Potential of palaeofluid analysis for understanding oil charge history

Fluid inclusion data, particularly the distribution of hydrocarbon fluid inclusions and their chemistry, can provide insights into oil charge in a petroleum-prospective region. Examples from the UK Atlantic margin show how we can understand thermal regime, timing and chemistry of oil charge. Data from the UK Atlantic margin based on fluid inclusion temperature profiles shows anomalously high temperatures which are highest at the top of the Triassic–Eocene sequence. This is interpreted as a product of hot fluid flow, probably reflecting hydrothermal activity related to intrusion of sills at depth. The preservation of high temperatures also implies rapid migration from depth through fracture systems. Ar–Ar analysis of oil-bearing K-feldspar cements, and petrographic studies of oil inclusion distribution help delimit timing and migration pathways for the hot fluid charge and later fluid migration events. Coupled with compositional data for oils measured destructively (organic geochemistry) or non-destructively (fluorescence), these approaches allow the development of oil charge histories based on real data rather than theoretical modelling.     

Sampling and analysis of geothermal fluids

Sampling of geothermal fluids presents some problems not encountered when sampling surface and nonthermal ground waters. Specific collection techniques are required to obtain representative samples because of the elevated temperature and boiling of these fluids, the effect of exposing them to the atmosphere and cooling of the samples. Sample treatment during collection depends on the analytical method to be used. When sampling wet‐steam wells, both the liquid and the vapour fractions should be collected at the same fluid separation pressure. When sampling fumarole steam, maximum information is obtained if the total discharge is collected into a single container without separating the gas and the steam condensate fractions. Silica polymerization affects the solution pH. The only way to obtain reliable pH measurement of a water sample supersaturated with respect to amorphous silica is to measure it on site, before the onset of polymerization. This paper provides an outline of the geothermal sampling techniques and analytical methods currently in use in Iceland. Sampling of hot‐water and wet‐steam wells is described, as is sampling of hot springs, fumaroles and gas bubbling through hot‐spring waters. Detailed procedures are given for the analysis of total carbonate carbon and total sulphide sulphur in geothermal water and steam condensate samples.     

On sampling the upper crustal reservoir of the NE Pacific Ocean

The oceanic upper crustal reservoir is a 600‐m thick layer of porous and permeable basaltic rock that forms the uppermost igneous basement underlying the global ocean. Pore spaces within this fluid aquifer contain a significant fraction of the global seawater, and active circulation through this reservoir has profound influence on the chemical composition of the ocean, strongly impacting the biological environment near the sea surface. Because of the relative inaccessibility of the deep seafloor, where hydrothermal fluid discharges and seawater re‐charges the oceanic crustal aquifer, our understanding of the dynamic physical, chemical and biological processes is strongly dependent on our ability to obtain uncontaminated samples from this challenging environment. Recent technological advances have addressed some, but certainly not all of these sampling problems, providing new data and samples that test our current hypotheses about the crustal fluid reservoir. Current scientific interest in the sub‐seafloor biosphere has focused on the uppermost igneous oceanic crust as likely to be one of the most habitable environments, because of its porosity and locus of hydrothermal circulation of chemical nutrients. Recent observations indicate that sub‐seafloor crustal environments harbor novel CO₂‐utilizing bacteria (primary producers) that could be a significant source of carbon‐fixation in the ocean, thus broadening possible habitable zones both on Earth and elsewhere where microbial life could exist independent of nutrient input from photosynthesis.     

Real‐time mud gas logging and sampling during drilling

Continuous mud gas loggings during drilling as well as offline mud gas sampling are standard procedures in oil and gas operations, where they are used to test reservoir rocks for hydrocarbons while drilling. Our research group has developed real‐time mud gas monitoring techniques for scientific drilling in non‐hydrocarbon formations mainly to sample and study the composition of crustal gases. We describe in detail this technique and provide examples for the evaluation of the continuous gas logs, which are not always easy to interpret. Hydrocarbons, helium, radon and with limitations carbon dioxide and hydrogen are the most suitable gases for the detection of fluid‐bearing horizons, shear zones, open fractures, sections of enhanced permeability and permafrost methane hydrate occurrences. Off‐site isotope studies on mud gas samples helped reveal the origin and evolution of deep‐seated crustal fluids.     

Hydraulic observations from a 1 year fluid production test in the 4000 m deep KTB pilot borehole

A long‐term pump test was conducted in the KTB pilot borehole (KTB‐VB), located in the Oberpfalz area, Germany. It produced 22 300 m³ of formation fluid. Initially, fluid production rate was 29 l min^?1 for 4 months, but was then raised to an average of 57 l min^?1 for eight more months. The aim of this study was to examine the fluid parameters and hydraulic properties of fractured, crystalline crusts as part of the new KTB programme ‘Energy and Fluid Transport in Continental Fault Systems’. KTB‐VB has an open‐hole section from 3850 to 4000 m depth that is in hydraulic contact with a prominent continental fault system in the area, called SE2. Salinity and temperature of the fluid inside the borehole, and consequently hydrostatic pressure, changed significantly throughout the test. Influence of these quantities on variations in fluid density had to be taken into account for interpretation of the pump test. Modelling of the pressure response related to the pumping was achieved assuming the validity of linear Darcy flow and permeability to be independent of the flow rate. Following the principle ‘minimum in model dimension’, we first examined whether the pressure response can be explained by an equivalent model where rock properties around the borehole are axially symmetric. Calculations show that the observed pressure data in KTB‐VB can in fact be reproduced through such a configuration. For the period of high pumping rate (57 l min^?1) and the following recovery phase, the resulting parameters are 2.4 × 10^?13 m³ in hydraulic transmissivity and 3.7 × 10^?9 m Pa^?1 in storativity for radial distances up to 187 m, and 4.7 × 10^?14 m³ and 6.0 × 10^?9 m Pa^?1, respectively, for radial distances between 187 and 1200 m. The former pair of values mainly reflect the hydraulic properties of the fault zone SE2. For a more realistic hydraulic study on a greater scale, program FEFLOW was used. Parameter values were obtained by matching the calculated induced pressure signal to fluid‐level variations observed in the KTB main hole (KTB‐HB) located at 200 m radial distance from KTB‐VB. KTB‐HB is uncased from 9031 to 9100 m and shows indications of leakage in the casing at depths 5200–5600 m. Analysis of the pressure record and hydraulic modelling suggest the existence of a weak hydraulic communication between the two boreholes, probably at depths around the leakage. Hydraulic modelling of a major slug‐test in KTB‐HB that was run during the pumping in KTB‐VB reveals the effective transmissivity of the connected formation to be 1 to 2 orders of magnitude lower than the one determined for the SE2 fault zone.     

Dating of modified femora of extinct dwarf Hippopotamus from Southern Madagascar: Implications for constraining human colonization and vertebrate extinction events

Two significant events in the late Holocene history of Madagascar were (a) the arrival of people, and (b) the loss of nearly two dozen species of land vertebrates in the socalled “subfossil extinctions”. The consensus is that the faunal losses occurred shortly subsequent to human arrival, but the timing of these events is poorly constrained. The minimum age for initial human presence on the island may now be set at approximately 2000 bp, on the basis of AMS ¹⁴C dates for human-modified femora of extinct dwarf hippos from SW Madagascar. Assuming that this date also marks the beginning of deleterious human interactions with the subfossil fauna, and assuming that this fauna became completely extinct by 900 bp, the width of the anthropogenic “extinction window” may have been as long as c. 1000 a. This estimate, nearly twice the length of previous ones, is close to the unadjusted minimum for the duration of the terminal Pleistocene extinction event in the Americas. Whether or not this length of time comports with theoretical expectations of a “blitzkrieg” pattern of losses is uncertain, but greater refinement in dating the end of the subfossil extinctions is unlikely to produce radically shorter estimates of duration.     

Geological setting, nature of ore fluids and sulphur isotope geochemistry of the Fu Ning Carlin-type gold deposits, Yunnan Province, China

Carlin‐type gold deposits in southern China are present in Palaeozoic to Mesozoic siliciclastic and carbonate rocks. The border region of Yunnan, Guizhou and Guangxi Provinces contains gold deposits on the south‐western margin of the Pre‐Cambrian South China Craton in south‐eastern Yunnan Province. The Fu Ning gold deposits host epigenetic, micron‐sized disseminated gold in: (i) Middle Devonian (D₁p) black carbonaceous mudstone at the Kuzhubao gold deposit and (ii) fault breccia zones at the contact between Triassic gabbro (β ) and the Devonian mudstone (D₁p) at the Bashishan gold deposit. The deposits are associated with zones of intense deformation with enhanced permeability and porosity that focused hydrothermal fluid flow, especially where low‐angle N‐S striking thrust faults are cut by NW striking strike‐slip and/or NE striking normal faults. Major sulphide ore minerals in the Fu Ning gold deposits are pyrite, arsenopyrite, arsenic‐rich pyrite, stibnite and minor iron‐poor sphalerite. Gangue minerals are quartz, sericite, calcite, ankerite and chlorite. Hypogene ore grades range from 1 to 7 g t^?1 Au and up to 18 g t^?1 Au at the Kuzhubao gold deposit and are generally less than 3 g t^?1 Au at the Bashishan gold deposit. Sub‐microscopic gold mineralization is associated with finely disseminated arsenic‐rich pyrite in the Stage III mineral assemblage. Two types of primary fluid inclusions have been recorded: Type I liquid–vapour inclusions with moderate‐to‐high liquid/vapour ratios, and Type II inclusions containing moderate liquid/vapour ratios with CO₂ as determined from laser Raman analysis. Temperature of homogenization (T_h) data collected from these primary fluid inclusions in gold‐ore Stage III quartz ranged from 180 to 275°C at the Kuzhubao gold deposit and 210 to 330°C at the Bashishan gold deposit. Salinity results indicate that there were possibly two fluids present during gold deposition, including: (i) an early fluid with 0.8–6.5 wt.% NaCl equivalent, similar to salinity in shear‐zone‐hosted gold deposits with metamorphic derived fluids; and (ii) a late fluid with 11.8–13.4 wt.% NaCl equivalent, indicating possible derivation from connate waters and/or brine sources. CO₂ and trace CH₄ were only detected by laser Raman spectrometry in gold‐ore‐stage primary fluid inclusions. Results of sulphur isotope studies showed that δ³⁴S values for pyrite and arsenopyrite associated with gold‐ore mineralization during Stage III at the Kuzhubao and Bashishan gold deposits are isotopically similar and moderately heavy with a range from +9 to +15 per mil, and also fall into the range of δ³⁴S values reported for Carlin‐type gold deposits. Sulphur isotopes suggest that the Fu Ning gold deposits were formed from connate waters and/or basinal brines. Fluid geochemistry data from the Fu Ning gold deposits suggest a Carlin‐type genetic model, involving fluid mixing between: (i) deep CO₂‐rich metamorphic fluids, (ii) moderately saline, reduced connate waters and/or basinal brines; and (iii) evolved meteoric waters.     

Experimental study of aqueous fluid infiltration into quartzite: implications for the kinetics of fluid redistribution and grain growth driven by interfacial energy reduction

To investigate the kinetics of interfacial energy‐driven fluid infiltration, experiments were carried out in a quartzite–water system at 621–925°C and 0.8 GPa. Infiltration couples were made by juxtaposing presynthesized dry quartzite cylinders and fluid reservoirs. The infiltration process was confirmed by the presence of pores at the quartzite grain edges. As predicted from theoretical considerations and previous experiments, wetting fluids such as pure water and NaCl aqueous solution infiltrated into quartzite, whereas nonwetting CO₂‐rich fluids did not. Newly precipitated quartz layers at the surfaces of the infiltrated sample proved that infiltration took place by a dissolution–precipitation mechanism. The enhancement of grain growth by fluid infiltration was observed over the entire range of experimental temperatures. The fluid fraction, gauged by the porosity of the run products, increases at the infiltration front and then decreases towards the fluid reservoir to form a high‐porosity zone with a maximum porosity of 2.3–2.9%. As infiltration proceeds, the high‐porosity zone advances like a travelling wave. This porosity wave is probably caused by a grain curvature gradient resulting from preferential grain growth in the infiltrated part of the quartzite, perhaps combined with other factors. The infiltration kinetics were modelled with a steady‐state diffusion model over the high‐porosity zone. The solubility difference between dissolving and precipitating grains was deduced to be 2 × 10^?2?3 × 10^?1 wt %. The experimentally obtained infiltration rate of aqueous fluid in the steady‐state diffusion regime (2 ± 0.5 × 10^?8 m sec^?1 at 823°C) is much faster than the estimated metamorphic fluid flux rates, so that interfacial energy‐driven fluid redistribution in quartz‐rich layers could significantly contribute to the fluid flux in high‐grade metamorphism, at least over a short distance. Cathodoluminescence observations of the run products revealed that the grain growth of quartzite in the presence of fluid proceeds extensively, which would promote the chemical equilibration between fluid and rock more effectively than would volume diffusion in quartz crystals.     

Retrograde P–T evolution and high temperature–low pressure fluid circulation in relation to late Hercynian intrusions: a mineralogical and fluid inclusion study of the Charroux-Civray plutonic complex (north-western Massif Central, France)

The calc‐alkaline plutonic complex from Charroux‐Civray (north‐western part of the French Massif Central) displays multiphase hydrothermal alteration. Plutonic rocks, as well as early retrograde Ca–Al silicate assemblages, which have crystallized during cooling and uplifting of the plutonic series, are affected by multiphase chlorite–phengite–illite–carbonate alteration linked to intense pervasive fluid circulation through microfractures. The petrographic study of alteration sequences and their associated fluid inclusions in microfissures of the plutonic rocks, as well as in mineral fillings of the veins, yields a reconstruction of the P–T–X evolution of the Hercynian basement after the crystallization of the main calc‐alkaline plutonic bodies. This reconstruction covers the uplift of the basement to its exposure and the subsequent burial by Mesozoic sediments. Cooling of the calc‐alkaline plutonic series started at solidus temperatures (～650°C), at a pressure of about 4 kbar (1 bar = 10⁵ N m^?2), as indicated by magmatic epidote stability, hornblende barometry and fluid inclusion data. Cooling continued under slightly decreasing pressure during uplift down to 2–3 kbar at 200–280°C (prehnite–pumpellyite paragenesis). Then, a hot geothermal circulation of CO₂‐bearing fluids was induced within the calc‐alkaline rocks leading to the formation of greisen‐like mineralizations. During this stage, temperatures around 400–450°C were still high for the inferred depths (～2 kbar). They imply abnormal heat flows and thermal gradients of 60–80°C km^?1. The hypothesis of the existence of one large or a succession of smaller peraluminous plutons at depth, supported by geophysical data, suggests that localized heat flows were linked to concealed leucogranite intrusions. As uplift continued, greisen mineralization was subsequently affected by the chlorite–phengite–dolomite assemblage, correlated with aqueous and nitrogen‐bearing fluid circulations in the temperature range of 400–450°C. In a later stage, a continuous temperature decrease at constant pressure (～0.5 kbar) led to the alteration of the dolomite–illite–chlorite type in the 130–250°C temperature range.