Mean Vitrinite Reflectance Values Research Articles

The influences of sedimentary environments on organic matter enrichment in fine-grained rocks of the Paleogene Shahejie formation in Nanpu Sag, Huanghua Depression, Bohai Bay Basin

The fine-grained rocks in the Paleogene Shahejie Formation in Nanpu Sag, Huanghua Depression, Bohai Bay Basin, are extremely important source rocks. These Paleogene rocks are mainly subdivided into organic-rich black shale and gray mudstone. The average total organic carbon contents of the shale and mudstone are 11.5 wt.% and 8.4 wt.%, respectively. The average hydrocarbon (HC)-generating potentials (which is equal to the sum of free hydrocarbons (S1) and potential hydrocarbons (S2)) of the shale and mudstone are 39.3 mg HC/g rock and 28.5 mg HC/g rock, respectively, with mean vitrinite reflectance values of 0.82% and 0.81%, respectively. The higher abundance of organic matter in the shale than in the mudstone is due mainly to paleoenvironmental differences. The chemical index of alteration values and Na/Al ratios reveal a warm and humid climate during shale deposition and a cold and dry climate during mudstone deposition. The biologically derived Ba and Ba/Al ratios indicate high productivity in both the shale and mudstone, with relatively low productivity in the shale. The shale formed in fresh to brackish water, whereas the mudstone was deposited in fresh water, with the former having a higher salinity. Compared with the shale, the mudstone underwent higher detrital input, exhibiting higher Si/Al and Ti/Al ratios. Shale deposition was more dysoxic than mudstone deposition. The organic matter enrichment of the shale sediments was controlled mainly by reducing conditions followed by moderate-to-high productivity, which was promoted by a warm and humid climate and salinity stratification. The organic matter enrichment of the mudstone was less than that of the shale and was controlled by relatively oxic conditions.

Effect of Maturation on the Dimethyl Naphthalene Indicator Used to Evaluate the Source Organic Type of Crude Oil

The dimethyl naphthalene composition is used as an indicator to estimate the source organic matter of crude oil. This study examined the effect of maturation on this indicator. Heating experiments were performed on dimethyl naphthalenes and immature coal. The composition of indicator naphthalenes in terrestrial organic matter decreased with maturation. This study concluded that the dimethyl naphthalene ratio can be used as an indicator of source organic matter at random mean vitrinite reflectance values of up to 1.3%–1.4%.

Geochemical Evaluation of the Cretaceous Mudrocks and Sandstones (Wackes) in the Southern Bredasdorp Basin, Offshore South Africa: Implications for Hydrocarbon Potential

The southern Bredasdorp Basin, off the south coast of South Africa, is only partly understood in terms of its hydrocarbon potential when compared to the central and northern parts of the basin. Hydrocarbon potential assessments in this part of the basin have been limited, perhaps because the few drilled exploration wells were unproductive for hydrocarbons, yielding trivial oil and gas. The partial integration of data in the southern Bredasdorp Basin provides another reason for the unsuccessful oil and gas exploration. In this study, selected Cretaceous mudrocks and sandstones (wacke) from exploration wells E-AH1, E-AJ1, E-BA1, E-BB1 and E-D3 drilled in the southern part of the Bredasdorp Basin were examined to assess their total organic carbon (TOC), thermal maturity, organic matter type and hydrocarbon generation potential. The organic geochemical results show that these rocks have TOC contents ranging from 0.14 to 7.03 wt.%. The hydrogen index (HI), oxygen index (OI), and hydrocarbon index (S2/S3) values vary between 24–263 mg HC/g TOC, 4–78 mg CO2/g TOC, and 0.01–18 mgHC/mgCO2 TOC, respectively, indicating predominantly Type III and IV kerogen with a minor amount of mixed Type II/III kerogen. The mean vitrinite reflectance values vary from 0.60–1.20%, indicating that the samples are in the oil-generation window. The Tmax and PI values are consistent with the mean vitrinite reflectance values, indicating that the Bredasdorp source rocks have entered the oil window and are considered as effective source rocks in the Bredasdorp Basin. The hydrocarbon genetic potential (SP), normalized oil content (NOC) and production index (PI) values all indicate poor to fair hydrocarbon generative potential. Based on the geochemical data, it can be inferred that most of the mudrocks and sandstones (wackes) in the southern part of the Bredasdorp Basin have attained sufficient burial depth and thermal maturity for oil and gas generation potential.

Burial and thermal history modelling of the Upper Carboniferous strata based on vitrinite refl ectance data from Bzie-Dębina-60 borehole (Upper Silesian Coal Basin, southern Poland)

Integrated modeling of the coal rank in the Upper Carboniferous sediments of the BzieDebina-60 borehole from SW of the Upper Silesian Coal Basin was performed based on a new vitrinite reflectance measurements from the Westphalian A. Microscopic investigations of Westphalian samples in the Bzie-Debina-60 well have shown that organic matter is dominated by vitrinite maceral group. The coal rank in the well shows an increase with depth. The mean vitrinite reflectance values in these samples range from 0.76 to 1.07 %. One-dimensional forward modelling of thermal maturity in the Bzie-Debina-60 borehole section has been performed in order to explain the burial and thermal history. The maximum temperature (c. 145 °C) causing the coalification occurred in the Latest Carboniferous (c. 300 Ma). These processes were likely related to signifi cant late Variscan burial heating in this area as indicated by previous authors.

Evaluation of Eocene source rock for potential shale oil and gas generation in north Cambay Basin, India

Source rock potential of 108 representative samples from 3 m intervals over a 324 m thick shale section of middle Eocene age from the north Cambay Basin, India have been studied. Variation in total organic carbon (TOC) and its relationship with loss on ignition (LOI) have been used for initial screening. Screened samples were subjected to Rock-Eval pyrolysis and organic petrography. A TOC log indicated wide variation with streaks of elevated TOC. A 30 m thick organic-rich interval starting at 1954 m depth, displayed properties consistent with a possible shale oil or gas reservoir. TOC (wt%) values of the selected samples were found to vary from 0.68% to 3.62%, with an average value of 2.2. The modified van Krevelen diagram as well as HI vs. Tmax plot indicate prevalence of Type II to Type III kerogen. Tmax measurements ranged from 425 °C to 439 °C, indicating immature to early mature stage, which was confirmed by the mean vitrinite reflectance values (%Ro of 0.63, 0.65 and 0.67 at 1988 m, 1954 m, and 1963 m, respectively). Quantification of hydrocarbon generation, migration and retention characteristics of the 30 m source rock interval suggests 85% expulsion of hydrocarbon. Oil in place (OIP) resource of the 30 m source rock was estimated to be 3.23 MMbbls per 640 acres. The Oil saturation index (OSI) crossover log showed, from a geochemical perspective, moderate risk for producing the estimated reserve along with well location for tapping the identified resource.

Petroleum source-rock evaluation of upper Eocene Kopili Shale, Bengal Basin, Bangladesh

The upper Eocene Kopili Shale occurs throughout the Bengal Basin, including in the northwestern Indian platform and deeper basin areas (e.g., Sylhet Trough) of Bangladesh. Mudrocks presumed to be equivalent to the Kopili Shale in India are known hydrocarbon source rocks. However, the source-rock potential of the Kopili Shale in Bangladesh is not well established, thus prompting the current study of abundance, character, and maturity of organic matter in Kopili Shale samples from the Bengal Basin.Organic petrologic observations and Rock-Eval pyrolysis data indicate that organic matter in the Kopili Shale is largely terrigenous, including an admixture of type I/II (liptodetrinite, cutinite, bituminite), type III (vitrodetrinite), and type IV (inertodetrinite) macerals. Mean vitrinite reflectance values (Ro=0.86–1.32%) and a single reliable Tmax value (433°C) indicate that organic matter from all sampled sections is thermally mature. Total organic carbon (TOC) contents of samples from core and outcrop are generally low (<0.6%) and thus reflect relatively poor hydrocarbon-source potential. However, TOC values of ~1.0% and S2 values from one section indicate that source potential is locally higher in the Sylhet Trough area.An understanding of differences in Kopili source-rock potential between India and parts of the Bengal Basin will require more comprehensive comparative facies analyses.

A petrological and organic geochemical study of Permian coal seams east of Maracajá, South Santa Catarina, Paraná Basin, Brazil

Coal samples from the Permian Rio Bonito Formation, collected from borehole MML-09 east of Maracajá, South Santa Catarina, Paraná Basin, Brazil, were characterized by petrological (vitrinite reflectance measurements, maceral analyses) and geochemical (Rock-Eval, Total Organic Carbon (TOC) and GC–MS) methods. The mean vitrinite reflectance values range from 0.73 to 0.87% Rrandom, indicating a rank level of high volatile B/A bituminous. The maceral composition showed predominance of vitrinite (49.4 to 61.2vol.%) and inertinite (9 to 24.5vol.%). The Rock-Eval analysis presented the following values: S1 and S2 ranging from 0.74 to 2.51mg HC/g coal and 34.1 to 108.87mg HC/g coal, respectively; Hydrogen Indices between 158 and 240mg HC/g TOC and Tmax (°C) between 440 and 448°C. The TOC contents of the coal samples present values ranging from 18.85 to 45.31wt.%. Vitrinite reflectance and Rock-Eval Tmax values indicate that the coals are in the beginning of the oil generation window. The potential for the formation of liquid hydrocarbons is rather low, due to the nature of the humic coal, whereas the rank level indicates that generation of gas from the coal has occurred. The biomarker analysis, using GC–MS and GC–MS–MS, allowed the interpretation of the depositional environment and provided another way of determining the degree of maturity of the coals, besides vitrinite reflectance and Tmax. The pristane/phytane (Pr/Ph) ratio is >1, mainly controlled by organic matter type, implied suboxic conditions during sedimentation. The vitrinite reflectance measurements, Tmax values and 20S/(20S+20R) and αββ/(αββ+ααα) ratios for C29 regular steranes revealed the marginally mature stage of the organic matter, although the distribution of hopanes indicated a mature stage of the organic matter, due to the absence of ββ hopanes and predominance of the 22S epimer over the 22R epimer for the hopanes from C-31 to C-35.

Hydrocarbon Source Potential and Depositional Environment of the Surma Group Shales of Bengal Basin, Bangladesh

Abstract: Surma Group is the most important geological unit of Bengal basin, Bangladesh, because petroleum resources occur within this group. It is mainly composed of alternation of shale and sandstone and the shale fraction has long been considered as source rocks and the sandstone fraction as reservoir. These source and reservoir rocks have been studied by different authors by different approach but none of them adopted organic geochemistry and organic petrology as a means of study of source rock and their possible depositional environment. A total of thirty shale core samples have been collected from eight different gas fields to fulfill the short coming. The collected samples have been subjected to Source Rock Analysis (SRA) and/or Rock-Eval (RE) followed by pyrolysis gas chromatography (PyGC), gas chromatography mass spectrometry (GCMS), elemental analysis (EA) and organic petrological study such as vitrinite reflectance measurement and maceral analysis. The analyzed organic matter extracted from the shales of Surma Group consists mainly of Type III along with some Type II kerogen. The studied shales are mostly organically lean (TOC ±1%) and the extracted organic matter is fair to moderate. Based on these results, the analyzed shales have been ranked as poor (mostly) to fair quality source rock. The organic matter of the analyzed shale samples is thermally immature to early mature for hydrocarbon generation considering their Tmax and measured mean vitrinite reflectance values. The hopane 22S/(22S + 22R), moretane/hopane ratio and sterane parameters are also in good agreement with these thermal maturity assessments. The predominance of odd carbons over even carbons (most common) and/or even carbons over odd carbon numbered n-alkanes, moderate Pr/Ph ratio, low to high Tm/Ts ratio, comparative abundance of sterane C29 (i.e., C29 &amp;gt;C27&amp;gt;C28), Pr/nC17 - Ph/nC18 values, C/S ratio and dominance of vitrinite macerals group with the presence of liptinite macerals demonstrate that the organic matter has derived mainly from terrestrial inputs with an insignificant contribution from the marine sources. The condition of deposition alternates from oxic to anoxic.

Hydrothermal fluids influence on the thermal evolution of the Stephanian sequence, the Sabero Coalfield (NW Spain)

In the present study, the thermal history of the Late Carboniferous (Stephanian) coal-bearing sediments of the Sabero Coalfield has been reconstructed in order to elucidate coal rank. The Sabero Coalfield is located in a small intramontane coal-bearing basin along the Sabero-Gordon fault zone, one of the major E-W trending strike-slip fault systems of the southern part of the Cantabrian Zone (NW Spain). The total thickness of the Stephanian succession is in excess of 2,000 m, and is composed of siliclastic rocks and intercalated coal seams with tonsteins. Mean vitrinite reflectance values in the Stephanian rocks in the Sabero Coalfield are in the range from 0.61 to 3.14% Rr, but most values are in the range from 0.8 to 1.5% Rr (based on 84 samples). Average vitrinite reflectance gradient is high (0.73% Rr/km), which suggests high value of average paleogeothermal gradient (52°C/km). The maximum paleotemperatures calculated from vitrinite reflectance values for the Stephanian rocks range between 89°C (top of the Stephanian) and 195°C (bottom of the Stephanian). Coalification of the organic matter in the Stephanian rocks was achieved in the Early Permian, and was most likely related to several almost simultaneous related to magmatic and hydrothermal activity during high subsidence period in the pull-apart basin. The primary, burial-related maturity pattern, was probably slightly overprinted by fluid migration event, which is supposed to have occurred in Early Permian time.

Trace element geochemistry of coals from the Southern Cantabrian Zone (NW Spain): preliminary results

Trace element geochemistry of coals from the Southern Cantabrian Zone (NW Spain): preliminary resultsBituminous to anthracite coals from three small Stephanian intramontane coal-bearing basins (La Magdalena, Cinera-Matallana and Sabero) located along the Sabero-Gordón fault line strike-slip systems of the Southern Cantabrian Zone (SCZ) were examined. Coal rank expressed as mean vitrinite reflectance values of these Stephanian coals is in the range 0.72-3.96%. The vitrinite maceral group exceeds 72 vol. % in all of the coals. The coals are characterized by relatively variable contents of mineral matter and coal-ash. The mineral matter comprises, in the main, clay minerals, carbonates, sulphides and quartz. The coals exhibit medium-high concentrations (see for comparison Ronov et al. 1990; Kabata-Pendias, Pendias 1999; Ketris, Yudovich 2009) of the following elements (in ppm): ΣREE (53-205), Ba (300-900), As (11-57), Zn (&lt;50-150), Cr (10-160), Rb (50-145), Co (5-26), Sc (2-24.6), Ce (17-99), Yb (1.3-4.5), Th (2.4-11.9) and U (1.1-8.1), Br (&lt;1-14), Cs (&lt;2-9), Eu (&lt;0,3-1.5), Lu (0.11-0.85) and Sb (0.8-4.8), and relatively low concentrations of Sm (0.6-6.6) and Ta (&lt;1-2). They are also characterised by relatively high Th/U values (1.31-2.29). LREE/HREE values fall in the range 24-44 (average - 30). In contrast, concentrations of Au, Ag, Hg, Ir, Ni, Se, Sn, Sr, and W are below detection limits for the applied INAA method. As the concentrations of elements are significantly higher in coal-ash, most are likely related to mineral matter in the coals.

Methane and carbon dioxide sorption on a set of coals from India

Abstract Complete sorption isotherm characteristics of methane and CO 2 were studied on fourteen sub-bituminous to high-volatile bituminous Indian Gondwana coals. The mean vitrinite reflectance values of the coal samples are within the range of 0.64% to 1.30% with varying maceral composition. All isotherms were conducted at 30 °C on dry, powdered coal samples up to a maximum experimental pressure of ~ 7.8 MPa and 5.8 MPa for methane and CO 2 , respectively. The nature of the isotherms varied widely within the experimental pressure range with some of the samples remained under-saturated while the others attained saturation. The CO 2 to methane adsorption ratios decreased with the increase in experimental pressure and the overall variation was between 4:1 and 1.5:1 for most of the coals. For both methane and CO 2 , the lower-ranked coal samples generally exhibited higher sorption affinity compared to the higher-ranked coals. However, sorption capacity indicates a U-shaped trend with rank. Significant hysteresis was observed between the ad/desorption isotherms for CO 2 . However, with methane, hysteresis was either absent or insignificant. It was also observed that the coal maceral compositions had a significant impact on the sorption capacities for both methane and CO 2 . Coals with higher vitrinite contents showed higher capacities while internite content indicated a negative impact on the sorption capacity.

Influence of a basic intrusion on the vitrinite reflectance and chemistry of the Springfield (No. 5) coal, Harrisburg, Illinois

The Springfield (No.5) coal, a Pennsylvanian-age high volatile B bituminous rank coal in the Carbondale Formation, is mined in the southeastern section of the Illinois Basin. Intruded into this coal is a 10.1-m-wide basic dike that is near vertical through the extent of the mined coal bed, and strikes N 22° W. The intrusion and the effects it had on the coal were studied petrographically and geochemically. Using a published heat flow model by Carslaw and Jaeger (1959) [Carslaw, H.S., Jaeger, J.C., 1959. Conduction of Heats in Solids. Oxford University Press] in conjunction with thin-section microscope analysis, a contact temperature of ∼600 °C was determined. Vitrinite reflectance was measured for a suite of coal samples collected distal to the intrusion. Vitrinite reflectance values rose uniformly from the ambient reflectance values of ∼0.69%, at just greater than one-dike thickness away, to ∼5% at the dike/coal contact. The increase in reflectance values for samples within ∼1.2 times dike thickness is concordant with data published by Bostick and Pawlewicz (1984) [Bostick, N.H., Pawlewicz, M.J., 1984. Paleotemperatures based on vitrinite reflectance of shales and limestones in igneous dike aureoles in the Upper Cretaceous Pierre shale, Walsenburg, Colorado. In: Woodward, J.G., Meissner, F.F., Clayton, J.L. (Eds.). Hydrocarbon Source Rocks of the Greater Rocky Mountain Region. Rocky Mountain Association of Geologists, pp. 387–392] on the effects of mafic intrusions into Cretaceous-age shales in Walsenburg, Colorado. Comparison of Colorado data with data from these sites indicates that, despite differences in intrusion temperature, timing, or coal rank at time of the intrusion, ambient mean vitrinite reflectance values remain unchanged at just greater than one-dike width from an intrusion.

97/01729 Evaporation enthalpies of higher boiling crude petroleum components

Barapukuria Coal Basin is situated in Dinajpur district of the northwestern part of Bangladesh. A total of eight coal samples have been collected from two different locations of the basin and analyzed using organic geochemical and organic petrological methods.The extracted organic matter of the studied samples is a mixture of Type-III and Type-II kerogen as evaluated by the source rock analyzer (SRA) and PyGC pyrograms. The measured total organic carbon (TOC) ranges from 61 to 74 wt. % and the recovered extractable organic matter (EOM) varies from 27,561 to 41,389 ppm. These results suggest that the coals are ranked as good quality source rock. The hydrocarbon yield has been calculated which is also high and it ranges from 12,192 to 20,799 ppm. The organic matter is thermally mature for hydrocarbon generation considering their Tmax and measured mean vitrinite reflectance values of 431 to 435 °C and 0.72 to 0.81%Ro respectively. The hopane 22S/(22S + 22R), moretane/hopane ratio and sterane parameters are also in support of these thermal maturity assessment.The maceral composition is dominated by the inertinite group with significant amounts of vitrinite and liptinite. The more dominant odd carbon numbered n-alkanes, high Pr/Ph ratio (4–8), high Tm/Ts ratio (13–18), predominant sterane C29 (i.e., C29 > C28 > C27) and Pr/nC17 – Ph/nC18 values, GI vs TPI cross-plot and dominance of inertinite macerals group clearly demonstrate that the organic matter has been derived from terrestrial inputs and the condition of deposition was oxic (i.e., dry forest swamp) which was also supported by the absence of alginite. It is most likely that the coals were deposited within a peat-swamp flood basin environmental setting.

97/00028 Fluorescence features of the aromatic hydrocarbons of peat and soft brown coal

An Upper Carboniferous coal sequence has been analysed by means of organic petrological, sedimentological and statistical methods in order to test the hypothesis of environmental influence on vitrinite reflectance values. Differences between the linear regression value and the vitrinite reflectance value along the depth profile have been taken as a measure of reflectance variation. Detailed maceral analyses have been carried out on forty-seven coal seams. Factor analyses of selected variables resulted in the recognition of common factors accounting for approximately 50% of the variation of the data matrix. Two factor analyses were performed including either maturity parameters (Tmax and %Rr), or variation in vitrinite reflectance. In both analyses, two factors were considered sufficient to represent the data. The factors have been attributed to coalification, degree of humification and oxidation. Clustering on the basis of detailed maceral analyses resulted in a two-fold subdivision of the coal seams. The two clusters are differentiated on the basis of the ratio of the vitrinite macerals telocollinite and desmocollinite, significant differences in hydrogen index, and marked differences in mean vitrinite reflectance values. The subdivision is related to differences in depositional environment. One cluster, interpreted to represent peat formation in a fluviatile influenced environment, accommodates samples with significantly lowered reflectance values. The other cluster is characterized by a deltaic setting of the coal seams. Petrographic and chemical differences are interpreted to be related to different levels of biochemical degradation in these two palaeo-environments. The fluviatile influenced coals have a marked lower maturity trend compared to those formed under deltaic conditions.

Environmental influence on maturity parameters in carboniferous coals of The Netherlands

An Upper Carboniferous coal sequence has been analysed by means of organic petrological, sedimentological and statistical methods in order to test the hypothesis of environmental influence on vitrinite reflectance values. Differences between the linear regression value and the vitrinite reflectance value along the depth profile have been taken as a measure of reflectance variation. Detailed maceral analyses have been carried out on forty-seven coal seams. Factor analyses of selected variables resulted in the recognition of common factors accounting for approximately 50% of the variation of the data matrix. Two factor analyses were performed including either maturity parameters ( T max and % R r), or variation in vitrinite reflectance. In both analyses, two factors were considered sufficient to represent the data. The factors have been attributed to coalification, degree of humification and oxidation. Clustering on the basis of detailed maceral analyses resulted in a two-fold subdivision of the coal seams. The two clusters are differentiated on the basis of the ratio of the vitrinite macerals telocollinite and desmocollinite, significant differences in hydrogen index, and marked differences in mean vitrinite reflectance values. The subdivision is related to differences in depositional environment. One cluster, interpreted to represent peat formation in a fluviatile influenced environment, accommodates samples with significantly lowered reflectance values. The other cluster is characterized by a deltaic setting of the coal seams. Petrographic and chemical differences are interpreted to be related to different levels of biochemical degradation in these two palaeo-environments. The fluviatile influenced coals have a marked lower maturity trend compared to those formed under deltaic conditions.

Thermal and Tectonic History of the Ordos Basin, China: Evidence from Apatite Fission Track Analysis, Vitrinite Reflectance, and K-Ar Dating

Apatite fission track analysis, vitrinite reflectance data, and K-Ar dating of Permian-Carboniferous and Mesozoic core samples have been successfully integrated to reconstruct the thermal and tectonic history of the Ordos basin, China. Apatite fission track ages of Carboniferous-Jurassic sedimentary rocks range between 3 and 137 Ma, and are significantly younger than the stratigraphic ages. Confined fission track lengths demonstrate exclusively mixed length distribution, indicating complex thermal history. The data suggest that the samples must have all experienced higher paleotemperatures in the past. Mean vitrinite reflectance values (R0) of the Triassic rocks range from 0.61 to 1.06%, giving a high coalification gradient of 0.36%/km and suggesting a high paleothermal gradient of 57°C/km. Permian-Carboniferous rocks have Ro values on the order of 1.0-3.0%, and locally up to 4.0-6.0%. Some high R0 values coincide with positive gravity and magnetic anomalies. K-Ar dating on Permian-Triassi samples reveals distinct illitization at 170-160 Ma, during which a thermal event occurred due to subsurface magmatic intrusion related to the early Yanshanian movement. The petroleum source rocks of the Upper Triassic experienced peak temperatures ranging from 90 to 160°C, corresponding to the oil window, and Permian-Carboniferous source rocks were heated to more than 150°C, passing through and out of the gas window. Due to rapid uplift and erosion in response to the rise of the Qinghai-Tibet plateau associated with the Asia-India collision and the Himalayan orogeny, cooling has taken place at least since approximately 23 Ma. The difference in the rate and amount of uplift between the eastern and western parts of the basin resulted in differential uplift and the present-day s ructural pattern of the basin.

Thermal evolution of the Tertiary Shimanto Belt, Muroto Peninsula, Shikoku, Japan

Abstract The Shimanto accretionary complex on the Muroto Peninsula of Shikoku comprises two major units of Tertiary strata: the Murotohanto Sub‐belt (Eocene‐Oligocene) and the Nabae Sub‐belt (Oligocene‐Miocene). Both sub‐belts have been affected by thermal overprints following the peak of accretion‐related deformation. Palaeotemperatures for the entire Tertiary section range from ∼ 140 to 315°C, based upon mean vitrinite reflectance values of 0.9–5.0%Rm. Values of illite crystallinity index are consistent with conditions of advanced diagenesis and anchimetamorphism. Illite/mica b0 lattice dimensions indicate that burial pressures were probably no greater than 2.5kbar. In general, levels of thermal maturity are higher for the Murotohanto Sub‐belt than for the Nabae Sub‐belt. The Eocene‐Oligocene strata also display a spatial decrease in thermal maturity from south to north and this pattern probably was caused by regional‐scale differential uplift following peak heating. Conversely, the palaeothermal structure within the Nabae Sub‐belt is fairly uniform, except for the local effects of mafic intrusions at the tip of Cape Muroto. There is a paleotemperature difference of ∼ 90°C across the boundary between the Murotohanto and Nabae Sub‐belts (Shiina‐Narashi fault), and this contrast is consistent with approximately 1200 m of post‐metamorphic vertical offset.Subduction prior to Middle Miocene probably involved the Kula or fused Kula‐Pacific plate and the background geothermal gradient during the Eocene‐Oligocene phase of accretion was ∼ 30–35°C/km. Rapid heating of the Shimanto Belt evidently occurred immediately after a Middle Miocene reorganization of the subduction boundary. Hot oceanic lithosphere from the Shikoku Basin first entered the subduction zone at ∼ 15 Ma; this event also coincided with the opening of the Sea of Japan and the rapid clockwise rotation of southwest Japan. The background geothermal gradient at that time was ∼ 70°C/km. Whether or not all portions of the inherited (Eocene‐Oligocene) palaeothermal structure were overprinted during the Middle Miocene remains controversial.

Geochemistry of Coal from Cretaceous Corwin and Chandler Formations, National Petroleum Reserve in Alaska (NPRA): ABSTRACT

The Cretaceous, coal-bearing Corwin and Chandler Formations accumulated in two river-dominated deltas on the North Slope. The larger Corwin delta (Corwin Formation), in the western portion of NPRA, prograded northeastward and eastward, and the smaller Umiat delta (Chandler Formation), in the southeastern part of NPRA, prograded northward. Ninety coal samples from these formations within NPRA were collected and analyzed in order to evaluate coal quality and elemental distribution. Their apparent rank ranges from lignite A in the northern part of NPRA to high-volatile A bituminous coal in the southern part. Mean vitrinite reflectance values range from 0.65 to 0.74%. Some Corwin Formation coal samples west of NPRA have coking potential with free-swelling indexes between 3.0 and 5.0. Compared to other western United States Cretaceous coal, NPRA coal is significantly lower in ash, volatile matter, 0, Si, Al, Ca, Fe, Ti, Cu, F, Li, Mn, Mo, Pb, Sb, Se, Th, and Zn. Statistical comparisons of element concentrations indicate that the mean content of Si, Al, K, Li, Sc, Y, and Yb increases as the mean ash content increases (correl tion coefficient at least 0.7). Sulfur values are extremely low (0.1%), and elements that normally show positive correlation with sulfur, such as Fe, As, Cd, Co, Cu, Mo, Pb, and Zn, are also low. Therefore, coal from NPRA can be characterized by low ash and sulfur contents and low contents of elements of environmental concern, such as As, Be, Hg, Mo, Sb, and Se. The elements found to have positive correlations with ash content are probably present as aluminosilicate or stable oxide minerals. Variations in element content and quality of NPRA coal were probably influenced by the geochemical conditions that existed in the Corwin and Umiat delta systems. End_of_Article - Last_Page 655------------