Nils Telnæs

The controls on the composition of biodegraded oils in the deep subsurface—part 1: biodegradation rates in petroleum reservoirs

Abstract Biodegradation rates in oilfields have been assessed conservatively using whole oil-column minimum rate estimates, diffusion-controlled oil column compositional gradient modelling and mixed oil kinetic models. Biodegradation rate constants (first order) are around 10−6–10−7 yr−1 for hydrocarbons in the degradation zones these corresponding well with zero order field-wide minimum rate estimates of about 10−8 kg hydrocarbons/kg oil/year for the whole oil column. With biodegradation induction times of around 1–2 Ma to perturb an entire oil column for light oil reservoirs and 10–20 Ma for heavy oil reservoir degradation the results indicate that where we see continuous gradients in the oil columns, degradation must have been occurring episodically for many millions of years. To remove the n-alkanes from an oil (i.e. about 10% of an oil) around ca 15 Ma is needed for a heavy oil (ca 5 Ma for a light N. Sea oil). The timescales of oilfield degradation and filling are thus very similar and consequently the degree of biodegradation will be substantially controlled by oilfield charge history. Assessment of mixed degraded/non-degraded oil occurrence provides an independent confirmation that these rates are realistic and that timescales of degradation and field charging are similar. The maximum effective rate constant of degradation, ultimately controlled by the limiting effect of diffusion of alkanes to the oil water contact (OWC) (ca 10−4 yr−1 for a 130 m thick oil column first order rate constant) is well above the estimated rate constants indicating oil biodegradation rate is not limited by electron donor supply (i.e. hydrocarbons) but by supply of nutrients or oxidants. This suggests that diffusive transport of nutrients and electron acceptors in the aquifer to the site of biodegradation may be adequate to maintain the low rate biosphere.

Organic geochemical characterisation of depositional palaeoenvironments of source rocks and oils in Brazilian marginal basins

Abstract An investigation of bulk, isotopic and molecular features of a wide selection of source rocks and oils from Brazilian marginal basins, ranging from Lower Cretaceous to Tertiary in age, provides a number of organic geochemical criteria that characterise and distinguish their depositional palaeo-environments. The methods employed include evaluation of organic carbon contents, Rock-Eval pyrolysis, vitrinite reflectance measurements, determination of carbon isotope ratios, elemental and visual kerogen analysis, assessment of oils and solvent extracts by liquid and gas chromatography, and quantitative biological marker investigations using GC-MS for aliphatic hydrocarbons and UV/Vis spectrophotometry for metalloporphyrins. Selected data were evaluated using principal component analysis. The results enable the recognition and differentiation of seven depositional regimes: lacustrine freshwater; lacustrine saline water; marine evaporitic; marine carbonate; marine deltaic; marine highly anoxic with a predominance of calcareous mudstone lithology, and marine anoxic with predominance of siliciclastic lithology. Diagnostic features in the biological markers from these depositional environments include: the carbon number distributions of n -alkanes, pristane/phytane ratios; absolute concentrations of β-carotane, higher acyclic isoprenoids, C 30 αβ hopane and steranes, and the abundance of the latter relative to 4-methylsteranes; the occurrence and abundance of particular biological markers such as gammacerane, tricyclic and tetracyclic terpanes, 18α (H)-oleanane, 28,30-bisnorhopane and 25,28,30-trisnorhopane, and the relative abundances of nickel and vanadyl porphyrins.

Maturity determination of organic matter in coals using the methylphenanthrene distribution

Abstract The distribution of phenanthrene and monomethylphenanthrenes in extracts of 15 different coals was determined by gas chromatography. The maturity as measured by vitrinite reflectance varied from 0.53% to 1.29% R 0 , covering the maturity range normally associated with the “oil window” ( 0.5–1.2% R 0 ). p ]Principal component analysis followed by factor rotation reveals that coal maturity correlates only with the distribution of the mono-methylphenanthrenes and not with the relative abundance of phenanthrene. This implies a methylphenanthrene distribution fraction (MPDF) linearly related to vitrinite reflectance for coal with maturity within the oil window.

Three series of rearranged hopanes in Toarcian sediments (northern Italy)

Abstract A suite of 36 shales and marls from the Toarcian Oceanic Anoxic Event section at three localities in northern Italy was found to contain variable contents of three series of rearranged hopanes: 17α-diahopanes (C 27 and C 29–34 ; and trace C 35 ), 18α-neohopanes (C 27 and C 29–30 ) and an unidentified early-eluting series (C 27 and C 29–35 ). Principal component analysis (PCA) shows the 17α-diahopanes and the early-eluting rearranged hopanes to closely covary, suggesting a related diagenetic mechanism for the formation of their rearranged skeletons. A general covariance with diasteranes is also noted. The 18α-neohopanes show no covariance with diasteranes or the other two series of rearranged hopanes. We also report 2α-methyl analogues of the 17α-diahopanes and early-eluting rearranged hopanes. We propose that the neohopanes may be at least partly derived from different biohopanoid precursors than the other two series of rearranged hopanes, and that the diagenetic origin of the neohopane structure differs from that of the 17α-diahopanes and the unidentified early-eluting series

Oleanane parameter: Verification by quantitative study of the biomarker occurrence in sediments of the Niger delta

Abstract The variation of the absolute concentration of oleananes in three selevted sedimentary columns of the Tertiary Niger delta is paralleled by the corresponding oleanane parameter (OP) profile, thereby confirming that the latter can be utilized for estimating the relative contribution of terrigenous organic matter. It is shown that OP features a dynamic range, especially at the oil-generative window (OGW), and can precisely delimit the top of the OGW which, for the Niger delta, is shown to coincide approximately with the 80°C isotherm and Ro% ∼0.55. This study provides the first direct evidence of the inadequacy of vitrinite reflectance as a maturity parameter for type III kerogen. Furthermore, based on the new findings, an alternative model for oleanene-oleanane interconversions in the geosphere is proposed.

Multivariate calibration of diffuse reflectance infrared spectra of coals as an alternative to rank determination by vitrinite reflectance

Abstract Christy, A.A., Velapoldi, R.A., Karstang, T.V., Kvalheim, O.M., Sletten, E. and Telnaes, N., 1987. Multivariate calibration of diffuse reflectance infrared spectra of coals as an alternative to rank determination by vitrinite reflectance. Chemometrics and Intelligent Laboratory Systems , 2:199-207. Sixty-four randomly selected, vitrinite-rich coal samples were subjected to petrological, spectrometric and multivariate data analysis. Diffuse reflectance Fourier transform infrared spectra in a Kubelka-Munk format were reduced by a maximum entropy reduction method and were calibrated against vitrinite reflectance using partial least squares regression. Two calibration models were calculated. One model was calculated for coal samples with vitrinite reflectance from 0.38 to 1.08, i.e., including most of the maturity range defined as the “oil window”. Outliers or non-population members were identified by an iterative process, leaving a total of 41 coal samples. The cross-validated model gave an absolute prediction error of ± 0.09, a value of the same order of magnitude as the average standard deviation of the vitrinite reflectance measurements (1 s = 0.06). The second model was calculated for eighteen samples with maturity from 1.32 to 3.54. An absolute prediction error of ±0.15 was obtained, reflecting the higher uncertainty in the vitrinite reflectance measurements for the higher-ranking coals.

Oil-oil correlation using multivariate techniques

Abstract Forty-five oils from the Norwegian sector of the North Sea were correlated using principal component analysis of the biomarker distributions and biomarker ratios. The results based on the metastable ion monitoring of the steranes and triterpanes were found to give a good separation between oils from the different fields and formations represented in the data set. The first three principal components could be correlated to geochemical processes.

Unusual triterpane distributions in lacustrine oils

Abstract A series of diahopanes and a second, unknown, early-eluting series of triterpanes are reported in lacustrine oil samples from three different basins. We suggest that these compounds, together with 18α(H) 30-norneohopane, show distributions which vary with the salinity of the depositional environment.

Visualizing information in multivariate data: Applications to petroleum geochemistry : Part 1. Projection methods

Abstract General procedures are developed for revealing and comparing the information carried by experimental data obtained for different petroleum fractions. Oblique rotation of principal components is used to obtain projections amenable to geochemical interpretation. A systematic method is given for selecting markers, i.e., compounds related to specific geochemical factors. By projecting samples onto axes defined by markers, their potential for describing a petroleum fraction can be evaluated. The similarity between samples or groups (classes) of samples is revealed by projecting samples onto “geochemicalrdquo; axes and by comparing their residual distances to class models. In class modelling, projections on markers are proposed as a method for eliminating differences induced by alteration processes not related directly to the source of the oils, e.g., maturity.

Visualizing information in multivariate data: Applications to petroleum geochemistry : Part 2. Interpretation and correlation of north sea oils by using three different biomarker fractions

Abstract Pentacyclic triterpanes, steranes and aromatic steroids of thirty-five North Sea crude oils are investigated for their usefulness in correlation and interpretation. Oblique components are necessary for obtaining data projections amenable to geochemical interpretation and for selecting molecular markers, i.e., compounds related to specific geochemical factors. Normoretane is shown to be a specific maturity parameter for the North Sea oils examined.