A new approach to interpreting Rock-Eval S2 and TOC data for kerogen quality assessment

A simple method for application in source potential mapping is used to assess the original oil and gas potentials in source rock horizons based upon Rock-Eval potential (S₂) and total organic carbon (TOC) values. The method assumes that kerogens consist of mixtures of end-members with assigned hydrogen index values. Based on suggested algorithms, the average amounts of oil-prone, gas-prone and inert organic material over source rock intervals are determined in TOC units. The method uses regression lines from plots of remaining hydrocarbon potentials (S₂) versus total organic carbon (TOC), and “quick-look” transparent overlays are used to read the appropriate kerogen mixture.Mineral matrix effects during pyrolysis, when strong, can cause erroneous results. This effect which occurs for oil-prone kerogens and adsorptive minerals can cause problems particularly for lean samples (S₂ = 0–3 mg HC/g rock) whilst the errors for richer samples are less.The method is applied on three sections of Upper Jurassic organic-rich rocks from the Danish North Sea sector, which are at different maturity stages. One of these sections is dominated by gas-prone material, one is dominated by oil-prone material and the third section contains a mixture of oil- and gas-prone material.The method has been compared with other methods that split kerogens in oil and gas generating potential and has given reasonable results.Experience using the method and a presented example suggest that sedimentological, system tract information may be derived from S₂ to TOC cross-plots. A constructed modelling example suggests that the end-member concept used in this approach may be used in forward type source rock prediction models when combined with sedimentological models. The resulting S₂–TOC plots can be used in order to check the forward modelling results against observed values.     

Isotopic anomalies of noble gases in meteorites and their origins—III. LL-chondrites

Nine LL-chondrites were studied by a selective etching technique, to characterize the noblegas components in three mineral fractions: HF-HCl-solubles (silicates, metal, troilite, etc.; comprising ～ 99% of the meteorite), chromite and carbon (～ 0.3–0.7%) and Q (a poorly characterized mineral defined by its solubility in HNO₃, comprising ～ 0.05% of the meteorite but containing most of the Ar, Kr, Xe and a neon component of ${}^{20}\text{Ne}{}^{22}\text{Ne}\text{= 10.9 ± 0.8}$). The ${}^{20}\text{Ne}{}^{36}\text{Ar}$ ratio in Q falls wi petrologic type and rising ³⁶Ar content, as expected for condensation from a cooling solar nebula, but contrary to the trend expected for metamorphic losses. Chondrites of different petrologic types therefore cannot all be derived from the same volatile-rich ancestor, but must have formed over a range of temperatures, with correspondingly different intrinsic volatile contents.The CCFXe (carbonaceous chondrite fission) component varies systematically with petrologic type. The most primitive LL3s (Krymka, Bishunpur, Chainpur) contain substantial amounts of CCFXe in chromite-carbon, enriched relative to primordial Xe as shown by high ${}^{136}\text{Xe}{}^{132}\text{Xe}$ (0.359–0.459, vs 0.310 for primordial Xe). These are accompanied by He and by Ne with ${}^{20}\text{Ne}{}^{22}\text{Ne}\text{≈ 8.0}$ and by variable amounts of a xenon component enriched in the light isotopes. The chromite in these meteorites is compositionally peculiar, containing substantial amounts of Fe(III). These meteorites, as well as Parnallee (LL3) and Hamlet (LL4) also contain CCFXe in phase Q, heavily diluted by primordial $\text{Xe}\text{(}{}^{136}\text{Xe}{}^{132}\text{Xe}\text{= 0.317–0.329)}$. On the other hand, LL5s and 6s (Olivenza, St. Séverin, Manbhoom and Dhurmsala) contain no CCFXe in either mineral. This deficiency must be intrinsic rather than caused by metamorphic loss, because Q in these meteorites still contains substantial amounts of primordial Ne.If CCFXe comes from a supernova, then its distribution in LL-chondrites requires three presolar carrier minerals of the right solubility properties, containing three different xenon components in certain combinations. These minerals must be appropriately distributed over the petrologic types, together with locally produced Q containing primordial gases, and they must be isotopically normal, in contrast to the gases they contain. On the other hand, if CCFXe comes from fission of a volatile superheavy element, then its decrease from LL3 to LL6 can be attributed to progressively less complete condensation from the solar nebula. Ad hoc assumptions must of the host phase Q, its association with ferrichromite and the origin of the associated xenon component enriched in the light isotopes.Soluble minerals in LL3s and LL4s contain a previously unobserved, solar xenon component, which, however, is not derived from the solar wind. Three types of ‘primordial’ xenon thus occur side-by-side in different minerals of the same meteorite: strongly fractionated Xe in ferrichromite and carbon, lightly fractionated Xe in phase Q, and ‘solar’ Xe in solubles. Because the first two can apparently be derived from the third by mass fractionation, it seems likely that all were trapped from the same solar nebula reservoir, but with different degrees of mass fractionation.     

Structure of sodium alumino-silicate melts quenched at high pressure; infrared and aluminum K-radiation data

Infrared and X-ray radiation data indicate that the effect of pressure on Na-Al-Si-O quenched melt is to change the coordination number of trivalent aluminum ions from four to six. This conclusion is based upon an observed decrease in the intensity of the infrared vibration involving a “bridging” oxygen in the polymer structure and a shift in both Al K_α (7 × 10^?4Å) and Al K_β (20 × 10^?4Å) radiation. The amount of Al^IV or Al^VI seems to be a continuous function of the pressure at which the melt was formed and is thus independent of the coordination change effected at high pressure in solids crystallized from the NaAlSi₂O₆ composition used in this study. The importance of the continuous shift of coordination number of aluminum ions in silicate melts at high pressure is discussed. The change in coordination of Al would also be expected in natural silicate melts (magmas) at high pressures.     

Radiogenic argon in glauconites as a function of mineral recrystallization

Natural glauconitic grains were subjected to hydrothermal treatment at a pressure of 2 kbars and temperatures of 200–414°C for the period of one month.The measurement of radiogenic argon before and after treatment shows that the mica-type glauconite mineral contains 92–96% of the argon initially present up to temperatures as high as 320°C.After recrystallization of the mixed-layered glauconitic minerals the new phases still contain significant amounts of argon in the structure. The mixed-layered glauconitic minerals are much more sensitive to temperature treatment at elevated pressure than glauconitic-type mica, they, however, still contain up to 84% of the initial argon of the untreated material at temperatures as high as 320°C, i.e., just below the thermal stability of glauconite mica. It is evident that low-temperature metamorphism will affect the apparent radiogenic age of glauconitic grains to an extent which depends upon their composition and the temperature of metamorphism.     

The Blomskog granite — a possible diapiric structure

A group of granitic intrusions occurs in western Sweden, showing a regular spacing between consecutive intrusions. Their spacing suggests solid-state doming of granitic material. However the textures of the rock are most easily interpreted in terms of crystallization from a magma. This interpretation is also consistent with contact observations and chemical characteristics of the rock. A hypothesis simultaneously explaining the textures and the spacing is presented, where doming of a light layer of potassium-rich migmatite is suggested to be followed by partial melting of the same rock. The magma collects at the top of the thus-formed sinusoidal waves and later intrudes into the country rock, where it crystallizes before reaching the surface.     

Seismic investigations in the Weddell Sea Embayment

Seismic multi-channel data collected during Norwegian Antarctic Research Expeditions in 1976–1977 and 1978–1979 outline aspects of the Cenozoic depositional environment in the Weddell Sea Embayment. Acoustic basement, probably representing the East Antarctic craton, is exposed in a 50–100 km wide swath along the ice barrier between 78°S–75.5°S on the eastern side of the Crary Trough. The shelf prograded westward and northward from the craton into a subsiding basin colinear with the Transantarctic Mountain Range. Measured sediment thicknesses exceed 5 km. During middle and late Tertiary times a submarine fan complex—the Crary Fan—developed on the southeastern margin of the Weddell Sea Embayment. The glacially eroded Crary Trough is located at the contact between the craton and a sedimentary basin to the west. The entire sedimentary section is undisturbed by faulting or folding, which indicates that any movements related to Cenozoic uplift of the Trans-Antarctic Mountains and/or relative motion of East Antarctica had little effect in the area north of the Filchner Ice Shelf east of 41°W.     

Clinopyroxenes in Mesozoic pillow lavas from the French Alps: influence of cooling rate on compositional trends

The clinopyroxene compositions determined in four spilite samples from the French Alps show trends with high Al and Ti contents. Two samples from the center and outer zones of a single pillow have different Al/Ti ratios as well as different evolution trends in the CaFeMg system. Two samples collected from two different pillows in another outcrop also show different Al and Ti contents.These variations can be correlated with texture and grain size of the rocks and can be seen to follow trends predicted from recent cooling rate experiments. Analyses of relict clinopyroxenes from a glaucophane-lawsonite-bearing pillow show that these metastable clinopyroxenes can survive an episode of low-temperature, high-pressure metamorphism.