Mean Random Vitrinite Reflectance Research Articles

Paleoenvironments, source rock potential and thermal maturity of the Upper Benue rift basins, Nigeria: implications for hydrocarbon exploration

The Upper Benue rift comprising the Gongola and Yola Basins in Nigeria consist of the Aptian-Albian Bima Formation, the Yolde Formation (Cenomanian-Turonian), Gongila/Pindiga/Dukul Formation (Turonian-Coniacian) and Gombe Formation (Campanian-Maastrichtian). To evaluate the maturity and source rocks potential, vitrinite reflectance, Rock-Eval pyrolysis and infrared spectroscopy were carried out on 52 shale samples collected from boreholes, mine quarries and outcrop sections. In the Gongola Basin, mean random vitrinite reflectance ( R om) values range from 0.45% in the Gombe Formation to 0.69% in the Pindiga Formation and to 0.82% in the Bima Formation. Reflectance values in the Yola Basin also increase with stratigraphic age ranging from 0.73% in the Dukul Formation to 0.94% in the Yolde Formation and up to 1.37% in the Bima Formation. Total organic carbon (TOC) values in the Pindiga and Gongila Shales are between 0.4 to 2.4% averaging 0.75%. TOC contents from 0.10 to 12.9 averaging 1.2% are contained in the Yolde Formation of the Yola Basin. T max values from the pyrolysis of shales in the Gongola Basin range from 419 to 435°C whereas for shales in the Yola Basin they range from 431 to 442°C. Plots of HI vs T max for kerogen classification indicate the prevalence of Type III kerogens in the Gongila and Pindiga Shales although there are some indications of Type II–III kerogens in the Yolde Shales of the Yola Basin. Our preliminary data suggest that Cretaceous successions in the Gongola Basin are thermally immature to marginally mature whereas source rocks in the Yola Basin are thermally mature with respect to hydrocarbon generation. The predominance of Type III kerogens in the Gongola Basin suggest their potential to generate gas in the deeply buried sections. The Dukul and Yolde formations with Type II–III kerogens may have generated some quantities of oil and gas in the deeper non-emergent sections.

Burial Metamorphism (Thermal Maturation) in Cretaceous Sediments of the Southern Benue Trough and Anambra Basin, Nigeria

Organic matter reflectance, illite crystallinity, and fluid-inclusion techniques have been used to evaluate burial metamorphic conditions for the lithostratigraphic successions that accumulated during the Cretaceous in the southern Benue trough and Anambra basin of southern Nigeria. These successions were invaded by intrusives, volcanic rocks, and vein-type lead-zinc minerals, especially in the Albian-Cenomanian section of the Abakaliki anticline. The sequence includes lower Maastrichtian subbituminous coals in the Anambra basin. In exposed Cretaceous deposits on a northwest-southeast section from Enugu to Abakaliki, mean random vitrinite reflectance (Rom) in oil ranges from 0.55 to 0.67% in the lower Maastrichtian coals and shales, approximately 0.91% in the Coniacian shales, 0.97% in the Turonian shales, and up to 4.31% in the Albian shales. A corresponding increase in the illite content of the illite-smectite mixed-layer clay fractions is reflected in the low values of illite crystallinity indices coupled with a decrease from 32 to 0% in the percentage of smectite from the northwest-southeast Maastrichtian-Albian section. Fluid-inclusion pressure-corrected temperatures from vein quartz in the Albian shales range from 170 to 250°C. The results of this work show that thermal maturation in the Cretaceous successions increases from the post-Santonian (Campanian-Maastrichtian) Anambra basin into the older Benue trough where strong diagenetic to anchimetamorphic (i.e., very low grade metamorphism) conditions were reached. The data suggest that these sediments at the present outcrop levels originally were buried at higher maturity levels. Maximum erosion appears to have taken place on the axis of the Abakaliki anticline. The presence of bitumen in fractures and pores of the exposed Maastrichtian units in the Anambra basin suggests that the matured sediments generated some unknown quantity of petroleum. This finding, coupled with reported gas finds and some oil in previous exploration wells of the Anambra basin, enhances the possibilities of Cretaceous targets in the downdip regions.

Chemical characteristics, mineralogical composition and rank of high sulphur coking coals of Middle Miocene age in the Gökler coal field, Gediz, Turkey

Abstract Most Miocene coals in Turkey are of subbituminous to lignite rank. The Gökler coal field in the western part of Turkey contains mainly high sulphur coals of bituminous rank. The chemical characteristics, mineralogical composition and rank of high sulphur coking coals of Middle Miocene age in the coal field are investigated for the first time. A total of 46 channel and core samples were collected from underground mine workings and from boreholes drilled in the coal field. The results of the proximate analyses as well as total sulphur analyses and calorific values on an air-dried basis show on average 1.2% moisture, 22.9% ash, 34.6% volatile matter, 6.9% total sulphur contents and 5850 kcal kg −1 calorific value. X-ray powder diffraction studies of the coal samples on an air-dried basis show quartz, pyrite and calcite to be the dominant minerals; kaolinite, hydromuscovite, dolomite, gypsum, iron sulphate hydrate and rarely illite/smectite and feldspar constitute the remainder. Secondary calcite in random fractures surfaces of the coals is especially abundant in samples obtained from an area adjacent to the fault zones. The mean random reflectance values (%Rr) of telocollinite vary between 0.50 and 0.95%. These values show that the rank can be determined as a high volatile bituminous stage. In addition, these coals can form isotropic coke. Fluorescence intensities of sporinite are weak to very weak. The mean random vitrinite reflectance values within the coal field generally increase towards southern parts of the coal field. It is thought that this increase can be related to the recent hydrothermal antimony mineralization in the southeastern parts of the coal field.

Clay Mineral Diagenesis and Thermal History of the Thonex Well, Western Swiss Molasse Basin

AbstractResults are presented of a diagenetic study from the 1300 m thick Oligocene Molasse deposits penetrated by the Thônex geothermal exploration well (Geneva, Switzerland). The x-ray diffraction (XRD) studies of fine-grained rocks indicate the following diagenetic changes: a decrease of illite/smectite (US) expandability from approximately 90% to 30% with depth, a decrease of the amount of US in the clay mineral fraction, and the appearance of corrensite at depths >750 m. The transition from random US to ordered I/S occurs at the base of the Thônex well (1200 to 1300 m) and is associated with a coal rank of about 0.7% Rr (mean random vitrinite reflectance) corresponding to paleotemperatures of 110 to 115 °C Corrensite appears at a vitrinite reflectance value of 0.6% Rr and a corresponding paleotemperature of 100 °C. The amount of post-Molasse erosion is estimated to be approximately 2 km. Thermal history modeling of the Thônex well suggests maximum paleotemperatures of 80 to 115°C and an average paleogeothermal gradient of 27 °C/km during Late Miocene maximum burial conditions.

A comparison of vitrinite reflectance measurements made on whole-rock and dispersed organic matter concentrate mounts

A comparison of mean-random vitrinite reflectance ( R v-r) values calculated from measurements on polished whole-rock and on concentrates of dispersed organic matter (DOM) mounts of the same samples generally show little difference. Compared to measurements on DOM mounts, R v-r measurements by the same microscopist on corresponding whole-rock samples are up to a few tenths of a percent lower over the range of 0.6–3.2% R v-r and have a slightly higher standard deviation. Whole-rock mounts have an advantage over DOM mounts in the identification of first-cycle vitrinite and differentiation of solid bitumen from vitrinite.

Thermochronological and Petroleum Generation Studies next to Dikes in the Upper Jurassic-Lower Cretaceous Strzelecki Group, Onshore Gippsland Basin, Australia: ABSTRACT

Fluid inclusion, apatite fission track analysis (AFTA[reg sign]) annealing, mean random vitrinite reflectance (Rv-r) data and heat flow modeling studies of nine basalt dikes ranging from 0.06 m to 40 in thick in the Strzelecki Group were used to constrain the conditions of contact metamorphism of coaly sedimentary organic matter (SOM) to investigate it as an extended-duration natural analog to hydrous pyrolysis experiments. Thermal history reconstruction suggests that at the time of dike intrusion the host rock was at a paleotemperature of 100 to 120[degrees]C. Reequilibrated fluid inclusions in the host rocks next to thin dikes (< 3.4 m thick) suggest maximum temperature (T[sub peak]) systematically increases towards the dike margin to around 550[degrees]C. These T[sub peak] data are in fair agreement with those predicted by a conductive heat flow model and measured Rv-r. AFTA indicates the time of uplift and cooling as mid to Late Cretaceous and that paleotemperatures were too high for heating duration information to be recorded by annealing of tracks in apatite near the dikes. The estimated T[sub peak] next to some larger dikes (20 to 40 m thick) as indicated by an extended zone of elevated Rv-r did not fit the pattern seen next tomore » the thin dikes. Heat flow modeling along with whole rock elemental and isotopic data suggest that the extended zone of increased Rv-r is caused by convection of heated fluids. This is in contrast to the apparent advection effects found adjacent to thin dikes. Laboratory hydrous pyrolysis compares well with contact metamorphism next to thin dikes with respect to the composition of the free hydrocarbons except in samples very close to the dike. Close to the dike, fluid inclusion evidence indicates a water vapor or super-critical phase in the rock making the organic geochemistry results diverge from.« less

Should fluorescence alteration replace vitrinite reflectance as a major tool for thermal maturity determination in oil exploration?

The fluorescence alteration multiple macerals (FAMM) technique is closely related to vitrinite reflectance. Until recently, fluorescence alteration study of dispersed organic matter (DOM) in sedimentary rocks has been regarded as technically difficult. With the introduction of optical microprobes equipped with small continuous wave gas lasers having a very stable output of blue radiation readily focused to 1–2 μm on the sample, this work has now become routine. FAMM has important advantages over vitrinite reflectance stemming from the possibility of resolving the data into two parameters, one related to the level of maturation, and the other related to the hydrogen-richness of a maceral. This enables the resolution of what are often difficult problems for the single parameter vitrinite reflectance technique, namely the identification of indigenous vitrinite in DOM, the recognition of caved and reworked organic matter, and the suppression of vitrinite reflectance. For these reasons, FAMM is a powerful tool for investigating vitrinite reflectance anomalies, especially those associated with unconformities. It is also well suited to investigate the tacit assumption in the vitrinite reflectance technique that all telocollinite matures along a single compositional pathway. The FAMM technique is not without problems. The range is restricted to equivalent mean random vitrinite reflectance Rmo = 0.4–1.2, the results are more sensitive to ambient temperature oxidation than vitrinite reflectance, and the technique demands a high level of technical expertise. The FAMM technique is calibrated on coals and it is therefore more likely to give high quality results on Types II and III kerogens rather than Type I kerogens which have not yet been intensively investigated. Although FAMM is unlikely to replace vitrinite reflectance, considerable confidence may be had in a maturation assessment when consistent results are obtained from these two closely related but independent techniques.

Should fluorescence alteration replace vitrinite reflectance as a major tool for thermal maturity determination in oil exploration?

The fluorescence alteration multiple macerals (FAMM) technique is closely related to vitrinite reflectance. Until recently, fluorescence alteration study of dispersed organic matter (DOM) in sedimentary rocks has been regarded as technically difficult. With the introduction of optical microprobes equipped with small continuous wave gas lasers having a very stable output of blue radiation readily focused to 1–2 μm on the sample, this work has now become routine. FAMM has important advantages over vitrinite reflectance stemming from the possibility of resolving the data into two parameters, one related to the level of maturation, and the other related to the hydrogen-richness of a maceral. This enables the resolution of what are often difficult problems for the single parameter vitrinite reflectance technique, namely the identification of indigenous vitrinite in DOM, the recognition of caved and reworked organic matter, and the suppression of vitrinite reflectance. For these reasons, FAMM is a powerful tool for investigating vitrinite reflectance anomalies, especially those associated with unconformities. It is also well suited to investigate the tacit assumption in the vitrinite reflectance technique that all telocollinite matures along a single compositional pathway. The FAMM technique is not without problems. The range is restricted to equivalent mean random vitrinite reflectance Rm o = 0.4–1.2, the results are more sensitive to ambient temperature oxidation than vitrinite reflectance, and the technique demands a high level of technical expertise. The FAMM technique is calibrated on coals and it is therefore more likely to give high quality results on Types II and III kerogens rather than Type I kerogens which have not yet been intensively investigated. Although FAMM is unlikely to replace vitrinite reflectance, considerable confidence may be had in a maturation assessment when consistent results are obtained from these two closely related but independent techniques.

An empirical determination of the minimum number of measurements needed to estimate the mean random vitrinite reflectance of disseminated organic matter

In coal samples, published recommendations based on statistical methods suggest 100 measurements are needed to estimate the mean random vitrinite reflectance ( R v−r) to within ±2%. Our survey of published thermal maturation studies indicates that those using dispersed organic matter (DOM) mostly have an objective of acquiring 50 reflectance measurements. This smaller objective size in DOM versus that for coal samples poses a statistical contradiction because the standard deviations of DOM reflectance distributions are typically larger indicating a greater sample size is needed to accurately estimate R v−r in DOM. However, in studies of thermal maturation using DOM, even 50 measurements can be an unrealistic requirement given the small amount of vitrinite often found in such samples. Furthermore, there is generally a reduced need for assuring precision like that needed for coal applications. Therefore, a key question in thermal maturation studies using DOM is how many measurements of R v−r are needed to adequately estimate the mean. Our empirical approach to this problem is to compute the reflectance distribution statistics: mean, standard deviation, skewness, and kurtosis in increments of 10 measurements. This study compares these intermediate computations of R v−r statistics with a final one computed using all measurements for that sample. Vitrinite reflectance was measured on mudstone and sandstone samples taken from borehole M-25 in the Cerro Prieto, Mexico geothermal system which was selected because the rocks have a wide range of thermal maturation and a comparable humic DOM with depth. The results of this study suggest that after only 20–30 measurements the mean R v−r is generally known to within 5% and always to within 12% of the mean R v−r calculated using all of the measured particles. Thus, even in the worst case, the precision after measuring only 20–30 particles is in good agreement with the general precision of one decimal place recommended for mean R v−r measurements on DOM. The coefficient of variation ( V = standard deviation/mean) is proposed as a statistic to indicate the reliability of the mean R v−r estimates made at n ⪡ 20. This preliminary study suggests a V < 0.1 indicates a reliable mean and a V > 0.2 suggests an unreliable mean in such small samples.

Geochemical evaluation of upper cretaceous fruitland formation coals, San Juan Basin, New Mexico and Colorado

Geochemical analyses of coal samples from the Upper Cretaceous Fruitland Formation in the San Juan Basin of New Mexico and Colorado were used to determine thermal maturity, type of kerogen, and hydrocarbon generation potential. Mean random vitrinite reflectance (% R m) of the Fruitland coal ranges from 0.42 to 1.54%. Rock-Eval pyrolysis data and saturated to aromatic hydrocarbon ratio indicate that the onset of thermal hydrocarbon generation begins at about 0.60% R m and peak generation occurs at about 0.85% R m. Several samples have hydrogen index values between 200 and 400, indicating some potential for liquid hydrocarbon generation and a mixed Type III and II kerogen. Pentacyclic and tricyclic terpanes, steranes, aromatic steroids and methylphenanthrene maturity parameters were observed through the complete range of thermal maturity in the Fruitland coals. Aromatic pentacyclic terpanes, similar to those found in brown coals of Australia, were observed in low maturity samples, but not found above 0.80% R m. N-alkane depleted coal samples, which occur at a thermal maturity of approx. 0.90% R m, paralleling peak hydrocarbon generation, are fairly widespread throughout the basin. Depletion of n-alkanes in these samples may be due to gas solution stripping and migration fromthe coal seams coincident with the development of pressure induced fracturing due to hydrocarbon generation; however, biodegradation may also effect these samples.

Biaxial vitrinite reflectance in coals of the Elk Valley coalfield, southeastern British Columbia, Canada

Ninety-nine grab coal samples collected from ten different diamond drill-cores from the Jurassic-Cretaceous Mist Mountain Formation in the Elk Valley coalfield in southeastern British Columbia were analyzed petrographically for mean maximum vitrinite reflectance ( R max ), mean random vitrinite reflectance ( R m ) and the lengths of the maximum, intermediate and minimum axes of the reflectance indicating surface or RIS of vitrinite ( R max R int and R min). Coal rank in the coalfield ranges from high-volatile A to low-volatile bituminous. Values of R max in the core samples range from 0.95 to 1.50%. R max , R m , R max , R int and R min are all strongly intercorrelated ( r>0.89 in all combinations). The vitrinite in all samples is biaxial, in most cases biaxial negative. As a consequence, R max is always less than R max, the relationship being R max = 1.044 R max . There is no relationship between degree of vitrinite reflectance anisotropy, as represented by the RIS shape, and rank. The results indicate that a secondary stress field, not perpendicular to bedding, existed during coalification.

Distribution of alkylated aromatic hydrocarbons and dibenzothiophenes in rocks of the Upper Rhine Graben

Distributions of alkylated naphthalenes, phenanthrenes, and dibenzothiophenes and of selected non-alkylated (parent) tri- and tetra-aromatic compounds were determined by capillary gas chromatography in extractable aromatic hydrocarbon fractions from 46 core samples from the middle Upper Rhine Graben. Samples of Palaeogene and Jurassic ages at 250–2400 m depth were derived from four boreholes in the Rastatt and Kehl Troughs. Assessment of thermal maturity based on the Methylphenanthrene Index (MPI 1) was in agreement with mean random vitrinite reflectance ( R r) only in the borehole closest to the Graben centre, where overmature stages were reached below 2000 m. Otherwise, early thermal evolution of the organic matter inferred from R r < 0.7% and Rock-Eval® pyrolysis data ( T max < 440°C), was in contrast to the postmature stages indicated by the predominance of more stable, β-type 2-methyl- and 3-methylphenanthrenes (MP's) over less stable α-type 1-MP and 9-MP. The 2- MP 1- MP ratios ⩾1.4 along with a predominance of phenanthrene, fluoranthene, and pyrene over alkylated tri- and tetra-aromatic hydrocarbons were attributed to pyrolytic-like polycyclic aromatic hydrocarbons probably associated with reworked organic matter. Minor impregnations of migrated oil were revealed by elevated contents of methylated phenanthrenes. Dimethylnaphthalenes (DMN's) were mostly generated at 0.8–0.9% R r, as indicated by (2,6- DMN + 2,7- DMN) 1,5- DMN ratios of 3–5, and then underwent extensive redistribution by upward migration. The exceptional predominance of 4-methyldibenzothiophene (MDBT) and dibenzothiophene together with very high 4- MDBT 1- MDBT ratios of 30–50 in certain samples from the Rastatt Trough were attributed to minor impregnations of hydrothermal petroleum.

Generation and distribution of aromatic hydrocarbons in coals of low rank

The generation of C 11+-aromatic hydrocarbons has been investigated in a total of thirteen coal samples with mean random vitrinite reflectance ( R r) between 0.45 and 1.00% mainly derived from the Lower Cretaceous German Wealden formation in the Lower Saxony Basin. Yields of total aromatics and selected dicyclic and tricyclic aromatic molecules which were extractable before and after heat treatment of the samples at 350 and 400°C have been determined by medium and high pressure liquid chromatography followed by capillary gas chromatography. Regular variations with increasing R r in the carbon-normalized yields demonstrated that aromatic hydrocarbon generation was strongly controlled by rank. In contrast, variations in maceral composition appeared to be of minor importance. Based on the conceptual model of “open” and “closed” poers, discrepancies in the quantitative distributions of extractable aromatics among unheated and heated coals were attributed to a restricted mobility of molecules contained in “closed” pores, which were accessible to solvent extraction only after the rigid physical coal structure had partially been disintegrated by heat treatment at 350°C.

Fluid-inclusion technique for determining maximum temperature in calcite and its comparison to the vitrinite reflectance geothermometer

Research Article| October 01, 1990 Fluid-inclusion technique for determining maximum temperature in calcite and its comparison to the vitrinite reflectance geothermometer Charles E. Barker; Charles E. Barker 1U.S. Geological Survey, Box 25046, MS 971, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar Robert H. Goldstein Robert H. Goldstein 2Department of Geology, University of Kansas, 120 Lindley Hall, Lawrence, Kansas 66045 Search for other works by this author on: GSW Google Scholar Author and Article Information Charles E. Barker 1U.S. Geological Survey, Box 25046, MS 971, Denver, Colorado 80225 Robert H. Goldstein 2Department of Geology, University of Kansas, 120 Lindley Hall, Lawrence, Kansas 66045 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1990) 18 (10): 1003–1006. https://doi.org/10.1130/0091-7613(1990)018<1003:FITFDM>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Charles E. Barker, Robert H. Goldstein; Fluid-inclusion technique for determining maximum temperature in calcite and its comparison to the vitrinite reflectance geothermometer. Geology 1990;; 18 (10): 1003–1006. doi: https://doi.org/10.1130/0091-7613(1990)018<1003:FITFDM>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Theory, laboratory experiments, and empirical observation suggest that many aqueous fluid inclusions in calcite reequilibrate during overheating, and therefore some homogenization temperatures (Th) record a temperature close to the maximum reached by the rock. This characteristic suggests that aqueous fluid inclusions in calcite can be used to establish maximum temperature (Tpeak). To test this hypothesis, we have compiled fluid inclusion Tpeak, mean random vitrinite reflectance (Rm), and present-day Tpeak from 46 diverse geologic systems that have been at Tpeak from 104 to 106 yr. Present Tpeak ranged from 65 to 345 °C, Th modes and means ranged from 59 to 350 °C, and Rm data ranged from 0.4% to 4.6%, spanning the temperature and thermal maturity range associated with burial diagenesis, hydrothermal alteration, and low-grade metamorphism.Plots of Th and Tpeak data for systems thought to be currently at maximum temperature demonstrate close agreement between Th and present Tpeak in sedimentary basins. Although caution should be applied, the relation suggests that Th of aqueous fluid inclusions in calcite may be a useful measure of maximum temperature. This study also compares Th to mean random vitrinite reflectance (Rm) to offer further support for the use of Th as a measure of Tpeak, and to provide a better understanding of Rm. Th correlates well with Rm and results in a curve similar to Rm vs. Tpeak calibrations determined by other workers. The strong correlation (correlation coefficient r = 0.93) between Tpeak and Rm in these systems suggests that maximum temperature is the major control on thermal maturation. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Renewed Petroleum Generation Related to Tertiary Intrusions and Increased Heat Flow, Western Permian Basin, Texas and New Mexico: ABSTRACT

Higher paleogeothermal gradients, commencing in the Tertiary after maximum burial, have caused renewed petroleum generation in the western Permian basin. Evidence for this reheating is two distinct trends in the mean random vitrinite reflectance (R/sub m/) and depth data compiled from over 40 wells. One group, with a 0.7% R/sub m//km gradient, is from the western edge of the basin; the other, with a 0.5% R/sub m//km gradient, is from the central and eastern portions. Post-Mississippian tilting produced greater subsidence and a thicker, mostly uneroded sedimentary section in the eastern portion of the Permian basin. Continued tilting prior to the Cretaceous caused uplift and erosion that exposed the Upper Permian section in the western part. Potassium-argon ages of igneous intrusions along the western edge of the basin show they were emplaced about 35 Ma, followed by Miocene to Holocene basin-and-range-type block faulting and associated high heat flow. Isopach-reflectance contours confirm this renewed heating is post-tectonic - that is, it occurred after eastward tilting and erosion had reduced burial depth. Maximum temperatures computed from R/sub m/-depth relationships infer that paleogeothermal gradients exceeded 40/degrees/C/km (2.2/degrees/F/100 ft) in the Tertiary. This reheating thermally matured rocks as young as Guadalupian in the western Permianmore » basin and apparently caused a second episode of petroleum generation. By this time, however, the potential reservoir rocks and evaporite seals had been deeply eroded, resulting in poor conditions for trapping the renewed pulse of petroleum.« less

Organic Thermal Maturity Applications to Source Rock Potential Analyses at Phillips Petroleum Company: ABSTRACT

The importance of determining accurate organic thermal maturity levels in potential hydrocarbon source rocks cannot be overstated. As important as organic quantity and quality, sufficient thermal alteration of the host rock must occur to convert the organics to hydrocarbons. In the author's laboratories the two most important criteria for determining the thermal alteration level of a rock sample are measuring its mean random vitrinite reflectance and estimating the spore coloration index (i.e., thermal alteration index, TAI) of plant spores and pollen present. An additional thermal maturity indicator is the fluorescence intensity of the oil-prone kerogen present. These three maturity criteria are microscopically derived measurements made on the concentrated kerogen (i.e., insoluble organics) recovered from rock samples after acidification. Source rock analysts have generally accepted a scale force relating vitrinite R{sub 0} (reflectance in immersion oil) and TAI to four thermal maturity stages: (1) immature for oil generation, (2) early oil generation, (3) peak oil generation, and (4) dry gas to overmature for hydrocarbon generation. These maturity data, when combined with the kerogen type and relative abundance, and then merged with the quantitative TOC and pyrolysis results, constitute Phillips' basic hydrocarbon source rock potential technique. This procedure is considered to bemore » an accurate measure of source rock potential and is further enhanced when all the data (as many as 24 values) are evaluated by their licensed artificial intelligence program, Source Rock Advisor. This PC-driven computer program generates a totally standardized and reproducible source rock evaluation.« less

Thermal Maturity Patterns and Time-Temperature Modeling of Cretaceous and Tertiary Rocks, San Juan Basin, Colorado and New Mexico: ABSTRACT

Mean random vitrinite reflectance (R{sub m}) values of coal beds in the Upper Cretaceous Fruitland Formation in the San Juan basin of Colorado and New Mexico range from 0.42% in the southern part of the basin to 1.54% in the northern part. R{sub m} profiles of wells drilled through approximately 8,000 ft of cretaceous and Tertiary rocks in the area of high thermal maturity are commonly nonlinear, with three or more segments. R{sub m} gradients through the coal-bearing part of the Fruitland are high, ranging from 1.0 to 1.5%/1,000 ft and 0.30%/1,000 ft, respectively. These gradients most likely represent the combined effects of contrasting thermal conductivities associated with lithologic variations and differences in heat transfer processes (conductive vs. convective). Time-temperature modeling of a nonlinear vitrinite reflectance well profile in the area of high thermal maturity indicates that the present heat flow of 96 m W/m{sup 2} is insufficient to account for the measured levels of thermal maturity. Furthermore, in order to match the nonlinear R{sub m} well profile, it was necessary to assign artificially high thermal conductivity values to nearly all of the stratigraphic units penetrated in the well. The authors interpret the unreasonably high thermal conductivity values as evidencemore » of convective heat transfer and conclude that the high level of thermal maturity in the northern part of the San Juan basin is due to a deep, localized source of high heat flow in conjunction with convective heat transfer associated with a strong component of vertically directed fluid flow.« less

Surface Thermal Maturity Map of Uinta, Piceance, and Eagle Basins Area, Utah and Colorado: ABSTRACT

Knowledge of thermal maturity is essential for basin analysis studies because heat and heat flow play major roles in mineral formation, hydrocarbon generation and destruction, ground-water flow, and petroleum reservoir quality. The authors studied the thermal maturity of several hundred outcrop samples of coal and carbonaceous shale from Pennsylvanian through Quaternary rocks in the area encompassing the Uinta, Piceance, and Eagle basins of Utah and Colorado. All samples were analyzed for mean random vitrinite reflectance (R/sub m/); thus, thermal maturity is defined by R/sub m/ for this study. Their data show that R/sub m/ generally increases with age of the rock, reflecting increasing burial depth and residence time at maximum temperature. Some coals show local anomalously high maturities because of proximity to Tertiary intrusives. Rocks of the same age show a general decrease in maturity from south to north in all three basins, which probably results from a decrease in geothermal gradients from south to north. In the Uinta basin, R/sub m/ values range from about 0.40 to 0.70% around the margins of the basin to 0.45% in the Altamont-Bluebell oil field near the basin center. In the Piceance basin, R/sub m/ values range from about 0.50 to 0.70% aroundmore » the margins to about 0.25 to 0.35% in the center. The Eagle basin shows the widest range of R/sub m/ values, from 0.30 to 0.50% around the margins to values in the 4.0% range near the center. Some vitrinite anomalies in the study area are best explained by sampling and analytical problems, such as choosing recycled vitrinite grains from organically lean shales or collecting weathered, oxidized samples.« less

Variations in Vitrinite Reflectance and Clay-Mineral Composition Within Vertical Sections of Belden Shale, Eagle Basin, Colorado--Problems in Determining Thermal Maturity: ABSTRACT

A major problem in defining true thermal maturity of shales by mean random vitrinite reflectance (R/sub m/) and clay-mineral geothermometry is determining the correct population of organic matter and/or interstratified illite/smectite (I/S) clay, respectively. Shales commonly contain a variety of both organic matter and I/S clay; the types of both the organic matter and the I/S clays vary widely, as do the interpreted values, reflecting the heterogeneous sources. The organic-rich, carbonate-bearing Belden Shale (Early Pennsylvanian) contains three distinct populations of organic matter. The vitrinite with the lowest reflectance is weathered-looking or porous, and is generally in the 0.90-1.30% R/sub m/ range. The next higher population of vitrinite R/sub m/ values is consistently in the 2.0-3.0% range. The highest R/sub m/ population is fusinite, measuring in the 4.0-5.0% range. Other indicators of thermal maturity (T/sub max/ and time-temperature index reconstructions, which show that these rocks have been subjected to relatively high temperatures, are in best agreement with the intermediate R/sub m/ values 2.0-3.0%). Although the ordered and highly illitic I/S is in good agreement with the other thermal indicators, randomly interstratified I/S typically does not exist in sedimentary rocks with R/sub m/ > 0.80%. Possible explanations for the problems stated aremore » (1) that weathering of precursor organic matter and minerals can produce lower values in R/sub m/ and highly expandable I/S, respectively; (2) suppression effects reported for vitrinite in other organic-rich marine shales and for the smectite-to-illite reaction in other calcium-bearing rocks; and (3) very late diagenesis at near-surface conditions producing highly expandable I/S.« less

Thermal Maturity of Organic Matter in Green River Formation, Piceance Creek Basin, Colorado: ABSTRACT

The thermal maturity of organic matter in the Green River Formation in the Piceance Creek basin was determined by vitrinite reflectance on coalified logs in the otherwise alginite-rich oil shale, marlstone, and sandstone. Only vitrinite from logs in sandstone and marlstone was used to determine thermal maturity because reflectance of vitrinite from alginite-rich oil shale generally is lower than that in associated other rock types. Mean random vitrinite reflectance (Ro) at the top of the Green River Formation ranges from about 0.30% around the perimeter of the basin, where maximum burial depth of the rocks was less than 1,000 m (3,300 ft), to 0.55% in the structurally lowest part of the basin, where maximum burial depth of the upper part of the Green River was ore than 1,500 m (4,900 ft). The Green River Formation is almost 1,200 m (3,900 ft) thick in the structurally lowest part of the basin, suggesting that the lower part of the formation in this area may have reached an Ro of 0.7%, generally accepted as the threshold for oil generation in alginitic rocks. Bitumin-filled fractures observed in core from this area of the basin support this conclusion. A lithologically similar lacustrine section of the Green River Formation in the adjacent Uinta basin, where maximum burial was as great as 5,600 m (18,400 ft), is producing large quantities of oil from overpressured, fracture-controlled reservoirs. Present-day maximum temperatures in the Green River Formation in the Piceance Creek basin are between 55 and 70°C (131 and 158°F) This temperature seems too low End_Page 492------------------------------ for hydrocarbon generation. However, temperatures in the past probably were high enough for hydrocarbon generation. Oil generated during this earlier, hotter period could have migrated into conventional stratigraphic and structural traps. End_of_Article - Last_Page 493------------