Showing papers on "Petrography published in 2015"

Capturing Key Attributes of Fine-Grained Sedimentary Rocks In Outcrops, Cores, and Thin Sections: Nomenclature and Description Guidelines

Abstract: An integrated nomenclature scheme is proposed to capture the inherent heterogeneity of fine-grained sedimentary rocks at the 102 to 10−3 mm scale and to assist the evaluation of these rocks as sinks of organic carbon, barriers to fluid flows, and reservoirs of oil and gas. This scheme incorporates previous knowledge and the latest field, petrographic, and laboratory observations. We propose to name fine-grained sedimentary rocks using a root term based on their texture (grain size), which is modified by description of bedding, composition, and grain origin. Regarding texture, we suggest the use of “mudstone” as a class name for the entire spectrum of fine-grained sedimentary rocks. We define mudstone as a rock in which more than fifty percent of its grains are mud (clay and silt) size (< 62.5 µm). Similar to the approach used for the description of sandstone texture, mudstone texture can be refined by a “coarse,” “medium,” or “fine” size-range term. Regarding bedding, we follow Campbell's (1967) genetic approach to define laminae, laminasets, and beds, and describe lamina geometry, continuity, and shape. Regarding composition, we propose terms such as “siliceous,” “calcareous,” “argillaceous,” and “carbonaceous” to capture differences in rock composition. The name of a mudstone can be further modified by additional attributes that detail the form and origin of the rock components. Application of this approach to the Cretaceous Eagle Ford Shale illustrates the variability typically present in mudstone successions and demonstrates how our detailed characterization can be used to decipher and predict rock properties of economic interest.

A Color Guide to the Petrography of Sandstones, Siltstones, Shales and Associated Rocks

Abstract: AAPG Memoir 109 is designed as a practical guide for students and professionals to learn the fundamentals of microscopic examination of sandstones, mudrocks, and associated rocks. With more than 1100 color illustrations, it covers the identification of grains, textures, and structures of clastic terrigenous rocks as well as their diagenetic alteration (compaction, cementation, dissolution, and replacement) and porosity reduction or enhancement. It also provides classification diagrams for formal description of those rocks and their porosity. Although the majority of the outcrop and subsurface examples come from the United States (35 states and Puerto Rico), there are representative photographs from 32 other countries, including many from the offshore areas. The foldout birefringence chart and an included DVD with Powerpoint files of all of the petrographic images provide additional aids for instructors and students.

Characterization of granitoid profiles in the Sila Massif (Calabria, southern Italy) and reconstruction of weathering processes by mineralogy, chemistry, and reaction path modeling

Abstract: This paper concerns the development of a multidisciplinary research on weathering profiles of granitoid rocks in the Sila Massif (Calabria, southern Italy), based on field investigations, chemical and minero-petrographic data, and geochemical modeling. It aims at evaluating modes and degree of weathering and focuses on the main transformation processes of the parent rock, integrating information obtained by direct observations/analyses on samples with results of simulations of pertinent water–rock interaction processes, carried out through the reaction-path-modeling approach. Physical responses to geological hammer hitting and morphological weathering features were used to characterize increasing weathering grade (I–VI weathering classes) of a representative weathering profile. Also, the main microfabric and minero-petrographic features were observed in thin sections; XRPD and SEM-EDS were used to identify primary minerals and neoformed products. Progressive dissolution of the granitoid rock was simulated using the software package EQ3/6, 8.0 and a modified version of the thermodynamic database data0.ymp.R5. A kinetic mode under closed-system conditions was applied with respect to a defined set of secondary solid phases, which are allowed to precipitate. The partial transformation of biotite and the partial destruction of feldspars, associated with the neoformation of secondary minerals and a substitution of the original rock fabric are the main mineralogical changes observed. Neoformed clay minerals and ferric products replaced feldspars and biotite during the most advanced weathering stage. Microfractures and morphological variations occur on the original rock. The final result of weathering process is a soil-like rock characterized by sand-gravel grain-size fraction. Results of geochemical modeling suggest that the chemical weathering processes occurring in the study area are relatively close to an iso-chemical transformation of the original granitoid rock, based on the small predicted changes in the concentrations of solutes from the initial to the final state of the simulation. The CO2-controlled dissolution of albite-rich plagioclase is the most important reaction, followed by the dissolution of K-feldspar, biotite, chlorite, and muscovite, in order of decreasing importance. The precipitating secondary minerals are ferrihydrite, saponite, vermiculite, illite, and kaolinite in order of decreasing pH values. The order of appearance and the quantities of secondary (product) minerals depend on the setup of geochemical modeling. Based on the standard thermodynamic properties of different illite endmembers, calculated in this work, it was possible to predict the varying compositions of the illite solid solution, which resulted to be similar to those observed in natural systems.

Provenance, diagenesis, tectonic setting, and geochemistry of Hawkesbury Sandstone (Middle Triassic), southern Sydney Basin, Australia

Abstract: The Hawkesbury Sandstone is an important groundwater reservoir in the southern part of the Sydney Basin, Australia. However, its diagenesis and provenance and its impact in reservoir quality are virtually unknown. The present study aims to reconstruct the parent rock assemblages of the Hawkesbury Sandstone, their tectonic provenance, and the physiographic conditions under which these sediments were deposited. Samples from the EAW 18a and EDEN 115 field representing the Middle Triassic Hawkesbury Sandstone were studied using a combination of petrographic, mineralogical, and geochemical techniques. The Hawkesbury Sandstone is yellowish brown in color, siliceous, and partly calcareous; it originated as sands were deposited in fluvial channels. Texturally, Hawkesbury Sandstone is medium- to coarse-grained, mature, and moderately well sorted. Scarcity of feldspars indicates that the rock is extensively recycled from a distant source. Hawkesbury Sandstone has an average framework composition of Q92.07F0.31R7.62, and 95.9% of the quartz grains are monocrystalline. The Hawkesbury Sandstone is mostly quartz arenites with subordinate sublithic arenites, and bulk-rock geochemistry supports the petrographic results. Petrographic and geochemical data of the sandstones indicate that they were derived from craton interior to quartzose recycled sedimentary rocks and deposited in a passive continental margin of a syn-rift basin. The cratonic Lachlan Orogen is the main source of Hawkesbury Sandstone. The chemical index of alteration, plagioclase index of alteration, and chemical index of weathering values (3.41-87.03) of the Hawkesbury Sandstone indicate low-moderate to high weathering, either of the original source or during transport before deposition, and may reflect low-relief and humid climatic conditions in the source area. Diagenetic features include compaction: kaolinite, silica, mixed-layer clays, siderite, illite, and ankerite cementation with minor iron-oxide, dolomite, chlorite, and calcite cements. Silica dissolution, grain replacement, and carbonate dissolution greatly enhance the petrophysical properties of many sandstone samples.

Petrography and characterization of microbial carbonates and associated facies from modern Great Salt Lake and Uinta Basin’s Eocene Green River Formation in Utah, USA

Abstract: Abstract Utah contains unique analogues of microbial hydrocarbon reservoirs in the modern Great Salt Lake and the lacustrine Eocene Green River Formation within the Uinta Basin. Characteristics of both lake environments include shallow-water, ramp margins that are susceptible to rapid widespread shoreline changes, as well as comparable water chemistry and temperature that were ideal for microbial growth and formation/deposition of associated carbonate grains. Thus, microbialites in Great Salt Lake and cores from the Green River Formation exhibit similarities in terms of microbial textures and fabrics. A detailed petrographic analysis provides unique insights into these modern and ancient deposits that can be used to determine reservoir characteristics in other microbial carbonate petroleum plays. Great Salt Lake is a hypersaline lake and carbonate ‘factory’, containing actively forming microbial mats, stromatolites, thrombolites and associated carbonate grains. Open constructional pores are common within a spectrum of microbial structures. Green River Formation cores display excellent examples of stromatolites and thrombolites that contain primary megascopic pore and microporosity, as well as carbonate grainstones composed of ooids, peloids and skeletal material with abundant interparticle and intraparticle porosity. West Willow Creek oil field produces from a Green River microbial buildup/mound, a feature not currently recognized in Great Salt Lake.

Petrography and geochemistry of Pharaonic sandstone monuments in Tall San Al Hagr, Al Sharqiya Governorate, Egypt: implications for provenance and tectonic setting

Abstract: Petrographic and major and trace element compositions of Tanis sandstones from Tall San Al Hagr, Al Sharqiya Governorate, Egypt, have been investigated to determine their source, provenance, intensity of paleoweathering of the source rocks, and tectonic setting. Tanis sandstones are yellowish brown in color, siliceous, partly calcareous, and originated from sands deposited in fluvial channels. Texturally, Tanis sandstones are medium- to very coarse-grained, mature, and moderately sorted. Scarcity of feldspars indicates that the sandstone is extensively recycled from a distant source. Their average modal composition (Q99.75F0.03L0.22) classifies them as quartz arenites (quartzite), which is consistent with the geochemical study. Chemical analyses revealed that sandstones have high SiO2 and Fe2O3t and low Al2O3 and TiO2 values, which are consistent with the modal data. Sandstone samples are enriched in most trace elements such as Zr and Ba, and they are depleted in V, Pb, Sc, Rb, U, and Th. The petrography and geochemistry results suggest that Tanis sandstones were deposited in an intracratonic basin or a passive continental margin of a synrift basin. They were mainly derived from deeply weathered granitic-gneissic sources, supplemented by recycled sands from an associated platform. The CIA and CIW values (60.2 and 87.74, respectively) of the Tanis sandstones indicate moderate to intensive weathering either of the original source or during transport before deposition, and may reflect low-relief and warm humid climatic conditions in the source area. The heavy-mineral and trace element results reveal that the Gebel Ahmar quarry is the probable source for the Tanis sandstones.

The effect of ore roasting on arsenic oxidation state and solid phase speciation in gold mine tailings

Abstract: Metallurgical pretreatment of As-bearing ores involves oxidation of sulphides (most often As-bearing pyrite, arsenopyrite or enargite) resulting in complex oxidized As-bearing products. We have evaluated roasting pretreatment of arsenic-bearing ores in a broad context and related this to the specific operations at the Giant mine, Yellowknife, NWT, Canada, which roasted arsenopyrite (FeAsS)-rich gold ore concentrates during 50 years of operations. A large portion of the As was collected and stored in underground vaults as As 2 O 3 dust; however, some of the As was also released with tailings which contain concentrations between 1000 to 5000 ppm. Powder X-ray diffraction (XRD) and sequential extractions have been completed on samples of mill products and various ages of tailings at the Giant mine. These data along with petrographic and synchrotron μXRD and μX-ray absorption near-edge spectroscopy (μXANES) indicate that the largely oxidized roaster products (calcine) and electrostatic precipitator (ESP) dust host most of the As in the tailings with a lesser component of sulphide arsenic. The fine-grained nature of these oxidized products has led to hydraulic sorting within the tailings impounds and dispersal to downstream creek and lake sediments.