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Petrography

About: Petrography is a research topic. Over the lifetime, 7449 publications have been published within this topic receiving 102018 citations.


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TL;DR: In this paper, the authors proposed that these dolomites were formed on an outer ramp setting with a maximum water depth of 50 m, during a period of nondeposition, with the dolombization process being promoted by the oxidation of organic material and the diffusion of Mg 21 from the overlying seawater.
Abstract: A significant proportion of oil production from the Kim- meridgian Arab-D strata in the Ghawar field, Saudi Arabia originates from dolomitized rocks. Stratigraphic, petrographic, and geochemical data suggest that at least four episodes of dolomitization affected these sediments. The lower portion of the Arab-D, Zone 3, is only partially dolomitized, with the dolomite frequently being associated with firm- grounds. We propose that these dolomites were formed on an outer- ramp setting with a maximum water depth of 50 m, during a period of nondeposition, with the dolomitization process being promoted by the oxidation of organic material and the diffusion of Mg 21 from the overlying seawater. The dolomites in Zone 2B are geochemically dis- tinct compared to those in Zone 3 in that they have relatively positive oxygen isotope compositions ( 2 1t o22‰ compared to 26.5 ‰). The relatively positive oxygen isotope composition and the geochemical sim- ilarity of Zone 2B to the dolomites in Zone 1, which are intimately associated with the overlying evaporites, has led us to conclude that the Zone 2 dolomites probably formed by the reflux of hypersaline fluids through the sediments. These hypersaline fluids bypassed Zone 2A by moving through the grain-dominated sediments. Early cemen- tation and dolomite formation made these units more susceptible to later fracturing that affected the entire Arab-D formation. This frac- turing allowed higher-temperature fluids to leach the dolostones, there- by removing any remaining calcite and partially resetting the oxygen isotope composition of some of the dolomites. As a result of this later dolomitization event, rocks that were only partially dolomitized were leached, creating units with extremely high permeability and porosity (super-k intervals). Dolomites in the lower Zone 3 were recrystallized during burial by the normal geothermal gradient, leading to the pre- sent negative oxygen isotope values. Zone 1 dolomites are petrograph- ically distinct from Zone 2 dolomite in that they are mimetic and fabric preserving, although they are geochemically similar. This mimetic style of dolomitization occurs immediately adjacent to the overlying anhy- drite and is interpreted to have formed very shortly after deposition from hypersaline brines. recovery, and their presence is inferred from a combination of production data and the diameter of the bore hole as measured using the caliper log. As a result of the importance of dolomite in the formation of the super-k intervals within the Arab-D, an understanding of the paragenesis of these zones is important not only in predicting super-k formation within the Arab-D but also as an analogue for understanding other dolomitized res- ervoirs with high permeability and porosity. Previous characterization work of dolostones from the Arab-D in the Ghawar field (Cantrell et al. 2004) has identified three types of dolomite with distinctive petrographic, stratigraphic, and geochemical characteristics: fabric-preserving, non-fabric-preserving, and baroque dolomite. Fabric-pre- serving dolomite is very finely crystalline in which petrographic details of the original limestone fabric are usually well preserved. Fabric-preserving dolomite contains low concentrations of Fe (average 247 ppm Fe) and relatively positive oxygen isotope values (average

115 citations

Journal ArticleDOI
TL;DR: In this article, thermochronological analyses on detrital grains, isotopic analyses on bulk rock, and petrographic and heavy mineral data indicate that the Palaeogene Indo-Burman Ranges contain a significant component of arc-derived material, interpreted as derived from the Burmese portion of the Mesozoic-Tertiary arc to the east.
Abstract: The Indo-Burman Ranges in western Myanmar extend along the Sunda Arc subduction zone and may be divided into a western portion of Neogene sedimentary rocks and an eastern portion of Palaeogene sedimentary rocks, separated by the Kaladan Fault Both Himalayan and Burman sources have been proposed for these sediments Our thermochronological analyses on detrital grains, isotopic analyses on bulk rock, and petrographic and heavy mineral data indicate that the Palaeogene Indo-Burman Ranges contain a significant component of arc-derived material, interpreted as derived from the Burmese portion of the Mesozoic–Tertiary arc to the east And older crustal component is also identifiable, which may have been sourced from the Himalaya or the Burmese margin By contrast, the Neogene Indo-Burman Ranges show dominant derivation from the Himalaya A minor arc-derived component may have been sourced from the Trans-Himalaya, or recycled from the arc-derived Paleogene Indo-Burman Ranges

114 citations

Journal ArticleDOI
TL;DR: In this paper, the authors identify 12 lithofacies based on sand-grain populations, cement types, and clay matrix content, and show that careful petrographic assessment of the heterogeneity can be upscaled by correlation with log properties to yield tools for field-scale reservoir quality prediction.
Abstract: Understanding the controls and distribution of reservoir quality is important for the economic success of tight-gas reservoirs in which diagenesis interacts with primary depositional variations in environment and texture to exert a strong control on pore networks, rock mechanical properties, and natural fractures. In the Upper Cretaceous Williams Fork Formation of the Piceance Basin, framework grain composition is a major control on compaction and the occurrence of authigenic phases. Alteration of volcanic grains in the upper Williams Fork led to grain-coating clay precipitation. Ferroan dolomite cement is found only in the deeper marine-influenced intervals in which dolostone fragments are present. This study shows that careful petrographic assessment of lithofacies heterogeneity can be upscaled by correlation with log properties to yield tools for field-scale reservoir quality prediction. Twelve lithofacies are identified based on sand-grain populations, cement types, and clay matrix content. Sandstones of the highest reservoir quality are those with grain-coating clays that inhibit quartz cementation; these sandstones can be identified based on high-density porosity log values. Sandstones with the poorest reservoir qualities are tightly cemented with carbonate and quartz cement or are rich in clay matrix. Carbonate-cemented intervals are identified by low-density porosity. Clay matrix–rich samples have high gamma-ray and low-density porosity values. The presence of abundant potassium feldspar in the upper intervals results in high gamma-ray readings even in the clean (clay matrix–free) sandstone.

113 citations

Journal ArticleDOI
TL;DR: In this article, a detailed study of the petrographic and chemical properties of carbonaceous chondrites was performed and it was shown that the four distinct chondrite subtypes may be related to one of two distinct chemical subdivisions.

113 citations

Journal ArticleDOI
TL;DR: In this paper, the diagenetic evolution of clay minerals in the Rotliegend sandstone reservoir under contrasting burial histories in the Broad Fourteens basin (Dutch sector, Southern North Sea).
Abstract: This paper describes the diagenetic evolution of clay minerals in the Rotliegend sandstone reservoir under contrasting burial histories in the Broad Fourteens basin (Dutch sector, Southern North Sea). The diagenetic modifications affecting the crystal structure of clay minerals (both kaolin-group and illitic minerals) were studied using X-ray diffraction (XRD). The XRD study includes oriented and random mounts of various size fractions, numerical processing (decomposition) of XRD profiles, and simulation of one-dimensional and three-dimensional XRD patterns. Petrographic observations, differential thermal analysis, K/Ar geochronology, and geochemical considerations complement the XRD study and allow determination of the sequence of mineral crystallization and the morphological evoluti n of clay minerals and place further constraints on the absolute timing of diagenetic events and on the nature of the fluids responsible for clay-mineral crystallization. From deposition time ( 275 Ma) to the Kimmerian orogeny ( 155 Ma), crystallization of kaolinite at the expense of K-feldspars was favored by acid fluids from the underlying Carboniferous Coal Measures source rocks; kaolinite crystallization is followed by a steady kaolinite-to-dickite transformation affecting both the structure and the morphology of kaolin-group minerals. The structural characteristics of kaolin-group minerals are related to the burial history of the sediments prior to the Kimmerian orogeny. During the Kimmerian orogeny, rapid illitization of kaolin-group minerals was favored both by increased heat flow in the sedimentary pile and by widespread presence of faults, which permitted significant fluid flow probably from the Zechstein Formation. The morphological and structural characteristics of illitic minerals, i.e., illite content of illite/smectite mixed layer (I/S), ratio of illite to I/S, and three-dimensional structure of illitic minerals, do not represent the progress of a smectite-to-illite transformation, but these characteristics clearly reflect the temperature during illitization of kaolin.

113 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
2023551
20221,098
2021370
2020344
2019310
2018291