Biological Marker Distributions Research Articles

Biological markers in lacustrine Chinese oil shales

Summary A major objective in molecular organic geochemistry is the assessment and characterization of sedimentary depositional environments from the occurrence of specific biological marker compounds related to particular source organisms. The aliphatic hydrocarbon distributions of petroleums and/or sediments from Chinese freshwater and hypersaline lacustrine environments are described and contrasted in the search for diagnostic features among their biological marker distributions. The Eocene Maoming oil shale, which crops out in Guangdong Province, shows major differences in hydrocarbon composition through its stratigraphic succession. These variations reflect changes in lithology and in the origins of the sedimentary organic matter, as seen from petrographic examination. The lower lignite and vitrinite layers contain significant contributions of biological markers from bacterial sources, plus components derived from terrigenous higher plants, such as C 29 steroids. In contrast, the overlying shales show a dominance of dinoflagellate-derived 4-methylsteroids and culminate in an horizon with botryococcane homologues as their dominant alkanes. Thus, the geochemical profile suggests that a swampy environment, in which peats accumulated, deepened to support populations of dinoflagellate and, finally, blooms of Botryococcus green algae. The aliphatic hydrocarbons of a series of sediments and petroleums from the Eocene hypersaline sequence of the Jianghan Basin in southern Hubei Province shows various features that may be typical of such facies. These include a dominance of even-numbered n-alkanes, high amounts of phytane and gammacerane, and an abundance of C 35 -hopanes. Similar characteristics are observed in shales from the Eocene Green River Formation and in Rozel Point crude oil, a petroleum thought to originate from sediments deposited under hypersaline conditions. The same features also appear in a lower Tertiary evaporitic mudstone from south of the Renqui oil field in Hebei Province, together with 4-methylsteranes, β-carotane and 18α(H)-oleanane, a marker for terrigenous higher plants. These data illustrate the potential of biological markers in the differentiation of different lacustrine environments and suggest that such characteristics may provide signatures for environmentally similar, but geographically distant, deposits.

Fractionation of biological markers in crude oils during migration and the effects on correlation and maturation parameters

A crude oil from the Kelamayi oilfield, China, has been used in a detailed laboratory study to examine fractionation effects on various classes of biomarkers in the oil. These experiments were undertaken on an alumina column and in general the fractionation effects were similar to those previously ascribed to migration in the natural situation. Observations included the elution of tricyclic terpanes prior to pentacyclic terpanes, αβ-hopanes and 22S αβ hopanes prior to the βα hopanes and 22R-αβ hopanes. In addition rearranged and ββ-steranes elute faster than the αα-steranes and the αα-20S-steranes elute faster than the αα-20R-steranes. These observations were subsequently applied to a series of oils and source rocks from the Kelamayi oilfield. It was possible to correlate many of the biological marker distributions in the oils with those observed in the laboratory experiments. Thus oils with high concentrations of ββ-steranes were considered to be early expulsion products and their related source rock had lower concentrations of the ββ-steranes. Gammacerane is a relatively late expulsion product from the laboratory experiments, hence oils with low gammacerane concentrations were derived from rocks with high gammacerane concentrations. A combination of laboratory fractionation experiments and characterisation of oils and source rocks permitted correlations to be made between various oils and source rocks in the Kalamayi Basin.

Oil generation in the michigan basin: A biological marker and carbon isotope approach

Palaeozoic strata in the Michigan Basin produced crude oils which, despite their rather small volume and economic value, are interesting because of the considerable age of the basin and because of the opportunity to study generation and migration of oils within a relatively simple but ancient geological setting. Based on n-alkane profiles, biological marker distributions and carbon isotope ratios, the oils belong to three main families of different genetic origin and a few less important mixed types. Silurian oils from Salina and Niagara Limestones have broad n-alkane distributions and abundant isoprenoid hydrocarbons. A strong phytane-over-pristane predominance and the lack of diasteranes indicate a carbonate source for these oils. Chemical maturity parameters show that they are more mature than the oils from the other main families from which they are also clearly distinguished by carbon isotope ratios of hydrocarbon fractions and single n-alkanes. Oils found in the calcareous Ordovician Trenton formation contain n-alkane, cyclohexylalkane and alkyl phenanthrene distributions typical of immature oils. Many oils from the Devonian Dundee reservoirs are very similar in overall composition to the Trenton oils. Devonian Traverse oils are considered to be mainly from a Devonian source with some contribution of Ordovician-type oil and are of intermediate maturity. Based on calculations using kinetic parameters of biological marker reactions, considerably deeper subsidence of the Devonian source rock in the past is implied.

Biological marker distribution and significance in oils and rocks of the Monterey Formation, California

The biological marker distributions of several oils, core extracts and solid bitumens of the Monterey Formation of California have been studied. Sterane, terpane and monoaromatic steroid hydrocarbons were analyzed in samples from the San Joaquin, Los Angeles, Ventura and Santa Maria Basins. The sterane patterns of both oils and extracts are characterized by 1. (a) low relative concentrations of diasteranes, 2. (b) low 20S/20R-5α,14α,17α-ethylcholestane ratios, 3. (c) relatively high concentrations of cholestane ( vs. methyl- and ethylcholestane) isomers. San Joaquin Basin samples contain significant amounts of the 5β isomer, which is generally absent in samples from other basins. The carbon number distribution of 5α,14α,17α,20R steranes is similar for all oils, regardless of API gravity, depth or basin location, and is suggestive of open marine depositional conditions for the source material involved. 17α(H),l8α(H),21β(H)-28,30-Bisnorhopane is present in almost all samples. Certain San Joaquin Basin oils and extracts contain 1. (a) a series of 25-nor hopanes, including 25,28,30-trisnorhopane, 2. (b) a distinctive monoaromatic steroid hydrocarbon distribution, 3. (c) an aliphatic hydrocarbon fraction devoid of n-paraffins. Biological marker characteristics suggest that the Monterey oils examined originated early in the maturational sequence, from elastics-poor source material. API gravities of the Monterey Formation oils examined vary monotonically with 1. (a) bisnorhopane/hopane ratios, 2. (b) aromatized/regular sterane ratios and 3. (c) the concentration of monoaromatized steranes relative to terpanes and regular steranes. These oil gravity correlations exist regardless of sample depth or basin location.

Correlation of source rocks and migrated hydrocarbons by GC-MS in the middle Triassic of Svalbard

Samples of Lower and Middle Triassic sediments from nine different locations on Svalbard have been analysed by GC-MS to study migration processes. Short range migration was investigated by analyses of interbedded shales and siltstones from three of the locations. In addition, shale samples from six locations were analysed to study long range migration. The maturity of the samples varied from immature through moderately mature to oil window maturity. The qualitative distribution of biological markers was seen to be fairly similar in all the analysed locations, irrespective of lithology. This implies that no source characteristic parameters from GC-MS analyses could be applied to exclude any of the analysed source rocks. Based on maturity considerations it was concluded that short range migration from shales to siltstones had probably been the most important process.

Unusual distribution of biological markers in an Australian crude oil

The stereochemical configuration of hopane-type triterpanes in source rock extracts or crude oils is normally determined by the maturity of the sample1. Naturally occurring precursors of the hopane hydrocarbons are dominated by the 17βH, 21βH stereochemistry with only one diastereomer at the C22 position in the C31 and higher homologues2. Note that the 17βH, 21βH-C28 homologue has never been reported in any sample. Mature crude oils in general contain hopane hydrocarbons dominated by the thermally more stable configuration of 17αH, 21βH stereochemistry and a mixture of the two diastereomers at the C22 position for C31 and higher homologues in the ratio of ∼3/2. We present here evidence for the presence of an unusual distribution of hopanes in an Australian crude oil. A study by computerized gas chromatography–mass spectrometry (C–GC–MS) designed to determine whether different oils from the same basin have similar sources has revealed the presence of the thermally unstable 17βH, 21βH isomers of hopane-type triter-panes in conjunction with C22 diastereoisomers in the unusual ratio of 1:9. We propose that there are two possible explanations for this unusual distribution of the triterpanes. (1) The oil is immature or (2) these compounds have accumulated in the oil as it migrated through coals in the region of the reservoir which are also known to contain the unusual hopane distribution.

GEOCHEMICAL CORRELATION OF AUSTRALIAN CRUDE OILS

Correlation of crude oils with either their suspected source rocks or other crude oils thought to be derived from similar sources is important for basin studies and their suspected source rocks or other crude oils thought to be derived from similar sources is important for basin studies and and source rock extracts using computerized gas chromatography — mass spectrometry (C-GC-MS). Biological markers are compounds that retain the structure of molecules occurring in living organisms during diagenesis and conversion of the original organic material to petroleum hydrocarbons. In general, the distribution of biological markers is identical for oils from similar sources and for their respective source rocks. Differences in sources or maturity of the oils lead to differences, sometimes very subtle, in the distribution of the biological markers.Results from C-GC-MS analyses performed on a series of oils from the Gippsland, Carnarvon and Surat Basins show how two classes of biological markers, namely steranes and triterpanes, can be used for correlation purposes. Oils from the Gippsland Basin can be divided into two groups differing in source and maturity. Similarly, the oils examined from the Dampier Sub-basin fall into two groups. It is proposed that they have at least one common source, with one group being distinguished by the presence of additional source material. Additional data are presented to show that the Barrow Jurassic and Windalia oils are derived from the same or similar sources and differ only as a result of the biodegradation of the Windalia oil. Results for the limited number of oils examined from the Surat Basin suggest the presence of at least two sources for the oils of this basin.