Detrital Laminae Research Articles

Deciphering the laminated botryococcus-dominated shales in saline lacustrine basin, Western Qaidam Basin, NW China: Implications for shale oil potential

Given the high lipid content and potentially huge biomass, the explosive occurrence Botryococcus is regarded as a significant marker of high-quality oil source rocks in petroliferous basins. However, they are rarely discovered in saline lacustrine source rocks, and the formation mechanism of these alga-rich shales remains uncertain. In this study, abundant Botryococcus fossils were firstly found, accounting for an average of 43.5% of the total organic debris from the upper Xiaganchaigou Formation in the Yingxiongling and Zhahaquan hydrocarbon-rich sags of the western Qaidam Basin. The Botryococcus-dominated palynofacies assemblages were usually interpreted as derived from a distal oxic shelf environment. These Botryococcus-dominated sediments were characterized by a perfect laminated texture and composed of detrital, carbonate, and clay laminae. The Botryococcus fossils exclusively occur in the detrital laminae with large-grained quartz and feldspar and low carbonate content, possibly reflecting increased river input under relatively heavy precipitation. The cyclothymic deposition between these Botryococcus-rich detrital laminae and other laminae probably indicates that sufficient terrestrial nutrient inputs and lake salinity decreases driven by periodic heavy precipitation result in Botryococcus bloom under fresh or brackish water environments. These Botryococcus-dominated shales in the western Qaidam Basin usually have a high oil-prone hydrocarbon generation potential, as evidenced by their organic geochemistry parameters. These sediments have biomarker features extremely similar to crude oil with a high relative abundance of C20–C24 tricyclic terpanes, phytane, C27ααR sterane, and a unimodal distribution of n-alkanes with the maximum at n-C18. Therefore, these Botryococcus-dominated shales are possibly important oil sources of these prolific oil sags. Additionally, these high-quality source rock sequences can reach a few tens of meters in thickness and evidently have significance for future shale oil resources.

Microstratigraphy and palaeoenvironmental implications of a Late Quaternary high‐altitude lacustrine record in the subtropical Andes

AbstractHigh‐mountain lake records in semiarid foreland settings, such as the central Andes of North‐western Argentina, are highly restricted and often deprived of well‐preserved microstratigraphic information to analyze palaeoenvironmental changes and their causes, particularly for periods prior to the Last Glacial Maximum. Laguna La Salada Grande (23°S/65°W, 4063 metres above sea‐level) is a closed shallow lake located at Cordillera Oriental, North‐western Argentina with a unique depositional record, including geomorphic and stratigraphic evidence of palaeoenvironmental changes since the Late Pleistocene. In order to understand the depositional dynamics of this mountain lacustrine system at different timescales, limnogeological multiproxy analyses together with a radiocarbon and 210Pb‐based chronology were applied on massive and laminated sediments from La Salada Grande. Laminated deposits were further analyzed using novel sub‐centimetric mineralogical, textural and geochemical automated methods (including a combination of micro‐X‐ray fluorescence and quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN®). Thick laminated microfacies at the beginning of the sequence record a deep and organic matter productive palaeolake prior to ca 34 ka (1 ka = 1000 years before 1950), that changed into a highly fluctuating shallower‐palaeolake with frequent detrital influxes after ca 34 ka. Microstratigraphy of the coarser‐grained detrital laminae between 31 ka and 25 ka reveals irregular and probably sub‐centennial high‐energy discharge events, pointing to convective atmospheric activity as the main trigger. After ca 21 ka the lake evolved to deeper water‐level conditions resulting in fine‐grained deposits with limited delivery of coarse‐grained sediments, followed by an abrupt lake level drop and incision. The multi‐millennial palaeoenvironmental reconstruction of La Salada Grande, and its correlation with other palaeoclimate records, shows a close link between the lacustrine depositional processes and palaeoclimate changes associated with the South American summer monsoon dynamics. This multifocal research in such an understudied environment provides key knowledge about lacustrine functioning and discharge events−climate interactions of mountain lakes of semi‐arid climates.

Hydroclimatic variability of opposing Late Pleistocene climates in the Levant revealed by deep Dead Sea sediments

Abstract. Annual and decadal-scale hydroclimatic variability describes key characteristics that are embedded into climate in situ and is of prime importance in subtropical regions. The study of hydroclimatic variability is therefore crucial to understand its manifestation and implications for climate derivatives such as hydrological phenomena and water availability. However, the study of this variability from modern records is limited due to their relatively short span, whereas model simulations relying on modern dynamics could misrepresent some of its aspects. Here we study annual to decadal hydroclimatic variability in the Levant using two sedimentary sections covering ∼ 700 years each, from the depocenter of the Dead Sea, which has been continuously recording environmental conditions since the Pleistocene. We focus on two series of annually deposited laminated intervals (i.e., varves) that represent two episodes of opposing mean climates, deposited during MIS2 lake-level rise and fall at ∼ 27 and 18 ka, respectively. These two series comprise alternations of authigenic aragonite that precipitated during summer and flood-borne detrital laminae deposited by winter floods. Within this record, aragonite laminae form a proxy of annual inflow and the extent of epilimnion dilution, whereas detrital laminae are comprised of sub-laminae deposited by individual flooding events. The two series depict distinct characteristics with increased mean and variance of annual inflow and flood frequency during “wetter”, with respect to the relatively “dryer”, conditions, reflected by opposite lake-level changes. In addition, decades of intense flood frequency (clusters) are identified, reflecting the in situ impact of shifting centennial-scale climate regimes, which are particularly pronounced during wetter conditions. The combined application of multiple time series analyses suggests that the studied episodes are characterized by weak and non-significant cyclical components of sub-decadal frequencies. The interpretation of these observations using modern synoptic-scale hydroclimatology suggests that Pleistocene climate changes resulted in shifts in the dominance of the key synoptic systems that govern rainfall, annual inflow and flood frequency in the eastern Mediterranean Sea over centennial timescales.

Using palynology to re-assess the Dead Sea laminated sediments – Indeed varves?

Lacustrine laminated sediments are often varves representing annual rhythmic deposition. The Dead Sea high-stand laminated sections consist of mm-scale alternating detrital and authigenic aragonite laminae. Previous studies assumed these laminae were varves deposited seasonally. However, this assumption has never been robustly validated. Here we report an examination of the seasonal deposition of detrital-aragonite couplets from two well-known Late Holocene laminated sections at the Ze'elim fan-delta using palynology and grain-size distribution analyses. These analyses are complemented by the study of contemporary flash-flood samples and multivariate statistical analysis. Because transport affects the pollen preservation state, well–preserved (mostly) air-borne transported pollen was analysed separately from badly-preserved pollen and fungal spores, which are more indicative of water transport and reworking from soils. Our results indicate that (i) both detrital and aragonite laminae were deposited during the rainy season; (ii) aragonite laminae have significantly lower reworked and fungal spore concentrations than detrital and flash-flood samples; and (iii) detrital laminae are composed of recycling of local and distal sources, with coarser particles that were initially deposited in the Dead Sea watershed and later transported via run-off to the lake. This is in line with previous carbon balance studies that showed that aragonite precipitation occurs after the massive input of TCO2 associated with run-off episodes. Consequently, at least for the Holocene Ze'elim Formation, laminated sediments cannot be considered as varves. Older Quaternary laminated sequences should be re-evaluated.

Beryllium isotopes as tracers of Lake Lisan (last Glacial Dead Sea) hydrology and the Laschamp geomagnetic excursion

The content of the cosmogenic isotope 10Be (t1/2=1.39 Ma) in lacustrine sediments that deposit in lakes with a large watershed is susceptible to both climate and cosmogenic production rate variations. In order to distinguish between these two controls, we measured 10Be and major elements in several sections of the annually laminated sediments of the Lake Lisan (the last Glacial precursor of the Dead Sea) that are composed of detrital sediments and primary (evaporitic) aragonites. The sections were selected to represent regional hydrology and climate as reflected by different lake configurations (level rise, drop and high-stands) and rapid change in the 10Be production rate during the Laschamp geomagnetic excursion.Since the short-lived cosmogenic “sister” of 10Be, 7Be (t1/2=53.3 d) has virtually no recycled component, the recycled 10Be in Lake Lisan detrital sediments was evaluated by measuring 7Be in their modern equivalents: modern flood suspended matter, dust and mud cracks.Our results demonstrate that although the recycled 10Be component is significant, secular variations in the 10Be concentration in Lake Lisan sediments correlate with hydrological variations and geomagnetic excursions.During periods of moderate variations in 10Be production rate, the 10Be concentration in the Lisan detrital sediments positively correlates with lake level, Al + Fe content and the (Al + Fe)/(Ca + Mg) ratio. These correlations suggest that the 10Be is adsorbed on the fine silicate component (probably clays) of the detrital laminae. The fine silicates together with carbonates were transported to Dead Sea drainage basin mainly as airborne dust that after a short residence time was washed into Lake Lisan as flood suspended matter. We suggest that preferential dissolution of carbonates in the flood suspended matter concentrated the residual fine component leading to the positive correlation between 10Be and the (Al + Fe)/(Ca + Mg) ratio. During periods of increased water discharge more carbonates were dissolved and hence the 10Be concentration in the detrital laminae increased.During periods of rapid increase in the 10Be production rate (e.g. the Laschamp excursion), 10Be showed a ∼2 fold increase, beyond the above-mentioned correlations (lake levels and Al + Fe contents). This observation suggests that Lake Lisan can serve as a potential high-resolution archive of 10Be production rate variations during periods of geomagnetic excursions.

The 1.5-ka varved record of Lake Montcortès (southern Pyrenees, NE Spain)

The karstic Lake Montcortès sedimentary sequence spanning the last 1548yr constitutes the first continuous, high-resolution, multi-proxy varved record in northern Spain. Sediments consist of biogenic varves composed of calcite, organic matter and detrital laminae and turbidite layers. Calcite layer thickness and internal sub-layering indicate changes in water temperature and seasonality whereas the frequency of detrital layers reflects rainfall variability. Higher temperatures occurred in Lake Montcortès in AD 555–738, 825–875, 1010–1322 and 1874–present. Lower temperatures and prolonged winter conditions were recorded in AD 1446–1598, 1663–1711 and 1759–1819. Extreme and multiple precipitation events dominated in AD 571–593, 848–922, 987–1086, 1168–1196, 1217–1249, 1444–1457, 1728–1741 and 1840–1875, indicating complex hydrological variability in NE Spain since AD 463. The sedimentary record of Lake Montcortès reveals a short-term relation between rainfall variability and the detrital influx, pronounced during extended periods of reduced anthropogenic influences. In pre-industrial times, during warm climate episodes, population and land use increased in the area. After the onset of the industrialization, the relationship between climate and human activities decoupled and population dynamics and landscape modifications were therefore mostly determined by socio‐economic factors.

Natural and human-induced environmental change in southern Albania for the last 300 years — Constraints from the Lake Butrint sedimentary record

A sediment core from Lake Butrint in southwestern Albania contains an annually-layered sequence covering the last ∼ 300 years. It provides thus an exceptionally well-dated time series to study past climate-driven environmental changes, as well as anthropogenic perturbations along the coast of the Ionian Sea. The varves are composed of organic-rich carbonate couplets and detritus-dominated clay layers. The first are deposited during spring-to-fall, and reflect the chemistry of the lake, which, in turn, is sensitive to 1) the relative importance of marine versus freshwater inputs, 2) relative evaporation rates, and 3) the productivity cycle within the lake. The detrital laminae are deposited during winter, reflecting precipitation and runoff conditions during the wet season. A 2–3‰ stable carbon isotope ratio shift in both bulk organics and authigenic carbonates was attributed to increasing eutrophication towards the end of the 20th century, and validated by historical and instrumental data. An increase in the δ 18O of authigenic carbonates by more than 8‰ indicates the progressive salinization of the lake, which can primarily be attributed to man-made perturbations that reduced the freshwater input to the lake and/or enhanced the exchange with seawater from the nearby Ionian Sea. A recent increase in the relative evaporation versus precipitation rates may have additionally contributed to the observed 18O enrichment in the Lake Butrint carbonates. The interdecadal cyclicity in the thickness of the detrital laminae seems to be at least partially controlled by NAO and/or ENSO-like phenomena that modulate precipitation patterns in the eastern Mediterranean. Thus, this study demonstrates the potential of combining microstratigraphic and stable isotopic tools to disentangle anthropogenic and natural environmental changes in Lake Butrint, validated by historical records.

Diatomaceous sediments from the Miocene Monterey Formation, California; a lamina-scale investigation of biological, ecological, and sedimentary processes

Finely laminated diatomaceous sediments from the Miocene Monterey Formation, Lompoc, Santa Barbara County, California, record the mutualistic coupling of life processes and environmental evolution at subannual and subseasonal resolution. In this study we present a new classification of lamina and couplet styles based on couplet bimodality, lamina thickness, compositional domination, lamina spacing, and cyclicity. We also describe five distinct lamina types with emphasis on paleoenvironmental settings, paleoecological associations, and biologically mediated sedimentary/taphonomic processes. Detrital laminae, consisting of silt, clay, and robust diatoms, were deposited from continental runoff during rainy seasons. Thin biosiliceous laminae consist of either moderately preserved high-diversity diatom assemblages, or well-preserved monogeneric phytoplankton assemblages. Most thick, continuous diatomaceous laminae are composed of well-preserved monogeneric and monospecific diatom assemblages that likely experienced biologically induced aggregation and rapid sedimentation without grazing. Thick, discontinuous diatomaceous laminae consist of either Thalassiothrix longissima mats or Chaetoceros setae. Mat laminae reflect stratified water conditions and high biomass conditions developed via vertically migrating diatom mats. Setae laminae are problematic to interpret. Macerated biosilica laminae, consisting of closely packed and highly fragmented biosilica from a variety of taxa, reflect intense zooplankton maceration and dissolution of diverse phytoplankton assemblages. Our results illustrate how different lamina types and associations can be used to proxy specific ecological and oceanographic conditions. This study provides a foundation for developing a high-resolution time series of paleoenvironmental variability recorded by biological event strata in hemipelagic sediments.

High-resolution record of geomagnetic secular variation from Late Pleistocene Lake Lisan sediments (paleo Dead Sea)

We measured geomagnetic secular variation in Lake Lisan sediments (paleo Dead Sea). More than 1500 oriented samples were collected from a 27.3-m section of alternating aragonite and detritus laminae in the Dead Sea basin ranging in age from 67 to 32 ka. The natural remanent magnetization (NRM) is carried by titanomagnetite in the detrital laminae whereas the aragonite is diamagnetic. The NRM is very stable and was acquired several hundred years after deposition. The mean direction of 878 horizons is D = 005°, I = 45° ( α 95=1°; κ=22). We observed three modes of directional geomagnetic variation as a function of (and by inference, time): very rapid inter-sample changes, slow variation in mean direction, and inclination shallowing of about 1°/m. The overall rate of change in direction is 0.57±0.57°/year, not significantly different from zero. For about 83% of the record the rate of change is less than 1°/year and comparable to historical values. High rates of change are observed more frequently in the Lisan than in historical records, and peak rates are up to ten times faster. A smoothed curve resulting in a maximum rate of change of 0.66°/year and a mean 0.10±0.10°/year may be a more realistic representation of the field behavior. No reverse NRMs were observed, but geomagnetic field excursions may be present where the VGPs deviate by more than 40° from the geographic north at about 52 and 41 ka; the latter may represent the Laschamp event.

Laminated cyanobacterial mats in sediments of solar salt works: some sedimentological implications

ABSTRACTFormation of microlaminated sediments in solar salt works along the Mediterranean coast in southern France only occurs within a restricted salinity range of 60–150 gl−1. These salinities are associated with development of a laminated cyanobacterial mat composed primarily of the filamentous cyanobacteria Microcoleus chthonoplastes interbedded with detrital laminae. Transplants of the cyanobacterial mat to a less saline zone (36–60 gl−1) indicated that the cyanobacterial mats failed to colonize the less saline waters due to herbivorous snails and competition for light from floating algal masses of Cladophora and Enteromorpha. Neither the snails nor the Cladophora and Enteromorpha masses are tolerant of salinities above 60 gl−1, and therefore the Microcoleus mats are restricted to those areas of the solar salt works with these higher salinities.Analyses of salinity, conductivity, dissolved oxygen and pH in shallow salt pans (with salinities of 60–150 gl−1) established a relationship between the daily development of oxygen supersaturation and cyanobacterial photosynthesis.Sediments are unlaminated in those portions of the solar salt works where there are no cyanobacterial mats. These mats are frequently drained of their overlying water, and thus desiccation cracks divide them into polygonal plates. The development and translocation of these plates is enhanced by gas bubbles which form under the surface of the mats. No correlation between the microlaminae in sections from two cores located approximately 1 m apart was observed. This was consistent with the hypothesis that the surface of the desiccation crack polygons can be removed by currents and redeposited on the top of other cyanobacterial mat polygons. This process results in a ‘patchwork quilt’of young and old cyanobacterial mat polygons with an irregular microlamination pattern. The presence of such an irregular pattern of laminae permits an important distinction to be made between sediments associated with stromatolite formation and those associated with the very fine and horizontal varved sediments of stratified (meromictic) water bodies. The sedimentological significance of these observations is reviewed in relation to the processes of stromatolite genesis.

Paleoclimate deduced from a multidisciplinary study of a half-million-year-old stalagmite from rana, northern Norway

A 7-cm-thick flowstone sequence has been dated by extended Uranium series techniques to less than 1.25 myr, with a probable growth interval of ≥350,000–≤730,000 yr B.P. The time span is in accordance with paleomagnetic results revealing normal polarity for the whole sequence, i.e., a depositional age of <730,000 yr B.P. Oxygen isotope variations suggest deposition during three warm periods, interrupted by two isotopically cold hiatuses characterized by bulk resolution and detrital laminae. Calculations suggest that climatic transitions may have involved a shift of 1.1–2.4°C in mean annual temperatures. Contemporary flooding of the cave due to adjacent glacier expansion provides evidence for the widely held view that speleothem deposition is halted by glacier proximity. The flowstone matrix contained appreciable amounts of pollen (pine, birch) as well as larger amounts of charcoal dust. The pollen assemblage suggests a climate comparable with the present, implying that Norway has experienced at least three glacial/interglacial transitions during the growth interval of the speleothem. The warmer periods were characterized by a forest environment, with ample evidence of fire.

Hypersaline brine diagenesis and evolution in the Dead Sea-Lake Lisan system (Israel)

Water soluble salts were extracted from 57 aragonite and detrital laminae, sampled from the Pleistocene Lake Lisan sediments. The solutions and solids were analysed for Ca, Mg, Na, K, Sr, Cl, SO 4 and bicarbonate. Average Na Cl , Mg Ca , Sr Ca , Mg Cl , Ca Cl , Sr Cl , K Cl , K Na , and K Mg ratios, calculated for seven stratigraphie elevations in the Lisan column, indicate that the Dead Sea Rift sediment brine system has remained practically closed for 70000 years. The soluble salts reflect both the hydrologie evolution of the lake as well as diagenetic processes. Mg Ca ratios systematically decrease in younger sediments, and this decline is accompanied by a 1.5–4 fold Sr enrichment in aragonite solubles relative to salts extracted from detrital laminae. The major process responsible for these observations is dolomitization of Sr-rich aragonite by interstitial brine. Diagenesis was triggered by slow desiccation of the lake's sediments after the Lisan waters started to recede due to a change in climate. Very sharp chemical gradients, which developed in the sediments after deposition, have been preserved because further diagenesis was quenched by the advancing dry-out. H 2 O-loss by evaporation induced difrusional migration of Na and Cl from aragonite laminae into adjacent detrital layers. This lowered the Na Cl ratio in the aragonite and raised it in the detritus relative to the initial (Dead Sea) Na Cl ratio of 0.26. The early Lisan brine was similar in its chemical composition and total salinity to that of the present Dead Sea.

Geologic Factors Influencing Reservoir Performance at Texaco's Salem Tertiary Recovery Project, Marion County, Illinois: ABSTRACT

A detailed lithologic reservoir study was conducted to aid Texaco personnel in designing and monitoring an experimental surfactant-polymer flood in the Mississippian Benoist Sandstone, one of several producing formations in the Salem field of south-central Illinois. Twelve elongated five-spot patterns are distributed over the 60-acre (24 ha.) project area. The Benoist Sandstone averages about 49 ft (15 m) of net pay at a depth of about 1,800 ft (550 m). Cores from eight wells were studied in detail. Particular attention was paid to variations in sedimentary structures, lithology, and mineralogy that could influence reservoir performance. Techniques employed in this study included examination of slabbed cores, thin-section petrography, X-ray diffraction (XRD) mineralogy, a d scanning electron microscopy/energy dispersive spectrometry (SEM/EDS). The Benoist Sandstone is one of several Late Mississippian deltaic sandstone units deposited in the subsiding Illinois basin. These sandstones are bounded above and below by fossiliferous marine limestone and shale. Delta-front sandstones, hereafter referred to as bar-finger sandstones, comprise the bulk of the formation. Channel-fill deposits are found near the base of the unit and nonreservoir, tidal-flat deposits near the top. The bar-finger deposits are moderately to well-sorted, fine to medium-grained sandstones with horizontal to inclined planar bedding and some ripple and planar cross-bedding. The planar bedding is accentuated by clay and mica-rich layers, one millimeter to several centimeters thick. These shale layers increase and thicken upward, and separate the bar-finger sa ds into several reservoir units. The Benoist sandstones are quartzose, containing 70 to 98% monocrystalline and polycrystalline quartz and small amounts of detrital feldspar and shale clasts. Cement is predominantly quartz in the form of syntaxial overgrowths, with minor calcite. Small amounts of clay occur as detrital laminae, authigenic pore fillings, and sand-grain coatings. The percentage of detrital and authigenic clay increases near the top of the bar-finger sandstone and significantly reduces permeability. Illite, the dominant clay, End_Page 445------------------------------ occurs as both grain coats and pore bridgings, kaolinite forms scattered pore fills, and Fe-chlorite coats a few grains. The channel-fill deposits are well to poorly sorted, very fine to medium-grained sandstones, commonly conglomeratic, and contain calcite-cemented zones. These deposits are mineralogically similar to the bar-finger deposits but contain abundant shale and fossil fragments at the base. Noncalcareous channel sandstones are characterized by scattered Fe-chlorite grain coatings and pore-filling illite. Intergranular porosity is well developed and has not been severely reduced by the pervasive quartz overgrowth cementation. The eastern part of the project area contains a higher quality reservoir section because of the sparsity of clay zones in the bar-finger sandstone and the thicker channel-fill deposits in this area. Secondary porosity, produced by the dissolution of feldspar grains, has slightly enhanced the overall quality of the Benoist Sandstone. End_of_Article - Last_Page 446------------

Some Applications of Scanning Electron Microscopy to the Study of Reservoir Rock

The scanning electron microscope (SEM) provides qualitative information about pore geometry through direct observation of a rock or a pore cast of the rock. This aids in understanding reservoir productivity capabilities. The SEM is useful for locating and identifying minerals, particularly clay minerals -an aid when designing drilling and completion programs. Introduction This paper discusses some applications of the scanning electron microscope (SEM) to the study of reservoir rocks. The SEM is used extensively to provide qualitative information about the pore geometry of rocks, either directly or indirectly through the examination of pore casts. Many of these studies were concerned with relating pore geometry to various tests or rock properties; specific applications also are discussed.The basic principles of the SEM were known in 1935. Experimental instruments were built in Germany and the U.S. before World War II. After the war, the British started a research program that eventually developed a commercial instrument in the early 1960's. Since then, the SEM has become an indispensible scientific tool in all fields of science and industry. Presently, there are approximately 10 SEM manufacturers, with prices ranging from about $17,000 to well over $ 100,000.The SEM provides several unique advantages for the geologist or engineer when compared with other types of microscopes:magnification range of 15 x to a practical upper limit of about 40,000 x for rocks,great depth of field,ease of sample preparation - samples made conductive by a coating of metal applied under vacuum,nondestructive sample study, andthree-dimensional image analogous to reflected light requiring no special techniques for interpretation. These features make the SEM ideal for studying irregular rock surfaces.An energy-dispersive X-ray attachment for the SEM provides qualitative information on chemical elements and aids in identifying minerals. Evaluation and Importance of Microporosity Micropores occur in sandstone and carbonate reservoirs and affect fluid flow properties. These micropores are seen and evaluated best with an SEM. Microporosity, for convenience, has been defined as pore apertures with a radius of less than 0.5 mum.In sandstone reservoirs, clay with associated micropores may occur as detrital laminae, pellets (grains) of silt-sized or coarser material, or as authigenic (that is, newly formed or regenerated) material (Fig. 1). In a water-wet sandstone, the micropores and high surface area associated with the clay minerals are conducive to holding significant amounts of bound water that can affect log calculations for water saturation. Because of improved log-calculation techniques, which take into consideration cation-exchange capacities of clay minerals, this is less of a problem than in the past.Micropores in carbonate reservoirs occur among calcite or dolomite crystals that typically are less than 4 mu m in diameter. JPT P. 1375＾