Near-surface Settings Research Articles

Fluid-rock interaction processes in the Te Kopia geothermal field (New Zealand) revealed by SEM-CL imaging

Scanning electron microscopy-cathodoluminescence (SEM-CL) imaging of hydrothermal quartz exposed by weathering in the Te Kopia geothermal field (New Zealand) has revealed a history of crystal growth, dissolution, overprinting and fracturing that cannot be detected using other observational techniques (e.g. transmitted or reflected light microscopy, back-scattered electron imaging or secondary electron imaging). The crystals initially grew as CL-dark quartz, at least 350 m below their present location on the Paeroa Fault scarp, in a neutral pH, 215±10 °C liquid reservoir (inferred from the analysis of primary liquid fluid inclusions: mean T h of 213 °C; 0.2–0.4 wt.% NaCl eq.). Relict quartz–adularia–illite alteration occurs at the surface, in the vicinity of the quartz crystals, and in drillcores from the nearby TK-1 exploration well. Repeated movement on the Paeroa Fault uplifted pyroclastic rocks hosting the quartz crystals, but also provided pathways for “pulses” of hot fluids to move through the system. Quartz precipitation occurred at the edge of the crystals as the reservoir fluids cooled, as indicated by micron-scale alternating CL-dark/CL-bright quartz growth bands, which contain fluid inclusions with T h values of 210±40 °C. Pressure fluctuations were the likely cause of dissolution, marked by corroded crystal edges, with subsequent precipitation of quartz into open space. SEM-CL imaging shows that the quartz crystals contain healed fractures, which trapped low salinity fluids with T h values of 201±6 °C. Low-pH fluids in the near-surface setting also rounded the quartz crystals, and coated them with kaolinite and CL-grey amorphous “silica residue”.

Fluid-rock interaction processes in the Te Kopia geothermal field (New Zealand) revealed by SEM-CL imaging

Scanning electron microscopy-cathodoluminescence (SEM-CL) imaging of hydrothermal quartz exposed by weathering in the Te Kopia geothermal field (New Zealand) has revealed a history of crystal growth, dissolution, overprinting and fracturing that cannot be detected using other observational techniques (e.g. transmitted or reflected light microscopy, back-scattered electron imaging or secondary electron imaging). The crystals initially grew as CL-dark quartz, at least 350 m below their present location on the Paeroa Fault scarp, in a neutral pH, 215±10 °C liquid reservoir (inferred from the analysis of primary liquid fluid inclusions: mean T h of 213 °C; 0.2–0.4 wt.% NaCl eq. ). Relict quartz–adularia–illite alteration occurs at the surface, in the vicinity of the quartz crystals, and in drillcores from the nearby TK-1 exploration well. Repeated movement on the Paeroa Fault uplifted pyroclastic rocks hosting the quartz crystals, but also provided pathways for “pulses” of hot fluids to move through the system. Quartz precipitation occurred at the edge of the crystals as the reservoir fluids cooled, as indicated by micron-scale alternating CL-dark/CL-bright quartz growth bands, which contain fluid inclusions with T h values of 210±40 °C. Pressure fluctuations were the likely cause of dissolution, marked by corroded crystal edges, with subsequent precipitation of quartz into open space. SEM-CL imaging shows that the quartz crystals contain healed fractures, which trapped low salinity fluids with T h values of 201±6 °C. Low-pH fluids in the near-surface setting also rounded the quartz crystals, and coated them with kaolinite and CL-grey amorphous “silica residue”.

4D seismic monitoring of the miscible CO2 flood of Hall-Gurney Field, Kansas, U.S.

Time-lapse seismic monitoring of enhanced oil recovery (EOR) programs has been described by several authors over the last 15 years. Advances in equalization techniques have allowed legacy data (in some cases 15 years prior to a monitoring survey) to be used as a baseline. Sensitivity of seismic data to subtle changes in pore composition has been notably improved with the development of dozens of new attribute analysis techniques in the last several years. Unique to our application is the use of nonamplitude, noninversion attributes for monitoring the effectiveness of EOR in thin, shallow (less than 1000 m) carbonates. Considering the very thin (< 5 m) reservoir interval, this first-time, high-resolution 4D survey needed to be highly repeatable, low-cost, high signal-to-noise, and include several repeat surveys prior to breakthrough. Devel-opment of a highly accurate map of CO2 progression through this field was complicated by difficulties propagating high-frequency signal in this near-surface setting, maintaining uniform fold coverage throughout the optimum offset range, vulnerability of signal to contamination by ground roll and air- coupled wave within the noise cone, weak fluid effects due to high carbonate stiffness, and complexity in porosity distribution. Primary and secondary production phases of the 70-year-old Hall-Gurney Field are nearly done. The Pennsylvan-ian Lansing-Kansas City (L-KC) groups have yielded 90 million bls of the 155 million bls cumulative production in the multipay field. Primary production was begun in 1931 and was followed by extensive waterflooding in the 1950s–60s. Waterfloods reached their economic limits in the 1970s–80s but bypassed oil represents a significant resource for CO2 miscible flooding, a third development phase. Reservoir rock of the Hall-Gurney oil field consists of shallow (about 900 m), thin (3.5-5 m), oomoldic limestone layers in L-KC with largely moldic porosities that decrease downward from around 35% to 12%. This reservoir is …

Determining Q of near-surface materials from Rayleigh waves

High-frequency (≥2 Hz) Rayleigh wave phase velocities can be inverted to shear (S)-wave velocities for a layered earth model up to 30 m below the ground surface in many settings. Given S-wave velocity ( V S), compressional (P)-wave velocity ( V P), and Rayleigh wave phase velocities, it is feasible to solve for P-wave quality factor Q P and S-wave quality factor Q S in a layered earth model by inverting Rayleigh wave attenuation coefficients. Model results demonstrate the plausibility of inverting Q S from Rayleigh wave attenuation coefficients. Contributions to the Rayleigh wave attenuation coefficients from Q P cannot be ignored when Vs/ V P reaches 0.45, which is not uncommon in near-surface settings. It is possible to invert Q P from Rayleigh wave attenuation coefficients in some geological setting, a concept that differs from the common perception that Rayleigh wave attenuation coefficients are always far less sensitive to Q P than to Q S. Sixty-channel surface wave data were acquired in an Arizona desert. For a 10-layer model with a thickness of over 20 m, the data were first inverted to obtain S-wave velocities by the multichannel analysis of surface waves (MASW) method and then quality factors were determined by inverting attenuation coefficients.

Petrogenesis of silicate inclusions in the Weekeroo Station IIE iron meteorite: differentiation, remelting, and dynamic mixing

The Weekeroo Station IIE iron meteorite contains a variety of felsic and mafic inclusions enclosed in an FeNi-metal host. Petrographic, EMP, and SIMS data suggest that the petrogenesis of the silicates was complex, and included differentiation, remelting, FeO-reduction, and dynamic mixing of phases. Differentiation produced a variety of olivine-free inclusion assemblages, ranging from pyroxene + plagioclase + tridymite with peritectic compositions, to coarse orthopyroxene, to plagioclase + tridymite and its glassy equivalent. Individual phases have similar trace-element abundances and patterns, despite large variations in inclusion textures, modes, and bulk compositions, probably as a result of mechanical separation of pre-existing phases in an impact event that dynamically mixed silicates with the metallic host. Trace-element data imply that augite and plagioclase grains in different inclusions crystallized from the same precursor melt, characterized by relatively unfractionated REE abundances of ∼20–30 × CI-chondrites except for a negative Eu anomaly. Such a precursor melt could have been produced by ∼2–5% equilibrium partial melting of an H-chondrite silicate protolith, or by higher degrees of partial melting involving subsequent fractional crystallization. Glass appears to have formed by the remelting of pre-existing plagioclase and orthopyroxene, in a process that involved either disequilibrium or substantial melting of these phases. During remelting, silicate melt reacted with the FeNi-metal host, and FeO was reduced to Fe-metal. Following remelting and metal-silicate mixing, inclusions apparently cooled at different rates in a near-surface setting on the parent body; glass- or pigeonite-bearing inclusions cooled more rapidly (≥2.5°C/hr between 1000–850°C) than pigeonite-free, largely crystalline inclusions. The results of this study point to two likely models for forming IIE iron meteorites, both involving collision between an FeNi-metal impactor and either a differentiated or undifferentiated silicate-rich target of H-chondrite affinity. Each model has difficulties and it is possible that both are required to explain the diverse IIE group.

Magnetic mineral assemblages and magnetic contrasts in diagenetic environments — with implications for studies of palaeomagnetism, hydrocarbon migration and exploration

Abstract Magnetic mineral assemblages in diagenetic environments are governed by microbial and inorganic/chemical processes. Empirical evidence and thermodynamic modelling of diagenetic environments ranging from near-surface settings to depths of about 6–7 km suggests that microbial processes are very important in surface and near-surface diagenetic settings, whereas inorganic diagenetic processes dominate in most deeper subsurface diagenetic settings. In all diagenetic environments, molecular oxygen and hydrocarbons are important because they control which types of bacteria can thrive and because they drive various redox reactions via changes in the redox potential. Iron oxides, such as hematite and goethite, are stable under relatively oxidizing/aerobic conditions. With decreasing redox potential, i.e. under relatively reducing/anaerobic conditions, magnetite and pyrrhotite are the most important stable magnetic minerals, whereas hematite is unstable. Other magnetic minerals, such as greigite, may be important in some instances. Pyrite and siderite are also important because they may form at the expense of magnetic minerals. A significant decrease in the redox potential in diagnetic settings, via the invasion of hydrocarbons during migration or seepage from traps, for example, almost invariability results in diagenetic remagnetization. As a result ‘positive’, ‘absent’ or ‘negative’ magnetic contrasts are generated relative to the total magnetization (or remanence, or susceptibility) before such a decrease, depending on the relative amounts of magnetic to non-magnetic minerals formed and destroyed. Thermodynamic modelling further suggests that magnetic contrasts are more probable and tend to become more ‘positive’ with depth and with proximity to hydrocarbon pools. Any significant change in the redox potential is likely to cause magnetic contrasts via the formation or destruction of magnetic minerals, thus generating secondary (chemical) magnetization and resetting original detrital or bacterial magnetization. The predicted magnetic contrasts are also likely to form as a result of hydrocarbon migration, and it appears possible to delineate migration timing and pathways by geomagnetic methods. Hence the predicted magnetic mineral assemblages can aid in the interpretation of palaeomagnetic studies, particularly via the recognition and proper interpretation of remagnetization. Magnetic contrasts, measured from the air, at the surface or on drill core, can be used for hydrocarbon exploration in association with other exploration methods.

Alteration of Early Dolomite by Meteoric Waters: Examples from Devonian and Mississippian Carbonates in Montana and Idaho

Regional studies of Devonian and Mississippian carbonates in Montana and Idaho illustrate meteoric alteration of early, near-surface dolomite. The early dolomites in these rocks formed in various near-surface settings. The best preserved early dolomites or remnants of them are nonstoichiometric, isotopically heavy, and trace element enriched. These dolomites also generally predate compaction fabrics and early calcite cements. Many of the early dolomites were altered by meteoric fluids in shallow subsurface environments (0-300 m burial depth); meteoric recharge occurred during long-term (10{sup 6}-10{sup 7} yr) episodes of regional subaerial exposure. Petrographically, the altered dolomites exhibit complex dissolution and/or replacement fabrics if precursor dolomites were incompletely replaced; homogeneous microfabrics characterize dolomites that have undergone total meteoric recrystallization or replacement. Dolomites with the strongest meteoric signature occur closest to recharge surfaces and typically are the most stoichiometric, have the most negative {delta}{sup 18}O values, and are trace element depleted. In some cases, meteoric flow paths can be mapped by contouring geochemical data from replacive dolomites. Depleted cerium contents ('negative Ce anomaly') also may characterize meteoric replacement dolomites. Retention of original fabrics and precursor geochemical signatures seems to be most prevalent in downflow regions, far from recharge surfaces. In downflow portions of paleoaquifers, groundwatersmore » most likely were saturated with respect to dolomite because of extensive dolomite dissolution near recharge areas. Early, pore-filling calcite cements in downflow regions also may seal off porosity, thus preventing reaction of the early, more soluble dolomites with later meteoric waters.« less

Sedimentology and Petrology of Tar Sands: ABSTRACT

The two largest tar sand provinces of the world are those of the Orinoco petroleum belt in Venezuela (estimated in-place reserves of 1,000 to 2,000 × 109 bbl) and the Mannville Group oil sand deposits of Alberta (1,350 × 109 bbl estimated inplace reserves). Possible additions to these ranks are the Russian deposits of the Melekess depression and the Olenek area (reserve figures very erratic), and the bitumen-bearing Paleozoic rocks of the Carbonate Triangle in Alberta (reserves tentatively estimated to be more than 1,000 × 109 bbl). Other significant accumulations including those in the United States (principally Utah and California), Madagascar, Albania, Romania, the Caribbean, and the Canadian Arctic are all markedly small r (reserves two or more orders of magnitude less). Elements common to most of the major accumulations include: stable tectonic settings, usually in large foreland basins; thermally immature and only moderately compacted reservoir sediments, commonly uncemented, with very high porosities and permeabilities; far-reaching hydrocarbon migration networks with access to organic-rich source beds; near-surface settings, within the realm of pervasive oil degradation by water washing and bacterial action; and complicated, often enigmatic trapping configurations, involving both stratigraphic and structural factors, with bitumen plugs perhaps serving locally as updip seals. The deposits in which the reservoirs are dominantly hydrophyllic (water wet), including those of both Venezuela and Alberta, are the most amenable to exploitation in that they can be processed by the inexpensive and relatively efficient hot-water extraction methods. Athabasca is the largest of Alberta's oil sand deposits. It encompasses a total area of 32,000 sq km and is estimated to contain 870 billion bbl of oil in place. The Lower Cretaceous McMurray Formation reservoir is 35 to 70 m thick of uncemented, very fine to fine-grained, moderately sorted quartz sand and associated shale. The most important control on the grade of the oil sands is the distribution of primary porosity and permeability in the McMurray Formation sediments. Achieving a cogent understanding of the facies patterns thus leads directly to a predictive capability regarding the geometry and character of the oil-bearing zones. Initial infilling of the McMurray depression appears to have developed in a wide variety of fluvial-deltaic environments, many of which are not yet fully understood. Subsequently, there developed a regime marked by the presence of deep channels, trending north and northwest, which locally incised the preexisting sedimentary sequences and deposited a characteristic, fining-upward cycle in many areas, particularly in the northern half of the deposit: trough cross-bedded channel-bottom sands at the base; giving way upward to solitary sets of epsilon cross-strata deposited on the sloping End_Page 1357------------------------------ flanks of channel-margin bars; passing upward into argillaceous sands of flood-plain origin. The channels which produced this sequence were up to 45 m deep and many hundreds of meters wide. Where they eroded and reworked the preexisting sedimentary pile, apparently along discrete meander-belt trends, they left behind a sand-dominated sequence that today constitutes some of the thickest and richest oil pay zones in the entire deposit. End_of_Article - Last_Page 1358------------