Changes In Pore Fluid Chemistry Research Articles

Reconstruction of the Diagenetic Environments of Tight Sandstone Reservoirs: A Case Study from the Tengger Formation in the Baiyinchagan Sag, Erlian Basin, Northern China

Abstract The Lower Cretaceous Tengger Formation located in the Baiyinchagan Sag of the Erlian Basin comprises mainly deeply buried tight sandstone. The identification of high-quality reservoirs in these thickly stacked and heterogeneous units requires a comprehensive understanding of the diagenetic environmental history of the rocks. This paper reports an integrated study involving thin-section petrography, scanning electron microscopy, X-ray diffraction, fluid-inclusion analysis, and vitrinite reflectance analysis of Tengger Formation sandstones with the aim of characterizing the diagenetic conditions of the reservoir rocks and providing guidance for future petroleum exploration. Observed mineral assemblages, the distribution of authigenic minerals, and the distribution and nature of pores suggest the presence of two types of diagenetic environment, acidic and alkaline, which have varied over time and vertically through the rock column. Acidic conditions are indicated by quartz overgrowths and dissolution of both feldspar and carbonate cement. In contrast, alkaline conditions are indicated by the precipitation of carbonate cement, feldspar overgrowths, quartz dissolution, and occurrences of authigenic illite and chlorite. Changes in pore fluid chemistry controlled the evolution of the diagenetic environment. The early diagenetic environment from 110 Ma to 107 Ma was syndepositional and thus controlled by the chemistry of water in depositional centers, which is interpreted to have been weakly alkaline. Significant burial that occurred at 107 Ma induced pulses of hydrothermal fluids and petroleum into the reservoir rocks, which caused a shift to an acidic diagenetic environment. From 103 Ma to 70 Ma, subsequent episodes of uplift and burial caused periodic alternation between acidic and alkaline diagenetic environments. Three distinct episodes of oil and gas charging interpreted from petrography and the homogenization temperatures of fluid inclusions played a critical role in the enhancement of porosity through time. From 70 Ma to the present, acidic diagenesis gradually weakened because of the consumption of organic acids during the process of interaction between rocks and fluids. This study demonstrates the importance of understanding the diagenetic history of reservoir rocks and provides the basis for improved reservoir characterization and optimized hydrocarbon exploration of the Tengger Formation.

How phyllosilicate mineral structure affects fault strength in Mg‐rich fault systems

AbstractThe clay mineralogy of fault gouges has important implications for the frictional properties of faults, often identified as a major factor contributing to profound fault weakness. This work compares the frictional strength of a group of Mg‐rich minerals common in the Mg‐Al‐Si‐O compositional space (talc, saponite, sepiolite, and palygorskite) by conducting triaxial frictional tests with water or argon as pore fluid. The studied minerals are chemically similar but differ in their crystallographic structure. Results show that fibrous Mg‐rich phyllosilicates are stronger than their planar equivalents. Frictional strength in this group of minerals is highly influenced by strength of the atomic bonds, continuity of water layers within the crystals, and interactions of mineral surfaces with water molecules, all of which are dictated by crystal structure. The formation and stability of the minerals studied are mainly controlled by small changes in pore fluid chemistry, which can lead to significant differences in fault strength.

Iron biomineralization controls on geophysical signatures of hydrocarbon contaminated sediments

The interpretation of geophysical data from mature hydrocarbon contaminated sites has relied on a conductive plume model where the conductivity of the subsurface contaminant volume is the result of microbial mediated changes in pore fluid chemistry. This conductive anomalous region is characterized by high total dissolved solids and occurs within the water table fluctuation zone where microbial activity is the maximum. Here we update this conductive plume model by providing new insights from recent laboratory investigations and geophysical data from hydrocarbon contaminated sites suggesting the unrecognized role of the impact that microbial-mediated metallic mineral precipitates have on geophysical signatures. We show that microbial redox processes (e.g., iron and sulfate reduction) during the biodegradation process involve mineralogical transformations and the precipitation of new minerals (e.g., magnetite, and pyrite) that can impact the electrical and magnetic properties of contaminated sediments. We provide examples from laboratory experiments and field studies and suggest that knowledge of the dominant redox processes occurring at hydrocarbon contaminated sites and the mineral phases formed is critical for a more robust interpretation of geophysical data associated with microbial-mediated changes at hydrocarbon contaminated sites. We also show that integration of both magnetic and electrical techniques may help reduce ambiguity in data interpretation.

Influence of Hyper-Alkaline pH Leachate on Mineral and Porosity Evolution in the Chemically Disturbed Zone Developed in the Near-Field Host Rock for a Nuclear Waste Repository

This paper evaluates the effect of hyper-alkaline (NaOH/KOH) leachate on the mineralogy and porosity of a generic quartzo-feldspathic host rock for intermediate- and low-level nuclear waste disposal following permeation of the cementitious repository barrier by groundwater. The analysis is made with reference to expected fluid compositions that may develop by contact of groundwater with the cementitious barrier to form a chemically disturbed zone (CDZ) in the adjacent host rock, as informed by relevant natural analogue sites. Theoretical analysis and numerical modelling is used to explore the influence of different host rock mineral assemblages on changes in pore fluid chemistry, multiple mineral dissolution and precipitation reactions and matrix porosity within the CDZ under these conditions. The numerical modelling accounts for kinetic and surface area effects on the mineral transformation and porosity development for periods of up to 10,000 years travel time from the repository and ambient temperature of \(20\,^{\circ }\hbox {C}\). The analysis shows that dissolution of quartz, feldspar and muscovite in the host rock, by the hyper-alkaline waste leachate, will create relatively high concentrations of dissolved Si and Al in the pore fluid, which migrates as chemical fronts within the CDZ. Precipitation of secondary mineral phases is predicted to occur under these conditions. The increase in matrix porosity that arises from dissolution of primary aluminosilicate minerals is compensated by a reduction in porosity due to precipitation of the secondary phases, but with a net overall increase in matrix porosity. These coupled physical and geochemical processes are most important for contaminant transport in the near-field zone of the CDZ and are eventually buffered by the host rock within 70 m of the repository for the 10,000 year travel time scenario. The predicted changes in matrix porosity may contribute to increased transport of radionuclides in the host rock, in the absence of attenuation by other mechanisms in the CDZ.

Relationships between fluid‐rock interactions and the electrical conductivity of sandstones

Batch and flow‐through experiments were performed on quartz‐feldspar granular aggregates and sandstone samples to investigate time‐dependent effects of fluid‐rock interactions on fluid and rock conductivity, respectively. The experiments were conducted at temperatures up to 164 °C, at confining and pore pressures up to 10 and 5 MPa, respectively, and for up to 136 days. It showed that changes in rock conductivity were unequivocally related to changes in pore fluid conductivity. It is inferred that these changes were dependent on kinetically controlled dissolution reactions between the mineral grains and the fluid. The time‐dependent signature of rock conductivity implied a detectable transition from initial dissolution toward some state of equilibrium. The response of rock conductivity to temperature changes followed an Arrhenius‐type behavior. An exploratory kinetic evaluation of the conductivity data for sandstone samples yielded an apparent activation energy of approximately 32 kJ/mol. A concurrent chemical fluid analysis showed that this is an integrated value over all reactions occurring in parallel within a sample. These reactions namely concern silica and silicate dissolution but also the dissolution of accessory salt minerals. It is concluded that measuring the evolution of rock conductivity in combination with chemical pore fluid analysis constitutes a powerful and quantitative tool for monitoring time‐dependent changes in pore fluid chemistry and thus fluid‐rock interactions in real time.

Coupled thermo-hydro-mechanical–chemical behaviour of cemented paste backfill in column experiments. Part I: Physical, hydraulic and thermal processes and characteristics

Cemented paste backfill (CPB) technology has been widely used to fill underground mine voids for underground mine support and/or tailings disposal. Once placed, the CPB structure is subjected to strong coupled thermal (T), hydraulic (H), mechanical (M) and chemical (C) processes. In this paper, an experiment with insulated-undrained high columns has been carried out to understand these THMC processes. A CPB mixture is loaded into two columns and instrumented with various sensors to monitor the evolution of temperature, pore water pressure and suction for a period of 150days. In addition, four other CPB columns are cured at 7, 28, 90 and 150days, and then extensive laboratory testing is carried out on the CPB samples with regards to their thermal conductivity, saturated hydraulic conductivity, water retention properties, and mechanical and physical properties. Also, the rate of evaporation is monitored in a cylindrical CPB sample for the entire period of study. The results obtained show that a higher temperature can result in faster microstructural refinement and hence lower fluid transport ability. Reduction in the degree of saturation decreases thermal conductivity. The hydraulic properties are strongly coupled to chemical and mechanical factors. Mechanical deterioration following surface shrinkage and cracks can increase the saturated hydraulic conductivity. Also, changes in pore fluid chemistry affect microstructural evolution and can influence physical properties, such as void ratio and degree of saturation. Moreover, the obtained results support that the THMC properties of CPB are strongly coupled due to several internal mechanisms, such as heat of hydration, self-desiccation, suction development and cement hydration. The findings can contribute to a better understanding on the behaviour of CPB and thus towards the designing of more cost-effective and durable CPB structures. The results of the evolution of the mechanical, chemical and microstructural processes are discussed in a companion paper (see Part II, this issue).

Evaluation of the impact of pore fluid chemistry on the hydromechanical behaviour of clay-based sealing materials

The results of an experimental program undertaken to evaluate the impact of pore fluid salinity on the hydromechanical performance of light and dense backfill materials are presented. Light and dense backfills are engineered barrier materials that are being examined in the Canadian concept for storage of spent fuel in a deep geological repository. The current research investigates the impact of pore fluid chemistry on the swelling, compressibility, stiffness, and hydraulic conductivity parameters of light and dense backfills that are required as material parameters for analysis and design. In these tests, pore fluid chemistry was selected to represent groundwater within potential host units including granite and limestone rock. Results show that the performance of light backfill is significantly affected by changes in pore fluid chemistry. The swell potential of light backfill decreases with increasing salinity of the solution. The hydraulic conductivity decreases with increasing effective montmorillonite dry density and specimens saturated with saline solution have higher hydraulic conductivity than those saturated with distilled water. Conversely, the behaviour of dense backfill is governed mainly by the crushed granite component and therefore changes to the pore fluid chemistry have relatively little effect. Results of dense backfill tests confirm the material performance as a sealing material.

Desorption response and degradation of buffering capability of bentonite, subjected to heavy metal contaminants

Changes in pore fluid chemistry due to leachate intrusion will change the physico-chemical properties and the attenuation performance of clay barrier materials. In the field, changes in the pore fluid chemistry will occur as a direct result of leachate intrusion from waste streams, discharges, leaking underground storage facilities, and waste disposal facilities – to name a few. This study investigates bentonite–contaminant interaction at different pH levels and heavy metals concentrations, with particular attention paid to the effect these have on the chemical behaviour of the bentonite soil. For chemical performance, the sorption characteristics of these specimens in relation to different pore fluid chemistries were studied. A series of batch equilibrium tests were performed at different concentrations of Zn 2+ and Pb 2+ and different pH levels to study the sorption properties of bentonite.

Contaminant‐induced mechanical damping in partially saturated Berea sandstone

We have measured mechanical damping in partially saturated Berea sandstone that is strongly dependent on the presence of a small amount of oil. This effect is observed as a function of water saturation and average strain amplitude. These observations are presented as evidence of a damping mechanism previously observed and characterized in artificial cracks. We conclude that this damping effect is due to surface chemistry changes in the rock, and infer that seismic attenuation can be used to monitor small changes in pore fluid chemistry under certain conditions.

Resistivity imaging of soil during electrokinetic transport

Electrical resistance imaging of soil specimens during electrokinetic treatment is reported. Column experiments were carried out on Speswhite kaolinite contaminated with lead nitrate to levels both above and below its cation exchange capacity (CEC). Post test chemical analyses of the specimens and their pore fluids show that resistivity variations correlate with changes in pore fluid chemistry but do not show the extent of decontamination. Regions of high resistivity correspond with precipitation zones within the specimens whereas regions of low resistivity correspond with regions of high pore fluid ionic strength. Where the contamination level is below the CEC, decontamination is slow as lead ions are mostly sorbed to the clay so most of the current is carried by electrolysis products and clay dissolution products. A broad resistive zone forms over the cathode half where hydroxyl and HCO − 3 ions formed in the cathode reservoir precipitate clay with dissolution products and other ions. Where the contamination level is above the CEC, lead ions are initially major charge carriers and decontamination over the bulk of the specimen is rapid. However, lead still precipitates immediately adjacent to the cathode reservoir to form a narrow resistive region.

A review of the influence of clay–brine interactions on the geotechnical properties of Ca-montmorillonitic clayey soils from western Canada

Natural Ca-montmorillonite clay soils or engineered clay barriers in western Canada are often used to contain brine generated from the surface disposal of potash tailings or from drilling activities associated with the oil and gas industry. The performance of these barriers has ranged from excellent to poor. The influence of salt brines on the geotechnical properties of these soils has been recognized as a potentially important factor for some time. It has been well documented in the literature that the behavior of clayey soils is strongly influenced by physicochemical interactions between clay particles and pore-fluid chemistry; consequently, the properties of these soils are sensitive to changes in the electrolyte concentration of the pore fluid. An increase in the concentration of the pore fluid to the levels of a concentrated brine can cause significant changes in the geotechnical properties of the soil. In this paper, the impact of brine contamination on the geotechnical properties of two Ca-montmorillonitic clayey soils of glacial origin from western Canada is reviewed. The influence of clay–brine interactions on the index properties (liquid limit, plastic limit, plastic index, mineralogy, density, grain size, and compaction characteristics), mechanical properties (volume change and shear strength), and hydraulic properties (hydraulic conductivity) is described. A quantitative explanation for the changes that occur in the hydraulic and mechanical properties of these soils as a result of brine permeation is also provided. This explanation relates the changes in pore-fluid chemistry to changes in an effective physicochemical stress state. This approach may be used to predict the changes in hydraulic conductivity, volume, or shear strength of a clayey soil as a result of brine contamination. Key words : clay–brine interactions, diffuse double layer, hydraulic conductivity, soil structure, physicochemical.

Mechanisms of osmotic flow and volume change in clay soils

The mechanical behavior of compressible clay soils may be strongly influenced by physicochemical effects when concentrated pore fluids are introduced to the soil. Conceptual models have been used to explain the influence of pore fluid chemistry on the mechanical behavior of clays in a qualitative way. In this paper an alternate macroscopic description of the osmotic volume change behavior of a clay soil undergoing changes in pore fluid chemistry is provided.Theoretical descriptions of two potential mechanisms of osmotic volume change (osmotic consolidation and osmotically induced consolidation) are presented. Osmotic consolidation occurs as a result of a change in the electrostatic repulsive-minus-attractive stresses, R — A, between clay particles. Osmotically induced consolidation occurs because of fluid flow out of the clay in response to osmotic gradients.A numerical simulation is used to demonstrate the characteristic behavior of a clay soil undergoing either of these volume change processes. The results of a laboratory testing program on two clay soils exposed to concentrated NaCl solutions are used to illustrate that the dominant mechanism of osmotic volume change in surficial clay soils is osmotic consolidation. Key words: physicochemical, osmosis, volume change, NaCl salt, montmorillonite, clay, stress state variables, R – A.

Petrology, Diagenesis, and Reservoir Quality of Lower Cretaceous Kuparuk River Formation Sandstone, Kuparuk River Field, North Slope, Alaska: ABSTRACT

The Kuparuk River formation consists of upper and lower members separated by an intraformational unconformity. Marine sandstone in each is distinct in terms of depositional environments, sand-body geometry, texture, composition, diagenesis, and reservoir quality. Sandstone in the upper member is very fine to very coarse-grained sublitharenite to lithic arenite with an average quartz-feldspar-lithic (QFL) of 75-1-24. Glauconite constitutes 10-50% of framework grains. Chert, muscovite, heavy minerals and mudstone, limestone, siderite, and metasedimentary rock fragments are less abundant. The diagenetic sequence is: aragonite or high-Mg calcite-collophane-pyrite-siderite-ankerite-calcite-(dissolution of carbonate cements and glauconite)-quartz-kaolinite-illite/smectite-pyrite. Sandstone in the lower member is very fine to fine-grained quartz arenite to subarkose with an average Q-F-L of 92-5-3. Mudstone fragments, chert, muscovite, heavy minerals, and glauconite are less abundant. The diagenetic sequence is: pyrite-siderite-ankerite-calcite-(dissolution of ankerite and feldspar)-quartz-kaolinite-illite/smectite-pyrite. Early diagenesis in upper and lower member sandstones is different, whereas burial diagenesis is similar. Early siderite cemented sandstones in the upper member but did not significantly affect sandstones in the lower member. Subsequent changes in pore fluid chemistry during burial resulted in precipitation of the cement sequence siderite-ankerite-calcite in both upper and lower member sandstones. Stable isotope trends in carbonate cements parallel those of cement texture and composition. Upper member porosity (mostly secondary) and permeability average 23% and 130 md, with upper limits of 28-33% and 500-1,500 md, respectively. Reservoir quality is heterogeneous and controlled by grain size, distribution of primary and secondary porosity, and fractures. Both horizontal and vertical permeability are similar except where fractures enhance horizontal permeability. Lower member porosity (mostly primary) and permeability average 23% and 100 md, with upper limits of 28-30% and 400-500 md, respectively. Reservoir quality is homogeneous. Ankerite locally eliminates porosity, and shale beds and laminations reduce vertical permeability. End_of_Article - Last_Page 664------------

Early Diagenetic Dolomite Cements: Examples from the Permian Lower Magnesian Limestone of England and the Pleistocene Carbonates of the Bahamas

ABSTRACT Early diagenetic dolomite cements are found in the Permian Lower Magnesian Limestone of eastern England and the Pleistocene carbonates of Great Abaco Island, Bahamas. In the Lower Magnesian Limestone the dolospar occurs as isopachous rinds that have grown both centripetally and centrifugally on the dolomitized micrite envelopes of hollow-centered allochems. Because compactional fractures penetrate both the allochem walls and the cement rinds, dolomite cementation is inferred to have predated any significant sediment accumulation. In the Great Abaco carbonates, limpid dolomite forms isopachous rinds around inter- and intragranular porosity, and is postdated by meteoric calcite cement. Both examples of early diagenetic dolomite cement textures developed as a consequence of changes in pore-fluid chemistry from marine, to mixed meteoric-marine, to meteoric waters. Therefore, the presence of this cement type may point toward aliagenetic environments resulting from marine regressions.

Chert-Chalk Diagenesis: The Miocene Island Slope of North Jamaica

ABSTRACT During late Miocene time the central part of the north Jamaican coast consisted of a carbonate bank bordered to the north by a slope on which pelagic chalk accumulated. Deformation between late Miocene and middle Pleistocene time uplifted the bank edge, subaerially exposing some of the slope deposits of cherry chalk, and down-faulted the lower part of the slope deeper into the Cayman Trench. Chalk from the down-faulted part of the Miocene slope has never been subaerially exposed, and has undergone very little chemical alteration. In contrast, subaerially exposed chalk has undergone varying degrees of chemical alteration. Except for depletion in 13C, vadose chalk diagenesis yields a chemical product nearly identical to some limestones recovered by JOIDES and produced during arine burial diagenesis. Chert from the down-faulted part of the slope consists of opal-CT porcellanite nodules. In contrast, subaerially exposed nodules are composed of quartz with a similar isotopic composition to the opal-CT precursor. It is hypothesized that the porcellanite quartz conversion was initiated either by temperature increase as the Miocene slope was uplifted through the warm surface waters of the ocean, or by changes in pore fluid chemistry once interstitial sea water was replaced with meteoric water. In either case, the isotopic composition of the microquartz nodules in this case appears to record the conditions of formation of the opal-CT precursor.