Adriana Dutkiewicz

Assessing the maturity of oil trapped in fluid inclusions using molecular geochemistry data and visually-determined fluorescence colours

Abstract The thermal maturity of oils extracted from inclusions and the fluorescence colours of oil-bearing fluid inclusions have been measured in 36 sandstone samples from Australasian oil fields. The inclusion oils were analysed using an off-line crushing technique followed by GC–MS. A maturity assessment was made for each inclusion oil using 25 molecular maturity ratios, including a newly defined dimethyldibenzothiophene ratio (DMDR). Each inclusion oil was placed in one of 4 maturity brackets, approximately equivalent to early, mid, peak and post oil generation windows. The fluorescence colours of oil inclusions were visually-discriminated into “blue”, “white” and “yellow plus orange” and their proportions estimated using point counting techniques. Sixteen samples have >85% of oil inclusions with blue fluorescence, whilst other samples have more variable fluorescence colours. One sample has 100% of oil inclusions with yellow plus orange fluorescence. The results show that samples containing mainly blue-fluorescing oil inclusions have thermal maturities anywhere within the oil window. In particular, the molecular geochemical data strongly suggests that oil inclusions with blue fluorescence can have relatively low maturities (calculated reflectance

Oil preserved in fluid inclusions in Archaean sandstones

Oil is generally thought to be geologically young, as it is thermodynamically unstable when subjected to elevated temperatures over long periods in open systems,. Indeed, almost all petroleum production comes from rocks younger than 400 million years (ref. 3). Although the oldest known oil occurs in rocks 1,650 Myr old, suitable source rocks were abundant in older geological successions and circumstantial evidence suggests that some of these generated hydrocarbons early in their history. Here, we report the discovery of oil preserved in fluid inclusions in sandstones dating back ∼3,000 Myr. Most inclusions lie within healed microfractures confined to individual detrital quartz grains, indicating that their oil was emplaced before Archaean or Palaeoproterozoic metamorphism sealed all voids and thus came from older (in some cases Archaean) sources. The fluid inclusions apparently acted as inert pressure vessels that protected the oil from subsequent degradation by circulating fluids and mineral catalysts. Because of its great age, this oil can potentially yield valuable information about the size and diversity of the early biosphere, particularly if it contains molecular fossils (biomarkers) of the primordial organisms from which it was derived.

Biomarkers, brines, and oil in the Mesoproterozoic, Roper Superbasin, Australia

Gas chromatography–mass spectrometry of oil inclusions from the ca. 1430 Ma marine Roper Group in the Roper Superbasin, Australia, provides a new source of information about the early biosphere and Proterozoic petroleum systems. Oil most likely derived from an overlying shale was trapped at ∼60 °C as abundant oil inclusions within transgranular microfractures in detrital quartz during Mesoproterozoic basin inversion. The oil is very mature and has a wide range of biomarkers, derived mainly from cyanobacteria, but lacks eukaryote biomarkers. Unlike associated solid bitumens, the inclusion oil is nonbiodegraded. Evidently, the inclusions remained closed systems, sheltered from postentrapment alteration and contamination. Because fluid inclusions have preserved biomarkers for >1000 m.y., they constrain the diversity of primordial ecosystems, whereas other forms of early Precambrian organic matter are usually absent or metamorphosed.

Past changes to isotopic and solute balances in a continental playa: clues from stable isotopes of lacustrine carbonates

In many salt lakes around the world, the relative abundance of preserved authigenic minerals is different from that predicted from solute mass balance calculations. Conventional mass balance models assume that chloride behaves conservatively over long periods of time and fail to take into account the role of diffusion, deflation and fractional crystallisation/dissolution of salts. An alternative approach is to use oxygen isotopes as these reflect directly water molecule rather than solute concentrations and have the added advantage of providing a palaeohydrological record in lacustrine carbonates. We present a steady-state, stable-isotope model, in conjunction with stable isotopic measurements of sub-surface brines, regional groundwaters and carbonate deposits from the Lake Malata–Lake Greenly playa complex in South Australia, to estimate the apparent leakage and palaeoleakage from these superficially closed playa lakes. The steady-state model calculations, using the present and compositions of the lake brines and inflowing groundwater, suggest that the apparent present-day leakage for the complex is 75 to 90% of inflow (∼35 times that calculated from Cl− and Br−). Under such conditions, only low magnesian calcite precipitates and the lake water experiences reduced effects of evaporation, gas and vapour exchange and, consequently, reduced isotopic and chemical enrichment. Further, our model shows that calcite becomes increasingly Mg-rich until leakage is reduced to ∼55 to 70% of inflow — a condition favourable for dolomitisation. and of the lacustrine carbonates show excursions on the order of 5‰ over the length of a 2.3 m core, indicating that the lake complex has varied from being throughflow dominated (presence of low Mg-calcite relatively depleted in and ) to evaporation dominated (high Mg-calcite/dolomite relatively enriched in and ) throughout late Quaternary. Our estimates of leakage fractions are consistent with the observed mineralogical suite, but there remains a discrepancy between apparent closure indicated by the presence of the highly saline brine reservoir (<1% leakage) and high rates of throughflow inferred from stable isotope data. We propose that the brine was formed by winter time re-solution of a seasonal halite crust which forms during summer dominated evaporative discharge. Recirculation of the secondary brine, and mixing with regional groundwater may decouple the solute cycle from the water cycle in many playa lakes. The end result is a partially mixed brine characterised by nearly conserved solutes but with isotopic signatures indicative of brine–rainfall–groundwater interactions.

Oil-bearing CO2–CH4–H2O fluid inclusions: oil survival since the Palaeoproterozoic after high temperature entrapment

The 2.45-Ga fluvial quartz arenites and conglomerates of the Matinenda Formation at Elliot Lake, Canada, contain evidence for two episodes of oil migration with entrapment of oil-bearing fluid inclusions. The first episode was during diagenesis, the second during a subsequent sub-greenschist facies metamorphic event at ca. 2.2 Ga at temperatures exceeding 250 °C. Remnants of the migrating hydrocarbon fluids are preserved as radiogenic pyrobitumen nodules occupying former inter-granular pore spaces and as complex fluorescing oil-bearing fluid inclusions. The latter include two dominant types: (1) aqueous-oil inclusions with a minor oil phase, located within original detrital grains and clasts and within rare quartz overgrowths, and (2) aqueous–carbonic inclusions with a minor oil phase, light hydrocarbons and solids, located within microfractures in detrital quartz grains and clasts. The majority of Type 1 inclusions were trapped with a high salinity fluid (up to 25 wt.% NaCl equivalent) at relatively low temperatures between 80 and 200 °C and pressures of less than 2 kbar early in the burial history of the Matinenda Formation. Type 2 inclusions were trapped during metamorphism with a much lower salinity aqueous–carbonic fluid (1.2 to 17 wt.% NaCl equivalent) at temperatures between 280 and 350 °C and pressures between 1 and 1.5 kbar. Under these conditions oil may have been miscible in the mixed fluid. δ13C of −25.5‰ and −24.9‰ for the pyrobitumen nodules and fluid inclusion hydrocarbon gas indicate a biogenic origin for the petroleum. δ13C of −16.2‰ for the bulk inclusion gas suggests a value of −12‰ for the carbonic phase, hence a mix of inorganic CO2 and organic CO2 from either thermal alteration of organic matter or in situ chemical oxidation of hydrocarbons. This study has implications for the depth and temperature at which oil may be present, suggesting that deep, old reservoirs may not be completely barren of petroleum. The fact that hydrocarbons in the Matinenda Formation fluoresce indicates that aromatic compounds are able to survive temperatures of up to 350 °C in a suitable geological environment.

The GPlates Portal: Cloud-Based Interactive 3D Visualization of Global Geophysical and Geological Data in a Web Browser.

The pace of scientific discovery is being transformed by the availability of ‘big data’ and open access, open source software tools. These innovations open up new avenues for how scientists communicate and share data and ideas with each other and with the general public. Here, we describe our efforts to bring to life our studies of the Earth system, both at present day and through deep geological time. The GPlates Portal (portal.gplates.org) is a gateway to a series of virtual globes based on the Cesium Javascript library. The portal allows fast interactive visualization of global geophysical and geological data sets, draped over digital terrain models. The globes use WebGL for hardware-accelerated graphics and are cross-platform and cross-browser compatible with complete camera control. The globes include a visualization of a high-resolution global digital elevation model and the vertical gradient of the global gravity field, highlighting small-scale seafloor fabric such as abyssal hills, fracture zones and seamounts in unprecedented detail. The portal also features globes portraying seafloor geology and a global data set of marine magnetic anomaly identifications. The portal is specifically designed to visualize models of the Earth through geological time. These space-time globes include tectonic reconstructions of the Earth’s gravity and magnetic fields, and several models of long-wavelength surface dynamic topography through time, including the interactive plotting of vertical motion histories at selected locations. The globes put the on-the-fly visualization of massive data sets at the fingertips of end-users to stimulate teaching and learning and novel avenues of inquiry.

Seawater chemistry driven by supercontinent assembly, breakup, and dispersal

Global oceans are known to have alternated between aragonite and calcite seas. These oscillations refl ect changes in the Mg/Ca ratios of seawater that control biomineralization and the composition of marine carbonates, and are thought to be caused mainly by the time dependence of crustal accretion at mid-ocean ridge crests and the associated high-temperature mid-ocean ridge fl uid fl ux. Here we use global ocean basin reconstructions to demonstrate that the fl uctuations in hydrothermal ocean inputs are instead caused by the gradual growth and destruction of mid-ocean ridges and their relatively cool fl anks during long-term tectonic cycles, thus linking ocean chemistry to off-ridge low-temperature hydrothermal exchange. Early Jurassic aragonite seas were a consequence of supercontinent stability and a minimum in mid-ocean ridge length and global basalt alteration. The breakup of Pangea resulted in a gradual doubling in ridge length and a 50% increase in the ridge fl ank area, leading to an enhanced volume of basalt to be altered. The associated increase in the total global hydrothermal fl uid fl ux by as much as 65%, peaking at 120 Ma, led to lowered seawater Mg/Ca ratios and marine hypercalcifi cation from 140 to 35 Ma. A return to aragonite seas with preferential aragonite and high-Mg calcite precipitation was driven by pronounced continental dispersal, leading to progressive subduction of ridges and their fl anks along the Pacifi c rim.

Seabed classification of the South Tasman Rise from SIMRAD EM12 backscatter data using artificial neural networks

We have developed an automated method for sea‐floor classification for the South Tasman Rise, based on a SIMRAD EM12‐backscatter (13 kHz) mosaic and 47 sea‐floor samples. The samples have been divided into 3 distinct groups representing: (i) thick blankets of foraminiferal ooze; (ii) mixed sediments comprising sand/silt/mud (turbidites/chalk); and (iii) outcrops of metamorphic basement and volcanic rocks. A total of 515 sub‐areas, each measuring 32 × 32 pixels (∼4 km2) and representing the different seabed types, were extracted from the image from areas 128×128 pixels large, centred on the sample locations. The texture of the sub‐images was analysed by calculating grey‐level run‐length features, spatial grey‐level dependence matrices, and grey‐level difference vectors in four directions. A total of 100 samples for each class and 18 feature statistics were chosen to train an artificial neural network to recognise the textural attributes and their variability for each class. The performance of the network w...

Census of seafloor sediments in the world’s ocean

Knowing the patterns of distribution of sediments in the global ocean is critical for understanding biogeochemical cycles and how deep-sea deposits respond to environmental change at the sea surface. We present the first digital map of seafloor lithologies based on descriptions of nearly 14,500 samples from original cruise reports, interpolated using a support vector machine algorithm. We show that sediment distribution is more complex, with significant deviations from earlier hand-drawn maps, and that major lithologies occur in drastically different proportions globally. By coupling our digital map to oceanographic data sets, we find that the global occurrence of biogenic oozes is strongly linked to specific ranges in seasurface parameters. In particular, by using recent computations of diatom distributions from pigment-calibrated chlorophyll-a satellite data, we show that, contrary to a widely held view, diatom oozes are not a reliable proxy for surface productivity. Their global accumulation is instead strongly dependent on low surface temperature (0.9–5.7 °C) and salinity (33.8–34.0 PSS, Practical Salinity Scale 1978) and high concentrations of nutrients. Under these conditions, diatom oozes will accumulate on the seafloor regardless of surface productivity as long as there is limited competition from biogenous and detrital components, and diatom frustules are not significantly dissolved prior to preservation. Quantifying the link between the seafloor and the sea surface through the use of large digital data sets will ultimately lead to more robust reconstructions and predictions of climate change and its impact on the ocean environment.

Oil migration in the Middle Proterozoic Roper Superbasin,Australia: evidence from oil inclusions and their geochemistries

Abstract The geochemistry and petrography of oil inclusions in the ca. 1.4 Ga dessie Creek Sandstone in the Roper Superbasin,Australia, provide insights into poorly understood Middle Proterozoic petroleum systems. Oil inclusions were trapped after the intrusion of ca. 1.3 Ga dolerite sills during basin reactivation and peak maturation. Biomarkers reflect the predominance of prokaryotes in the Mid-Proterozoic biosphere and the most likely source rock is the directly overlying Velkerri Formation.