Dag A. Karlsen

Hydrocarbon composition of authigenic inclusions: Application to elucidation of petroleum reservoir filling history

Abstract Geochemical analysis of petroleum inclusions trapped in authigenic feldspar and quartz in the Ula Formation in the North Sea Ula oil field revealed a petroleum of markedly different composition than the oil presently in the reservoir. Using microthermometry and the burial history as a dating tools, it is concluded that the petroleum in the K-feldspar inclusions was present in the more porous and permeable parts of the Ula Formation as early as 45–75 My Bp when the field was at a depth of about 1.0–1.5 km, as compared with the current depth of 3.4 km. This early petroleum, which was trapped as inclusions in authigenic K-feldspar, shows a distinctly different distribution of tricyclic terpanes and pentacyclic triterpanes from that of the current petroleum charge in the Ula Formation, which was derived from the Mandal Formation source rock in late Neogene time. Molecular parameters show that the oil in the K-feldspar inclusions is significantly less mature than the crude oil in the present reservoir. The approximate 90°C temperature increase occurring after entrapment of the early petroleum in Kfeldspar (the field is currently at 143°C) appears not to have reset the low maturity signature of the oil in the K-feldspar inclusions. This could suggest that the temperature in the inclusions is too low for isomerization/selective thermal degradation to occur (lack of catalysts?), or that there are other controls on the ratio of some of these parameters. Still, parameters like the ratio of C21 to C28 triaromatic steroids, and those based on dimethyl- and trimethyl-naphthalenes, are comparatively similar in both the inclusions and in the reservoir crude. The oil inclusions in authigenic quartz and albite, formed from about 10 My BP (burial depth ≈ 2.5 km) until the present (burial depth = 3.4 km), are interpreted as representing a palaeo-petroleum charge having a composition intermediate between the oil found in K-feldspar inclusions and the oil charge in the present reservoir. Our data suggest strongly that the petroleum trapped in the inclusions in quartz and plagioclase represents progressive dilution of the early reservoired oil, now found only in K-feldspar inclusions, with oil from the Mandal Formation. Analysis of solution gas in these inclusions suggests that the first petroleum-associated gas to arrive at the trap from the progressively maturing Mandal Formation had a much wetter composition than the current reservoir solution gas. To explain the present comparatively drier and isotopically lighter reservoir solution gas composition, the inference is made that there has been a recent ( During formation of K-feldspar inclusions (45–75 My BP), maturation of the Upper Jurassic Mandal Formation, the main source rock in this region, was limited possibly to the Graben axis. This suggests that the petroleum found in the K-feldspar inclusions was most likely derived from a deeper post Caledonian Palaeozoic formation.

Analysis of petroleum fractions by TLC-FID: applications to petroleum reservoir description

Abstract Thin-layer chromatography and flame ionization detection (Iatroscan TLC-FID) offers a rapid and accurate method for quantifying saturated and aromatic hydrocarbons, and resin/asphaltene fractions, in solvent extracts of petroleum source rocks, reservoir rocks and crude oils. While the instrument is superficially simple, special precautions need to be taken in calibration, and parameter setting, due to complexities arising from the operational characteristics of the detector system. Nevertheless, the technique offers superior speed and precision to conventional MPLC analysis of petroleum fractions. This paper describes the choice of operational parameters, provides comparison with the traditional group type analysis technique (MPLC) and presents some geochemical applications.

Relationship between reservoir diagenetic evolution and petroleum emplacement in the Ula Field, North Sea

Abstract The diagenetic and petroleum filling histories of the Ula Field have been studied by analysing aqueous and petroleum inclusions occurring in authigenic cements. This study shows that diagenesis continues actively after the arrival of petroleum in the sandstones, although the reaction rates and petroleum saturation remain obscure. Microthermometric measurements of fluid inclusions in authigenic quartz suggest an onset of extensive quartz cementation at temperatures around 110°C (i.e. ≈ 2.5 km) and that this cementation has continued to the present day. Synchronously with quartz cementation, authigenic albite was formed as inter- and intragranular cements. Large amounts of petroleum inclusions locally occur in the albite and quartz cements. These inclusions were trapped in the cements as the main charge of petroleum arrived at the structure. Diffusion, through thin water layers between the petroleum and the mineral surface, would probably have dominated mass transfer of solutes for the precipitation of authigenic quartz and albite. It is evident from analysis of the petroleum inclusions in early formed K-feldspar overgrowths that some secondary migrated petroleum probably arrived in the reservoir at a shallow burial depth, before the rapid subsidence in the last 25 my. Compared with petroleum in the free porosity of the Ula Formation, the petroleum in the K-feldspar inclusions is evidently sourced from a different source facies and most maturity parameters testify to the latter being less mature. Later trapped petroleum inclusions, in quartz and albite, have characteristics found both in K-feldspar and in the Ula Formation DST oil and, thus, is likely to reflect the progressive change in the Ula Field petroleum charge which occurred during the time period of quartz diagenesis.

Geochemically driven exploration models in uplifted areas: Examples from the Norwegian Barents Sea

The Norwegian Barents Sea with multiple source rock intervals represents a prime example of an overfilled petroleum system. However, several episodes of uplift and erosion from the Paleocene until the Pliocene-Pleistocene have caused the depletion of hydrocarbon accumulations in the region. These uplift events were not only potentially catastrophic but have also caused the redistribution of the remaining oil and gas over laterally large distances in the Barents Sea region. This redistribution directed petroleum to distal parts of the various hydrocarbon-generating basins, thus charging traps, which otherwise would not have been reached. Therefore, we expect that discoveries will be made in distal basin settings, particularly in traps with partly leaking cap rocks, which can bleed off gas and thereby retain oil. Many oil accumulations in the region represent various mixtures of oils from several different stratigraphic source intervals. This suggests that Triassic and Paleozoic oils may be trapped below the presently drilled targets, which are mostly Jurassic in the Hammerfest Basin and older to the north and east. Deeper exploration targets also stand a higher chance of containing oil because the amount of gas being released from oil during uplift, erosion, and subsequent pressure release is lower. Uplift and erosion are followed by a reduction in temperature. This is why hydrocarbon generation is believed to have ended in uplifted areas. Some discoveries in this data set suggest, however, a significant fresh gas charge.

Lipids as indicators of eutrophication in marine coastal sediments

Total organic carbon (TOC) and sedimentary lipid contents were investigated in the Bunnefjord, the most inner part of the Oslofjord (Norway). The Bunnefjord is an intermittent anoxic basin and has undergone major eutrophication since the early 1800s. A core from this fjord was collected at 100 m depths under anoxic remnant waters. The first 15 cm corresponding to deposits from 1500 to present were considered for analysis. Lipid classes were quantified by TLC-FID and the molecular composition of selected lipid classes was investigated by GC and GC-MS. Lipids were dominated by two main classes, phospholipids and hydrocarbons. The hydrocarbons represented up to 7.4% of total lipids in the sediment layers covering the period when the most extensive cultural eutrophication took place (1900 to 1970). The higher fluxes of organic carbon produced during this period may have controlled hydrocarbon inputs into the sediments, due to the hydrophobic character of these pollutants. The hydrocarbon concentration reversed toward pre-industrial levels in the more recent layers, which suggests an improvement of the water quality, possibly in response to improved treatment of the sewage in the cities around Bunnefjord. The second most abundant pool of lipids consists in phospholipids, mostly contributed by bacteria. Even though the concentration decreased with depth, their relative proportions to total lipids remained high, mainly in the deepest layers (>80% of total lipids). A rapid decrease of the polyunsaturated fatty acid methyl esters (FAME) from the phospholipid fraction in the upper 4 cm suggests a rapid biodegradation of planktonic inputs and meiofauna. Odd branched fatty acids were more probably contributed by bacteria linked to the high sedimentary hydrocarbon content. The down core distribution of 16:1omega7, 18:1omega7, 18:1omega5 esterified to phospholipids suggests a vertical zonation of the microbial community in relation to redox conditions and available organic matter. In addition to bacterial sulphur biomass, the presence of hopanoic acids in the phospholipids fraction suggests the contribution of bacteria growing on methane. According to the sterol composition, dominated by 4alpha(H)-methylsterols, dinoflagellates represent the major contributors to the organic matter produced in the water column, particularly during the period of extensive eutrophication. Long-chain diols (1,13-C(26), 1,15-C(30) and 1,15-C(32)) and long-chain keto-ols (1,15-C(30) and 1,15-C(32)) are reported for the first time at high latitudes. Their relative distributions (diol and keto-ol indexes of Versteegh et al. [Org. Geochem. 27 (1997)]) have allowed depicting a particular event during the eutrophication period, a freshwater intrusion with inputs of land-derived organic matter. This is in accordance with the downcore distribution of freshwater/terrestrial markers as sitosterol, dehydroabietic acid and iso- and anteiso-pimaric acids. The diol and keto-ol indexes have also underlined the general transition trend from marine to more brackish waters in the Bunnefjord. These last observations provide confidence into the use of these compounds in paleoenvironmental reconstruction.

Estimating the availability of polycyclic aromatic hydrocarbons for bioremediation of creosote contaminated soils.

Abstract Bioremediation of soil contaminated by organic compounds can remove the contaminants to a large extent, but residual contamination levels may remain which are not or only slowly biodegraded. Residual levels often exceed existing clean-up guidelines and thereby limit the use of bioremediation in site clean-up. A method for estimating the expected residual levels would be a useful tool in the assessment of the feasability of bioremediation. In this study, three soil types from a creosote-contaminated field site, which had been subjected to 6 months of bioremediation in laboratory column studies, were used to characterize the residual contamination levels and assess their availability for biodegradation. The soils covered a wide range of organic carbon levels and particle size distributions. Results from the biodegradation studies were compared with desorption rate measurements and selective extractability using butanol. Residual levels of polycyclic aromatic hydrocarbons after bioremediation were found to be strongly dependent on soil type. The presence of both soil organic matter and asphaltic compounds in the soil was found to be associated with higher residual levels. Good agreement was found between the biodegradable fraction and the rapidly desorbable fraction in two of the three soils studied. Butanol extraction was found to be a useful method for roughly estimating the biodegradable fraction in the soil samples. The results indicate that both desorption and selective extraction measurements could aid the assessment of the feasability for bioremediation and identifying acceptable end-points.

Determination of Petroleum Accumulation Histories: Examples from the Ula Field, Central Graben, Norwegian North Sea

The application of coupled geological and geochemical studies of oilfields can provide information on the maturity of the source rocks that fed the field, the directions of filling and estimates on the timing of accumulation development. In this ongoing study, detailed three-dimensional compositional mapping of the Ula Field permitted the selection of samples for more detailed molecular analysis, providing data on both source facies and maturity. It was observed from these data that, in addition to the expected lateral variations in chemical composition, vertical variations are also evident. Furthermore, local polar compound concentrations (small tar mats that are chemically relatable to the accumulation) are definable on a stratigraphic basis. Despite these chemical compositional variations and maturity differences, the petroleum column in the Ula Field appears mechanically stable. Indeed, the observed chemical compositional variations and the mechanical properties of the field are consistent with Late Tertiary filling as indicated by maturation modelling of maturity parameters.

Petroleum biodegradation in the Tertiary reservoirs of the North Sea

Abstract A suite of 17 oil samples from the Tertiary reservoirs (Palaeocene-Eocene) of the North Sea was subjected to routine geochemical analytical techniques in order to evaluate the degree of biodegradation. The methods included latroscan TLC-FID, gas chromatography of the saturated and aromatic hydrocarbon fractions, gas chromatography-mass spectrometry of the saturated hydrocarbon fraction and stable carbon and hydrogen isotope analyses. In addition, API gravities and weight percentages of asphaltene were also determined. Based on these data it was found that oils in the suite were derived from the Jurassic aged Kimmeridge Clay Formation originating from source rock horizons at similar maturity levels. The oils in the suite were taken from reservoirs with present day temperatures 49–114°C. Differences in the chemical composition of the oils (e.g. reduced relative amounts of saturated hydrocarbons, increased polar amounts and reduced relative concentrations of n -alkanes), were attributed to secondary alteration processes of which the most likely process to occur at temperatures less than 80°C and consistent with the observed chemical characteristics of the oils is biodegradation. The differences in chemical composition are thus attributed to different degrees of biodegradation. The oils were ranked and classified in terms of the degree of biodegradation using a range of restricting parameters based on the gas chromatographic concentrations of different alkane species. Based on these parameters, oils were divided into four classes. These classes are (a) non to slightly biodegraded; (b) mildly biodegraded; (c) moderately biodegraded; and (d) severely biodegraded. In cases where the degree of biodegradation was found to have reached levels where unambiguous identification of alkanes was not possible, unresolved complex mixture (UCM) parameters were utilised to assess the extent of biodegradation. The degree of biodegradation was subsequently compared to the type of reservoir sandstone facies from which oils in this study were taken. It was found that at similar temperatures, oils in the Balder Formation (Upper Late Palaeocene) were biodegraded to a higher degree compared to oils from either The Forties (Lower Late Palaeocene) or Sele (Middle Late Palaeocene) Formation. The higher degree of biodegradation in Balder reservoirs is attributed to relatively greater openness to meteoric water invasion in these reservoirs.

A rapid correlation method for petroleum population mapping within individual petroleum reservoirs: applications to petroleum reservoir description

While oil-oil and oil-source rock correlation is a routine part of most exploration programmes, it is only rarely that detailed comparison of oil compositional variations within reservoirs are studied. Although many mechanisms serve to homogenize reservoir fluids, petroleum populations of varied composition can be defined and related to reservoir contiguity and the history of field filling from diverse source rock assemblages. The definition of these heterogeneous petroleum populations, while of great significance to exploration problems related to field filling and definition of prospective sub-traps, is also an under-exploited development tool. Conventional approaches to reservoir definition rely heavily on sedimentological and downhole geophysical analysis of formations coupled with flow testing. Petroleum population mapping may allow direct definition of filling/production conduits within fields if mixing processes can be understood and quantified.

Processes controlling water and hydrocarbon composition in seeps from the Salton Sea geothermal system, California, USA

Water-, mud-, gas-, and petroleum-bearing seeps are part of the Salton Sea geothermal system (SSGS) in Southern California. Seeps in the Davis-Schrimpf seep fi eld (~14,000 m 2 ) show considerable variations in water temperature, pH, density, and solute content. Water-rich springs have low densities ( 98 vol%). Halogen geochemistry of the waters indicates that mixing of deep and shallow waters occurs and that near-surface dissolution of halite may overprint the original fl uid compositions. Carbon isotopic analyses suggest that hydrocarbon seep gases have a thermogenic origin. This hypothesis is supported by the presence of petroleum in a water-dominated spring, composed of 53% saturated compounds, 35% aromatics, and 12% polar compounds. The abundance of polyaromatic hydrocarbons and immature biomarkers suggests a hydrothermal formation of the petroleum, making the SSGS a relevant analogue to less accessible hydrothermal seep systems, e.g., the Guaymas Basin in the Gulf of California.