Rock-Eval Analysis Research Articles

Oxidation of Organic Matter in the Transition Zone of the Zechstein Kupferschiefer from the Sangerhausen Basin, Germany

Thirteen samples of the Permian Kupferschiefer (Sangerhausen Basin, Germany) were studied by microscopy, stable isotope analysis, Rock-Eval analysis, Fourier transform infrared (FT-IR) spectroscopy, and additional geochemical methods in order to understand the oxidation processes of organic matter in a profile of mineralized Kupferschiefer. Relatively intense oxidation occurred at the bottom of the profile, where bitumens were oxidized by ascending brines, and altered to pyrobitumen and “kir” bitumen. Because of organic matter oxidation Corg-values, extract yields, and HI values decrease at the bottom of the profile. The oxidation resulted in release of δ12C-rich organic constituents and enrichment of the residual organic matter in δ13C organic carbon in the extracts of the oxidized Kupferschiefer. In the bottom sample, major IR absorption bands between 1207 and 900 cm-1 (C−O−C, C−O−H) were caused by oxidation of saturated hydrocarbons. A reduced absorption at 2955−2869 cm-1 relative to less oxidized samples indicates oxidation of aliphatic compounds. The intensity of the carbonyl band at 1700 cm-1 (CO) relative to the band at 1600 cm-1 (CC) decreases from the bottom to the top of the profile, indicating a preferential oxidation in the bottom part of the profile. Results from the analysis of extracts by gas chromatography−mass spectrometry (GC-MS) indicate that these major absorption bands are due to the oxidation of long-chain saturated hydrocarbons and alkylated aromatic hydrocarbons. The oxidation products from alkylated aromatic compounds include naphthalene, biphenyl, phenanthrene, and dibenzofuran. The oxidation products of saturated hydrocarbons include aliphatic acids, alcohols, and esters. The oxidation may have occurred after Kupferschiefer deposition.

Potential of emission Fourier transform infrared spectroscopy for in situ evaluation of kerogen in source rocks during pyrolysis

Eleven petroleum source rock samples have been examined by Emission Fourier transform infrared spectroscopy (FTIRES) and by Rock-Eval thermal analyses. FTIRES has been used to estimate the amount of C–H aliphatic and C–H aromatic signal lost during thermal treatment at Rock-Eval temperatures. In particular, the loss of signal from 150°C to 300°C has been monitored and compared to the yield of oil obtained over this range by Rock-Eval analysis. Likewise, signal loss between 300–500°C and 500–700°C was monitored. Correlations of FTIRES changes with Rock-Eval pyrolysis derived parameters were not found because FTIRES reflects changes in composition of undistilled material during pyrolysis. However, by relating Rock-Eval and FTIRES derived parameters a source rock quality index can be derived. This index can be useful as a quick routine method for scanning samples to determine if the free oil in the source rock was generated by thermal maturation of the same source rock.

Chemical characteristics of Upper Cretaceous (Turonian) jet of the Gosau Group of Gams/Hieflau (Styria, Austria)

Jet and coaly sediments within the Upper Cretaceous (Gosau) Schoenleiten Formation were collected from two outcrops near Gams/Hieflau (Styria, Austria). For comparison, additional jet and coal samples from different Gosau localities were included in the study. The identification of jet as bituminous driftwood (collotelinite) has been provided by microscopical examination of organic matter [Kollmann, H.A. and Sachsenhofer, R.F., Mitt. Ref. Geol. und Paläont. Landesmuseum Joanneum SH 2 (1998) 223]. Rock–Eval analyses revealed enhanced HI and lower T max values of jet compared with the coals and coaly shales. The results reflect the higher contents of bituminous organic matter compared to vitrain-rich coals of similar maturity. In comparison with the coals and coaly shales, high amounts of liberated hydrocarbons (mg HC/g C org) are in contrast to generally lower yields of soluble organic matter during extraction with dichloromethane, indicating that portions of the hydrocarbons are liberated from lipids loosely bound to kerogen during pyrolysis. Further differences in n-alkane distribution patterns, odd over even predominance, pristane/phytane, and pristane/ n-C 17 ratios exist between jet, coals and coaly shales. The microbial origin of short-chain n-alkanes presumably resulted from the degradation of the predominant odd-numbered long-chain n-alkanes within the jet. The occurrence of biological markers (sesqui and diterpenoids) within the sediments (coaly shales), coals and jet samples indicates the origin of terrestrial organic matter from conifers. Biomarker composition of jet samples indicates enhanced aromatisation of sesqui and diterpenoids. These results correspond to enhanced aromatisation of steranes, as reflected by generally higher relative contents of triaromatic steroids in the jet. Because of comparable maturation (0.5–0.6% R r) of the coals, coaly shales and the jet samples, confirmed by the isomerisation of αβ C 31-hopanes and ααα C 29-steranes, the increase in aromatisation in the jet is thought to reflect increased contribution of bacteria to organic matter degradation. The lack of enrichment of the jet samples in hopanes relative to steranes suggests that enhanced biomarker aromatisation is caused by anaerobic rather than by aerobic bacteria.

Sulfate Reduction by Organic Matter in Colombian EmeraldDeposits: Chemical and Stable Isotope (C, O, H) Evidence

Colombian emeralds occur within carbonate-silicate-pyrite veins and breccias in black shale-limestone– hosted deposits. They are contained within two zones, the eastern and western emerald zones, in the Eastern Cordillera. Mineralizing fluids were derived from the interaction of low-salinity fluids with primary halite and anhydrite. The source of beryllium is sedimentary and of sulfide sulfur is evaporitic. The presence of organic matter, as bitumen, in both the black shales and the emerald-bearing hydrothermal veins invites discussion of the role played by organic matter in thermochemical sulfate reduction, the mechanism proposed for H 2 S production. Organic matter is found in the orebodies in two forms: altered and unaltered. It is also found in primary fluid inclusions of emeralds. Rock-Eval analyses (hydrogen index <1; 19 < oxygen index < 61) as well the elemental H/C ratio (hydrogen index <1) confirmed the intense cracking suffered by the original organic matter. In hydrothermal bitumen, infrared microscopy revealed the absence of aliphatic and aromatic bands and the disappearance of oxygen functional groups. The eastern emerald zone contains 13 C-depleted carbonates (mean δ 13 C = –11.3 ± 1.3‰, n = 14) with an homogeneous oxygen isotope composition range ( δ 18 O = 17.8 ± 0.3‰). The western emerald zone contains carbonates with relatively heavy δ 13 C (mean δ 13 C = –5.1 ± 0.5‰, n = 25) and notable δ 18 O variation (17–23.8‰). In both emerald zones, 13 C-depleted calcite points to thermochemical sulfate reduction by oxidation of organic matter, but most δ 13 C values are heavier than predicted by thermochemical sulfate reduction. A model that mixes heavy CO 2 evolved from dissolution of local limestone and light CO 2 evolved by oxidizing organic matter explains the observed δ 13 C range of carbonates. Variations in δ 18 O of carbonate also reflect the local heterogeneity of the host rocks. Redox reactions produced HCO 3 – and H 2 S; these compounds then reacted with the predominant cations (Fe 2+ and Ca 2+ extracted from the host black shale and limestones by the hydrothermal fluid) and induced calcite and pyrite precipitation. The oxygen and hydrogen isotope compositions of fluid inclusions and structural waters of the gangue minerals and emeralds constrain the origin of the mineralizing fluids. The composition of these fluids was compared with the isotope composition of fluids in the Zipaquira halite-bearing salt intrusions, which are near the emerald deposits. The calculated fluid δ 18 O in equilibrium with calcite and dolomite (eastern emerald zone: 7.7 δ 18 O Fluid < 12.8‰; western emerald zone: 8.2 δ 18 O Fluid < 17.8‰) and emerald (eastern emerald zone: 15.5 δ 18 O Fluid < 17.1‰; western emerald zone: 17.5 δ 18 O Fluid < 23.6‰) are typical of metamorphic waters or highly evolved sedimentary formation waters (temperature of formation = 300°C). The high δ 18 O H 2 O values of the minerals suggest that the fluid may have been continuously enriched in δ 18 O owing to exchange with limestones and black shales. In both emerald zones, the δ D values of carbonates and quartz parental waters (–64 δ D < –40.7‰) are similar to those found for halite (–66.4 δ D < –41.2‰), whereas δ D values of emerald, muscovite and albite are –51.4 to –7.1 per mil. The increase in δ D is interpreted as a consequence of mixing of fluids of different isotope composition during the mineralizing event. Two observations can be made about the O and H isotope ratios of fluid inclusions and structural waters from salt, muscovite, albite, and emerald. In the eastern emerald zone, they indicate that the fluids from halite and aluminosilicates are the same and are the result of mixing of a meteoric water ( δ 18 O = –12.7‰, δ D = –91.6‰) with a fluid representing the previous mixture of a water with the Early Cretaceous evaporite brines. In the western zone, the isotope ratios do not show the covariation seen in the eastern zone. The 18 O H 2 O shift found in emerald and muscovite is interpreted to be the result of oxygen isotope exchange reactions with sedimentary formations during the precipitation of the silicate phase.

The Drama Lignite Deposit, Northern Greece: Insights from Traditional Coal Analyses, Rock-Eval Data, and Natural Radionuclides Concentrations

In the present study the results of traditional methods for characterizing 25 lignite samples, representing a continuous sequence of the Drama deposit, Northern Greece, are presented. The moisture, the volatile matter, and the ash contents are high, while the sulfur content remains at low levels. Rock-Eval analysis was performed on the same samples. The Tmax values obtained do not correlate with coal type and rank and are higher than those expected for lignites. The reflectivity values are lower than 0.2%. In values <0.5% Rm and >1.5% Rm, Tmax does not correlate with Rm. It is also noticed that the difference in hydrogen contents is not related to the Tmax values as would be expected. The concentrations of 238U, 235U, 226Ra, 210Pb, 228Ra, 228Th, and 40K, determined in samples D2 and D5, are similar to those reported in the literature for Greek lignites. The concentrations of uraniumseries isotopes are among the higher for lignites worldwide.

Preliminary Hydrocarbon Source Rock Assessment of the Paleozoic and Mesozoic Formations of the Western Black Sea Region of Turkey

Source rock maturity and potential of Paleozoic and Mesozoic formations in the Eregli, Zonguldak, Bartin, Ulus, and Eflani subregions of the western Black Sea region (WBSR), have been investigated by rock-eval pyrolysis, reflected-light microscopy, and palynofacies analyses. The % Ro values of dispersed organic matter of the Paleozoic formations primarily range from 0.72 to 1.8 %, but values as high as 2.6 % occur locally in the Silurian Findikli Formation in the Eregli subregion. The % Ro values of Namurian-Westphalian coal seams in the K20 H well drilled in the Zonguldak subregion range from 0.87 to 1.52 %, with increasing depth consistent with sedimentary depth of burial. Most Cretaceous age samples have reflectance values ranging from 0.44 to 1.6% Ro that indicates they are marginally mature to mature with respect to the oil window. Rock-eval pyrolysis demonstrates that the Paleozoic formations have limited oil-generation potential (HI values < 200 mg HC/gCo rg), but good gas potential (TOC values up to 3 %). Cretaceous formations have better petroleum source rock characteristics, but they too are primarily gas prone. Variations in the source rock maturity probably reflect variable burial histories in different localities of the WBSR.

Geochemical Characteristics and Origin of Tar mats from the Yaha Field in Tarim Basin, China

Tar mats were firstly discovered and determined accurately in terrestrial oil and gas reservoirs associated with Lower Tertiary sandstone reservoirs in the Yaha field of the Tarim Basin, China, by thin-layer chromatography-flame ionization detector (TLC-FID) and Rock-Eval analysis. The relative content of asphaltene in gross composition of tar mat extracts accounts for more than 30 %, that in the corresponding oil leg less than 20 %. In the geochemical description profile of oil and gas reservoirs, drastic changes in asphaltene contents between tar mats and oil legs could be discovered. This is an important marker to determine tar mats. Distribution characteristics of saturated and aromatic hydrocarbons from reservoir core extracts and crude oils in the Yaha oil and gas reservoirs in the Tarim Basin are described systematically in this paper, and the results show there are similarities among n-alkane distribution characteristics, biomarker distribution characteristics and their combined characteristics of saturated hydrocarbons, and the geochemical characteristics of aromatic hydrocarbons for tar mats, oil leg, asphaltic sand and crude oil. These characteristics suggest the hydrocarbons in these samples were originated from the common source rocks. However, the geochemical characteristics of tar mats revealed that the mechanism of formation of tar mats is the precipitation of asphaltene from crude oils in petroleum reservoirs caused by increased dissolved gas in oil legs (gas injection).

99/01220 Hydration and strength development of binder based on high-calcium oil shale fly ash : Freidin, C. Cem. Concr. Res., 1998, 28, (6), 829–839

This study provides coal-quality, organic-petrographic and organic-geochemical data on Tertiary subbituminous coal of the Hafik area, northwestern part of the Sivas Basin, Turkey. Coal-petrological studies along with proximate and ultimate analyses were undertaken to determine the organic-petrographic characteristics of the Hafik coals. Huminite reflectances were found to be between 0.38 and 0.48% (corresponding to an organic-material-rich and coal layers), values characteristic of low maturity. This parameter shows a good correlation with calorific values (average 21,060 kJ/kg) and average Tmax (422 °C) mineral-matter diagenesis, indicating immaturity. The studied coals and organic material underwent only low-grade transformation, a consequence of low lithostatic pressure. Therefore, the Hafik coals are actually subbituminous in rank. Rock-Eval analysis results show types II/III and III kerogens. The organic fraction of the coals is mostly comprised of humic-group macerals (gelinites), with small percentages derived from the inertinite and liptinite groups. In this study, organic-petrographic, organic-geochemical and coal quality data were compared. The Hafik deposit is a high-ash, high-sulfur coal. The mineral matter of the coals is comprised mainly of calcite and clay minerals.

Organic facies characteristics of the Sivas Tertiary Basin (Turkey)

The Sivas Tertiary Basin is located on top of the Late Cretaceous Neotethys ophiolitic nappes which were abducted onto both the Pontic belt and the Tauric platform. Organic facies characters of Tertiary units in this basin have been determined by means of total organic carbon (TOC) analysis, rock-eval pyrolysis, vitrinite reflectance data and organic petrography investigation. The total organic carbon content (TOC) of fine grained sediments from the Eocene (Kozluca and Bozbel Formations) are between 0.05 and 2.26% indicating that these formations are poor to moderate in organic matter. Rock-eval analysis yield low hydrogen index (HI) and high oxygen index (OI) values for all samples. The dominant organic matter is type III kerogen and the hydrocarbon-generating potential is thus very low. The vitrinite reflectance ( R o) value is between 0.30 and 0.93%. In the Sivas Tertiary Basin, the Oligocene sediments from the Selimiye Formation are characterized by a low TOC content (0.05–1.72%) and hydrogen index (21–47 mg HC/g TOC). The vitrinite reflectance of Oligocene sediments is 0.38–0.64%. The total organic carbon content (TOC) for Miocene sediments (Hafik and Karacaören Formation) is 0.07–1.46%. The organic matter content is richer in the Miocene sediments of the Karacaören Formation marine shelf than in the sediments of the Hafik Formation terrestrial.

Characterization and origin types of immature and low-mature amorphous kerogen in terrestrial source rocks

The microfluorescent technique was applied to investigate the fluorescence alteration patterns of amorphous kerogen of the terrestrial source rocks of China. Two fluorescence parameters, I54620 andR were suggested, which can be taken to divide the amorphous kerogen into five types: algae amorphinite, biodegraded amorphinite A, B, C, and humic amorphous. On the basis of this, the Rock-Eval analysis results of source rocks were used to investigate the hydrocarbon generating potential of these amorphinite.

Oxygen index as an indicator of early organic maturity

Most maturity indices can be used during catagenesis and metagenesis but there are only a few indicators to show the degree of organic maturation within the diagenesis zone. A previous study has already demonstrated that the oxygen index (OI) measured by Rock-Eval analysis provides reliable information about the early evolution stages of coals and type III kerogens. The main purpose of this study was to investigate the conditions of OI application in both natural and artificial evolution. The results of experiments presented here revealed that the accuracy of OI in estimating the evolution level depended on the amount and chemical composition of the organic matter. Natural series of low-rank coals showed OI to be a valuable rank parameter not only during diagenesis but in early catagenesis also. OI appeared to be a sensitive indicator during both simulated diagenesis and the entire oil window for gas-prone coal having a substantial humic contribution to its precursor material. In contrast, the OI of oil-prone coal and coal with mixed oil and gas potential proved to be a valuable rank parameter only up to the beginning of the oil window. Though it could be utilized successfully for describing depth profiles in organic-rich intervals of early maturity in wells, its use in determining natural maturity was restricted by the organic-richness of rocks, as well as by the effect of the mineral matrix. Providing humic material is present, OI can be used to follow the artificial thermal maturation of type II kerogen in the absence of mineral matrix.

97/02599 Scanning electron microscopy analysis of coal products from different comminution methods

This study provides coal-quality, organic-petrographic and organic-geochemical data on Tertiary subbituminous coal of the Hafik area, northwestern part of the Sivas Basin, Turkey. Coal-petrological studies along with proximate and ultimate analyses were undertaken to determine the organic-petrographic characteristics of the Hafik coals. Huminite reflectances were found to be between 0.38 and 0.48% (corresponding to an organic-material-rich and coal layers), values characteristic of low maturity. This parameter shows a good correlation with calorific values (average 21,060 kJ/kg) and average Tmax (422 °C) mineral-matter diagenesis, indicating immaturity. The studied coals and organic material underwent only low-grade transformation, a consequence of low lithostatic pressure. Therefore, the Hafik coals are actually subbituminous in rank. Rock-Eval analysis results show types II/III and III kerogens. The organic fraction of the coals is mostly comprised of humic-group macerals (gelinites), with small percentages derived from the inertinite and liptinite groups. In this study, organic-petrographic, organic-geochemical and coal quality data were compared. The Hafik deposit is a high-ash, high-sulfur coal. The mineral matter of the coals is comprised mainly of calcite and clay minerals.

97/02608 Supercritical fluid extraction of a high-ash Brazilian mineral coal

This study provides coal-quality, organic-petrographic and organic-geochemical data on Tertiary subbituminous coal of the Hafik area, northwestern part of the Sivas Basin, Turkey. Coal-petrological studies along with proximate and ultimate analyses were undertaken to determine the organic-petrographic characteristics of the Hafik coals. Huminite reflectances were found to be between 0.38 and 0.48% (corresponding to an organic-material-rich and coal layers), values characteristic of low maturity. This parameter shows a good correlation with calorific values (average 21,060 kJ/kg) and average Tmax (422 °C) mineral-matter diagenesis, indicating immaturity. The studied coals and organic material underwent only low-grade transformation, a consequence of low lithostatic pressure. Therefore, the Hafik coals are actually subbituminous in rank. Rock-Eval analysis results show types II/III and III kerogens. The organic fraction of the coals is mostly comprised of humic-group macerals (gelinites), with small percentages derived from the inertinite and liptinite groups. In this study, organic-petrographic, organic-geochemical and coal quality data were compared. The Hafik deposit is a high-ash, high-sulfur coal. The mineral matter of the coals is comprised mainly of calcite and clay minerals.

Chemical structure of the organic matter in a Pliocene maar-type shale: Implicated Botryococcus race strains and formation pathways

A maar-type Pliocene oil shale from Pula (Hungary) was examined by Scanning and Transmission Electron Microscopy (SEM, TEM) bitumen analysis, spectroscopic analyses (FTIR, solid state 13C NMR) and pyrolytic analyses of insoluble material. The investigated samples, corresponding to the “massive section” of Pula deposit, exhibit TOC values ranging from 18 to 42%, and Rock-Eval analysis indicated that such samples are comprised of low maturity type I kerogen. Electron Microscopy observations confirmed that recognizable microfossils in this organic matter-rich material almost exclusively correspond to Botryococcus braunii colonies, the morphology of which is perfectly retained. Identification of bitumen and pyrolysate constituents (hydrocarbons, ketones, fatty acids), along with morphological and spectroscopic features, indicate (1) that B. braunii provided a major input to the most organic matter-rich section of Pula oil shale; (2) that both the n-alkadienes-producing and the lycopadiene-producing strains ( A and L, respectively) were present; (3) that the selective preservation of the insoluble and non-hydrolysable macromolecules building up B. braunii outer walls was by far the main process in the formation of this material; (4) that such a process, along with B. braunii prolific growth in the favourable environment that occurred in the crater lake, accounts for the very high TOC and oil potential of Pula deposit; and (5) that the condensation of high molecular weight ether lipids from the A race and of some bacterial lipids also likely contributed to Pula kerogen formation.

Kerogen as a geochemical marker of the thermal history of rocks

The study presents examples of the application of oxyreactive thermal analysis to the geological prospecting of bitumen deposits. Thermal analysis performed under properly chosen analytical conditions allows determination of characteristic features of organic matter and, at the same time, estimation of the thermal history of rocks. Thermal oxyreactivity curves document whether rocks were heated before or not. The results of the oxyreactive thermal analysis correlate well with the results of routine chemical analyses of organic matter (e.g. the composition of kerogen and bituminous fraction, Rock-Eval analysis and others). Our study clearly demonstrates that the composition of organic matter in facially, lithostratigraphically, and biogenetically identical rocks depends on their thermal history.

KITCHENS, KETTLES AND CUPS OF HYDROCARBONS, VICTORIAN OTWAY BASIN

Evaluation of Early Cretaceous source rocks within the onshore Victoria Otway Basin has revealed that thick, mature shales containing predominantly gas-prone and in local concentrations, oil-prone macerals exist northwest of Portland, in the Tyrendarra Embayment, and around the Port Campbell region.Current results of Rock-Eval, bulk composition, gas chromatography, and biomarker analyses, coupled with geohistory and hydrocarbon generation interpretations, indicate that at least three phases of oil generation and expulsion occurred within the basin. The earliest phase, which coincided with the maximum heatflow in the crust around 100 Ma, resulted in the charging of the existing stratigraphic/shoestring traps of the basin. The second and third phases occurred in the eastern end of the basin at around 85 and 60 Ma. There is also evidence to suggest that structural traps of the eastern areas were formed later, during Oligocene time, and that these traps are probably still receiving late-stage charges of hydrocarbons.Although the sparse well density in the basin has resulted in limited, non-uniforin sampling opportunities, several regions with good Early Cretaceous source rocks can be recognised. Some of these good source rock areas are in close proximity to the several known hydrocarbon shows and producing fields. These current studies, which also include a source rock risk analysis indicating source rock adequacy, show that locations for future exploration could include the Casterton-Portland-Mt Gambier western region, the Peterborough-Port Campbell eastern region, and the prospective close peripheries and offshore extensions of these regions.

Geology, Thermal Maturation, and Source Rock Geochemistry in a Volcanic Covered Basin: San Juan Sag, South-Central Colorado

The San Juan sag, concealed by the vast San Juan volcanic field of south-central Colorado, has only recently benefited from oil and gas wildcat drilling and evaluations. Sound geochemical analyses and maturation modeling are essential elements for successful exploration and development. Oil has been produced in minor quantities from an Oligocene sill in the Mancos Shale within the sag, and major oil and gas production occurs from stratigraphically equivalent rocks in the San Juan basin to the southwest and in the Denver basin to the northeast. The objectives of this study were to identify potential source rocks, assess thermal maturity, and determine hydrocarbon-source bed relationships. Source rocks are present in the San Juan sag in the upper and lower Mancos Shale (including the Niobrara Member), which consists of about 666 m (2184 ft) of marine shale with from 0.5 to 3.1 wt. % organic carbon. Pyrolysis yields (S1 + S2 = 2000-6000 ppm) and solvent extraction yields (1000-4000 ppm) indicate that some intervals within the Mancos Shale are good potential source rocks for oil, containing type II organic matter, according to Rock-Eval pyrolysis assay. Oils produced from the ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received August 23, 1995; revised manuscript received September 4, 1996; final acceptance February 4, 1997. 2Priority Oil & Gas, LLC, 633 17th Street, Suite 1520, Denver, Colorado 80202. 3U. S. Geological Survey, P.O. Box 25046, MS 977, Denver Federal Center, Denver, Colorado 80225. 4Platte River Associates, Inc., 2000 W. 120th Avenue, Suite 10, Denver, Colorado 80234. Use of product names is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey. We are grateful to Waggoner-Baldridge Energy Co., Meridian Oil, and UP Resources for supplying samples and data for this study. Lynn Gries provided hours of picking clean shale samples and manuscript typing. T. A. Daws and A. Warden, U.S. Geological Survey, are gratefully acknowledged for performing Rock-Eval and carbon isotope analyses, respectively. We thank R. Walker, U.S. Geological Survey, and Ann Priestman, GeoGraphics, for drafting the figures. Reviews of earlier versions of the manuscript by Vito Nuccio, Chuck Chapin, P. Lillis, N. Bostick, P. Wagner, and an anonymous reviewer are appreciated.

Relationship between metal enrichment and organic composition in Kupferschiefer hosting structure-controlled mineralization from Oberkatz Schwelle, Germany

Base metal mineralization is observed in some Kupferschiefer localities where Rote Fäule (oxidized front of hematite-bearing fluids) is not present, and where contemporaneous Mesozoic to Tertiary tectonism controlled the (vein-type) metal enrichment processes after diagenesis (structure-controlled mineralization). One of these localities is the Spessart-Rhön-Treischfelder-Oberkatz Schwellen region in the Hessian Depression, Germany. In order to clarify the mechanism of base metal precipitation under these particular conditions, a closely-sampled Kupferschiefer sequence from the Oberkatz Schwelle was investigated using organic and inorganic geochemical methods. The structure-controlled mineralization is enriched in As, Mo and U in the bottom part of the Kupferschiefer and in the Weissliegendes sandstones. Further elements such as Cu, Ag, Co, Sb, Zn and Pb show a redox-controlled pattern of zonation in the Kupferschiefer section. These elements were precipitated or redistributed in the final stage of Kupferschiefer mineralization. Participation of organic matter in base metal deposition is suggested by the depletion of H equivalents in the organic matter. A similar phenomenon has been observed in Rote Fäule-controlled base metal enrichment in the North-Sudetic Syncline of SW Poland. However, observed differences in the composition of extractable organic matter might be due to the oxidizing potential of the migrating metal-bearing fluids. In the North-Sudetic Syncline, the ascending fluids were more oxidizing compared with the Oberkatz Schwellen area. Additional S content and depletion of Corg are observed in the bottom section of the profile. The H Index (HI) values from Rock-Eval analyses provide further evidence for the depletion of H equivalents in the organic matter. Reconstruction of the thermal regime by the measurement of vitrinite reflectance and determination of molecular rank parameters indicate a maximum temperature in the Kupferschiefer of around 150°C.

Examples and New Applications in Applying Organic Geochemistry for Detection and Qualitative Assessment of Overlooked Petroleum Reservoirs: ABSTRACT

Techniques of organic geochemistry can provide speck information on the presence and quality of petroleum in payzones that may be overlooked by exploration and production geologists. This paper presents case histories where the use of basic geochemical techniques on rock and/or fluid samples were used to find reservoir intervals and predict the quality of the reservoired petroleum on both active and previously drilled wells. Two examples are from wells drilled in the San Joaquin basin in California. TOC, Rock-Eval, and Thermal Extract Gas Chromatography analyses on cuttings from an actively drilling well were used to successfully identify in missed reservoirs, qualify the hydrocarbons present, and identify the oil/water contact. The second San Joaquin Basin example used the same techniques on old cuttings samples to discover by-passed shallow reserves of over 200,000 barrels of oil. The use of Gas Chromatography on a RFT liquid sample successfully identified a gas condensate reservoir in a Gulf of Mexico Eocene Yegua trend well that was drilled with a diesel mud system. The diesel in the mud prevented the conventional evaluation techniques from identifying any liquid hydrocarbons in the well. A 50 API gravity liquid was tested from the sand interval. Additional examples will bemore » presented to illustrate other geochemical techniques and their usefulness in finding by-passed pay zones and assessing the quality of reservoired hydrocarbon.« less

Differences in the depositional environment of basal Zechstein in southwest Poland: implication for base metal mineralization

Base metal enrichment in the Permian basal Zechstein sediments occurs in two major areas in southwest Poland, the North-Sudetic Syncline and the Fore-Sudetic Monocline. Two profiles of Kupferschiefer and adjacent limestones from the Konrad mine and from the Rudna mine were studied by petrographic and geochemical methods in order to recognize similarities, and differences, in the comparative depositional environment of both areas. In the North-Sudetic Syncline basal Zechstein was deposited in a shallow marine environment under slightly oxidizing sedimentary conditions. It is composed of 1.2m thick mergelkalk, which might be the equivalent of Werrakalk (Ca1). In the Fore-Sudetic Monocline the basal Zechstein is a black shale, the so-called Kupferschiefer (T1) with a thickness of 0.2–0.5 m. Microscopical investigations have shown that a substantial contribution of higher continental plant material is observed only in the North-Sudetic Syncline because of its location near the Zechstein sea-shore. Pyrite is present in small amounts only. In the Fore-Sudetic Monocline, apart from the former shoreline of the Zechstein sea, the organic matter is preferentially composed of degraded algae due to its deposition under deeper anoxic bottom waters. Abundant pyrite is also present. Differences in the chemical composition of the respective kerogens are reflected in the results obtained from Rock-Eval analyses. Hydrogen indices (HI) are significantly higher in the T1-section of the profile from the Rudna mine, in comparison to the basal Zechstein of the Konrad mine profile. A depletion of hydrogen is only observed in the highly mineralized section and in Rote Fäule of the Konrad mine. This indicates that hydrogen equivalents from organic matter participated in metal sulfide precipitation by thermochemical sulfate reduction (TSR). This is further confirmed by the occurrence of pyrobitumen, saddle dolomite and calcite spars. In contrast, in the mineralized section of the Rudna mine profile a depletion of hydrogen is not observed, pyrobitumen is missing and saddle dolomite and calcite spars are very rare. Instead, abundant pyrite originating from bacterial sulfate reduction (BSR) could have been the source of reduced sulfur species, available for base metal precipitation after deposition of the sediment (pyrite replacement). The results allow one to conclude that in both areas base metals were precipitated from ascending solutions due to the interaction with the reducing Zechstein barrier. Several observations argue for a post-depositional mineralization process at a temperature of less than 130°C in both areas.