Sterane Isomerization Research Articles

Application of Fully Integrated Quantitative Basin Analysis Model: Oseberg Area, Norwegian North Sea: ABSTRACT

A new deterministic dynamic basin model was developed to quantify basin evolution in terms of geologic, hydrodynamic, geothermal, and geochemical histories. Quantification of the processes occurring in sedimentary sequences was based on series of equations defining fluid flow, grain velocity, conductive and convective heat flow, maturation of organic matter, isomerization of steranes and hopanes, hydrocarbon generation, and potential migration directions. One-dimensional studies on 12 wells were used to optimize (1) the conceptual model in terms of tectonic history, basin formation, stratigraphy, lithology, paleoclimate, and paleodepositional environments and (2) the parameters of the equations used. Two-dimensional studies were undertaken on three seismic profiles from the region. The model results were checked against all the available data on geology, hydrodynamics, geothermics, seismics, and geochemistry. The modeling results showed that the crust was not thinned much, contrary to the amounts cited in literature. The maximum amount of heat flow was 1.20 heat-flow units (HFU) during Late Jurassic. The heat flow steadily decreased, reaching a value of 1.0 HFU at the beginning of the Cretaceous. A slight increase in heat flow during middle to late Eocene was computed. Petroleum generation started 66 Ma and continued at different rates until the present. Petroleum then migratedmore » eastward and southward into the Oseberg structures. The modeling results indicated two major petroleum types in the area. All the modeling results indicated two major petroleum types in the area. All the modeling results have been confirmed by geochemical data in terms of pyrolysis, biomarkers, and isotopes.« less

長岡周辺新第三系ガス田の生物指標化合物

Cuttings materials from Katagai Gas Field in the vicinity of Nagaoka City, Niigata Prefecture, were analyzed for biological markers (triterpenoid and steroid hydrocarbons) in order to evaluate their validity as maturity indicies, with the following results.(1) Isomerization of both steranes and triterpanes are recognized from immature to oil generation zone.(2) The most reliable criteria in the immature zone and the oil generation zone are inferred to be the isomerization of hopane nucleus and the epimerization of stigmastane side chain, respectively.(3) Change of monoaromatic steroid hydrocarbons into triaromatics seems to occur from the immature to oil generation zone.(4) Cracking reactions in the side chains of C26, 27, 28-triaromatic steroid hydrocarbons into C20, 21-triaromatics appear to take place from the late stage of oil generation zone to the overmature zone. This reaction may be useful as a maturity index of crude oils or gas condensates, which cannot be measured for vitrinite reflectance.

Terpenoid biomarker distributions in low maturity crude oils

Twenty-seven heavy crude oils of diverse origin were geochemically assessed with respect to both bulk and mlecular composition for the purpose of identifying and quanttfying valid biomarker parameters for low maturity oils. The low thermal maturity level of many of these oils is evident from the bulk and alipathic chromatographic data, and oil sourced from both marine and terrigenous organic matter are represented. Selective metastable ion monitoring (SMIM) was employed to measure separately the distribution of C 27, C 28, and C 29 sterane isomers. The useful maturity indicators include the C 29 5α(H) 20S/20R ratio, the relative quantity of the biological sterane configuration in each of the total normal C 27, C 28, and C 29 steranes, and the rearranged to normal sterane ratio. In addition, C 27 rearranged steran es appear to form at a faster rate than C 28 or C 29 rearranged steranes. However, the isomerization of the C 27 biological component appears to occur at a slower rate than the C 29 counterpart suggesting that the former may be used as a maturity parameter at higher levels of thermal maturation. In the triterpane distributions, the C 27 trisnorhopane isomers and the moretane to hopane ratios appear to be both source and maturity related and cannot be used as successful maturity parameters in oils unless they share a common source. The C 31+ hopane 22S/22R equilibrium ratio appears to increase with increasing molecular weight (C 31–C 34).

Steranes and triterpanes generated from kerogen pyrolysis in the absence and presence of minerals

Steranes and triterpanes generated from pyrolysis of immature Monterey Formation kerogen in the presence and absence of calcite, illite and montmorillonite reveal results that are both consistent and divergent with published data that reflect the use of these biological markers as maturation indicators. The extent of isomerization of biomarkers generated from pyrolysis of kerogen at 300°C for 2 hours, at C-20 in 14α(H),17α(H)-steranes, at C-22 in 17α(H),21β(H)-hopanes and of 17β(H),21β(H)-hopanes correspond to early diagenetic stages in rock extracts from sedimentary basins. Isomerization increases with heating time and, after 1000 hours, attains values which correspond to the catagenetic stage in sedimentary basins, or equivalent to that of mature oil. Stepwise pyrolysis of the kerogen indicates faster isomerization rates for steranes and triterpanes in the bitumen than for those retained in the kerogen structure, confirming earlier studies. Presence of a mineral matrix can influence the isomerization of steranes and triterpanes considerably. Comparisons with results from kerogen heated alone, for a given maturation stage, show that calcite inhibits, illite catalyzes slightly and montmorillonite has a pronounced catalytic effect on these reactions. This effect results in early isomerization of steranes and hopanes corresponding to the catagenetic stage in the presence of montmorillonite, while kerogen or kerogen with calcite held at the same temperature (300°C) and time (10 hours) only yield isomerized products which correspond to a diagenetic stage. Further, illite and montmorillonite affect various isomerization reactions differently. The fastest reaction is the isomerization at C-20 in 14α(H),17α(H)-steranes followed by that at C-22 in 17α(H),21β(H)-hopanes and the slowest is the formation of 14β(H),17β(H) steranes. These results show that maturation measurements of rock or oil samples from sedimentary basins which use biological markers have to take into account the mineral matrix effects, which have been largely ignored until present.

Sterane isomerisation and moretane/hopane ratios in crude oils derived from Tertiary source rocks

The extent of sterane isomerisation reactions and the moretane/hopane ratios of 234 crude oils, taken world wide, from a wide variety of source rocks of differing geological ages, have been measured. This data indicates that in 78 crude oils derived from Tertiary source rocks, sterane isomerisation reactions as determined by the 20S/(20S + 20R) ration of the C 29 5α(H), 14α(H), 17α(H) normal-steranes and the C 29 iso/(iso + normal) ratio [iso = 5 α (H), 14 β(H), 17 β(H)] are mainly incomplete and sometimes considerably so. In addition, the same crude oils have 17β(H), 21α(H)-moretane/17α(H), 21β(H)-hopane ratios which are significantly greater (predominantly in the range 0.10–0.30) than those of crude oils derived from older, mature source rocks (mainly less than 0.1). This data, for crude oils, lends support to the hypothesis, proposed by Mackenzie and McKenzie (1983) for source rock extracts, that the time/temperature constraints of sterane isomerisation reactions are such that the time available for isomerisation in Tertiary sediments is generally insufficient, despite generation of crude oil at relatively high temperatures. An alternative hypothesis is that the incomplete sterane isomerisation of Tertiary crude oils may be due to generation of these crude oils from their deltaic, land plant-containing source rocks under low heating conditions. A third hypothesis proposes that the Tertiary crude oils may have picked up the incompletely isomerised steranes from immature sediments during migration. Although possible in particular instances, such a mechanism does not appear to be generally applicable since, in that case, the phenomenon would then appear to be restricted to the Tertiary. The higher moretane/hopane ratios of the Tertiary crude oils could suggest that constraints, similar to those applying in sterane isomerisation, also operate in the conversion of moretane to 17α(H)-hopane.

Thermal Maturity of Devonian Oils and Paleozoic Strata in Michigan Basin: Organic Geochemical Parameters: ABSTRACT

Devonian oils from the Michigan basin exhibit subtle differences in several geochemical maturity parameters. These differences may be related to source rock variations, migration, or post-reservoir alteration of the oils. Sterane isomerization (20S/20S + 20R) is variable and generally higher than predicted using established kinetic parameters and equations. The oils exhibit diasterane distributions expected of oils generated in the presence of clay minerals. Triterpene distributions are typical for mature oils. Small maturity differences in the Dundee oils may reflect variations in the depth to reservoir and related higher temperatures. Methylphenanthrene index values have been converted to calculated vitrinite reflectance equivalents (% R/sub c/). Dundee oils average 0.57% R/sub c/, whereas Detroit River and Traverse oils average 0.70% R/sub c/. Carbon isotope measurements on aliphatic and aromatic fractions indicate a source rock containing marine organic matter and an earlier migration for some of the Devonian oils. Potential source rocks and related strata have been subjected to reflectance (% R/sub o/), kerogen pyrolysis, and biomarker maturity measurements. These independent maturity measurements have been examined and compared to those from the Devonian oils, to define the past geothermal gradient in this Paleozoic basin and to correlate the oils to their source rocks.

Steroid biomarker-clay mineral adsorption free energies: Implications to petroleum migration indices

Enrichments of 14β,17β steranes over 5α,20R steranes in petroleums relative to shale source rocks have bee discussed inthe literature and ascribed to possible migrational geochromatographic processes involving mineral surfaces. Liquid−solid chromatographic (LSC) effects arise from adsorption free energy (AFE) differences between various chemical compounds and an active surface. If nonmaturational concentration differences of 14β,17β/5α,20R sterane isomers in petroleum arise frommigrational geochromatographic effects, then those differences must be reflected by sterane-mineral AFE differences. Accordingly, AFE values for 23 selected steroid biomarkers, including saturated-isomerized, mono-,di-, and triaromatized and partially cracked structures were measured on montmorillonite clay using a chromatographic technique. Those data conform to LSC theory: laboratory migration rates for biomarkers on montmorillonite of increasing water content can be calculated from AFE values. Biomarker AFE values are shown to depend on specific structural chemical features, i.e. aromatic carbon number and stereochemistry, in a systematic way. Quantitative contributions of those features to AFE values are calculated and rules are developed which permit the prediction of relative chromatographic migration rates for related biomarker compounds. Finally, AFE valuess are used to calculate 253 possible biomarker pair concentration ratio variations (measurable by GC-MS) expected for clay mineral LSC-type geochromatographic effects.

Biological marker distribution in coexisting kerogen, bitumen and asphaltenes in Monterey Formation diatomite, California

Organic-rich (18.2%) Monterey Formation diatomite from California was studied. The organic matter consist of 94% bitumen and 6% kerogen. Biological markers from the bitumen and from pyrolysates of the coexisting asphaltenes and kerogen were analyzed in order to elucidate the relationship between the various fractions of the organic matter. While 17α(H), 18α(H), 21α(H)-28,30-bisnorhopane was present in the bitumen and in the pyrolysate of the asphaltenes, it was not detected in the pyrolysates of the kerogen. A C 40-isoprenoid with “head to head” linkage, however, was present in pyrolysates of both kerogen and asphaltenes, but not in the bitumen from the diatomite. The maturation level of the bitumen, based on the extent of isomerization of steranes and hopanes, was that of a mature oil, whereas the pyrolysate from the kerogen showed a considerbly lower maturation level. These relationships indicate that the bitumen may not be indigenous to the diatomite and that it is a mature oil that migrated into the rock. We consider the possibility, however, that some of the 28,30-bisnorhopane-rich Monterey Formation oils have not been generated through thermal degradation of kerogen, but have been expelled from the source rock at an early stage of diagenesis.

Origin of solid bitumens, with emphasis on biological marker results

Solid bitumens are allochthonous, non-disseminated organic matter found associated with sedimentary rocks. Common worldwide examples include gilsonite, grahamite and ingramite. Their occurrence in petroliferous basins makes knowledge of their origin of interest to organic geochemists and petroleum explorationists. Using current analytical methods, it is usually possible to assign an origin to these materials, based on chemical characteristics of the whole material and its extractable fraction. In this study, 27 solid bitumens and their extracts have been studied, and the following analyses were performed: elemental (H, C, N, Ni, and V) and isotopic ( σ 13C) analyses of whole bitumen and extracts thereof; pyrolysis-gas chromatography of whole bitumen, and gas chromatography of extractable aliphatic hydrocarbons; carbon-13 NMR aromaticity measurements of the whole bitumen, using CP-MAS techniques; and GCMS studies of biological markers in the distillable (400–900°F) hydrocarbons. Results which have a bearing on the genesis of these materials are presented and discussed. Results indicate that (a) the classical generic names assigned to each bitumen, based on solubility, fusibility and H/C ratios, are neither source-distinctive nor indicative of genesis; (b) the relative amounts of n-hydrocarbons in chromatograms and pyrograms are useful in assessing both origin and the nature of secondary alteration (e.g. biodegradation); (c) the solid bitumens examined can be successfully classified using a genetic scheme, based largely upon molecular and bulk maturation parameters. Using NMR-derived ƒ a values, H/C atomic ratios, moretane/hopane ratios, and ratios of sterane isomers, solid bitumens associated with oil source rocks can be genetically classified as either pre-oil or post-oil. Pre-oil solid bitumens are early-generation (immature) products of rich source rocks, probably extruded as very viscous fluids, which have migrated minimal distances to fractures. In contrast, post-oil solid bitumens are products of the alteration of a once-liquid oil, which was generated and migrated from a mature source rock. Following generation and expulsion, both pre-oil and post-oil solid bitumens are subject to the same modification processes, namely biodegradation, devolatilization and thermal alteration.

The kinetics of specific organic reactions in the zone of catagenesis

Recently it has been shown that the measured extent of reactions in sedimentary biological marker compounds during catagenesis (for example, steroid aromatization, sterane isomerization) can assist in providing information about the extent of maturation of sedimentary organic matter before and during oil generation and also the thermal history of sediments. In the present study, aromatization of a C 27 C-ring aromatic steroid and isomerization at the chiral centres of an isoprenoid alkane, 6( R ),10( S )-pristane, have been brought about under laboratory conditions, elemental sulphur being used as the source of radicals. Precise rate laws have been determined and rate coefficients measured at different temperatures. The pre-exponential factors and activation energies were found to be 6.7 x 10 12 s -1 and 145 kJ mol -1 for the aromatization, and 2.1 x 10 7 s -1 and 120 kJ mol -1 for the configurational isomerization of pristane. These values reflect the relative behaviour of the two types of reactions in sediments, the aromatization being the more temperature-dependent reaction.

Biological marker characteristics of oils and asphalts from carbonate source rocks in a rapidly subsiding graben, Dead Sea, Israel

A detailed GC/MS study of biological marker compounds in the saturated and aromatic hydrocarbon fractions of oils and asphalts from the Dead Sea area, Israel, provided decisive information to the solution of a long-lasting controversy by showing that the asphalts are products of early generation in an immature stage from the same type of carbonate source rock which generated more mature oils. The asphalts are not biodegraded residues of the oils. Oils from six different wells, and asphalts from wells, outcrops, and a floating block from the Dead Sea all have very similar sterane and triterpane patterns. They all lack rearranged steranes (diasteranes) indicating a carbonate source matrix and compare reasonably well with a sample of Upper Cretaceous bituminous chalk from Nebi Musa. The main difference between the oils and the asphalts is a significantly higher triaromatic to mono- plus triaromatic steroid hydrocarbon ratio in the former. This is explained as a result of rapid subsidence and heating of their source rock close to the deep parts of the Dead Sea graben. The oils thus were generated in the more deeply buried source rock blocks under the graben fill, whereas the asphalts either originate from an immature source rock section closer to the graben rims or represent an earlier phase of generation and expulsion. This study also provides general information on the evolution of biological markers in carbonate source rocks. Low-activation-energy processes, like isomerisation of steranes, appear to occur much faster at low temperatures than in shales. The high sulfur content and less cross-linking of the biogenic organic matter into a complex kerogen structure are suggested to be responsible for this. Care should be taken when using only sterane isomerisation to assess the maturity of hydrocarbons from carbonate rocks and of carbonate-derived oils.

Isomerization and Aromatization of Hydrocarbons and the Paleothermometry and Burial History of Alberta Foreland Basin

The hydrocarbon fractions extracted from 34 Cretaceous shales from a cross section of the Alberta basin have been analyzed using gas chromatography-mass spectrometry. Particular attention has been paid to the extents to which three temperature-dependent geochemical reactions have occurred: isomerization of steranes at C-20; isomerization of hopanes at C-22; and aromatization of C-ring monoaromatic steroid hydrocarbons. Estimates of the kinetic constants of these reactions have been published previously; therefore, constraints may be put on the maximum temperature experienced by the sediments and the time for which this temperature existed. The subsidence history of the Alberta basin is simulated using a lithospheric flexural model in which the lithosphere is downwarped by the loading of overthrusts in the adjacent Cordilleran orogene. When overthrusting ceases, erosion of the overthrusted region lightens the load and the lithosphere rebounds, causing uplift and erosion of the foreland basin. Major overthrusting and therefore deep burial occurred during the Laramide orogeny of Late Cretaceous to mid-Tertiary time. The extent of chemical reactions suggests that the present maturity of the Cretaceous shales is almost entirely a reflection of burial and heating during the Laramide orogeny in a manner that is consistent with the lithospheric flexural model for the basin. The chemical reactions suggest that the paleogeothermal gradient increased systematically away from the foothills of the Rocky Mountains along the profile studied. That a similar trend exists today suggests this situation has persisted at least since the early stages of the Laramide orogeny. This variation is best explained by advective heat transfer. Water entering the basin in the faulted and topographically higher foothills is heated at depth, thereby depressing the geothermal gradient. This hot water then migrates updip and away from the foothills to enhance the geothermal gradients in the Edmonton area and beyond. The route of the migrating hot water is similar to that taken by hydrocarbons from Devonian and Cretaceous source rocks in the deep basin, updip to the oil and tar-sand accumulations of Alberta.

Biological markers in bitumens and pyrolyzates of Upper Cretaceous bituminous chalks from the Ghareb Formation (Israel)

The sterane and triterpane distributions of three bituminous chalks from the Upper Cretaceous Ghareb Formation (Israel) were investigated both in the original extractable bitumens and in extracts obtained after pyrolysis of whole rock and isolated kerogen samples at 450°C. Pyrolysis was performed in a closed system under hydrous (whole rock) and anhydrous conditions (isolated kerogens). The carbon number distributions of steranes and triterpanes differ significantly between original bitumen and pyrolyzates. Unlike the bitumens in which diasteranes were not detected, the anhydrous pyrolyzates contain small amounts of diasteranes. The presence of water during pyrolysis leads to an increase of sterane isomerization, the abundant formation of diasteranes and an increase of the 18α( H)- trisnorneohopane 17α( H)- trisnorhopane ratio. Sterane isomerization maturation parameters show a closer match between original bitumen and pyrolyzates after pyrolysis in a closed system when compared with an open system.

Formation of immature asphalt from organic-rich carbonate rocks—II. Correlation of maturation indicators

The present study focuses on some discrepancies among maturation indicators of the Senonian bituminous rocks (SBR) in Israel. These rocks are mainly carbonates that were deposited under reducing conditions and contain heteroatom-rich organic matter that has been severely reworked by bacteria. In two cases, heavy asphaltic oils are associated with the SBR and a genetic relation between them has been established. Whereas the time-temperature index (TTI < 3) and vitrinite reflectance ( R 0 = 0.3−0.4%) indicate early stage maturation, the bitumen content (up to 700 mg g C org −1) and the asphaltic oils imply an oil generation zone in the SBR. Other indicators such as isomerization of hopanes and steranes and CPI values indicate even higher maturation levels. The discrepancies between these maturation indicators are explained by the different kinetic parameters of the maturation processes in the various constituents of the organic matter.

Isomerization and aromatization of hydrocarbons in sedimentary basins formed by extension

Summary. The reactions involved in oil generation are of great economic importance, but remain to be studied in detail. We have investigated the rates of three reactions which occur before and during the early stages of oil formation, and have used the predicted thermal and subsidence history of stretched basins to estimate the six reaction constants. Two of the reactions are isomerization reactions, at C-20 in a sterane and at C-22 in a hopane hydrocarbon. The third reaction converts C-ring monoaromatic to triaromatic steroid hydrocarbons. No single measure of maturity can describe the progress of these reactions. In old basins, such as the North Sea, both isomerization reactions are almost complete before appreciable aromatization has occurred, whereas in young basins, such as the Pannonian Basin in Hungary, aromatization is almost complete before appreciable isomerization of the steranes has occurred. We show that the calculated progress of these reactions agrees well with that observed in both basins if the frequency factors and activation energies are 6 x 10-3 s-1 and 91 kJ mol-1, 0.016 s-1 and 91 kJ mol-1, 1.8 x 1024 s-1 and 200 kJ mol-1 for the isomerization of steranes, of hopanes, and the aromatization of steroid hydrocarbons respectively. The rate of conversion of the R to the S form was taken to be 1.174 and 1.564 times that of the reverse reactions for sterane and hopane isomerizations respectively, and the aromatization reaction was assumed to be irreversible. All three reactions were assumed to be first order and unimolecular. The aromatization rate is consistent with laboratory observations. The rate of hopane isomerization is not, and different reaction mechanisms probably dominate at different temperatures. The same constants can be used to predict the progress of the reactions in basins which have been uplifted by inversion, such as the Lower Saxony Basin in West Germany. The geochemical observations provide estimates of the amount and time of uplift which agree with those from geological studies. Geochemical observations from the eastern part of the Paris Basin suggest that this region has also been uplifted by between 1 and 2 km.

A biological marker investigation of petroleums and shales from the Shengli oilfield, The People's Republic of China

The distributions of the alkanes and aromatic steroid hydrocarbons of eleven oils and six shales from the Shengli oilfield (Zhanhua depression), have been examined by gas chromatography and gas chromatography-mass spectrometry. Particular attention has been paid to the so-called biological marker compounds (steranes, triterpanes and aromatic steroid hydrocarbons). Where metalloporphyrins are present, their distributions have been examined by high-performance liquid chromatography and mass spectrometry. The shales show progressive increases in the extent of aromatisation of aromatic steroid hydrocarbons and the extent of configurational isomerisation in the triterpanes which correlate well with present burial depth. They are less mature than all of the oils when maturity assessment is based on the degree of sterane configurational isomerisation. One oil (Yi-18) is less mature than all the other oils from most of the measurements. A number of ratios and compound distributions thought to be indicative of either maturity or palaeoenvironment (or both) suggest the Upper Shahejie (Oligocene) as the most appropriate source for the Yi-18 oil. The same ratios and distributions rule out the Lower Shahejie (as represented by the sample examined here) as a possible source for any of the oils (even in a more mature state). The main source(s) for all the other oils is more mature (by extent of sterane isomerisation) than all the shales examined. Suitable sources for these oils have not been found in this study. There is no correlation between oil maturity and reservoir depth, suggesting that the present maturity of the oils relates mainly to maturation effects in the source rocks and not the reservoir. All oils pooled at depths of ⪝ 1500 m (∼70°C) show the effect of biodegradation ( n-alkanes removed) whilst those pooled at depths ⪞ 1500 m appear to be unaffected.

Ion subtraction — a computer-aided step in the geochemical analysis of fossil fuels

The subtraction of the signal intensities of the ion fragments m/e 149 and m/e 151 in the GLC-MS computer analysis of the saturated hydrocarbons of a fossil fuel, reveals the distribution of the 5α and 5β steranes. This technique is a convenient method of establishing the efficacity of the thiourea clathration step, and simplifies routine geochemical analysis. Subtraction of the signal intensities of the ion fragments m/e 218 and m/e 217 reveals the distribution of 14α and 14β sterane isomers, and is a further refinement in the study of geological steroids.