Mixed-layer Clays Research Articles

Evidence of a Precursor in the Neoformation of Palygorskite — New Data by Analytical Electron Microscopy

AbstractThe rocks of the palygorskite deposit at Bercimuel (Segovia, Spain) have been studied by transmission electron microscopy and microanalysis. These rocks correspond to the zone of convergence of two alluvial fan systems that have filled the small basin of the River Riaza and would originally have been composed of illite and quartz silts. Among the accumulations of palygorskite it is possible to observe surrounded particles of micromicas that have undergone dissolution and opening thereby giving rise to disordered illite-smectite mixed-layer clays. This process continued up to the individualization of structural relics formed of units of 1–5 layers. At the same time, the chemical composition was modified with a loss of K and Al and a relative increase in Si and Mg, progressively evolving towards the composition of palygorskite. The paleogeographic position of the deposits, and the climatic conditions (arid environment), appear to be the dominant factors in the neoformation of palygorskite by alteration of the original sediments. The basic mineralogical process could be referred to as ‘early diagenesis’ in the formation of calcretes.

The Cretaceous‐Tertiary fireball layer, ejecta layer and coal seam: Platinum‐group element content and mineralogy of size fractions

Abstract— Grain size separates of the Cretaceous‐Tertiary (K‐T) fireball layer at marine sites and the entire K‐T boundary interval at nonmarine sites (fireball layer, ejecta layer, coal seam) were analysed for platinum‐group elements (PGE: Ru, Ir, Ft, Pd, Rh) and Au using inductively coupled plasma mass‐spectrometry. X‐ray diffractometry and scanning electron microscopy were performed on each fraction to establish the mineralogy. It was determined that the most abundant minerals in the finest fraction were a smectite‐group mineral at marine K‐T sites and illite‐smectite mixed layer clay at nonmarine K‐T sites. Positive correlation of PGE content and mineralogy indicates that the PGE are presently associated with these clay minerals which were likely formed by alteration of a condensed mafic phase (e.g., olivine or pyroxene). The correlation of clay mineral abundance with PGE content is imperfect because not all of the clay minerals in the fireball layer are derived by alteration of the original condensed ejecta. In addition, undetected PGE host(s), present in trace amounts, are likely also present.

SWELLING BEHAVIOUR OF MIXED-LAYER CLAYS IN HYDROCARBON RESERVOIRS

The freshwater sensitivity of a variety of mixed-layer illite/smectite and chlorite/smectite clays has been studied with an Environmental Scanning Electron Microscope (ESEM) in order to determine whether all hydrocarbon reservoirs hosting these clays should be regarded as freshwater sensitive. Experimental procedures involved closely comparing in situ mixed-layer clay within selected areas at high magnification before and after prolonged freshwater treatments.The response of smectite-rich illite/smectite (two samples; 10–20 per cent illite interlayers) to freshwater immersion varied greatly. One sample rapidly swelled to many times its original volume to form a pervasive gel that greatly reduced porosity and permeability, whereas the other sample underwent only a subtle morphological change that had little or no adverse effect on reservoir quality. Illite-rich illite/smectite (three samples; 65—>85 per cent illite interlayers) and chlorite-rich chlorite/smectite (two samples; 50 per cent and >70 per cent chlorite interlayers) underwent no morphological change when immersed in freshwater for up to three months, indicating that these clays in sandstones are unlikely to cause formation damage due to swelling if exposed to freshwater-based fluids.

Porosity Reduction and Major Controls on Diagenesis of Cretaceous-Paleocene Volcaniclastic and Arkosic Sandstone, Middle Park Basin, Colorado

ABSTRACT Thin-section and X-ray diffraction analyses of 31 outcrop samples show that the early diagenesis of heterogeneous volcaniclastic and arkosic sandstone and conglomerate of the Middle Park Formation (Upper Cretaceous- Paleocene) was strongly influenced by detrital mineralogy. Rocks with abundant andesitic rock fragments experienced the most compaction and are characterized by an early generation of authigenic chloritic mixedlayer clays and relatively minor illite-smectite and chlorite, whereas rocks with moderate to minor percentages of volcanic rock fragments experienced less compaction than volcanic-rich rocks and are characterized by earlygeneration authigenic illite and lesser illite-smectite, chlorite, and chloritic mixed-layer clay. Compaction and cementation occluded most macropo osity in those samples richest in volcanic detritus, thereby sealing the rocks and preventing the development of later-stage cements or dissolution porosity. Unlike that of clay minerals, the distribution of authigenic heulandite was not influenced by variations in detrital mineralogy. This suggests significant calcium-ion transport during early diagenesis and shows that intervals rich in unstable constituents can influence the diagenesis of adjacent intervals without large percentages of plagioclase or volcanic rock fragments. Late diagenesis was strongly influenced by structural deformation that fractured, folded, and subaerially exposed the rocks. Fracturing may have caused local drops in pore-fluid pressure, causing laumontite to replace heulandite as the stable calcic zeolite. Penetration of the fractured rocks by oxidizing, slightly acidic, dilute meteoric water altered the pore-fluid chemistry and caused precipitation of minor percentages of illite-smectite, kaolinite, and hematite. Macroporosity is less than 5% in all but two samples. The Middle Park Formation illustrates several factors that can adversely affect porosity during diagenesis of volcaniclastic sandstone: (1) lack of early leaching by fresh meteoric water, (2) a prolonged episode of compaction before the first authigenic minerals precipitated, (3) cementation by first-generation clays and heulandite occluding most pores that had survived compaction, (4) minor burial dissolution, (5) minor late-stage cements that further reduced porosity and occluded multigrain fractures, and (6) absence of dissolution of framework grains or cement during uplift and subaerial exposure.

Silica activity and the smectite-illite reaction

Early silicate diagenesis is characterized by the precipitation of minerals with high silica activity, such as opaline silica, smectite, and clinoptilolite, and by pore-water silica activities exceeding quartz saturation. With burial, this early, high-silica activity diagenesic facies is succeeded by a low-silica activity facies distinguished by quartz precipitation and progressive development of illite as the nonexpandable component of mixed-layer clays. We suggest that the smectite-illite reaction occurs as a consequence of the reduction of silica activity at the onset of quartz precipitation. Data from oceanic and sedimentary basins confirm that the smectite-illite reaction does not control aqueous silica activities. Where the smectite-illite reaction occurs at higher temperatures, aqueous silica activities likewise reach quartz saturation at higher temperatures.

An experimental evaluation of the reaction of granite with streamwater, seawater and NaCl solutions at 200°C

Abstract Experiments reacting granite with streamwater, seawater, and NaCl solutions have been conducted at 200°C, 50 MPa under batch conditions using direct-sampling autoclaves for durations up to 80 days. Granite-streamwater experiments at 10:1 and 2:1 water/rock ratios produced fluids of low TDS ( The reaction of granite with seawater produced a fluid of low pH (≈ 3.5), and principal changes in the fluid chemistry as follows: a gain of SiO2; and losses of sulphate, magnesium, and calcium. Appreciable mobilisation of heavy metals into the fluid phase such as Fe (up to 15 mg/l) and Mn (up to 2 mg/l) were noted. Solid precipitates consisted of anhydrite, caminite (magnesium hydroxide sulphate hydrate) and a mixed-layer smectite-chlorite clay. The low pH of the evolved fluid is attributed to the precipitation of the hydroxyl-consuming mineral phases, caminite and the smectite-chlorite clay. The reaction of granite with NaCl solutions (0.008 and 0.028 M) produced fluids which were similar to those of the streamwater experiments. However, pH was lower in the NaCl experiments (=7.4–7.8), so that the steady-state concentrations of a number of the cationic constituents of the fluids were slightly greater in these experiments compared with the streamwater experiments. The dominant constituent of the product fluids (excepting NaCl) was SiO2, the concentration of which was buffered by quartz solubility. The sole secondary solid product identified was a sparsely-developed illitic clay. The injection of a Li-Rb-Cs-Na-Cl fluid into the experiments after 1000 hours demonstrated that the concentrations of the rare alkalis in the fluids were controlled by non-equilibrium processes. Other chemical components perturbed by dilution returned to the steady states developed prior to the injection process, demonstrating buffering by metastable reactions with the rock. The steady-state composition of the fluids was adequately predicted using an assumption of chemical equilibrium between the principal minerals of the granite and thermodynamic data for pure mineral compositions.

Interstrat-An Expert System to Help Identify Interstratified Clay Minerals from Powder XRD Data: I. Description of the Program

AbstractThe INTERSTRAT program has been specifically designed to aid the geologist in the identification of the phyllosilicates from powder XRD data. The program achieves this by comparing the experimental00ld-spacings with a knowledge base of diffraction parameters; a set of likely solutions is then displayed. The method employed by INTERSTRAT is analogous to that used by the clay mineralogist whereby the clays are initially identified down to the group level from the d(OOl) data alone. This procedure also utilises thed-spacings collected from clays that have been subjected to standard clay mineral treatments, i.e. glycolation and heating to 300~ and 500~ One of the most important features of INTERSTRAT is its knowledge base of d-spacings for the interstratified clay minerals in which calculatedd(00l)spacings for an individual mixed-layer clay are recorded with respect to its Reichweite, the state under which the d-spacings were collected and the percentage of the mixed-layer components.

Geochemical and mineralogical characteristics of a power station feed-coal, Eggborough, England

Chemical and mineralogical analyses are presented for a power station feed-coal in the Yorkshire Coalfield, U.K. The mineralogy of the whole coal consists of quartz, kaolinite, illite, mixed-layer clay, chlorite, pyrite and carbonates. Pyrite, carbonates and some of the kaolinite are diagenetic. In separated, low-density fractions of the coal both pyrite and kaolinite are concentrated relative to the detrital minerals. Compared with the high density fractions the detrital clay mineralogy of these low density fractions contains less chlorite, and the mixed-layer clay to illite ratio is higher, which is consistent with a grain size control during sedimentation. The major element analyses are comparable with mudrocks in the stratigraphic sequence, but there is evidence of Na 2O enrichment related to connate brinnes and the Fe 2O 3 content shows significant enrichment, particularly in the ashes of the low density fractions. The dissolution of oxhydroxide grain coatings associated with detrital sediment in the coal, and also in the adjacent strata, are probable sources of the iron in the diagenetic pyrite. There is evidence of iron mobility in the sedimentary sequence and introduction into the coal itself during diagenesis. Within the coal, trace elements are also enriched (on an ash basis) relative to the mudrocks and their origin is also probably very similar to that of iron. A noteworthy feature of the trace element enrichment is that the enrichment factors for a number of elements are comparable with those determined for marine black shales in the sequence. This suggests a common origin for the elements during the diagenetic processes. Analysis of the coal density fractions, after correcting for detrital mineralogy, suggests Pb, Cu, Ni, Zn and possibly Mn are associated with pyrite and V, Sr, Ba, Zr, and Nb are either directly or indirectly associated with organic components. Possibly Zr and Nb originate from dispersed volcanic ash.

Composition, Diagenesis, and Morphology of Chlorite and Illite/Smectite Mixed-Layer Clays in the Cherry Canyon Formation, Delaware Mountain Group, Screwbean Field, Reeves County, Texas

Oil and gas production in the Screwbean field of Reeves County, Texas, is predominantly from the subarkosic Bell Canyon (Ramsey sand member) and upper Cherry Canyon sandstones of the Permian (Guadalupian) Delaware Mountain Group. Authigenic clays compromise up to 10% of the bulk rock and can seriously degrade the production potential and performance of reservoir rock. The chlorite and illite/smectite mixed-layer clays can have several effects on the reservoir: loss of permeability as a result of swelling, formation damage because of acid sensitivity, and high irreducible water saturations (bound water) caused by microporosity. Twenty-five powdered samples from whole core taken in the Cherry Canyon Formation from a well in Reeves County were analyzed by x-ray diffraction. The actual x-ray diffraction patterns from the powdered samples were compared to simulated x-ray diffraction patterns generated by a microcomputer. Once matched, the computer models give the fractional clay composition of that particular sample. The prominent morphology of the authigenic clays is also very important. Photomicrographs taken with a scanning electron microscope were employed to delineate the clay morphologies and illustrate the intergranular habits of these clay minerals. The failure to recognize the aforementioned problems can lead to prematurely abandoning and bypassing possible productivemore » zones. The data generated by this study will allow us to better use these reservoirs and more effectively explore future zones.« less

Dissolution-precipitation processes induced by hot water in a fractured granite Part 1: Wall-rock alteration and vein deposition processes

Mass transfer and fluid-rock interactions have been investigated in zoned alteration halos around hydrothermal veins which crosscut the La Peyratte granite (Armorican Massif, France). Temperature conditions have reached 340°C in the veins, according to a previous fluid-inclusion study. The width of the vein varies from 300 μm to 20 μm and the mineral composition of the deposit changes : phengite (300 to 200 μm); adularia (50 μm); quartz and feldspar overgrowths ( 50 μm wide) and plengite around the larger ones (300 μm wide). Both chlorite and albite determine sharp fronts inside the altered rock. The width of zone 1 is 4 mm for the plengite vein and 1 mm for the adularia one. Zone 2 extends from the chlorite front the unaltered rock; only dissolution voids are observed in oligoclase crystals. The width of zone 2 is 12 mm for the phengite vein and 10 mm for the adularia one. Illite/smectite mixed-layer clays have precipitated in some of these voids. Considering zones 1 and 2, two observations have been made : 1, the smectite percentage increases from 40% to 70u toward the vein; 2, the volume of oligoclase replaced by white mica or dissolved decreases exponentially with increasing distance from the vein. The widths of the chloritized-biotite zone (W CH ) and of the zone with white mica in oligoclase (W WM ) increase with the width of the vein (W V ). W CH and W WM are linearly correlated indicating that the formation of chlorite and white mica are interdependent processes. Combining average chemical compositions and quantities of primary and secondary minerals, we have calculated the chemical mass balance in the two altered zones for a reference volume of rock (40 dm 3 ). Results obtained by this way are consistent with the mass balance calculated with Gresens' method. In spite of the strongly altered aspect of the rock, the bulk chemical composition is little changed, with addition of water and small amounts of sulphur, and loss of Si and Ca. The major part of the elements leached out of the primary minerals is consumed by the crystallization of secondary minerals in the altered zone and inside the fracture, through a sequence of interdependent mineral reactions. The end-product of this sequence is phengite or adularia (or overgrowths of primary minerals in narrow veinlets) which seal progressively the open fracture. After the sealing of the fracture, the temperature decreased and illite/smectite mixed-layer minerals precipitated in the oligoclase dissolution voids (<100°C)

Ground water chemistry and geochemical modeling of water-rock interactions at the Osamu Utsumi mine and the Morro do Ferro analogue study sites, Poços de Caldas, Minas Gerais, Brazil

Surface and ground waters, collected over a period of three years from the Osamu Utsumi uranium mine and the Morro do Ferro thorium/rare-earth element (Th/REE) deposits, were analyzed and interpreted to identify the major hydrogeochemical processes. These results provided information on the current geochemical evolution of ground waters for two study sites within the Poços de Caldas Natural Analogue Project. The ground waters are a KFeSO 4F type, a highly unusual composition related to intense weathering of a hydrothermally altered and mineralized complex of phonolites. Tritium and stable isotope data indicate that ground waters are of meteoric origin and are not affected significantly by evaporation or water-rock interactions. Recharging ground waters at both study sites demonstrate water of less than about 35 years in age, whereas deeper, more evolved ground waters are below 1 TU but still contain in most cases detectable tritium. These deeper ground waters may be interpreted as being of 35 to 60 or more years in age, resulting mainly from an admixture of younger with older ground waters and/or indicating the influence of subsurface produced tritium. Geochemical processes involving water-rock-gas interactions have been modeled using ground water compositions, mineralogic data, ion plots and computations of speciation, non-thermodynamic mass balance and thermodynamic mass transfer. The geochemical reaction models can reproduce the water chemistry and mineral occurrences and they were validated by comparing the results of thermodynamic mass transfer calculations (using the PHREEQE program, Parkhurst et al., 1980). The results from the geochemical reaction models reveal that the dominant processes are production of CO 2 in the soil zone through aerobic decay of organic matter, dissolution of fluorite, calcite, K-feldspar, albite, chlorite and manganese oxides, oxidation of pyrite and sphalerite, and precipitation of ferric oxides, silica and kaolinite. Gibbsite precipitation can be modeled for the shallow (recharge) water chemistry at Morro do Ferro, consistent with known mineralogy. Recharge waters are undersaturated with respect to barite and discharging waters and deeper ground waters are saturated to supersaturated with respect to barite demonstrating a strong solubility control. Strontium isotope data demonstrate that sources other than calcium-bearing minerals are required to account for the dissolved strontium in the ground waters. These may include K-feldspar, smectite-chlorite mixed-layer clays and goyazite [SrAl 3(PO 4) 2(OH) 5·H 2O].

Transformation of coal mineral matter in relation to coal metamorphism in the Rajhara sub-basin, Daltonganj Coalfield, India

The nature and composition of silicate/clay minerals present in coals of the Rajhara sub-basin, India, and their changing pattern with progressive metamorphism is presented. Earlier studies had already revealed a remarkable range of metamorphism: the existence of a wide spectrum of coals varying in rank from 79% to 94% carbon level in the same seams within a distance of barely 600 m. In addition, higher rank coals, including semi-anthracitic and anthracitic ones (84–94% C), occur almost exclusively within an anticlinal zone. Outside of this zone the same coal seams contain only lower rank bituminous coals (79–84% C). Mineralogical studies of the associated silicate minerals of both the high and low rank coals have revealed a distinctive compositional pattern. The silicate minerals associated with the lower rank coals (exclusively situated outside the anticlinal zone) are mainly kaolinite, free silica and muscovite. Similar studies of higher rank coals, especially semi-anthracitic and anthracitic ones (which are the metamorphic products of the same low rank coals), indicate that a new phase of a three or mixed-layer type clay mineral is present in addition to the kaolinite, free silica and muscovite. The development of this new phase seems to be at the expense of the original kaolinite and free silica. It thus appears that the change in mineralogical composition of the Rajhara coals and its enhancement in rank are closely related to the conditions of metamorphism.

Morphology, Texture, and Microstructure of Halloysitic Soil Clays as Related to Weathering and Exchangeable Cation

AbstractThis paper aims at characterizing the morphology, texture, and microstructure of three hydrated kaolin rich clays (f &lt; 0.2 μm) from volcanic soils. These clays represent a weathering sequence in which CEC, halloysite content with respect to kaolinite, as well as smectite content in the halloysite-smectite mixed-layer clays decrease with increased weathering. The clay samples were made homoionic (K+ or Mg2+) and hydrated under a low suction pressure (3.2 kPa). After replacing water by a resin, ultrathin sections were cut and examined by TEM. Particle shape varies with increased weathering, as follows: spheroids → tubes → platelets. Higher aggregation and dispersion are observed by TEM after Mg2+ and K+ saturation, respectively, at two levels of the clay-water system organization: intraparticle and interparticle. The microstructure variations induced by the nature of the exchangeable cation become less pronounced with decreasing layer charge of the 2:1 layers. They are thus related here to the presence of smectite layers localized in the halloysite habitus, mostly at the particle periphery. These results show that small amounts of smectite largely affect the organization of clays rich in kaolins at a high water content, and that K+ behaves here as a dispersing ion.

Ferripyrophyllite and related Fe3+-rich 2:1 clays in Recent deposits of Atlantis II Deep, Red Sea

AbstractIron-rich dioctahedral 2:1 clays were sampled in metalliferous sediments from Atlantis II Deep (Red Sea), and studied by XRD, TEM, FTIR and Mössbauer spectroscopy. One sample with the structural formula (Si3·94Fe3+0·06)(Fe3+1·90Mg0·10)K0·1(Mg, Ca)0·03 is closer to the theoretical end-member ferripyrophyllite Si4Fe3+2O10(OH)2 than any other Fe-rich clay previously described. Other samples appear as mixed-layered clays: ferripyrophyllite-hydroferripyrophyllite or hydroferripyrophyllite-celadonite.

Randomly interstratified clay minerals in Plio-Pleistocene sediments of North-West Peloponnesus, Greece: origin and distribution

Four clay mineral assemblages are recognized in the Plio-Pleistocene fluviolacustrine marine and terrestrial sediments of Northwestern Peloponnesus. The four assemblages contain varying proportions of illite, chlorite-smectite, illite-smectite, vermiculite-smectite and chlorite-vermiculite, the mixed-layer clays being poorly crystalline and randomly interstratified. A predominantly detrital rather than diagenetic origin is suggested for most of the days. The similarity of the mineral assemblages observed to those encountered in the flysch of the Pindos and Gavrovo zones indicates derivation from the latter sediments. No distinct areal and stratigraphic variation of the clay mineral assemblages was recognized. However, certain mineral assemblages are considered to be indicative for a certain depositional environment either because of their abundance or their frequence of appearance. The nature of the irregular mixed-layer clays is variable: there is a progressive sequence between smectitic terms in the west and chloritic ones in the east. This may be attributed to variations in primary clastic clay composition and/or to an eastwards advancing degree of burial diagenesis.

Relation between the macroscopic behavior of clays and their microstructural properties

This paper shows the basis for establishing a relation between the mineralogical characteristics and the macroscopic properties of clays for radioactive waste disposal. Clays prepared as a paste were submitted to decreasing suction pressures to obtain the curves for the retention of water and changes in volume. The powders from these samples were compacted from 30 to 120 MPa. Comparison of the porosities and the water contents obtained shows that the compaction ability of materials depends on their mineralogical properties. The cation exchange capacity (CEC) of a clay and its specific surface area determined by ethylene glycol monoethyl ether (EGME) are the two best indicators of the mineralogical properties. They provide data on the texture and indicate an order of magnitude of the extent of desiccation required to reach the air-entry point. The existence of a three phase water-air-solution system in the clay matrix plays a decisive role in the physical stability of clays. For clays with low CEC (illites), rapid wetting produces a complete disintegration of the material. For smectites a high physical stability is confirmed, but significant macroscopic swelling is obtained only with dilute wetting solutions and with a monovalent cation such as Na+. It is concluded that the choice of smectitic materials is prefereable to clays that are partly illitic, like the mixed layer clays, where the poorer physical stability appears to be detrimental to the performance of engineered barriers.

Paleosols as an Indicator of Climatic Change at the Early-Late Cretaceous Boundary, Southwestern New Mexico

ABSTRACT Cretaceous (Aptian to Lower Campanian) alluvial paleosols in southwestern New Mexico suggest that a paleoclimatic change from arid or semi-arid to humid or subhumid occurred near the boundary between Early and Late Cretaceous. Lower Cretaceous paleosols of the Hell-to-Finish (Lower Aptian) and Mojado (Upper Albian) formations are primarily Aridisols, characterized by a dark reddish brown, pedoturbated Bw horizon underlain by a Bk horizon composed of calcareous nodules and tubules or a massive calcareous Bkm horizon. Retention of carbonate in the paleosol profiles suggests that annual precipitation was less than 60 cm. In contrast, Upper Cretaceous paleosols consist of non-calcic Alfisols and Inceptisols. Inceptisols consist of two types: 1) a gleyed variety displaying A, Bg, and BC or Cg horizons in the Satten Formation (uppermost Albian to lowermost Cenomanian) and in the Crevasse Canyon Formation (Upper Coniacian to Lower Campanian); and 2) a non-gleyed variety consisting of A, Bw, and C horizons in the Moreno Hill and Tres Hermanos formations (Middle Turonian) and in the Crevasse Canyon Formation. In addition, Alfisols in the Tres Hermanos and Crevasse Canyon formations are composed of A, E, Bt, BC, and C horizons. The argillic B horizons (Bt) contain kaolinite and mixed-layer clay and display blocky peds, vertical, clay-filled root traces, and argillans, indicative of relatively well-drained woodland soils. Modern counterparts of the Upper Cretaceous Alfisols exist in humid and subhumid regions receiving greater than 75 cm/yr precipitation. Comparison of the data resented in this study with paleosol and paleobotanical data from other locations in the Western Interior of the U.S. suggests that the paleoclimatic change proposed for southwestern New Mexico may have been of regional extent.

Shaly Sand Analysis as an Indicator of Hydrocarbon Production Potential in the Olmos Formation, Las Tiendas Trend, South Texas

ABSTRACT The sandstones of the Olmos Formation in the downdip Las Tiendas trend of Webb County, Texas originated on a storm-dominated shelf seaward of a major deltaic complex. Stratigraphically trapped gas is produced along a northwest-southeast trend 40 mi long and up to 8 mi wide. The reservoir comprises thin-bedded sandstones separated by non-productive thin siltstones and stacked into two to six, 20- to 50-ft-thick productive intervals. Porosity and permeability sufficient for gas production are preserved in the bases of individual storm-event sandstones below the depth to which bioturbation has completely homogenized sand with overlying silt and clay. Due to the presence of chlorite and illite-smectite mixed-layer clays in the Olmos sandstones, shaly sand analysis was required to determine accurate hydrocarbon pore-feet thicknesses. Because of the diversity of logging suites, the same types of porosity logs were not available from all wells and effective porosities had to be calculated in different wells using a variety of porosity-log types. In addition effective water saturations had to be calculated by both the Fertl and the Automatic Compensation methods. A volume of clay cutoff of less than 30% and an effective porosity cutoff of 10% were used to determine net pay intervals. Hydrocarbon pore-feet thicknesses were determined using shale-corrected effective porosities and water saturations. Cross plots of hydrocarbon pore-feet thickness versus cumulative gas production have a correlation coefficient of +0.79. The cross plots also reveal that wells that do not contain net pay in both major productive intervals, the Olmos A2 and Olmos B2 zones, and that have hydrocarbon pore-feet thicknesses less than 1.5 ft, typically have cumulative productions of less than 0.5 Bcf. Notwithstanding the diverse logging suites that were run on wells in the Las Tiendas trend, hydrocarbon pore-feet thicknesses can be related to well performance using detailed shaly sand analysis.

Paleocene and Eocene kaolinite distribution in the South Atlantic and Southern Ocean: Antarctic climatic and paleoceanographic implications

Clay mineral associations of Paleocene and Eocene age have been examined in six drilled sequences from the southern South Atlantic and Southern Ocean. These sites are located to provide three depth transects across intermediate water masses. Widespread abundance of pedogenic smectite at all locations suggests globally warm climates and alternating wet and dry seasons on the adjacent continental margins, including Antarctica as was the case during the Late Cretaceous. Remarkably, kaolinite is observed only in Paleocene/Eocene sediments at the highest latitudes of these sites and must have originated from Antarctica. The restriction of kaolinite to the Antarctic region is inferred to have resulted from especially higher rainfall there during the Paleocene-Eocene relative to the lower latitude areas examined. This inferred high rainfall is considered to have resulted from globally warm temperatures that increased poleward latent heat transport, compensating for a strong heat deficit in the Antarctic region relative to the tropics. Kaolinite was not observed in the latest Cretaceous (Maastrichtian) sediments of Maud Rise, Antarctica. Kaolinite first appears in the clay mineralogical assemblages in the earliest Paleocene (∼ 64 to 64.5 Ma) at a time of relatively cool intermediate waters and before a warming trend of surface waters. Currents at intermediate water depths transported fine terrigenous sediments from the Antarctica during the latest Paleocene and early Eocene (∼ 59 to 52 Ma) at a time when the oxygen isotopic strongly in East Antarctica during the latest Paleocene and early Eocene (∼ 59 to 59 Ma) at a time when the oxygen isotopic data indicate maximum warmth of surface and intermediate waters of the Southern Ocean. These are interpreted to reflect a Paleocene maximum of Antarctic rainfall. This development peaked at 57.3 M, close to the Paleocene-Eocene boundary, a time of maximum warmth, based on oxygen isotopic data. Increased illite content (and associated chlorite and random mixed-layer clays) at the end of the middle Eocene (41 Ma) suggest local development of poorly weathered soils on Antarctica, favored by a cooling of the Southern Ocean. Coeval extension of kaolinite into shallower water depths, and similar clay mineral associations in shallower and deeper sites, indicate the development of cooler conditions led to an increasing influence of Antarctic environments upon intermediate water masses at high latitudes.

Mixed-Layer Clay Geothermometry in the Wairakei Geothermal Field, New Zealand

AbstractMixed-layer clays of variable composition and structure occur in core samples from two drillholes (WK207 and WK210) drilled into the Te Mihi sector of the Wairakei geothermal field. These were identified by X-ray diffraction analysis of glycolated and oriented sample fractions at less than 2 μm and less than 0.2 μm.Low permeability lacustrine sediments encountered by drillhole WK207 contain a well-developed sequence of mixed-layer clays. The shallowest downhole appearance of mixed-layered illite/smectite (I0.6/Sm) occurs at 146 m depth where temperature is only 100°C. Discrete illite is present only below 297 m (200°C) in the finer size fraction (less than 0.2 ¼m). Chlorite first appears downhole, in association with illite-smectite, at 177 m depth (110°C).Drillhole WK210 encountered predominantly ignimbrites and rhyolites, and fluid flow here is mainly in channels. Within these rocks, a sequence of interlayered clays is poorly developed. Discrete illite and chlorite are present in core from only 244 m (180°), but the measured temperatures where interlayer clays occur ranges from 140 to 209°C.Differences in the identity of clay minerals present in the Wairakei reservoir, where conditions are otherwise the same, demonstrate the strong control that the type of fluid flow has on their formation. In poorly-permeable sediments, where diffuse fluid flow prevails, a clearly-defined sequence of mixed-layer clays occurs. These are absent where channel flow dominates, the discrete chlorite and illite deposit directly from solution.