Thermal Alteration Index Research Articles

The biostratigraphy and palaeobiogeography of Cambrian and Ordovician acritarchs and chitinozoa from the Simeh-Kuh, NW Damghan City, the Alborz Mountains, northern Iran

A study in Simeh-Kuh, northern Iran, focuses on Ordovician strata, documenting acritarchs and chitinozoa, and has helped to understand ancient environments. The aim is chronostratigraphical calibration, using palaeontological data including brachiopods, trilobites, and graptolites. Ninety-three surface samples were collected and analysed, yielding a rich collection of 117 morphotypes, including 59 acritarch species and 58 chitinozoan species. Eight acritarch assemblage biozones (A–H) and 14 chitinozoan biozones (1–14) were established. Biozones A–F indicate Late Cambrian to Middle Ordovician ages in the Lashkarak Formation, while biozones G–H indicate the Late Ordovician in the Ghelli Formation. The Eremochitina brevis-Linochitina pissotensis biozones suggest late Floian to late Darriwilian ages, and the Belonechitina robusta-Armoricochitina nigerica biozones support a Late Ordovician age. These findings place the Alborz Mountains in the North Gondwana Domain during the Ordovician. Palynomorphs with a thermal alteration index of 4.0–4.5 indicate overmature organic matter in Lower Palaeozoic deposits, potentially linked to the Ordovician metamorphic event in the southern Caspian Sea. Additionally, four new species were identified: Cyathochitina sp. A, Tanuchitina hosseini-nezhadi sp. nov., Dactylofusa kazzazii sp. nov., and Inflatarium alborzensis sp. nov.

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.

PALYNOFACIES AND ORGANIC GEOCHEMISTRY OF LACUSTRINE SOURCE ROCKS: THE POTRERILLOS – CACHEUTA SOURCE ROCK SYSTEM IN THE TRIASSIC CUYO BASIN, WEST‐CENTRAL ARGENTINA

This study presents an integrated investigation of the Upper Triassic Potrerillos – Cacheuta lacustrine source rock in the Cuyo Basin of western Argentina. Data came from palynofacies analyses, organic petrography, Rock‐Eval pyrolysis and mineralogical studies based on X‐ray diffraction analyses. An 80 m thick outcrop section was studied and is interpreted to represent the transition from shallow‐lacustrine sediments influenced by fluvial discharges (uppermost Potrerillos Formation) to the deposits of a deep, permanent lake (Cacheuta Formation). Three palynofacies were defined. Palynofacies I is characterized by shallowing‐upward cycles with abundant woody material, and was deposited under an oxic, disturbed water column. Palynofacies II and III occur in laminated shales rich in amorphous organic matter (AOM) and freshwater algal material (Botryococcus) respectively, which were deposited under oxygen‐depleted conditions. In general, the detrital material present suggests an input derived from fluvial discharges; however, interbedded tuffs altered to analcime and smectite suggest the transformation of vitric material in pyroclastic ash under saline to alkaline water conditions. Kerogen Types II/III and III with high total organic carbon values indicate a moderate oil‐ and gas‐prone source rock whose thermal maturity varies from immature to the early oil window (Tmax: 430‐438 °C; vitrinite reflectance: 0.59‐0.67 % VRo; and thermal alteration index: 2‐2+).This study demonstrates the importance of palynofacies analyses for the interpretation of depositional changes and associated controls in lacustrine shale successions. When integrated with data from organic geochemistry, palynofacies analysis is an important tool in the evaluation of a source rock's thermal maturity and hydrocarbon generation potential.

Integrated organic, inorganic geochemical and palynofacies analyses to characterize the paleoenvironment of the Tortonian (Late Miocene) Wakar Formation, offshore Nile Delta, Egypt

The subsurface Wakar Formation (Fm) has been identified in all wells of the offshore concessions in the eastern Mediterranean in Egypt. This formation is important because it represents the end member of the Late Miocene (Tortonian) siliciclastic sedimentary rock of a vertical thickness between 500 and 900 m in these fields and is a promising formation for gas production in Egypt. There are no comprehensive studies on the geochemistry and palynofacies of this Wakar Fm in literature. Therefore, sixteen collected ditch samples of the entire thickness of the Wakar Fm from a PFM-SE -1 well were investigated. The main objective is to reveal the history of depositional events that prevailed in North Africa at this time. TOC content (average = 0.81 wt%) and generative potential of the Wakar Fm are of poor and fair quantity. The organic matter (OM) belongs to types IIIC and III of coastal and delta settings as revealed by the palynofacies and geochemical data. The Tmax of the samples does not exceed 435 °C (419–432 °C) of the immature stage, which corresponds to the carbon preference index (CPI >1) (1.15–1.4). The thermal alteration index range between 2 and 2+, indicating immature to marginally mature OM. GC of the two extracted samples shows a predominance of short-chain and mid-chain n-alkanes derived mainly from algae and seagrass; the Pr/Ph ratio (3.24–3.02; suboxic) and low terrigenous/aquatic ratio (TAR; 0.23–0.47) indicate that the OM bearing sediments were accumulated in a transitional setting of the suboxic aqueous medium. The dominance of terrestrial OM and low amorphous organic matter also suggests deposition in a marginal basin under dysoxic-anoxic to suboxic-anoxic conditions. Pr/n-C17 versus Ph/n-C18 diagrams confirm that preservation occurred in weakly oxidized environments from a source of kerogen type III gathered from a swamp and transitional marine zones. The inorganic geochemical analyses (XRD and XRF) and the calculated paleoenvironmental indices (i.e; log (Fe2O3/K2O) versus log (SiO2/Al2O3)) indicate that the main composition of the Wakar Fm is a combination of iron shale and sandstone facies. The Fe-sandstone facies occupies the middle part of the formation with a thickness of 170 m, which is the pay zone in the present concession. The parent protolith of Wakar Fm is a combination of intermediate and mafic igneous rocks. The prevailing paleoclimate is arid and semi-arid conditions synchronize the base, and top of the Wakar Fm, which is responsible for weak to moderate chemical weathering as revealed from chemical index of alteration values (CIA average = 67.99). Discriminant functions exhibit a depositional setting belonging to an active continental margin and oceanic island arc between marine and terrestrial zones under oxic-dysoxic environments.

The sedimentary filling differences of subsags and their impacts on oil shale and coal accumulation in a rift lacustrine sag: A case study from the Lower Cretaceous in the Zhangqiang sag, southern Songliao Basin, Northeast China

In order to characterize the sedimentary filling differences of two rift lacustrine subsags and their control factors and to construct the depositional model for coal and oil shale, the Lower Cretaceous Jiufotang and Shahai formations in the Qijiazi and Zhanggutai subsags, Zhangqiang sag, Songliao Basin, one of the most petroliferous basins in Northeast China have been analyzed. Seven facies were identified in the Lower Cretaceous strata of two subsags and were grouped into three facies associations, namely alluvial fan, fan delta and lacustrine. During the deposition of Jiufotang Formation, the Qijiazi subsag is mainly characterized by alluvial fan deposits, while the Zhanggutai subsag is dominated by fan delta and littoral-sublittoral lacustrine deposits. During the deposition of Shahai Formation, the Qijiazi subsag is characterized by fan delta, littoral-sublittoral lacustrine, and deep lacustrine deposits, whereas the Zhanggutai subsag is filled with fan delta and littoral-sublittoral lacustrine deposits. Coal and oil shale deposits are only preserved in the Shahai Formation of the Qijiazi subsag. Total organic carbon content, organic matter type, vitrinite reflectance, thermal alteration index and total reduced sulfur content also show that there are differences in organic matter preservation and redox conditions between two subsags. In the Jiufotang Formation, the total organic carbon contents are relatively low, the kerogen type is dominated by type III and the source rocks entered a mature phase in the Zhanggutai subsag, and the total reduced sulfur contents show a suboxic to oxidized environment in the Zhanggutai subsag and an oxidized environment in the Qijiazi subsag. In the Shahai Formation, the total organic carbon contents are relatively high, the kerogen type is dominated by type II and the source rocks entered an immature to mature phase in the Zhanggutai subsag, whereas the kerogen type is dominated by type III and the source rocks display an overall mature phase in the Qijiazi subsag. The total reduced sulfur contents of the Shahai Formation show the reduced environment in two subsags. Tectonics, sediment supply and redox conditions control the temporal and spatial differences in sedimentary filling and distribution of organic-rich rocks in two subsags. A comparison of rift lacustrine Qijiazi subsag and Fushun Basin shows that the rift basins are both characterized by deepening-upward to shallowing-upward successions controlled by tectonic subsidence and climate changes. The relatively high subsidence along with warmer climate resulted in the enhanced input of lake plankton and the decrease in higher plant input and triggered the transition from coal to oil shale formation.

Sedimentology, Palynofacies, and Hydrocarbon Potential of the Early Cretaceous Sarmord Formation from Khabbaz and Kirkuk Oil Fields, Northern Iraq

The Early Cretaceous Sarmord Formation of northern Iraq has been studied from two subsurface sections, Kirkuk Oil Field (well K-229) and Khabbaz Oil Field (well KH-14), through petrographic, palynolofacies and organic geochemical analyses. The study reveals presence of two groups of microfacies settings. The first is the inner ramp, and the second represents the middle to outer ramp setting. The palynofacies study indicates that the amorphous organic matter is the predominant components (87-96%), followed by phytoclasts (3-11%) and palynomorphs (0-3 %). One main palynofacies type has been identified, which includes three secondary palynofacies types (PF1 A, PF1B, and PF1C) and refers to suboxic to anoxic basin condition. Thermal alteration index measured for the KH-14 well, indicated an immature to mature (TAI= 2, +2, 3) level, while, for the K-229 well mostly immature (TAI= -2, 2). The Rock-Eval analysis indicates that the TOC wt.% content of the samples range between 0.11% and 0.82% (average 0.29%) and 0.17% and 0.56% (average 0.35%) for K-229 well and KH-14 well, respectively. The Tmax values show that the most of analyzed samples of the both sections located in the immature zone, while the few samples from K-229 refer to mature zone. Another immature stage evidence is the shape of pyrograms with minor S1 and the large S2 peak, Tmax value less than 435°C, and PI around 1.0; all these criteria support the immature stage for these rock samples.

An evaluation of the hydrocarbon potentials of the gboko formation, middle benue trough, nigeria

A total of seventeen (17) rock samples from the Albian Gboko Formation were collected from the quarry of Benue Cement Company for organic geochemical analysis, in order to evaluate its hydrocarbon potential . Total Organic Carbon (TOC) values for the samples ranges from 0.11-0.5wt%. The rock-eval data recorded S2, hydrogen index (HI), Oxygen index (OI), Tmax, and production index (PI) of 0.04-0.11, 25-54, 50-400, 310-4610C, and 0.4-0.6, respectively. These implies that the samples has a fair to poor source rock quality with a type IV kerogen. Optical methods (R0, TAI & VKF) conducted on three selected samples of shales reveal vitrinite reflectance (R0) of 0.77%, 1.11% and 1.54%, while thermal alteration index (TAI) recorded 2.7, 3.3 and 3.7. Kerogen fluorescence indicated weak to non fluorescence. These results infer that the sediments had evolved from oil to gas window due to severe thermal effects . This indicates their potential to generate hydrocarbon gas

Burial and thermal history of the eastern transform boundary of the central western carpathians based on 1D basin modeling

This paper represents the first data on the thermal and burial history of Mesozoic and Cenozoic sedimentary rocks in the Maruszyna IG-1 deep borehole from the transpressional shear zone of the Western Carpathians. Representative, organic-rich mudstone and claystone samples were selected to determine thermal maturity using mean random vitrinite reflectance (Ro) and thermal alteration index (TAI) data. The reconstruction of the thermal history of fold-and-thrust units was carried out using 1D modeling. This study uniquely distinguishes various thermal maturity levels in the deep profile determined by strata repetition, different absorption of clay and carbonate mineral settings and structural thermal changes in humic particles and palynomorphs. Reflectance of autochthonous vitrinite varies from 0.77 to 1.95% indicating thermally mature to overmature organic matter. Different subsidence-uplift models were tested during modeling in order to find a plausible scenario of tectonic development and heat flow history. The thermal maturity of the individual thrust sheets was modeled separately allowing their individual thermal histories to be investigated before and after the main and final thrusting phase. The best-calibrated scenario assumes a low paleo heat flow ranging from 45 to 54 mW.m-2, affecting the eastern part of the Western Carpathians. The maximum temperature which lead to maturation of the organic matter seems to be related to its successive burial during Cretaceous due to compressional deformation. Individual thrust sheets reached thermal maturity prior to the final thrusting phase. The overthrust was accompanied by 3000–4000 m burial/erosion resulting in different thermal pathways during wedge growth.

Optical and geochemical assessment of petroleum source rock potential and palaeodepositional environments of Cretaceous mudstones, Koum Basin, North Cameroon

Limited organic matter (OM) characterisation studies on Cretaceous mudstone successions in the Koum Basin have resulted in a significant knowledge gap in the source rock potential; a petroleum system prerequisite. Field mapping and lithofacies analysis of outcrop sections point to fluvial channel and marginal to profundal lake deposits. Representative mudstone samples were evaluated using multiproxy organo-petrographic and bulk geochemical analyses to assess the kerogen type, thermal maturity, hydrocarbon generation potential, and deduce the paleodepositional environment. Results reveal that ∼90% of the samples are organic-lean and ∼10% organic-rich with TOC content of 2.81–6.91 wt%. Rock-Eval pyrolysis S2 yields of 14.50–54.18 mg HC/g rock and hydrogen index (HI) 784–846 mgHC/gTOC, suggest excellent potential for oil generation. High S1 values of 1.18–1.45 mg HC/g rock and low production indices (PI) of 0.02–0.03 indicate in situ generated free hydrocarbons. PI and Tmax of 438°C–443 °C, consistent with measured vitrinite reflectance (Ro% = 0.28–0.53%), Spore Colour Index (SCI = yellow to golden brown) and Thermal Alteration Index (TAI = 1.5 to 2.6) on the Chevron scale, portraying an immature to marginally mature OM. Oil-prone kerogen Type I-II dominates and to a lesser extent gas-prone Type III, derived from algal-terrestrial organic facies, typified by the predominance of amorphous OM 51.2%, liptinites 24.6%, vitrinite 16.4% and inertinite 7.8%. Based on lithofacies, and palynological evidence, a fluvio-lacustrine depositional setting with limited marine influence, under fluctuating warm humid to hot semi-arid paleoclimates, is inferred. A comparison with surrounding petroleum-producing basins further enhances the understanding of the Koum Basin's hydrocarbon potential within the West Central African rift system context.

Paleomagnetism and Rock Magnetism of Permian sedimentary rocks and Early Jurassic Karoo (Large Igneous Province) sills intersected in the KWV-1 borecore, Eastern Cape Province, South Africa

The nearly 100 Ma record of almost continuous shallow marine to non-marine sedimentation from the Late Carboniferous to the Early Jurassic recorded in the Karoo Basin, South Africa, includes coal deposits and potentially significant shale gas occurrences. As such, numerous studies have addressed the thermal history of the Karoo Basin, using a range of approaches. We use paleomagnetic and rock magnetic data to evaluate the paleotemperature history of the host Permian strata (Ecca Group and lowermost Beaufort Group) intersected in the ∼2320 m deep KARIN KWV -1 drilling project core, obtained in 2015. Where cored, the Karoo sedimentary sequence has been intruded by several Early Jurassic mafic sills, belonging to the Karoo Large Igneous Province, of thickness ranging from less than 10 cm to ∼150 m. The continuous core provides a unique natural laboratory for studying the effects of Karoo magmatism on the basin's sedimentary sequence. The KWV-1 drilling project was spudded ∼10 km east of Willowvale town, Eastern Cape Province.The spatial variation in thermal effects in host sedimentary strata, relative to proximity to Karoo sill contacts, is well-defined on the basis of anisotropy of magnetic susceptibility (AMS) data, paleomagnetic demagnetization response, and thermal alteration index (A40) values based on temperature vs. magnetic susceptibility tests. A zone of pervasive remagnetization of host sedimentary strata characterizes the contact aureoles, the spatial extents of which are to a first order dictated by the thickness of each Karoo sill. A40 data are interpreted to show that ambient paleotemperature estimates increase from ∼150 °C in lower Beaufort/upper Ecca strata to ∼285° in Lower Ecca strata that are sufficiently removed from Karoo sills. However, lower Ecca Group rocks in close proximity to Karoo sills were likely heated to ∼500 °C. Natural remanent magnetization (NRM) intensities of rocks in proximity to sills range from ∼17 to ∼ 438 mA/m, and susceptibilities range from 1.3 to 3.2 × 10−3 SI volume. In contrast, for strata sampled at distances greater than the width of the nearest sill, NRM intensities range from 0.056 to ∼4 mA/m, and susceptibility values range between 193 and 488 X 10−6 SI volume. For host sedimentary rocks, remanence is completely unblocked at temperatures ranging from ∼350 °C to ∼630 °C and acquisition and backfield DC demagnetization of isothermal remanent magnetization (IRM) indicate that specimens are fully saturated in fields of ∼200–600 mT, with coercivity of remanence of ∼30–100 mT. All of the eleven Karoo sills examined in this study yield remanence inclinations that are moderate to steep negative, and are inferred to have recorded a normal polarity field, possibly during a single chron. Our results are interpreted to suggest that substantial thermal effects of Karoo sills on the host sedimentary strata are largely confined to the contact aureoles, with a width that is dependent on sill thickness. We hypothesize that parts of the Ecca sedimentary sequence have remained at ambient, basinal burial temperatures below about 150 °C, and thus that an early-acquired Permian remanence may possibly be preserved in these sedimentary rocks at sufficient distance from thermal contact aureoles. Alternatively, much of the sedimentary section may have been subject to a combination of thermoviscous/chemical remagnetization.

Integrated organic geochemical/petrographic and well logging analyses to evaluate the hydrocarbon source rock potential of the Middle Jurassic upper Khatatba Formation in Matruh Basin, northwestern Egypt

The present national and international pursuit of converting Egypt into a regional hub of natural gas in the East Mediterranean region resulted in conducting extensive exploration activities in several areas of northern Egypt. Therefore, organic petrographic, geochemical, and geophysical analyses were carried out on the upper Khatatba Formation from the OBA 2-2A well in the Matruh Basin.Restoration of the original sedimentological and geochemical characteristics of the upper Khatatba Formation suggest deposition of this formation in deltaic settings. Few shale and partly calcareous shale units of the Upper Safa and the uppermost Zahra members accumulated in TST in prodelta settings under dysoxic-anoxic conditions. These shale units had an original gas/oil-prone organic facies BC. Other shales and partly calcareous shales of the Upper Safa and Zahra members accumulated in HST in deltafront settings under relatively oxic conditions and had an original lower gas-prone organic facies C.The Upper Safa Member had an original active fair to excellent organic richness (0.76–7.35, avg. 3.38 TOCo(live) wt%) and hydrocarbon source rock potential (S2o: 2.22–17.97, avg. 8.05 mg HC/g org. C). It was characterised mainly by original active mixed gas/oil-prone kerogen type IIIo(live)-IIo(live) (HIo(live): 200.27–297.34, avg. 238.03 mg HC/g TOCo(live)) and subordinately by gas-prone kerogen type IIIo(live) (HIo(live): 181.50 mg HC/g TOCo(live)). The Upper Safa Member had original poor to fair proportions of active oil-prone (0.15–1.47, avg 0.68 TOCo(live, oil) wt%) and higher fair to very good proportions of active gas-prone (0.61–5.88, avg. 2.70 TOCo(live, gas) wt%) kerogens. The Upper Safa Member originally possessed poor to very good oil (S2o(oil): 0.44–3.59, avg. 1.16 mg HC/g org. C) and good to very good gas (S2o(gas): 1.77–14.37, avg. 6.44 mg HC/g org. C) generation potential. The carbonates and partly calcareous shales of the uppermost Safa and the Zahra members possessed poor oil and gas generation potential due to their organic-lean status.The present-day organic geochemical characterization indicates that the Upper Safa Member still shows active fair to excellent organic richness (0.68–6.75, avg. 3.11 TOCpd(live) wt%). However, this member shows a relatively lower present-day good to very good/excellent source rock potential (S2pd: 1.33–10.78, avg. 4.83 mg HC/g rock) and lower active kerogen quality IIIpd(live) (HIpd(live): 115.97–198.21, avg. 151.07 mg HC/g TOCpd(live)). This is related to the current high thermal maturation (gas-window: Tmax: 459–468, avg. 469 °C, thermal alteration index: TAI of 4-) and conversion of the original kerogen IIIo(live)-IIo(live) into oil and gas. The present-day high organic richness, high remaining generative potential, high shale volume of the thermally mature gas-prone kerogen, classify the Upper Safa Member as an active fair to excellent shale gas play in the Matruh Basin. Intrabasinal and interbasinal correlations across northwestern Egypt indicate that the upper Khatatba Formation shows locally and regionally dual source and reservoir rock characteristics due to the lateral facies changes.

Discovery of the Lower-Middle Jurassic high potential source rocks based on the geochemical investigation from the Indus Basin, Pakistan

The Lower to Middle Jurassic sedimentary succession of the Indus Basin, Pakistan represents thick strata of shale, siliciclastic and carbonate rocks. The shale is subjected to the experimental methods of total organic carbon contents (TOC), rock-eval™, vitrinite reflectance, and x-ray diffraction with clay mineralogy for identification of high potential source rock. This set of black shale having well-defined kerogen types I and II, an abundance of organic matter ≥2%, HI ≥ 150 or 200 mg HC/g TOC, Tmax ≤ 465 °C, good maturity such as 1.3–2.0 Ro%, thermal alteration index ≥2, and spore color index ≥5, and illite-smectite clay is strong enough (60–80%) to produce hydrocarbon. Furthermore, the stratigraphic thickness is ≤ 100 m in studied strata in a depth of 3500–4500 m in the Indus Basin feasible for future exploration. The chemical composition of black shale is siliceous mudstone, calcareous mudstone, argillaceous mudstone, and siliceous marlstone. The cross plot of OI versus HI and Tmax versus HI shows mature types II and I kerogen while the cross plots of TOC versus S1 + S2 with TOC versus generative potentials (GP) showing good to high potentials of indigenous hydrocarbons. The plotting of the Tmax versus productive index (PI) exhibits that the kerogen mostly lies in the mature zone, having the capacity of oil generation. Besides the geochemical properties, the studied rock is thick and widely disturbed across the Indus Basin deposited under the dysoxic-anoxic setting. Moreover, the Lower to Middle Jurassic strata are correlated with the equivalent stratigraphic formations in India and China for their importance in the regional context.

Source Rocks’ Potentiality of the Sargelu Formation (Middle Jurassic) in the Taq Taq Oilfield, Kurdistan Region, Iraq

Thirty rock samples were selected from the well Tq-1 that penetrated the Jurassic beds in the Taq Taq Oilfield to be studied the source rock potentiality of the Sargelu Formation. The formation is characterized by three types of microfacies, namely, foraminiferal packstone, grainstone microfacies, fossiliferous packstone microfacies, and foraminiferal wackestone which were deposited in an environment extending from middle to outer carbonate ramp. An average of 3.03 wt.% of total organic carbon was obtained from a Rock Eval pyrolysis analysis carried out on 24 selected rock samples. The petrographic analysis for such organic matters revealed that they are of kerogen types III and IV and they are currently in a post-mature state. Pyrolysis parameters showed that limited generation potential was remained for these sources to expel generated hydrocarbons. The palynological study showed that Amorphous Organic Matter forms the highest percentage of organic matter components with more than 70%, followed by phytoclasts with 10 – 25 % and palynomorphs of less than 10%. The organic matters within the Sargelu Formation are deposited at the distal part of the basin under suboxic to anoxic condition. The color of the organic matter components, examined under transmitted light, showed Thermal Alteration Index values between 3+ and 4-. Such values may indicate that these organic matters are thermally at the end of the liquid oil generation zone and beginning of condensate-wet gas generation zone. The thermal maturity of the Sargelu Formation depending on the calculated VRo% revealed that the formation in the studied oilfield is currently at the peak of the oil generation zone. The Sargelu Formation in the studied field is considered as an effective source rock, as it has already generated and expelled hydrocarbons.

Assessing Thermal Maturity through a Multi-Proxy Approach: A Case Study from the Permian Faraghan Formation (Zagros Basin, Southwest Iran)

This study focuses on the thermal maturity of Permian deposits from the Zagros Basin, Southwest Iran, employing both optical methods (Thermal Alteration Index, Palynomorph Darkness Index, Vitrinite Reflectance, UV Fluorescence) and geochemical analyses of organic matter (Rock Eval Pyrolysis and MicroRaman spectroscopy) applied to the Faraghan Formation along two investigated Darreh Yas and Kuh e Faraghan surface sections. Furthermore, an integrated palynofacies and lithofacies analysis was carried out in order to integrate the few studies on the depositional environment. The Faraghan Formation, which is widely distributed in the Zagros area, generally consists of shale intercalated with sandstones and pebble conglomerates in the lower part, followed by a succession of sandstone, siltstone and shaly intercalations and with carbonate levels at the top. The integrated palynofacies and lithofacies data confirm a coastal depositional setting evolving upwards to a shallow marine carbonate environment upwards. Rock Eval Pyrolysis and Vitrinite Reflectance analysis showed that the organic matter from samples of the Darreh Yas and Kuh e Faraghan sections fall in the mature to postmature range with respect to the oil to gas generation window, restricting the thermal maturity range proposed by previous authors. Similar results were obtained with MicroRaman spectroscopy and optical analysis such as Thermal Alteration Index and UV Fluorescence. Palynomorph Darkness Index values were compared with Rock Eval Pyrolysis and vitrinite reflectance values and discussed for the first time in the late stage of oil generation.

Optical (visual) Kerogen assessment of Enugu Shale, Anambra Basin, Southeastern Nigeria: Implications for source rock potential and thermal maturation

Palynological analysis was carried out on Ten (10) samples from outcrops of the Campanian Enugu Formation, a component lithostratigraphic unit of the Anambra Basin, using the acid maceration techniques for recovering acidinsoluble organic-walled microfossils. Two main lithological units were encountered, which include: carbonaceous shale and siltstone. Result from kerogen laboratory examination reveals two (2) main groups of palynofacies association namely; palynofacies (A and B), based on the change in particulate organic matter constituents of organic residue extract. Palynofacies A is characterized by abundant opaques debris with common terrestrial phytoclasts, which occupy the southwestern and northwestern parts of the studied area, whereas palynofacies B dominates in the northeastern part, characterised by abundant phytoclasts followed by frequent opaques debris. Kerogen type III with gas-prone material is suggested for both palynofacies. The examined exine of spore/ pollen grain are pale yellow – yellow, with Thermal Alteration Index TAI of 1+ to 2- and Vitrinite Reflectane (R o) (0.3 % - 0.4 %) in palynofacies A, and yellow – yellow brown, with Thermal Alteration Index TAI of 2- to 2, and Vitrinite Reflectane (R o) of 0.3% - 0.5% in palynofacies B. These revealed source rock that is thermally immature to slightly mature but has potential to generate mainly gas. The kerogen data generated using transmitted light microscopy correlated well with geochemical data obtained using rock-eval pyrolysis method, and this shows the method a reliable tool for assessing petroleum potential in any given sedimentary basins.

Organic petrological characterisation and facies interpretation of the Upper Triassic (Carnian – Julian) black limestone in the Lesno Brdo area (External Dinarides, Central Slovenia)

Upper Triassic (Carnian - Julian) bedded dark limestone in the Lesno Brdo area, more precisely in the Drenov Grič quarry (Central Slovenia), was investigated in this study to describe its carbonate facies, its geochemical characteristics of the inorganic and organic matter, and to estimate its hydrocarbons generative potential. The site was chosen as a well referenced location of previous lithological and paleontological studies with the aim to fill a gap in the up-to-date knowledge on organic petrology of the investigated rocks termed as the Julian “Carbonate Member” within the Lesno Brdo stratigraphic sequence. The study represents the first multi-analytical organic petrological investigation of Mesozoic carbonate rocks in Slovenia. In the Drenov Grič (DRG) section, 7 microfacies were defined. A total of 38 DRG samples of micritic limestones, marlstones, and a coaly sample were collected from the section. The average total organic carbon (TOC) content of the samples is 0.70 wt% with one outstanding sample DRG-28 with a TOC content of 15.56 wt.%. According to the TOC content, DRG samples are considered as a fair to good source rock for hydrocarbons generation. In addition, the samples with TOC >0.5 wt.% were characterised by the Rock-Eval S1, S2 and S3 peaks, hydrogen and oxygen indices (HI, OI), Tmax and production index (PI) values. According to these data, typical “black” Julian carbonate beds from the Drenov Grič locality are poor source rocks. Based on pyrolysis data (HI versus OI), the type of the organic matter could not be determined, so a micropetrographic examination was performed. . The predominant and the only clearly visible maceral in the DRG samples is vitrinite within the amorphous organic matter. Given the low S2 peaks, thermal maturity was determined by vitrinite reflectance. Vitrinite reflectance results (between 2.71 ± 0.19 %Ro and 3.89 ± 0.17 %Ro) indicate that the organic matter in the DRG samples is in a thermally mature to overmature stage, which is also consistent with the Thermal Alteration Index (TAI) (4- - 4). All 38 samples were also analysed on main and trace elements, focusing on the redox sensitive trace elements (Ni, Mo, Th, U, V and Zn) to supplementary interpret depositional conditions. Overall, the “black” carbonate rocks of the Drenov Grič section show a negligible potential for the generation of hydrocarbons. Their original dispersed organic matter was thermally strongly transformed. Therefore, the investigated carbonate rocks almost completely exhausted their ability to generate a considerable quantity of hydrocarbons. They can therefore be classified as “spent source rocks”, which could conditionally generate some dry gas.

Very low-grade metamorphism of the Dezadeash Formation (Jura-Cretaceous): Constraints on the tectonometamorphic history of the Dezadeash flysch basin and implications regarding the tectonic evolution of the Northern Cordillera of Alaska and Yukon

<abstract> <p>Mesozoic convergence of the Wrangellia composite terrane with the western margin of North America resulted in the collapse of intervening flysch basins. One of these basins, the Jurassic-Cretaceous Gravina-Nuzotin belt, comprises from south to north, the Gravina sequence and Gravina belt in southeastern Alaska, the Dezadeash Formation in Yukon, and the Nutzotin Mountains sequence in eastern Alaska. Previous work shows that the Gravina sequence and Gravina belt were underthrust > 20 km beneath the margin of North America in mid-Cretaceous time, culminating in amphibolite facies metamorphism. This tectonometamorphic scenario was subsequently applied to the entire Gravina-Nutzotin belt, despite any detailed studies pertaining to the tectonometamorphic evolution of the Dezadeash Formation. The present analysis of the Dezadeash Formation reveals that metamorphic mineral assemblages in sandstone and tuff document subgreenschist, high temperature zeolite facies metamorphism; Kübler indices of illite and Árkai indices of chlorite in mudstone record diagenetic to high anchizone metapelitic conditions; and the color of organic matter (i.e., the Thermal Alteration Index of palynomorphs and the Conodont Alteration Index) and pyrolysis of organic matter in mudstone and hemipelagite beds document thermal maturation at catagenesis to mesogenesis stages. Collectively, the mineralogic and organic thermal indicators in the Dezadeash Formation suggest that strata experienced maximum pressure-temperature conditions of 2.5 ± 0.5 kbar and 250 ± 25 ℃ in the Early Cretaceous. The inferred tectonometamorphic evolution of the Dezadeash Formation does not support the northern part of the Gravina-Nutzotin belt being underthrust > 20 km beneath the western margin of North America in mid-Cretaceous time, thus contrasting sharply with the Gravina sequence and Gravina belt in the southern part of the Gravina-Nutzotin belt. The diverse tectonometamorphic histories recorded by the southern and northern parts of the Gravina-Nutzotin belt may be a manifestation of oblique collision and diachronous south-to-north accretion of the Wrangellia composite terrane to North America.</p> </abstract>

Palynostratigraphy, Paleoenvironments and Kerogen Evaluation of the Campanian-Maastrichtian Enugu Shale, Anambra Basin, Nigeria

Palynological and palynofacies analyses were used to investigate the paleoenvironments and hydrocarbon potential of the Campanian-Maastrichtian Enugu Shale, Anambra Basin, Nigeria. The palynological analysis of twenty-three outcrop samples revealed diverse and abundant palynomorph assemblages, which consisted of pteridophyte spores 46.82%, angiosperms 36.9%, gymnosperms 1.48 %, algae 5.65%, dinoflagellate cysts 5.1%, fungal elements 1.17%, acritarchs 0.34%, microforaminiferal wall linings 0.01% and miscellaneous palynomorphs (Charred graminae cuticle, incertae sedis) 2.53%. Seven informal palynological assemblage zones (PAZ) with corresponding eustatic sea level changes were delineated. Palynofacies analysis revealed four palynofacies types I to IV based on the percentage relative abundances of the sedimentary organic matter (SOM). Palynofacies types P-I and P-II suggest proximal fluvio-deltaic setting, while P-III and P-IV reflect a heterolithic-oxic shelf (proximal shelf) environment, indicative of deposition in an oxidizing condition. Results of the Spore Colour Index (SCI) and Thermal Alteration Index (TAI) analyses indicate that the Enugu shales are thermally immature. Keywords: Enugu, Campanian, Maastrichtian, Palynomorphs, Phytoclasts

Hydrocarbon Generation Potential of the Campano-Maastrichtian Dark Mudstone Lithofacies, Benin Flank, South West Anambra Basin, Nigeria

The dark mudstone lithofacies of Mamu Formation was deposited during the Campano-Maastrichtian flooding episode. It is laterally heterogeneous, and has been subdivided into marsh, bay and central basin subenvironments in order of proximality. Arising from recommendation from a previous study, we evaluated its hydrocarbon generating potential using multidisciplinary tools involving visual kerogen analysis, as well as bulk and isotope geochemistry. Seventy-seven sample materials were taken from 3-outcrop sites at Uzebba, Okpekpe and Imiegba locations, Benin flank, SW Anambra Basin, Nigeria. The results show that bulk of the samples have good organic richness. Kerogen quality is dominantly of gas prone Type III kerogen. However, visual kerogen analysis indicates the presence of an oil prone Type II/III kerogen in the central basin subenvironments. An immature thermal maturity is inferred based on spore colour index (SCI) of less than 6 on the SCI chart (thermal alteration index of <2.5). In addition, we hypothesize that the dark mudstone lithofacies possesses biogenic gas potential based on its organic richness, kerogen quality and thermal maturity. Shale gas prospectivity is further enhanced by the low dip of the Mamu Formation, shallow burial as well as high silica content. Worth mentioning is the proximal marsh mudstone (Uzebba location) with suitable microfabric, very high silica as well as >10m of combined (continuous) outcropping and subcropping thickness
 Keywords: Kerogen, palynofacies, stable isotope geochemistry, shale gas,

Application of the TAI (Thermal Alteration Index) and palynofacial analysis for determining the degree of organic matter termal maturity in the Upper Permian and Triassic deposits in the northern part of the Nida Basin (central Poland)

Application of the TAI (Thermal Alteration Index) and palynofacial analysis for determining the degree of organic matter termal maturity in the Upper Permian and Triassic deposits in the northern part of the Nida Basin (central Poland)