Padmanabhan Sundararaman

Sensitivity of biomarker properties to depositional environment and/or source input in the Lower Toarcian of SW-Germany

Abstract A sequence of Lower Toarcian sediments which are highly contrasting in their depositional environment over a 5 m depth interval has been investigated in detail for the variability of geochemical properties which are used for source characterization and oil-to-source correlation. Lower Liassic marls are representative of a well-aerated shallow-water environment in contrast to the immediately overlying bituminous shales which are deposited under extremely reducing conditions. Various properties have been found to vary considerably within these two units. Amongst the most important are carbon isotopic composition of the kerogen, the pristane/phytane ratios, nickel as opposed to vanadyl porphyrins, and the C27 dia-/regular steranes. Although maturation within the profile does not change, some of the maturation-dependent biomarker properties such as the monoaromatic steroid side-chain cracking and the Tm/Ts ratio exhibit large changes which can be assigned to diagenetic processes. Another maturation-dependent property, the 20S/20R epimerization of C29 steranes, exhibits smaller changes which could also be due to early diagenetic processes. The study suggests that reducing and oxidizing conditions, i.e. Eh and pH in the sediment, exert an influence on several biomarker precursor-product pathways. In maturation studies initial variations due to depositional conditions have therefore to be taken into account.

Effects of hydrous pyrolysis on biomarker thermal maturity parameters : Monterey Phosphatic and Siliceous Members

Hydrous pyrolysis of immature Monterey Phosphatic or Siliceous rock at progressively higher temperatures causes systematic changes in biomarker thermal maturity parameters of the generated hydrocarbons. Biomarker ratios based on proposed carbon-carbon cracking or aromatization reactions increase during hydrous pyrolysis along similar pathways for both Siliceous and Phosphatic members. An increase in these biomarker ratios is also observed for oils of increasing thermal maturity from the offshore Santa Maria Basin, although the rates of changes for each parameter differ between the hydrous pyrolysis and natural samples. Changes in some cracking parameters during maturation appear to result from differential thermal stability of the compounds rather than conversion of precursors to products. The behavior of isomerization-based biomarker ratios in these experiments is more complex than ratios based on carbon-carbon cracking or aromatization reactions. During heating, kerogen-bound precursors generate steranes and hopanes showing lower levels of thermal maturity based on isomerization ratios than those extracted from the unheated rock. Asymmetric centers in the kerogen-bound steroids and hopanoids appear to be protected from isomerization compared to those of free steranes or hopanes in the bitumen. The Phosphatic and Siliceous rocks can show different sterane or hopane isomerization ratios when heated under the same time/temperature conditions. Further, these isomerization ratios unexpectedly decrease at high hydrous pyrolysis temperatures (> 330°C) for the Phosphatic, but not for the Siliceous samples. This could be caused by the combined effects of isomerization and differential destruction of epimers, apparently mediated by rock mineralogy. Differences between the biomarker compositions of bitumens and expelled oils in these experiments mimic those caused by natural primary migration. Heavier, more polar compounds are preferentially retained in the bitumen. For example, bitumens are enriched in tri- over monoaromatic steroids, hopanes over tricyclic terpanes, and regular steranes over diasteranes compared to expelled oils. No significant fractionation of stereoisomers was observed, such as 22S vs 22R C32 17α(H),21β(H)-homohopanes or 20S vs 20R and ββ vs αα C29 5α(H)-steranes.

Vanadylporphyrins, indicators of kerogen breakdown and generation of petroleum

Abstract Attempts at understanding the transformation of organic material into fossil fuels often relies on biomarkers, organic compounds present in the geological record that can be related to naturally-occurring molecules from specific organisms. While porphyrins were the first biomarkers identified in fossil fuels, no significant use has been made of them in geochemical correlation studies. We believe that one limitation to their usage has been inaccurate models proposed for the fate of porphyrins during catagenesis. Using laboratory pyrolysis experiments we show that the changes observed in vanadylporphyrin distribution during catagenesis is due to increasing dilution of preexisting DPEP vanadylporphyrins by ETIO vanadylporphyrins released from kerogen. In conjunction with a quantitative expression describing the change in vanadylporphyrin distribution during maturation called the Porphyrin Maturity Parameter (PMP), a basis is now provided for the use of vanadylporphyrins as indicators of onset of petroleum generation.

Relationship of biomarker distribution to depositional environment: Phosphoria Formation, Montana, U.S.A.

From biomarker distributions it is possible to distinguish extracts from four different members/facies (Meade Peak, Rex, Retort, and Tosi) of the Permian Phosphoria Formation at Little Sheep Creek, Montana. These distributions relfect changes in floral and faunal distribution in response to a changing depositional environment and suggest that the Retort and Tosi Members (the second cycle of deposition) were deposited under more anoxic conditions and at higher salinity than Meade Peak and Rex Members (the first cycle). Correlations between biomarker indicators of anoxia and salinity suggest that anoxia was in part the result of a chemocline separating normal marine waters above from more saline bottom waters. Anoxia and salinity in the bottom waters increased with time making conditions in the basin progressively more hostile to benthic organisms. Development of extreme environmental conditions in continental shelf habitats (such as those suggested by biomarker parameters at Little Sheep Creek) may have compounded spatial problems created by continental suturing suggested to be a cause of Permian extinctions. Factor analysis indicates that four factors for 75.3% of the variance in biomarker ratios. These factors are suggested to be source organisms (types of eukaryotic plankton), salinity, Eh, and mineralogy. The Grandeur Member of the Phosphoria, which is a typical reservoir rock for Phosphoria derived oils, yields an extract containing a novel C30 nitrogen-containing steroid. This unique steroid is not abundant in any other known extracts or oils, which along with the extremely low Porphyrin Maturity Parameter of all the extracts (PMP < 0.01), confirms the absence of migrated oils in the Phosphoria at Little Sheep Creek. The biomarker ratios C29αββ/(gaββ + αααa steranes; Ts/(Ts + Tm); triaromatic/(triaromatic + monoaromatic) steroids; short/(long + short chain) monoaromatic steroids; and short/(long + short chain) triaromatic steroids are commonly used to determine the thermal maturity of oils and source rock extracts. However, these ratios show considerable variation in core extracts of the same thermal maturity from different facies. In contrast, the thermal maturity parameters, C29 ααα 20S/(20S + 20R) steranes and Porphyrin Maturity Parameter (PMP) show little variation, indicating that these parameters are less affected by changes in depositional environment and may be most useful for thermal maturation determinations of Phosphoria related oils and source rocks. This study supports previous work suggesting environmental effects must be considered if correct determination of thermal maturity of an oil or extract is to be made. This is especially true of oils and extracts from source rocks with high sulfur concentrations and/or those deposited at elevated salinities.

Source Rock Quality Determination from Oil Biomarkers II--A Case Study Using Tertiary-Reservoired Beaufort Sea Oils

Biomarkers (molecular fossils) in 26 Beaufort Sea Mackenzie delta oils reveal three genetic oil groups and suggest descriptions of their probable sources. Group 1 (21 Tertiary-reservoired oils) was soured from Tertiary deltaic sediments. Group 2 (three Cretaceous- and one Devonian-reservoired oil) and group 3 (one Lower Cretaceous-reservoired oil) derive from two high-quality sources deposited in open-marine environments. Geographical variations in the geochemistry of the 21 group 1 oils suggest that their deltaic source was substantially more oil-prone in the distal deltaic portions than in the region closer to the paleoshoreline. Although these data do not address source rock volumes, they do indicate that poor source quality is not a cause of underfilled traps in the o fshore. The map pable lateral variations in group 1 oil compositions illustrate how, in basins with vertically drained sources, lateral source rock facies changes can bc inferred from regional variations in the geochemistry of the overlying oils. Furthermore, the data suggest that geochemical studies attempting to deduce genetic relationships between geographically separated oils should carefully consider the potential effects of source rock facies changes on the oil compositions.

Comparison of nickel and vanadyl porphyrin distributions of sediments

Abstract The distributions of nickel and vanadyl porphyrins of Toarcian shale, northern Germany, and the Monterey Formation, California, were examined. The compositions of nickel and vanadyl porphyrin fractions of extracts from the same sample show substantial differences. The nickel porphyrin distribution is more complex than the vanadyl porphyrin distribution. While the vanadyl porphyrin distributions of samples from a core are the same, their nickel porphyrin compositions show wide variations. The nickel porphyrins are enriched in “etio” and 3-nor C30DPEP structures. The vanadyl porphyrins are enriched in the C33DiDPEP structure. Methylpropano porphyrins are found only in the vanadyl porphyrin fraction, whereas the cycloheptano structural type is predominant in the nickel fraction. C31DPEP and C32DPEP are found both in the nickel and vanadyl fractions. Their relative abundance in the two fractions is proportional to the Ni VO porphyrin ratio. The observed differences in the compositions of nickel and vanadyl porphyrin fractions are due to differences in the rate constants for their formation from the pheopigments.

Effect of biodegradation on vanadylporphyrin distribution

Oils, tars, and degraded oils of varying degrees of biodegradation occur in the Permian Phosphoria Formation along the eastern flank of the Wind River mountains, Wyoming, US. Biodegradation has altered the isomer distributions of steranes, the hopanes, and the mono- and triaromatised steroids in these oils. The conventional maturity parameters based on these biomarkers are of limited use in estimating their maturity. On the other hand, even in severely biodegraded oils the vanadylporphyrin distributions are unaltered. Because of this, the Porphyrin Maturity Parameter (PMP) derived from the vanadylporphyrin distribution is an ideal parameter for estimating the maturity of these oils. This study also confirms the previous observation that T[sub s] and T[sub m] are unaffected by even severe biodegradation. 14 refs., 5 figs., 2 tabs.

Vanadyl porphyrins in exploration: maturity indicators for source rocks and oils

Abstract The Porphyrin Maturity Parameter ( PMP ), which is derived from the vanadyl porphyrin distribution, is an excellent parameter for: (1) identifying the zone of hydrocarbon generation from marine source rock extracts; and (2) determining from oils the thermal maturity of their source rocks at expulsion. The PMP is measured using a methodology which is inexpensive, reliable and faster than earlier methods, allowing it to be used as a routine exploration tool. The PMP may be a more reliable maturity indicator for marine organic matter than some conventional methods such as vitrinite reflectance. Unlike most conventional maturity parameters guided by processes other than kerogen conversion, the reactions causing PMP evolution directly monitor the generation of bitumen and the concurrent thermal degradation of kerogen. Measurements on hydrous pyrolyzates from the Monterey Formation (offshore California), source rock bitumens from the Devonian-Mississippian Bakken Shale (Williston Basin), and Miocene Monterey equivalent source strata (San Joaquin Basin, California) illustrate the method. In all cases reviewed so far, PMP begins increasing at the onset of hydrocarbon generation and increases systematically and predictably as kerogen decomposition proceeds. In oils generated from high-S marine kerogens, PMP reflects the maturity of the source rock at the time of oil expulsion, provided that the oil does not undergo subsequent reservoior maturation or mixing with in-situ bitumen.

Vanadyl 3-nor C30DPEP: Indicator of depositional environment of a lacustrine sediment

Vanadyl 3-nor C{sub 30}DPEP (Fig. 1) was first isolated as the nickel complex by Fookes (1983) and is presumably formed from chlorophyll during early diagenesis. Porphyrin analyses of the extracts from the Bucomazi Formation, Onshore Cabinda, West Africa, show that the concentration of 3-nor C{sub 30}DPEP increases systematically relative to C{sub 32}DPEP with depth. This systematic change is not due to maturity and has been interpreted to indicate that the pH of the depositional environment of Bucomazi Formation increased with time. Superimposed on this pH change was change in oxicity, which is indicated by the change in VO/(VO + Ni) porphyrin ratio.

Depositional environment, thermal maturity and irradiation effects on porphyrin distribution: Alum Shale, Sweden

Abstract The Swedish Alum Shale, which was deposited from Middle Cambrian to Lower Ordovician time in the shallow marine waters of the Lapetus Ocean, contains abundant uranium. The sedimentary organic matter associated with these shales has undergone irradiation, which has affected both the insoluble and soluble portions. Rock extracts from the Alum Shale contain exclusively vanadyl porphyrins indicating deposition under a marine anoxic environment. The decrease in the concentration of porphyrins with increasing uranium content indicates that irradiation has destroyed porphyrins. Comparison of the detailed vanadyl porphyrins distribution does not show any preferential destruction of particular vanadyl porphyrin, i.e., DPEP or ETIO, due to irradiation. Hence the porphyrin maturity parameter, which is based on the changes in the relative concentration of ETIO and DPEP porphyrins, can be used for estimating the maturity of the Alum Shale regardless of its uranium concentration. This also suggests that the effect of irradiation on porphyrins is different from that of heating.