Petrographic Factors Research Articles

Geological and Petrographic Factors Affecting Formation of Active Landslides in the North of Biga Peninsula, NW Turkey

The study includes landslide movements that take place on Eocene and Middle-Upper Miocene aged the terrestrial and marine sedimentary rock units on the Canakkale basin in the Biga Peninsula. In this study, the active Ambaroba Landslide, Şevketiye and Adatepe Landslides located on the northern coasts of Biga Peninsula were investigated.The geological studies in the Ambaroba landslide have shown the integrity of the Bayramiç formation and the Şapcı volcanics in this area have shown their influence in the Miocene period.The sandstones in the Bayramiç formation are not very well paste and are yellow, dirty yellow and gray in color.The sliding surface units in the moving masses consist of non-hardening, very large blocky, pebbly and loose cement-bonded pebbles. In the Şevketiye and Adatepe Landslides, a sliding movement has occurred on the sandstone and conglomerate units with yellowish brown colors in the Fıçıtepe formation. The sandstones in these landslide areas are massive and well cemented. Conglomerates are weaker than sandstones and are fond of a gray-beige color that does not harden. Precipitation data and GPS measurements of the landslide areas were evaluated together with the lithological properties of the rocks.Loose sandstones and pebbles in the entire landslide areas studied can easily move due to seasonal precipitation.This movement was mostly seen in the Şevketiye landslide area and then in the Adatepe and Ambaroba areas, respectively. Although their effectiveness has decreased, the movements will continue, as the lithological features will not change in these areas. As a result, the measures specified in this study should be taken into account in order to eliminate the destructive effects of landslides.

Petrographic Controls on Pore and Fissure Characteristics of Coals from the Southern Junggar Coalfield, Northwest China

The productive potential of coalbed methane projects is controlled by pore and fissure characteristics, which are intrinsically related to coal petrology. This work attempts to identify the influence of petrographic factors on the development of pore and fissure systems in the southern Junggar Coalfield, Northwest China. Here, Middle Jurassic coal (lignite and subbituminous) petrology in coal seam No. 45 of the southern Junggar Coalfield (SJC) is studied with respect to the characteristics of pore and fissure structure with the aid of optical microscopes, scanning electron microscopy, mercury intrusion porosimetry, and nuclear magnetic resonance analysis. Maceral analysis shows coals at the SJC are dominated by vitrinite (38–87 vol %), with moderate quantities of inertinite (1–28 vol %) and liptinite (0.5–30 vol %). Decomposition of plants occurs under slightly oxic–anoxic conditions, with good tissue retention. Four types of coal facies are classified using petrographic indices, comprising (1) lower delta plain marsh, (2) lower delta plain fen, (3) upper delta plain wet forest swamp; and (4) piedmont plain moor. Pores and fissures are generally observed in telinite, collotelinite, fusinite, and semifusinite in SJC coals, indicating that the generation of pores and fissures is strongly influenced by coal macerals. Pore and fissure structures of coals in coal facies (1) appear weakly connected, whereas those in coal facies (2) reveal good connectivity. Coals in coal facies (3) and (4) show moderate connectivity between pore and fissure structure. Therefore, pore and fissure structures are significantly controlled by coal facies. This work provides practical recommendations and implementation methods for petrological studies in future coalbed methane exploration/development in the SJC. This study also serves to predict the physical properties of pores and fissures and interpret the control mechanism of coalbed methane production using coal petrology.

Building stones used in the architectural heritage of Morelia (México): quarries location, rock durability and stone compatibility in the monument

Four different rocks have been used in the architectural heritage of Morelia city (Michoacan state, Mexico). Piedra Vieja (PV) is the original building rock whilst Tlalpujahua, Cointzio and Jabalina stones (TL, CO and JA, respectively) are the replacement varieties used in the restoration works during the last decades. All of them correspond to rhyolitic ignimbrites quarried close to Morelia. The new varieties were selected exclusively after aesthetical similarity, and no petrophysical criterion was considered. In this paper, a deep analysis of the weathering process of these rocks during wet–dry and salt crystallization cycles is carried out. Rock decay is defined in terms of linearity and homogeneity: JA and TL show nonlinear decay mode and heterogeneous behaviours, whilst samples of CO and PV weather after linear and homogeneous modes. Moreover, each rock variety shows different decay patterns, such as differential erosion (PV and TL), fractures (TL), scaling (JA) and granular disintegration (CO). All these obtained results are discussed according to: (1) petrographic factors; (2) hydric and mechanical properties; and (3) the partial effective pressures reached inside the porous system of each rock during salt crystallization. Finally, a review about the quality of previously published durability estimators is carried out. In general terms, results reveal that theoretical estimators best fit the visual weathering suffered by the rocks than its mass loss. Concluding this paper, a petrophysical and aesthetical evaluation of the compatibility between both original and replacement building stones used in the architectural heritage of Morelia is carried out in order to offer technical recommendations for future restoration works. CO ignimbrite offers the highest chromatic compatibility with the original building rock (PV), but its durability is extraordinarily low and consequently its use is not recommended. JA and TL are advisable replacement stones, although from a petrophysical point of view, TL results the most convenient. These results highlight the importance of carrying out the selection of building stones for restoration works according to petrophysical criteria, instead of using exclusively an aesthetic valuation.

Estimation of petrographic factors of Deccan basalt using petro-physico-mechanical properties

Basaltic rocks of the Deccan Traps are the major aggregate sources in Western India, and quarrying of these rocks has been ongoing in several quarries for many decades. The aggregate is the major component of the construction material. Therefore, quality assessment of aggregates by various physico-mechanical tests is essential. Many classification systems have been proposed for rocks/aggregates based on petrographic factors (PF). PF is defined by a range of values which indicate the quality of rock in term of excellent, good, fair and poor. However, PF depends upon the physical characteristics of the rock and geological characteristics of the particular area. The present study is the evaluation of rocks based on petro-physico-mechanical characterizations of the aggregate before the start of quarrying. To evaluate PF of aggregate, petrographic examination including visual and microscopic observations together with physico-mechanical tests, such as water absorption, point load index, slake durability index and impact strength index. Furthermore, a modified classification system was proposed for the PF of different basaltic rocks of the Deccan Traps which includes an equation based on the importance of above-mentioned parameters.

Assessment of mineralogical and petrographic factors affecting petro-physical properties, strength and cracking processes of volcanic rocks

Microtextural features influence the petrophysical and mechanical properties of rocks, including the elastic properties and cracking processes under axial loads. Volcanic rocks with varying microtextural characteristics and compositions were evaluated to assess the effect of the main constituents' mineralogical, petrographic and micro-structural features on the petrophysical properties, elastic properties and strength. Furthermore, detailed analyses were also used to understand the cracking processes under axial loads. The analyses of the cracking processes were completed on thin sections obtained from the mechanically tested specimens.The results from the quantitative mineralogical and petrographic studies and observations from thin sections revealed that the mineral mass fractions have an effect on the specific gravity and loss-on-ignition (LOI) values. On the other hand, unconfined compressive strength (UCS) and elastic properties are mostly affected by petrographic variables (e.g., mineral content). The UCS values tend to decrease with a relative increase in the groundmass with respect to the phenocryst content and vice versa. Biotite is the only mineral that influences the UCS individually. Geometric features (e.g., Feret's diameter and perimeter) of opaque minerals and biotite are determined to be important constituents influencing the Young's modulus.Under axial loads, crack propagation is strongly dependent on the proportional distribution of the phenocryst and groundmass. It was observed that increasing groundmass content leads to predominantly axial cracks. The cracks tend to bend or propagate as a boundary crack when they reach the boundary of an unaltered phenocryst. Thus, axial and shear cracks co-exist with increasing amounts of phenocrysts, which hinders crack propagation. Occasionally, cracks can penetrate the altered or opaque phenocrysts, depending on the degrees of orientation (with respect to applied load) of the same minerals. Furthermore, the synthesis of the measurements obtained from UCS tests and thin section observations reveals that an increase in the amount of phenocrysts, which promote the formation of more shear cracks in addition to axial cracks, causes an increase in the radial strain and Poisson's ratio. The other factors investigated had minor effects on physical and mechanical properties of studied rocks.

Ti distribution in grain-size fractions of Apollo soils 10084 and 71501

Much work has been devoted to the correlation between the remotely sensed UV–VIS slope and the TiO2 concentration of the lunar surface, and this correlation has been used to map the lunar surface TiO2 distributions using data obtained from various missions. However, additional work is needed to fully evaluate the UV–VIS–TiO2 correlation. Such work would help ongoing efforts to improve TiO2 mapping (e.g., as currently underway with LROC Wide Angle Camera (WAC) data). To evaluate the UV–VIS–TiO2 correlation, we are investigating soil petrographic factors (e.g., modal abundances of Ti bearing minerals, lithic, and glass hosts, ilmenite morphology, grain size, and maturity) that may influence the spectra. This “ground truth” approach will be useful in comparing between sample information and laboratory spectra to investigate the influence of petrographic factors on the spectra. In this work, we report the quantitative results of a systematic laboratory investigation of three size fractions (210–100μm, 100–48μm, 48–20μm) of two high Ti lunar soils 10084 and 71501 using a combined digital imaging (backscattered electron image and X-ray maps) method. For each size fraction, the results include: (1) the modal abundances for single phase minerals and lithologic components; (2) the Ti distributions among Ti host components; and (3) the shape of ilmenite grains. We compile and compare the data together for the three size fractions of the two high Ti soils with different maturities as well as data from previous studies. Future work will include the investigation of finer size fractions (<20μm) and more samples (Apollo 12, Apollo 15) covering a larger range of Ti concentration and maturity, and comparison with their spectra.

Grus weathering mantles—problems of interpretation

Grus is an ill-defined product of deep weathering of coarse-grained rocks whose relationships to other weathering changes remain unclear. This paper attempts to address this issue by reviewing a number of examples of coarse saprolites from a variety of climatic and topographic settings. Grus is the category of weathering mantle that possesses the following characteristics: sand+gravel 75–100%; silt+clay <25%; clay <10%; Chemical Index of Alteration (CIA) 60–70; Chemical Weathering Index (CWI) 15–20. The origin of grus is connected with weakening of rock fabric by development of microcracks, biotite expansion, and initial alteration of plagioclase. It may originate either beneath the surface or at greater depths within a weathering profile. The climatic approach to the formation of grus mantles offers limited explanation of field occurrences, as these materials are widespread across climatic zones, from the humid tropics to cool temperate areas, although rates of grusification are likely to be influenced by climatic parameters. By contrast, topographic and also petrographic factors appear to play key roles in the development of grus, which may be regarded as a response of weathering systems to rapid relief differentiation. Grus mantles are preferentially associated with moderate to high relief; hence, they are essentially azonal and their development is under way in many areas of the world.

Evaluation of Dolomite and Related Aggregates Used in Bituminous Overlays for Indiana Pavements

To allow vehicles to stop in a reasonable distance, adequate friction resistance is needed to prevent pavement slipperiness. In stone-masticasphalt surfaces, friction resistance is mainly caused by the interaction between aggregates exposed at the surface and vehicle tires. Performance of aggregates is reduced with time through wear and polishing by vehicular traffic. A method to investigate aggregate performance using physical, chemical, and petrographic factors was evaluated. The objective was to develop a laboratory test for Indiana dolomite, limestone, sandstone, and gravel aggregates to predict field friction resistance and to determine causes for the range of polish resistance of these aggregates. Dolomite and slag are specified aggregates for wearing surfaces for high-traffic roads in Indiana, but some limestones and crushed gravels may prove acceptable. Gravel sources were assessed on the basis of percentages of rock types included in the gravel. Initial friction values and polished values were determined in the laboratory using the British wheel and pendulum procedures, and field values were determined with the towed friction trailer (ASTM E274-97; average friction numbers), which yielded friction number smooth (smooth tire) and friction number ribbed (ribbed tire). Correlations between parameters predict friction resistance from laboratory test data. Also polished values of 25 may be equivalent to a friction number smooth of 27.7 and friction number ribbed of 30.7. Further refinement is required to establish performance specifications for individual aggregate selection.

Thermochemical history in the genetic path of coal: derivation of a novel correlation for heat of combustion

A plot of the heat of combustion of coal ( Q) vs sum-total of the C and H contents (on Parr's basis), for bright coals, has been found to show three clear-cut linear phases in the transition from lignite to anthracite, marked by two turning points, one at about 84–85 and the other at about 92 wt% C. Significantly, analogous curves showing exactly the same pattern of variation of Q vs rank, have also been found to emerge from the plots of Q vs C or Q vs O. The first long linear step (AB) encompasses the high-volatile (hv) low rank coals (C: 65–84 wt%), the second (BC), the middle rank coals (C: 84–92 wt%) and the third (CD), the highest rank coals (C: 92–97 wt%), tending to end up linearly at the point for graphite. Attention is drawn to the one-to-one correspondence of the above thermochemical paths with what had been observed earlier in the “genetic path” of coal, traced out from a plot of atomic H/C′ ratio vs rank for the same range and types of coals, showing the same three linear steps, demarcated by the same two turning points. The Q vs (C+H) relationship is considered more significant because of its direct relevance to the C–H matrix of coal and its progressive structural change as shown by the sequential changes of its atomic H/C′ ratio. Such interpolation of the thermochemical and genetic paths has led to the development of the following novel correlation, holding good from lignite to anthracite: Q( MJ/kg)=0.512( C+H)−11.19+6.75( H/C′−0.78) which, in turn, simplifies to Eq. (7): Q=0.512 ( C+H)+81.07 H/C−16.46, with atomic H/C′ ratio replaced by its weight ratio (H/C). Parallel correlations have also been developed based on Q vs C or Q vs O relationships but the Q vs (C+H) has been found to be the most accurate, and is recommended for general use. Allowing for contributions of heat due to S and mineral-matter (MM) transformation to ash, it has also been possible to logically deduce a derivative (Eq. (8)) giving calorific value on dry basis, directly from dry coal data: Q d =0.512 ( C d + H d )+(81.07 H d +C d −16.46)×(100− MM)/100+12 S d −0.007 Ash d The “genetic” correlations have been found to apply excellently well to all coals irrespective of their rank, region and variable petrographic composition, predicted values seldom deviating from experimental values by more than 0.5%. Its (Eq. (8)) potential accuracy is likely to be far better as assessed from a special test conducted on US coals. The success of the newly found correlations, is attributed mainly to the role of its H/C term, significance of which is discussed in full vis a vis rank and petrographic factors of coal.

Determination of petrographic factors controlling permeability using petrographic image analysis and core data, Satun Field, Pattani Basin, Gulf of Thailand : M.C. Bowers, R. Ehrlich & R.A. Clark, Marine & Petroleum Geology, 11(2), 1994, pp 148–156

Determination of petrographic factors controlling permeability using petrographic image analysis and core data, Satun Field, Pattani Basin, Gulf of Thailand : M.C. Bowers, R. Ehrlich & R.A. Clark, Marine & Petroleum Geology, 11(2), 1994, pp 148–156

Determination of petrographic factors controlling permeability using petrographic image analysis and core data, Satun Field, Pattani Basin, Gulf of Thailand

Miocene sandstones of the Pattani Basin, Gulf of Thailand, undergo progressive diagenesis with depth. Permeability varies from the millidarcy to darcy range over the short depth interval of 6000–9000 ft (1828–2748 m) with a tendency for deeper samples to be less permeable. Image analysis of representative thin sections showed that these rocks contain five pore types, three of which are strongly related to permeability. At shallower depths, less than 7500 ft (2285 m), the relative abundance of the two largest pore types are major controls on enhanced permeability. Below 7500 ft (2285 m) the median throat sizes of these pore types are halved (10 versus 20μm) and permeability is greatly reduced. A small intergranular pore type, unimportant in the upper section, has an increased throat size from about 2 to 6μm below 7500 ft (2285 m). Physical models of permeability allow the construction of ‘artificial’ rocks where the contributions to permeability of each pore type are calculated at various porosity values. These models can be used to assess the maximum permeability likely to be encountered even if such maxima are not observed in core.

Porosity Trends of the Lower Cretaceous J Sandstone, Denver Basin, Colorado

ABSTRACT This study examines relationships between porosity and time-temperature history, and the influence of rock properties upon porosity, for the Lower Cretaceous J sandstone in the Colorado portion of the Denver basin. The J sandstone is classified as a quartzarenite to litharenite and was deposited in nearshore-marine, deltaic, and fluvial-estuarine (valley-fill) settings. Principal elements of its paragenetic sequence include quartz cementation and pressure solution, carbonate cementation and dissolution, dissolution of feldspar and rock fragments, and formation of authigenic clays. Porosity versus vitrinite reflectance (R0) regression lines of the form = A(R0)B (where B is a negative number) depicting the 10th, 25th, 50th, 75th, and 90th porosity percentiles of the J sandstone were derived from 963 core-plug measurements representing 31 wells. The data span a thermal maturity range of R0 = 0.41%-1.14%. Porosity distributions at different locations within the basin can be estimated as a function of thermal maturity on the basis of these regression lines. Porosity trends of the J sandstone, if considered as a function of R0, are similar to those of broad, composite data sets representing sandstones in general. The petrographic factors that most affect J sandstone porosity variability at a given level of thermal maturity are carbonate cementation and clay content. Carbonate cement, where present, reduces porosity. If previously more widespread, carbonate cement could also introduce porosity heterogeneity by temporarily preserving the pore network relative to uncemented intervals. Abundant detrital and authigenic clay reduces porosity by occupying pores. Low clay content indirectly reduces porosity because the inhibiting effects of clay upon quartz cementation and pressure solution are largely absent.

Petrographic Factors Affecting Jointing in the Banded Series, Stillwater Complex, Montana

Joint spacings for two mutually perpendicular joint sets were measured in each of five sample sites in gabbronorites and anorthosites. Joint density ranges from $$3-13 joints/m^{2}$$ for the mafic rocks and from $$1-6 joints/m^{2}$$ for the felsic rocks. Joint spacing varies from 1-128 cm in mafic rocks (average, 4-65 cm) and from 1-300 cm in felsic rocks (average, 9-144 cm). Layer thickness ranges from 1 to 6 m. Mineral mode and clump size consistently show strong relations with joint density and average joint spacing; the higher the mafic mode and the larger the mafic clumps, the greater the joint density and the smaller the average spacing. Felsic grain size decreases with increasing joint density. Layer thickness shows no relation with average spacing for either joint set, with joint density, or with mineral mode. Cooling and unloading could account for most of the differences in joint density because volumetric thermal expansion for pyroxene is about 50% more than for calcic plagioclase, but volumetric compressibility of pyroxene is about 50% less. The initiation of microcracks leading to joints may thus be favored by petrographic contrasts within the rock.