically during thermal maturation of a crustal magmatic system, with large eruptions due to rapid wholesale remobilisation of mushy reservoirs being more likely in thermally mature systems.

Experimental investigation of kerogen structure and heterogeneity during pyrolysis

Kerogen in shale microstructure plays a vital role in hydrocarbon generation, retention and accumulation. However, characterization of kerogen structure is challenging arguably due to a complex microporous structure and chemical heterogeneity. Thus, the role of kerogen molecular structure in controlling porosity and its heterogeneity, as well as its evolutionary processes, remains unclear. In this study, we investigate the chemical and pore structure of heated kerogen samples by employing a range of characterization tools including gas adsorption experiments, high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FTIR) spectroscopy, and laser Raman experiments for a broad range of pyrolysis temperatures (420–850 °C). Multifractal theory and peak fitting techniques were used to evaluate the controlling mechanisms of kerogen pore structure and heterogeneity. The results suggest that with increasing thermal maturity, the content of oxygen functional groups and long-chain aliphatic groups in kerogen significantly decreases, while the length of aromatic fringes increases and gradually becomes oriented. Furthermore, with increasing thermal maturity, the pore volume of kerogen exhibits a non-monotonic trend, i.e., a decrease, followed by an increase, and then finally a decrease. This is primarily influenced by processes such as hydrocarbon production weakening, secondary hydrocarbon cracking, and carbonization. The heterogeneity and dispersion of meso to macro-sized pores in kerogen increase with temperature, while the connectivity gradually decreases. The chemical structure and spatial distribution of kerogen significantly affect the heterogeneity, connectivity, and dispersion of meso to macro-sized pores. Specifically, high aromaticity, structural disorder, and short aliphatic chains result in enhanced heterogeneity and dispersion of meso to macro-sized pores, reducing pore connectivity. This study thus contributes to a deeper understanding of the evolution of organic matter pores.

The application of diamondoids indices for oils from Block 4 of the Muglad Basin in the middle of Africa

The abundances and distribution diamondoids were studied with a series of crude oil samples from Block 4 of the Mugald Basin in Sudan, to investigate the validity of various diamondoid parameters on the oil source, thermal maturity and biodegradation information. All crude oils of Block 4 were generated by lacustrine source in lower part of the Abu Gabra Formation as a whole, and can be further divided into two crude oil groups: oil group of the Neem-Azraq Uplift and oil group of K-3 well and B-1 well. They are all mature oils within oil window and four crude oils from the Neem-Azraq Uplift were slightly biodegraded. The concentrations of diamondoids in oils of K-3 well and B-1 well, especially the concentration of diamantanes, are higher than those of oils from the Neem-Azraq Uplift. The concentrations of diamondoids are mainly controlled by source of oil, while the influence of thermal maturity and biodegradation may be limited. The isomerization ratios as indicators of thermal maturity are inapplicable at the mature stage, but DMDI-2, 4,8/3,4-DMD and 1-EA/3-EA can clearly distinguish the two groups of crude oils. The concentration ratios of oils from K-3 and B-1 wells are significantly lower than these of oils from Neem-Azraq Uplift, and these concentration ratios including newly proposed parameters such as MAs/DMAs, DMAs/TMAs, D/MDs, MDs/DMDs and so on, are effective indicators on organofacies and oil group division within oil window.

Evolution of pore structure and diagenetic stages of late Permian shales in the Lower Yangtze Region, South China

This study examines the evolution characteristics of shale pores in the Late Permian Longtan Formation located in the Lower Yangtze region of South China. The complete process of thermal evolution of the samples was achieved through thermal simulation experiments while identifying qualitatively the pore types and their development characteristics using field emission-scanning electron microscope (FE-SEM). Quantitative analysis of pore size distribution was conducted through mercury intrusion capillary pressure (MICP), nitrogen (N2) and carbon dioxide (CO2) adsorption experiments. The paper comprehensively analyzed the pore evolution characteristics and controlling factors of shale in the study area, with an analysis of diagenetic differences. Findings reveal that during the pore evolution process, both morphology and pore size are influenced by thermal maturity. The pore volume is dominated by mesopores and macropores, while the specific surface area is mainly dominated by mesopores and micropores. Thermal evolution promotes the formation of micropores and mesopores but hinders the development of macropores. Moreover, clay minerals transformation and mineral dissolution make certain contributions to the development of micropores. The diagenesis in the study area is controlled primarily by the pyrolysis of organic matter. Pyrite and clay minerals are the first to dissolve, followed by calcite and quartz. Five stages of evolution characterization have been identified from low-mature stage (Ro = 0.88 %) to overmature stage (Ro = 3.35 %) in combination with diagenesis. The development of pore structure and its influencing factors vary across different stages. The influencing factors mainly include hydrocarbon generation from organic matter, compaction, mineral transformation, and dissolution. The process of hydrocarbon generation in organic matter occurs throughout the entire pore evolution process, resulting in the development of numerous micropores and mesopores. Compaction primarily impacts pore development during the early diagenetic stage, causing a substantial transition of primary mineral pores from macropores to mesopores. Mineral transformation and dissolution take place during and after the middle diagenetic stage. The former governs the development of mesopore-sized clay interlayer pores, while the latter primarily generates micropores.

Acritarch Palynomorph Darkness Index (PDI) as an indicator of thermal maturity in Silurian sections from Saudi Arabia

Palynomorph Darkness Index (PDI) measurements on acritarchs from Silurian strata in Saudi Arabia exhibit an irregular but potentially useful relationship to other vitrinite reflectance proxies. Four distinct stages in PDI evolution can be recognized with thermal maturity. In Stage 1, acritarch PDI is <10% and shows no increase with increasing maturity. A very rapid increase in PDI towards the end of the oil window from <10% to ca. 25% characterises Stage 2, which is followed by a gradual increase in Stage 3 from ca. 25% to approximately 80%. PDI Stage 4 is represented by postmature rocks, with acritarch PDI >82% and showing little change with increasing maturity. Acritarch fluorescence is totally extinguished within the oil window.