M. Feely

Characterization of crude oils using fluorescence lifetime data

The average fluorescence lifetimes of nine North Sea crude oils with API gravities of between 20 and 51 were measured using a modular, filter based, instrument developed in-house. Two pulsed light emitting diode (LED) excitation sources (460 and 510 nm) were used to excite fluorescence, the lifetime of which was measured at a range of emission wavelengths. Fluorescence lifetimes were found to vary from 1.8 to 8.2 ns with confidence intervals of +/- 0.11 ns. The average lifetimes at all emission wavelengths were linearly correlated with API gravity and with aromatic concentration with the best results being obtained with the 460 nm excitation source. Predictive models with an accuracy of +/- 7.6 API degrees were generated using partial least-squares methods from average fluorescence lifetimes measured at an emission wavelength of 500 nm using 460 nm excitation. A better correlation was found between the aromatic concentration of the oils and the ratio of the average fluorescence lifetimes at measured at 550 and 650 nm using 460 nm excitation. This led to a quantitative model with an accuracy of +/- 5.4% for aromatic concentration.

The Variscan thermal history of west Clare, Ireland

Vitrinite reflectance data from Namurian rocks in west Clare suggest that high maturation levels, corresponding to palaeotemperatures of 340–370 °C, were attained prior to Variscan deformation. Fluid inclusions in syntectonic quartz veins homogenize between 330 °C and 50 °C with an accompanying decrease in salinity from 27 to 5 eq. wt % NaCl. Maximum fluid inclusion entrapment temperatures ranged from more than 300 °C to 250 °C during Variscan folding in County Clare. The observed maturation levels (c. 7.5% Rmax) far exceed values for simple burial maturation based on the estimated burial history and ‘normal’ geothermal gradients, and do not increase with depth in the Doonbeg No. 1 exploration well. Fluid advective heating is suggested as the most likely mechanism consistent with the Clare reflectance and thermometric data. Vein and shear zone dimensions preclude rapid vertical movements of hot fluids through the section, and extensive lateral fluid migration from sedimentary basins undergoing tectonically driven dewatering to the south or west is therefore proposed.

Time-resolved fluorescence microspectroscopy for characterizing crude oils in bulk and hydrocarbon-bearing fluid inclusions.

Time-resolved fluorescence data was collected from a series of 23 bulk crude petroleum oils and six microscopic hydrocarbon-bearing fluid inclusions (HCFI). The data was collected using a diode laser fluorescence lifetime microscope (DLFLM) over the 460–700 nm spectral range using a 405 nm excitation source. The correlation between intensity averaged lifetimes (τ̄) and chemical and physical parameters was examined with a view to developing a quantitative model for predicting the gross chemical composition of hydrocarbon liquids trapped in HCFI. It was found that τ̄ is nonlinearly correlated with the measured polar and corrected alkane concentrations and that oils can be classified on this basis. However, these correlations all show a large degree of scatter, preventing accurate quantitative prediction of gross chemical composition of the oils. Other parameters such as API gravity and asphaltene, aromatic, and sulfur concentrations do not correlate well with τ̄ measurements. Individual HCFI were analyzed using the DLFLM, and time-resolved fluorescence measurements were compared with τ̄ data from the bulk oils. This enabled the fluid within the inclusions to be classified as either low alkane/high polar or high alkane/low polar. Within the high alkane/low polar group, it was possible to clearly discriminate HCFI from different locales and to see differences in the trapped hydrocarbon fluids from a single geological source. This methodology offers an alternative method for classifying the hydrocarbon content of HCFI and observing small variations in the trapped fluid composition that is less sensitive to fluctuations in the measurement method than fluorescence intensity based methods.

Re–Os molybdenite dates from the Ballachulish and Kilmelford Igneous Complexes (Scottish Highlands): age constraints for late Caledonian magmatism

Abstract: New Re–Os molybdenite geochronology is presented from the Ballachulish Igneous Complex (433.5 ± 1.8 Ma) and the Kilmelford Igneous Complex (425.8 ± 1.7 Ma) of the SW Scottish Highlands. The sulphide mineralization in the Ballachulish Igneous Complex is related to the latest phase of granite magmatism, and therefore the new Re–Os age provides a minimum crystallization age, c. 3.7 Ma earlier than previous crystallization age estimates. This Re–Os age overlaps U–Pb ages obtained from subduction-related granitic and appinitic magmatism north of the Great Glen Fault, and an origin related to active subduction rather than slab breakoff is proposed for the Ballachulish Igneous Complex. Molybdenite mineralization in the Kilmelford Igneous Complex is spatially and genetically associated with porphyry Cu mineralization, which is consistent with the rapid ascent of volatile-rich magma during early rebound following the breakoff of subducted oceanic lithosphere. The Kilmelford Igneous Complex and the coeval Lorn Lava Pile may represent the earliest of the igneous bodies predicted by slab breakoff, indicating that slab breakoff occurred at c. 426 Ma.

Diorite-granite magma mingling and mixing along the axis of the Galway Granite batholith, Ireland

The late Caledonian Galway Granite batholith is bisected by a WNW-trending magma mingling and mixing zone (the MMZ). This trans-batholithic feature marks the entrainment and ascent of hydrous dioritic magmas in anatectic silicic magmas. Deep in the zone, flowage and hydraulic stress stretched out alternating mafic and hybrid granitoid sheets. At progressively shallower levels, highly elongate and deformed enclaves led up to discrete clouds of ovoid enclaves. Magmatic end-members are identified as quartz diorite and calc-alkaline granite, from which a mixing continuum was produced despite a persistent degree of immiscibility among the various hybrids. A generally west-directed flowage along the MMZ locally induced detachment and upthrusting of ductile blocks of deeper-seated mingled rock. The detailed plan of the MMZ reveals left offsets, supporting field evidence for emplacement in a broad dextral NW-SE shear zone. The Maam and Clifden fault systems provide the partners to model this emplacement within a crustal pull-apart.

Long-lived granite-related molybdenite mineralization at Connemara, western Irish Caledonides

New Re–Os age determinations from the Galway Granite (samples: KMG = 402.2 ± 1.1 Ma, LLG = 399.5 ± 1.7 Ma and GBM = 383.3 ± 1.1 Ma) show that in south Connemara, late Caledonian granite-related molybdenite mineralization extended from c. 423 Ma to c. 380 Ma. These events overlap and are in excellent agreement with the published granite emplacement history determined by U–Pb zircon geochronology. The spatial distribution of the late-Caledonian Connemara granites indicates that initial emplacement and molybdenite mineralization occurred at c. 420 Ma (that is, the Omey Granite and probably the Inish, Leterfrack and Roundstone granites) to the N and NW of the Skird Rocks Fault, an extension of the orogen-parallel Southern Uplands Fault in western Ireland. A generally southern and eastward progression of granite emplacement (and molybdenite mineralization) sited along the Skird Rocks Fault then followed, at c. 410 Ma (Roundstone Murvey and Carna granites), at c. 400 Ma (Errisbeg Townland Granite, Megacrystic Granite, Mingling Mixing Zone Granodiorite, Lough Lurgan Granite and Kilkieran Murvey Granite) and at c. 380 Ma (Costelloe Murvey Granite, Shannapheasteen and Knock granites). The duration of granite magmatism and mineralization in Connemara is similar to other sectors of the Appalachian–Caledonian orogeny and several tectonic processes (e.g. slab-breakoff, asthenospheric flow, transtension and decompression) may account for the duration and variety of granite magmatism of the western Irish Caledonides.

New perspectives on the order and style of granite emplacement in the Galway Batholith, western Ireland

The late Caledonian Galway Batholith is cut by two major faults which divide it into three separate areas: the western, central and eastern blocks. The upthrown and more deeply eroded central block is bounded by these faults, in the west by the north–northeast trending Shannawona Fault and in the east by the north–northwest trending Barna Fault. We present new granite field relations from part of the central block (Inveran sector) which are fundamental in establishing the order and style of emplacement for the granites of the central block and the batholith as a whole. Unequivocal field evidence from the Inveran sector indicates upward movement of early central block granites which then became the solid roof rocks to subsequent intrusions. In the case of the Knock Granite these earlier intrusions were block stoped. We use this field evidence to review the geology of the central block in a 200 km 2 area that incorporates the previously mapped Costelloe and Spiddal areas. Sharp intrusive contacts are a predominant feature of this sector of the central block and are in marked contrast to the gradational contacts recorded elsewhere in the batholith. Whereas juxtaposition of plutons in the western block occurred as the granites were partly crystallized, the central block reveals earlier, deeper level granites that were consolidated by the time they were intruded by late-stage higher level granites.

Influence of chemical composition on the fluorescence lifetimes of crude petroleum oils

Fluorescence based methods are increasingly being used for the analysis of crude oils because they offer high speed, low cost, non-contact, and non-destructive testing options. The fluorescence of crude oils is due to the presence of a wide range of cyclic aromatic compounds, the intensity and temporal behaviour of this fluorescence is directly related to chemical composition. In particular, we have found that the Fluorescence Lifetime (FL) measured at a range of different emission wavelengths is correlated with the concentrations of the aromatic and polar fractions of petroleum oils, and the density (API gravity). We have analysed 22 different crude oils from around the world with API gravities of between 10 to 50. The fluorescence steady-state emission spectra and fluorescence lifetimes at a range of emission wavelengths were measured for 380 nm excitation. It was found that the correlations between the chemical and physical characteristics of the crude oils and the measured fluorescence parameters are highly non-linear. Furthermore, there is a wide degree of scatter in the observed data for medium oils, which have similar physical properties but widely varying chemical compositions. We discuss these findings in the context of developing quantitative methods of analysis for crude petroleum oils based on fluorescence lifetime measurements.

Fluid inclusion studies of well samples from the hydrocarbon prospective Porcupine Basin, offshore Ireland

Abstract Transmitted light and ultraviolet (UV) light microscopy combined with microthermometry of fluid inclusions in sandstonesfrom 10 wells in the Irish Porcupine Basin reveal the presence of aqueous and oil-bearing fluids in cements and detrital quartz grains. Jurassic sandstone samples contain two-phase oil inclusions trapped during syn- and post-cementation and homogenise to the liquid state at ∼50 to 125 °C. Furthermore, early trapping in cement of less mature relatively heavy oil (°API gravity∼25–35) followed by ingress of lighter more mature oil (°API gravity∼45–50) is indicated. Primary two-phase aqueous fluid inclusions trapped in cements are generally of low salinity (

Hydrocarbon migration in the Porcupine Basin, offshore Ireland: evidence from fluid inclusion studies

ABSTRACT A petrographic and microthermometric study of fluid inclusions in Jurassic and Cretaceous sandstones from the Porcupine Basin, offshore Ireland was integrated with innovative fluorescence lifetime measurements of hydrocarbon-bearing fluid inclusions to determine the compositions of the fluids associated with diagenesis and post-diagenetic fluid migration and the extent of hydrocarbon and aqueous fluid migration pathways. Petrographic analyses indicate that Jurassic strata were the main fluid migration pathways for hydrocarbon fluids and that hydrocarbon migration occurred relatively late in the diagenetic history of these sandstones. UV fluorescence and fluorescence lifetime measurements have recognized at least two chemically distinct hydrocarbon groups (Types 1a and 1b) with dissimilar lifetime-wavelength (τ-λ) profiles, consistent with at least two petroleum charges derived from different sources. Primary aqueous inclusions in authigenic cements show that cementation in Cretaceous sandstones occurred at relatively shallow levels at low temperatures (<50°C), while inclusions in authigenic cements in Jurassic sandstones were trapped at higher temperatures (70–120°C) and deeper levels. Aqueous fluid inclusions in intergranular trails indicate that post-cementation fluid migration occurred at high temperatures (up to 175°C). These high temperature fluid migrations are interpreted to be associated with plume-related activity during the opening of the North Atlantic.