1,720,983 research outputs found
Effects of Diagenetic Alterations on Hydrocarbon Reservoirs and Water Aquifers
Full text linkReservoir quality (porosity and permeability) and heterogeneity in carbonate and siliciclastic hydrocarbon reservoirs and groundwater aquifers are significantly constrained by diagenetic processes, such as biological, chemical, biochemical, and mechanical changes, that occur in sediments subsequent to deposition and prior to low-grade metamorphism. Diagenesis, which has a variable but overall important impact on reservoir quality evolution, is controlled by several inter-related parameters. These parameters include the depositional composition of the sediments, depositional facies, sequence stratigraphy, pore water chemistry, burial history and tectonic setting of the basin, and paleoclimatic conditions.Carbonate and siliciclastic sediments often undergo multiple stages of diagenesis, which are related to complex patterns of burial-thermal history (subsidence and uplift) that are controlled by the tectonic evolution of the basin. Tectonic evolution of the basin is controlled by the position of the basin with respect to the type and activity along the plate boundaries. The episodes of burial and uplift may result in profound modifications in the pressure–temperature regimes and in the extent of mineral–water interaction, and hence in various phases of compaction, as well as mineral dissolution, recrystallization, transformation, and cementation. Diagenesis impacts reservoir quality in the following ways: (i) destruction by mechanical compaction and extensive cementation, (ii) preservation by prevention of mechanical and chemical compaction, or (iii) generation by dissolution of labile framework grains and intergranular cements
Diagenesis and Reservoir-Quality Evolution of Deep-Water Turbidites: Links to Basin Setting, Depositional Facies, and Sequence Stratigraphy
No full textA study of the distribution of diagenetic alterations and their impact on reservoir-quality evolution in four deep-water turbidite successions (Cretaceous to Eocene) from basins in active (foreland) and passive margins revealed the impact of tectonic setting, depositional facies, and changes in the relative sea level. Diagenetic modifications encountered in the turbiditic sandstones from the passive margin basins include dissolution and kaolinitization (kaolin has δ18OV-SMOW = +13.3‰ to +15.2‰; δDV-SMOW = -96.6‰ to -79.6‰) of framework silicates, formation of grain coating chloritic and illitic clays, cementation by carbonates and quartz, as well as the mechanical and chemical compaction of detrital quartz. Kaolinitization, which is most extensive in the lowstand systems tracts, is attributed to meteoric-water flux during major fall in the relative sea level. Preservation of porosity and permeability in sandstones from the passive margin basins (up to 30% and 1 Darcy, respectively) is attributed to the presence of abundant rigid quartz and feldspar grains and to dissolution of carbonate cement as well as mica and feldspars. Diagenetic modifications in turbidites from the foreland basins include carbonate cementation and mechanical compaction of the abundant ductile rock fragments, which were derived from fold-thrust belts. These diagenetic alterations resulted in nearly total elimination of depositional porosity and permeability. The wide range of δ13CV−PDB values of these cements (about -18‰ to +22‰) in passive margin basins is attributed to input of dissolved carbon from various processes of organic matter alterations, including microbial methanogenesis and thermal decarboxylation of kerogen. The narrower range of δ13CV−PDB values of these cements (about -2‰ to +7‰) in the foreland basins suggests the importance of carbon derivation from the dissolution of carbonate grains. The generally wide range of δ18O values (about -17‰ to -1‰) of the carbonate cements reflect the impact of oxygen isotopic composition of the various fluid involved (including marine depositional waters, fluxed meteoric waters, evolved formation waters) and the wide ranges of precipitation temperatures. Results of this study are anticipated to have important implication for hydrocarbon exploration in deep-water turbidites from passive and active margin basins and for pre-drilling assessment of the spatial and temporal distribution of reservoir quality in such deposits
Chemical, Mineralogical and Isotopic Studies of Diagenesis of Carbonate and Clastic Sediments
Full text linkDiagenesis of carbonates and clastic sediments encompasses the biochemical, mechanical, and chemical changes that occur in sediments subsequent to deposition and prior to low-grade metamorphism. These parameters which, to a large extent, control diagenesis in carbonates and clastic sediments include primary composition of the sediments, depositional facies, pore water chemistry, burial–thermal and tectonic evolution of the basin, and paleo-climatic conditions. Diagenetic processes involve widespread chemical, mineralogical, and isotopic modifications affected by the original mineralogy of carbonate and clastic sediments. These diagenetic alterations will impose a major control on porosity and permeability and hence on hydrocarbon reservoirs, water aquifers, and the presence of other important economic minerals. In this Special Issue, we have submissions focusing on understanding the interplay between the mineralogical and chemical changes in carbonates and clastic sediments and the diagenetic processes, fluid flow, tectonics, and mineral reactions at variable scales and environments from a verity of sedimentary basins. Quantitative analyses of diagenetic reactions in these sediments using a variety of techniques are essential for understanding the pathways of these reactions in different diagenetic environments
Modelling of reservoir quality in quartz-rich sandstones of the Lower Cretaceous Bentheim sandstones, Lower Saxony Basin, NW Germany
No full textThe Lower Cretaceous Bentheim sandstones of the Lower Saxony Basin in Northwest Germany (one of the main onshore oil fields of Western Europe) are mature quartz arenites in terms of texture and mineralogy. This study shows that the mineralogical maturity to a large degree is the result of diagenetic processes. Hence, the present day detrital composition and texture is not what they were at the time of deposition. The sandstones are highly porous and permeable and show presence of quartz cement as overgrowths which cement the sandstone, partly dissolved detrital feldspar grains, oversized pores caused by complete feldspar dissolution and authigenic kaolinite-dickite booklets. The porosity and permeability have been measured in the laboratory. The results were combined with the observations made by conventional optical microscopy and quantification of detrital and diagenetic components-textures by point counting, cathodoluminescense petrography, and back scattered electron imaging. These techniques have been undertaken in order to assess the impact of the presence of quartz overgrowths and feldspar dissolution on porosity and permeability. XRD analyses were done for identifying the mineralogy of the main detrital and diagenetic components, in particular for detrital and authigenic clay minerals. Quartz cement in the form of syntaxial overgrowths on detrital quartz grains played an important role in determining the petrophysical properties in the Bentheim sandstones. This kind of cement usually causes harmful reductions in porosity and permeability. However, in the Bentheim sandstones quartz cementation possibly had a positive impact in terms of porosity preservation. Cement precipitated in limited amounts and probably helped the reservoir framework to withstand the destructive effect of overburden pressure and consequent mechanical compaction. The study shows that detrital quartz grains were important, and that possibly the different types of quartz grains had different susceptibility in terms of acting as host grains for precipitation of authigenic quartz. The presence of detrital feldspar and its composition was another major parameter during diagenesis and consequent porosity-permeability modification. Dissolution of feldspars may have been an important internal (local) source of silica and aluminium for kaolinite-dickite authigenesis and possibly also for quartz cementation, and created oversized secondary pores.Popular summary: Hydrocarbons, ground water and economic minerals occur in pores (voids) between sediment grains, which later may form more solid and less porous sedimentary rocks. Thus porosity is a term that describes the abundance of these pores and can be defined as the quantity of oil, gas, water and economic minerals a rock can hold and it is expressed as percentages (%). The other important parameters in any oil, gas or water reservoir rock (mostly sedimentary rocks such as sandstones and limestone) is permeability. Permeability is the ability of a rock to transmit oil, gas and water and can depends on the interconnectedness of the pores. The more interconnected pores will lead to more efficient production of oil in oil wells and water in ground water wells. Not all sedimentary rocks are characterised by sufficient porosity and permeability. Accordingly not all rocks can be considered as good reservoirs. Each well (especially oil wells) costs huge amounts of money. When oil companies invest they naturally want to minimise the risk of not striking oil. In other words the ratio of success in striking (finding) oil is directly dependent on the amount of porosity and permeability. Exploration geologists always try to predict porosity and permeability ahead of drilling, by examining adjacent exposed bedrock. They play a profound roll in estimating the reserves (the amount of oil or gas which can be produced) and production rates in suspected oil fields. Porosity and permeability can be accounted for and evaluated through the direct and indirect parameters which control them. One of the many factors controlling porosity and permeability is cementation, i.e., the process of forming new minerals inside the pores, which leads to diminishing porosity and permeability (negative effect). In many quartz grain rich sandstones, such as the Cretaceous Bentheim sandstones and the Jurassic North Sea reservoir sandstones, the cement mineral is quartz minerals which precipitate on already-existing quartz grains. Sandstones consist of different kinds of quartz grains such as monocrystalline quartz (which consists of one single crystal) and polycrystalline quartz (which consists of more than one crystals). Different quartz types have different susceptibilities for hosting quartz cements. Hence, if we know which type of detrital quartz grain is abundant in a certain reservoir rock, so we can presuppose the degree of cementation which in its turn the degree of porosity and permeability reduction. It is believed that monocrystalline quartz grains are more susceptible for quartz cements (secondary quartz) than polycrystalline quartz. This suggestion is tested in this study. In some cases the formation of quartz cement is strengthening the framework of the reservoir rocks and thus help the entire reservoir to withstand the destruction effect of mechanical compaction (mechanical compaction is resulted from the pressure of overlying bed rocks which compresses the pores, thus leading to less pore ratios within the rocks). In this project, it is concluded that the quartz cements were precipitated in limited amounts, supporting the reservoir sand grains against negative effects of mechanical compaction. It was proved in this study that some minerals dissolve and produce new pores and largely enhance the total amount of porosity. This project stresses that multitude factors control porosity and permeability, and that each reservoir is unique and must be studied and examined separately. Hence, conclusions should not necessarily hold for all reservoirs because of the multitude of geological factors influencing porosity and permeability
Impact of diagenesis on the spatial and temporal distribution of reservoir quality in the Jurassic Arab D and C members, offshore Abu Dhabi oilfield, United Arab Emirates
Full text linkThis study is based on petrographic examination (optical, scanning electron microscope, cathodoluminescence, backscattered electron imaging, and fluorescence) of 1,350 thin sections as well as isotopic compositions of carbonates (172 carbon and oxygen and 118 strontium isotopes), microprobe analyses, and fluid inclusion microthermometry of cored Jurassic Arab D and C members
from 16 wells in a field from offshore Abu Dhabi, United Arab Emirates. The formation was deposited in a ramp with barrier islands and distal slope setting.
Petrographic, stable isotopic and fluid-inclusion analyses have unraveled the impact of diagenesis on reservoir quality of Arab D and C within the framework of depositional facies, sequence stratigraphy, and burial history. Diagenetic processes include cementation by grain rim cement and syntaxial calcite overgrowths, formation of moldic porosity by dissolution of allochems, dolomitization and dolomite cementation, cementation by gypsum and anhydrite, and stylolitization.
Partial eogenetic calcite and dolomite cementation has prevented porosity loss in grainstones during burial diagenesis. Dolomitization and sulphate cementation of peritidal mud are suggested to have occurred in an evaporative sabkha setting, whereas dolomitization of subtidal packstones and grainstones was driven by seepage reflux of lagoon brines formed during major falls in relative sea level.
Recrystallization of dolomite occurred by hot saline waters (Th 85–100°C; and salinity 14–18 wt% NaCl). Anhydrite and gypsum cements (Th 95–105°C; fluid salinity = 16–20 wt% NaCl), were subjected to extensive dissolution, presumably caused by thermal sulfate reduction followed by a major phase of oil emplacement.
The last cement recorded was a second phase of anhydrite and gypsum (Th 95–120°C; 16–22 wt% NaCl), which fills fractures associated with faults
Dolomitization and Hypogenic Dissolution of the Eocene Avanah Formation, Iraqi Kurdistan
Full text linkThis study constrains the mechanism of extensive dolomitization and its impact on reservoir quality of the shallowwater marine ramp carbonates of the Avanah Formation (Eocene), Iraqi Kurdistan. The presence of shoal deposits, which semiisolate a lagoon water body from the open marine, suggests that dolomitization was by seepage reflux of brines. Nevertheless, the absence of eogenetic gypsum/anhydrite in the dolostones succession indicates that the dolomitizing fluids were mesohaline/penesaline brines formed during cycles of relative sea level (RSL) fall. Dolomitization resulted in the formation of abundant intercrystalline and moldic/vuggy pores. Restriction of dolomitization and related reservoir quality improvement to the lower part of the formation is attributed to an overall 3rd order fall in the RSL. Conversely, the lack of dolomitization in the upper part of the formation is attributed to deposition during 3rd order marine transgression, which prevented severe restriction and evaporation of the inner ramp and, consequently, inhibited the development of dolomitizing brines. It is suggested that hypogenic dissolution (karstification) by upward flow of aggressive fluids along faults and fractures during the Zagros Orogeny caused dissolution and considerable porosity and permeability improvement of the dolostones. A greater extent of dolostones dissolution in the flanks, which was accompanied by calcite cementation, compared to the crest, reflects the role of oil emplacement in the retardation of diagenetic reactions
The role of multiple weak lithologies in the deformation of cover units in the northwestern segment of the zagros fold-and-thrust belt
No full textThe geometry, kinematics and dynamics of fold-and-thrust belts are strongly influenced by the mechanical behaviour of the basal decollement. However, many fold-and-thrust belts also include mechanically weak lithologies such as evaporites and marls or mudstones at different levels within the shortened stratigraphy. The kinematics and dynamic evolution of these thrust belts are controlled by the mechanical behaviour both of the basal decollement and of the weak units embedded within the overlying stratigraphic succession. In the Zagros fold-and-thrust belt (ZFTB), the shortened sedimentary cover is between 7 and 12 km thick and mechanically weak lithologies compartmentalize the stratigraphic column at shallow and intermediate levels. In this paper, satellite, field and seismic data from the Kurdistan Region of Iraq are used to identify structures of different sizes and surface traces. The observations are used to underline the role of mechanically weak horizons within the Zagros stratigraphy and the decoupling of deformation both laterally and with depth in the belt. The decoupling between shallow and deeper structures observed in seismic profiles from the Kurdistan Region of Iraq is also reported from field observations from the Iranian part of the Zagros fold-and-thrust belt, where folds with different surface traces occur. Decoupling between shallow and deep layers by incompetent lithologies at intermediate depths (e.g. marls, mudstones and evaporites) results in the formation of disharmonic folds. The geometry, size and location of such folds may differ between outcropping and subsurface structures. Decoupling may have a significant impact on hydrocarbon exploration in different parts of the Zagros fold-and-thrust belt due to potential offsets between outcropping and subsurface structures and their associated traps
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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