Geochemistry and kinetic models od Cretaceous samples

The source rock potential of Cretaceous organic rich whole rock samples from deep sea drilling project (DSDP) wells offshore southwestern Africa was investigated using bulk and quantitative pyrolysis techniques. The sample material was taken from organic rich intervals of Aptian, Albian and Turonian aged core samples from DSDP site 364 offshore Angola, DSDP well 530A north of the Walvis Ridge offshore Namibia, and DSDP well 361 offshore South Africa. The analytical program included TOC, Rock-Eval, pyrolysis GC, bulk kinetics and micro-scale sealed vessel pyrolysis (MSSV) experiments. The results were used to determine differences in the source rock petroleum type organofacies, petroleum composition, gas/oil ratio (GOR) and pressure-volume-temperature (PVT) behavior of hydrocarbons generated from these black shales for petroleum system modeling purposes. The investigated Aptian and Albian organic rich shales proved to contain excellent quality marine kerogens. The highest source rock potential was identified in sapropelic shales in DSDP well 364, containing very homogeneous Type II and organic sulfur rich Type IIS kerogen. They generate P-N-A low wax oils and low GOR sulfur rich oils, whereas Type III kerogen rich silty sandstones of DSDP well 361 show a potential for gas/condensate generation. Bulk kinetic experiments on these samples indicate that the organic sulfur contents influence kerogen transformation rates, Type IIS kerogen being the least stable. South of the Walvis Ridge, the Turonian contains predominantly a Type III kerogen. North of the Walvis Ridge, the Turonian black shales contain Type II kerogen and have the potential to generate P-N-A low and high wax oils, the latter with a high GOR at high maturity. Our results provide the first compositional kinetic description of Cretaceous organic rich black shales, and demonstrate the excellent source rock potential, especially of the Aptian-aged source rock, that has been recognized in a number of the South Atlantic offshore basins.

Observations of plankton production in Kiel Bay 1957 to 1975

This data volume presents a series of planktological observations carried out over a 19-year-period in Kiel Bight in the Western Baltic Sea. Three fixed stations were visited at monthly intervals, and the planktion standing stock was investigated in relation to depth and environmental factors, employing a standard observation programme. This consisted in the measurements of temperature, salinity, density, oxygen, phosphorus, seston, protein and chlorophyll a. Additional measurements comprised in the caloric content of seston, particulate organic carbon and nitrogen, as well as dry weight and organic matter of plankton, sampled by vertical hauls of three plankton nets of different mesh size.

Alkenone composition, UK'37 index, and carbon, sulphur, nitrogen and element oxides of ODP Site 186-1151

Ocean Drilling Program (ODP) Site 1151 (Sacks, Suyehiro, Acton, et al., 2000, doi:10.2973/odp.proc.ir.186.2000) is located in an area where the surface water mass is influenced by both the Kuroshio and Oyashio Currents. The site also receives a relatively high flux of detrital materials from riverine input from Honsyu Island and eolian input from Central and East Asia. We analyzed alkenones and alkenoates in the sediments to reconstruct alkenone unsaturation index (Uk'37)-based sea-surface temperature (SST), total organic carbon, and total nitrogen to estimate the terrigenous contribution by the C/N ratio during the last glacial-interglacial cycle. The major elements were also analyzed to examine the variation in terrigenous composition.

Geochemistry, lithology and maceral analysis at DSDP Hole 71-511

The quantity, type, and maturity of the organic matter in Recent through Upper Jurassic sediments from the Falkland Plateau, DSDP Site 511, have been determined. Sediments were investigated for their hydrocarbon potential by organic carbon and Rock-Eval pyrolysis. Kerogen concentrates were prepared and analyzed in reflected and transmitted light to determine vitrinite reflectance and maceral content. Total extractable organic compounds were analyzed for their elemental composition, and the fraction of the nonaromatic hydrocarbons was determined by capillary column gas chromatography and combined gas chromatography/mass spectrometry. Three main classes of organic matter can be determined at DSDP Site 511 by a qualitative and quantitative evaluation of microscopic and geochemical results. The Upper Jurassic to lower Albian black shales contain high amounts of organic matter of dominantly marine origin. The content of terrigenous organic matter increases at the base of the black shales, whereas the shallowest black shales near the Aptian/Albian boundary are transitional in composition, with increasing amounts of inert, partly oxidized organic matter which is the dominant component in all Albian through Tertiary sediments investigated. The organic matter in the black shales has a low level of maturity and has not yet reached the onset of thermal hydrocarbon generation. This is demonstrated by the low amounts of total extractable organic compounds, low percentages of hydrocarbons, and the pattern and composition of nonaromatic hydrocarbons. The observed reflectance of huminite and vitrinite particles (between 0.4% and 0.5% Ro at bottom-hole depth of 632 m) is consistent with this interpretation. Several geochemical parameters indicate, however, a rapid increase in the maturation of organic matter with depth of burial. This appears to result from the relatively high heat flow observed at Site 511. If we relate the level of maturation of the black shales at the bottom of Hole 511 to their present shallow depth of burial, they appear rather mature. On the basis of comparisons with other sedimentary basins of a known geothermal history, a somewhat higher paleotemperature gradient and/or additional overburden are required to give the observed maturity at shallow depth. A comparison with contemporaneous sediments of DSDP Site 361, Cape Basin, which was the basin adjacent and to the north of the Falkland Plateau during the early stages of the South Atlantic Ocean, demonstrates differences in sedimentological features and in the nature of sedimentary organic matter. We interpret these differences to be the result of the different geological settings for Sites 361 and 511.

Carbon chemistry of cretaceous marine organic matter

Geochemical studies of Cretaceous strata rich in organic carbon (OC) from Deep Sea Drilling Project (DSDP) sites and several land sections reveal several consistent relationships among amount of OC, hydrocarbon generating potential of kerogen (measured by pyrolysis as the hydrogen index, HI), and the isotopic composition of the OC. First, there is a positive correlation between HI and OC in strata that contain more than about 1% OC. Second, percent OC and HI often are negatively correlated with carbon isotopic composition (delta13C) of kerogen. The relationship between HI and OC indicates that as the amount of organic matter increases, this organic matter tends to be more lipid rich reflecting the marine source of the organic matter. Cretaceous samples that contain predominantly marine organic matter tend to be isotopically lighter than those that contain predominantly terrestrial organic matter. Average delta13C values for organic matter from most Cretaceous sites are between -26 and -28 per mil, and values heavier than about -25 per mil occur at very few sites. Most of the delta13C values of Miocene to Holocene OC-rich strata and modern marine plankton are between -16 to -23 per mil. Values of delta13C of modern terrestrial organic matter are mostly between -23 and -33 per mil. The depletion of terrestial OC in 13C relative to marine planktonic OC is the basis for numerous statements in the literature that isotopically light Cretaceous organic matter is of terrestrial origin, even though other organic geochemical and(or) optical indicators show that the organic matter is mainly of marine origin. A difference of about 5 per mil in delta13C between modern and Cretaceous OC-rich marine strata suggests either that Cretaceous marine planktonic organic matter had the same isotopic signature as modern marine plankton and that signature has been changed by diagenesis, or that OC derived from Cretaceous marine plankton was isotopically lighter by about 5 per mil relative to modern plankton OC. Diagenesis does not produce a significant shift in delta13C in Miocene to Holocene sediments, and therefore probably did not produce the isotopically light Cretaceous OC. This means that Cretaceous marine plankton must have had delta13C values that were about 5 per mil lighter than modern marine plankton, and at least several per mil lighter than Cretaceous terrestrial vegetation. The reason for these lighter values, however, is not obvious. It has been proposed that concentrations of CO2 were higher during the middle Cretaceous, and this more available CO2 may be responsible for the lighter delta13C values of Cretaceous marine organic matter.

Lipid analyses of ODP Leg 117 samples

The solvent-extractable organic fractions of sediment samples from six Ocean Drilling Program Leg 117 sites were investigated by gas chromatography and gas chromatography-mass spectrometry. Sediments deposited in the Indus Fan (Site 720) as well as Miocene sediments from the Owen Ridge (Sites 722 and 731) contain almost exclusively organic matter of terrigenous origin. The organic matter in sediments from the Oman Margin (Sites 723, 725, and 728) and in the Pliocene/Pleistocene sections from the Owen Ridge is mainly of a marine origin with variable admixtures of terrigenous material. In these latter samples strong variations of the lipid composition and distribution are noted. However, the interpretation of the relation to potential biological sources is hampered by a lack of information on the possible lipid composition of appropriate organisms.

Environmental analyses of cyclic Middle Eocene sediments and palynomorph records of ODP Hole 189-1172A

The influence of orbital precession on early Paleogene climate and ocean circulation patterns in the southeast Pacific region is investigated by combining environmental analyses of cyclic Middle Eocene sediments and palynomorph records recovered from ODP Hole 1172A on the East Tasman Plateau with climate model simulations. Integration of results indicates that in the marine realm, direct effects of precessional forcing are not pronounced, although increased precipitation/runoff could have enhanced dinoflagellate cyst production. On the southeast Australian continent, the most pronounced effects of precessional forcing were fluctuations in summer precipitation and temperature on the Antarctic Margin. These fluctuations resulted in vegetational changes, most notably in the distribution of Nothofagus (subgenus Brassospora). The climate model results suggest significant fluctuations in sea ice in the Ross Sea, notably during Austral summers. This is consistent with the influx of Antarctic heterotrophic dinoflagellates in the early part of the studied record. The data demonstrate a strong precessionally driven climate variability and thus support the concept that precessional forcing could have played a role in early Antarctic glaciation via changes in runoff and/or precipitation.

Concentrations of dinoflagellate cysts in the surface layer of bottom sediment from the White Sea

Dinoflagellate cysts were studied in 42 samples from surface sediments of the White Sea. Total concentration of dinocysts varies from single cysts to 25000 cyst/g of dry sediments, which reflects biological productivity in White Sea waters and regional particular features of sedimentation processes. The highest concentrations are observed in silts; they are related to the regions of propagation of highly productive Barents Sea waters in the White Sea. Generally, spatial distribution of dinocyst species in the surface sediments corresponds to distribution of the major types of water masses in the White Sea. Cysts of relatively warm-water species (Operculodinium centrocarpum, Spiniferites sp.) of North Atlantic origin that dominate in the sediments indicate an intensive intrusion of Barents Sea water masses to the White Sea along with hydrological dwelling conditions in the White Sea favorable for development of these species during their vegetation period. The cold-water dinocyst assemblage (Islandinium minutum, Polykrikos sp.) is rather strictly confined to inner parts of shallow-water bays, firstly, those adjacent to the Onega and Severnaya Dvina river mouths.

Organic geochemistry of ODP Leg 107 sediments

The Rock-Eval pyrolysis of rock samples and the elemental analysis of kerogens show clear differences between Messinian black shales and Pliocene-Pleistocene sapropels recovered during ODP Leg 107. The Messinian black shales are characterized by a large variety of compositions which probably reflects a great diversity of depositional and diagenetic paleoenvironments. In contrast, the Pliocene-Pleistocene sapropels, occurring as discrete layers in nannofossil oozes barren of organic carbon, constitute a rather homogeneous group in terms of organic content. A considerable contribution of terrestrial organic matter in the sapropels could mean that an identical phenomenon of terrestrial input has been periodically reproduced in the basin. The maturity and the nature of the organic matter are discussed with respect to anomalous values recorded for Tmax parameter.

Paleomagnetic of ODP Leg 133 holes

Paleomagnetic and rock-magnetic analyses from discrete samples of carbonate sites on the Queensland Plateau were used to determine magnetic polarity reversal stratigraphy and the nature of magnetization in these sediments. Magnetic polarity zones were correlated with the geomagnetic polarity time scale in the upper portions of cores at Sites 812 through 814, usually back to a late Pliocene age. Loss of reliable directional data was coincidental with a major decrease in magnetic intensity, below which, no stable polarity zones could be identified. The intensity reduction is either an in-situ alteration of magnetic grains, or an input signal representing progressive increase in the magnetic component of Queensland Plateau sediments. Although not conclusive at this point, the geochemical conditions and differing age of intensity reduction support the former hypothesis. Rock-magnetic analysis of carbonate sediments suggests that ultrafine-grained magnetite or maghemite crystals is an important carrier of remanence and may be biogenic in origin. Application of a recently calibrated anhysteretic remanent magnetization test to assess configuration of single-domain crystal within a natural matrix indicates that cementation (ooze-chalk-limestone) may be important in post-depositional changes affecting magnetostatic grain interaction.

C1-C8 hydrocarbons at DSDP Site 93-603

Small amounts of C1-C8 hydrocarbons were detected in continental rise sediments from DSDP Site 603. Organiccarbon- lean sections contained only C1-C3 compounds believed to have migrated from organic-carbon-rich sections. Heavier (C4-C8) hydrocarbons were found only in organic-carbon-rich sections. Restricted and sporadic distribution of C4-C6 compounds in 0-1100 m sub-bottom sediments suggest low-temperature (<20°C) biological/chemical generation processes. Increased C4-C8 concentrations and complexity, including unusually high levels of xylene, were detected in two deeper Cretaceous sections (603-34-2, 134 cm and 603-81-3, 120 cm). This behavior, which was not observed in 17 other samples from sub-bottom depths greater than 1100 m, is similar to that observed in immature surface sediments from the geothermally active Guaymas Basin (Gulf of California) area.

Organic carbon geochemistry of ODP Leg 121 samples

Organic petrological and geochemical analyses were performed on samples cored on Broken Ridge and Ninetyeast Ridge in the Central Indian Ocean during Leg 121. Organic carbon (Corg) contents are less than 1% in each individual sample and average Corg values calculated for larger stratigraphic units are less than 0.2%. Generally, there is more organic matter in Cretaceous sediments than in Tertiary. In the Cretaceous, the bulk of the organic matter consists of terrigenous debris, but a significant contribution of marine-derived organic matter was found in some samples, especially in the early Maestrichtian on Broken Ridge (Site 754). The youngest Pliocene-Pleistocene sediments at Site 758 (northern part of Ninetyeast Ridge) contain a significant amount of clastic material transported to the site by the (distal) Bengal Fan. In these sediments, Corg contents of up to 0.9% were measured and are due to the inflow of terrigenous organic debris. Corg values are positively correlated with bulk sediment accumulation rates (i.e., sediments contain more organic matter at times of faster deposition). The size of terrigenous organic particles is generally small in all sediments. The extremely small number of particles in the Cretaceous sediments at Site 758 and their smaller grain size, compared to the Cretaceous sediments on Broken Ridge, indicate that Cretaceous surface water paleocurrents flowed from southeast to northwest in the Proto-Indian Ocean. In the central Indian Ocean, sediments deposited above the carbonate compensation depth consist of nannofossil and foraminiferal oozes. In contrast to Corg values, calcite contents in the sediments are negatively correlated with bulk sediment accumulation rates (i.e., carbonate oozes were deposited only during times of extremely slow sedimentation). Therefore, older sediments deposited in the young and still narrow Indian Ocean accumulated faster and are less carbonate-rich than Neogene sediments, although carbonate accumulation rates were higher.