Effects Of Diagenetic Processes Research Articles

Effects of diagenetic processes on the velocity evolution in carbonate reservoirs

Carbonate rocks are geologically complex due to the diagenetic processes they experience before and after lithification. Diagenetic processes alter their matrix and pore structure leading to the modification in their sonic velocities. Understanding the effect of these diagenetic features on the seismic velocities is crucial to have a reliable image of the subsurface. The dataset used in this study comprises well logs, and core data. Core data were analyzed using different methods (i.e., thin section analysis, X-ray diffraction (XRD) analysis, and scanning electron microscopy (SEM images)) to investigate the presence or absence of different diagenetic processes in each depth interval of the Sarvak formation. In order to minimize porosity effects on velocity variations, we divided all porosity data into five equal porosity classes and performed bar chart analysis in each class. The results indicated that bioturbation (through stiff pore creation and infilling with stiff minerals) and compaction (through pore space volume reduction) increase velocities, but dissolution increased velocities only for the low porosity samples (through moldic and vuggy pores creation) while reduced it in the high porosity samples (through the interconnection of the isolated pores). Furthermore, porosity enhancement (through increasing pore space volume), micritization (through porosity reduction inhibition during compaction), open fracture (through creation of soft pores and cracks), and neomorphism (through the creation of microporosity during compaction) reduce sonic velocities.

Geochemistry of REY-Enriched Phosphorites in Zhijin Region, Guizhou Province, SW China: Insight into the Origin of REY

The rare earth elements and yttrium (REY)-enriched phosphorites in the Zhijin region, southwest China, have attracted much attention, yet its origin has not been sufficiently addressed. The geology, mineralogy and geochemistry of samples from four sections of the early Cambrian age from the Zhijin region were studied to attain the redox condition and origin of REY. Data from in situ analysis show that REY are mainly contained in francolite, and the two types of francolite (bioclastic and granular) have no distinct difference in REY content (ΣREY). A Ce anomaly indicates an oxic condition in primary seawater. An Eu anomaly and ratios of redox sensitive elements (RSEs) indicate a fluctuating redox condition during phosphorite deposition. Shale-normalized REY patterns of whole rock and francolite show a hat-shaped pattern, indicating the effect of diagenetic processes. The Y/Ho ratio of the selected sections ranges from 38.13 to 61.93, and together with the Y anomaly, LaN/NdN ratio, LaN/SmN ratio, and LaN/YbN ratio, this indicates a seawater origin. This is supported by the Ce/Ce* of the phosphorite that ranges from 0.32 to 0.52 and the Eu/Eu* that ranges from 0.88 to 1.82, which is similar to the characteristics of seawater and deep-sea mud. We also propose a minor contribution of terrigenous debris and influence of diagenesis and hydrothermal processes.

Impact of diagenetic processes on the petrophysical properties and quality of the Miocene “Reefal limestone” reservoir in Al-Hamd field, Gulf of Suez, Egypt

Unconventional reservoirs have burst with a great force in the oil and gas worldwide production. Reefal limestone reservoir is one of them. The present study deals with the reefal carbonate Hammam Faraun Member in Al-Hamd oil field. This study focus on the interpretation of sedimentological and petrophysical data to show the effect of diagenetic processes on middle and lower Miocene reefal carbonate reservoirs in Al-Hamd oil field. Comprehensive petrophysical analysis for Hammam Faraun Member was performed using wireline logs and core data to determine the proper petrophysical parameters. Of the reservoir intervals. The analytical reservoir rock achieving the main reservoir parameters as shale content (Vsh), effective porosity (Oeff), water and hydrocarbon saturation (SW and Shr) as well as the net-pay thickness. The Formation Micro Imager (FMI) was also applied to detect the variation, types, and directions of the fracture systems that present in the lower and middle Miocene reefal limestone reservoir. Petrographic analysis was done, indicating the presence of a clear effect of diagenetic factors on the quality, productivity, and efficiency of the reefal limestone reservoirs behavior. The diagenetic factors include dolomitization, slicification, recrystallization, and solution process which affect greatly the petrophysical characteristics of the reefal carbonate Hammam Faraun reservoir in the study area. The core analyses were also used to detect the paleo environment, associated textural/ lithological and sedimentological characteristics to gain and understand the factors responsible for the productivity behavior of the reservoir in the Al-Hamd oil field. Rock facies have been divided as specific characteristics and were further described considering their color, texture, grain-size characteristics, and the depositional and biogenic sedimentary structures.

Microfacies and diagenetic overprints in the limestones of Middle Jurassic Fort Member (Jaisalmer Formation), Western Rajasthan, India: Implications for the depositional environment, cyclicity, and reservoir quality

The present study investigates the microfacies and the effects of diagenetic processes on the reservoir quality of limestones of the Fort Member, Jaisalmer Formation, Western Rajasthan. The study interpolates an integrated approach including field studies, petrography, staining, and X‐ray diffraction techniques. The Fort Member is composed of medium‐ to thick‐bedded, nodular highly fossiliferous limestones, which are fine‐ to medium‐grained, poorly‐ to very well‐sorted, and comprise angular‐ to well‐rounded grains of low to high sphericity. These limestones contain abundant skeletal (bivalves, brachiopods, echinoderms, foraminifera, algae, gastropods, sponge, coral, and calcispheres) and non‐skeletal grains (intraclasts, ooids, and peloids). On the basis of detailed petrographic investigations, eight microfacies have been recognized including: bioclastic wackestone, whole‐fossil wackestone, bioclastic lithoclastic packstone, aggregate grainstone, peloidal grainstone, ooidal grainstone, bioclastic grainstone, and coated bioclastic grainstone. Based on the microfacies analysis, the limestones of the Fort Member are inferred to have been deposited in shoal, lagoon, and open marine environments. The diagenetic overprints of the Fort Member limestones is characterized by the several diagenetic features such as micritization, cementation, dolomitization, compaction, dissolution, silicification, neomorphism (aragonite to calcite transformation and development of microspar), and secondary porosity. Such main diagenetic features affecting the carbonates are developed in marine phreatic, meteoric phreatic, mixed meteoric phreatic and burial diagenetic environments. Micritization of allochems, dolomitization of lime muds, and cementation by isopachous calcite, granular calcite cement, neomorphism, and recrystallization of bioclasts are developed in meteoric‐phreatic diagenesis. The meteoric‐vadose diagenesis led to prevalence of dissolution process and sparry calcite infilling the fractures. Dolomitization, blocky calcite cement, and silicification processes were developed during the burial diagenesis. These diagenetic processes have direct effects on the modification of both intergranular and intragranular/crystalline porosity. In this manner, it is found that the calcite cementation, micritization, and compaction were responsible for the reduction of the pore spaces, whereas dolomitization, dissolution, formation of moldic pores, and fracturing increased the pore volumes. Therefore, the enhanced pore volumes improved the carbonate reservoir characteristics, suggesting the limestones of the Fort Member to be a better carbonate reservoir for hydrocarbon accumulation.

The effects of diagenetic processes and fluid migration on rare earth element and organic matter distribution in seep-related sediments: A case study from the South China Sea

Methane seepage is an important factor governing microbial activity in marine sediments, although its impact on the distribution of rare-earth elements (REEs) and organic matter in sediments is unclear. Here, we report isotope signatures of organic (δ13CTOC) and inorganic (δ13CTIC) carbon as well as total REEs contents of core sediments collected from a well-characterized seep area on the northern South China Sea. Previous studies have identified sediments corresponding to three methane-release events (MREs) in the studied core: MRE I at 4.0–5.5 m, MRE II at 7.4–8.2 m, and MRE III at 10.0–11.5 m depth intervals. Increased methane flux and reducing bottom waters during MRE I resulted in a higher accumulation of marine-sourced organic matter, as indicated by lower C/N ratios and higher δ13CTOC values compared with those of MREs II and III. Sediments deposited during MRE I have relatively low total REE contents, which may be related to reduced REE adsorption on Mn oxyhydroxides. However, the absence of Ce anomaly at MRE I is more likely due to the detrital input that obscures the original Ce anomaly. In the MRE III sediments, a weak negative Eu anomaly relative to that in the MRE I and II may be attributed to the reduction of Eu3+ to Eu2+ under strongly reducing conditions that caused by intense methane seepage. These observations demonstrate that fluid flow may have an important effect on the distribution of both organic matter and REEs in methane-rich marine sediments.

Genetic Characterisation of Ironstone Deposits from the Agbaja Formation Southern Bida Basin Central Nigeria: Evidence From Rare Earth Elements

The oolithic and pisolithic Agbaja ironstone deposit of the Campano- Maastrichtian southern Bida Basin around Lokoja districts is one of the most studied ironstone formations within the Nigerian Inland basins where controversies still surrounds the origin of the ironstone deposits. Though analyses of Rare Earth Element (REE+Y) distributions in chemical sedimentary rocks has been proved to offer an opportunity to gain new insights into likely ancient palaeo-environmental conditions based on the distinct differences in the rare earth element geochemistry between different types of water and their respective precipitates, but this has not been properly utilized in unraveling the mystery surrounding the origin of this deposit. The present study therefore, is an attempt to provide clues to further understand the genetic characterization of these ores from geochemical data based on the rare earth elements. Geological field mapping was carried out in the study area and representative samples from different rock exposures and outcrops were collected. Seventeen (17) representative samples of the various rock types collected from the field were selected for geochemical analysis. The results reveal a total REE contents of the oolithic and pisolithic ironstone samples range from 189.2 to 471.3ppm with an average of 332.56ppm, with all of the samples having positive Gd anomalies and characterized by the unique absence of Ce anomaly (Ce/Ce*=0.95–1.01), a slight positive Eu anomaly (Eu/Eu*=0.99–1.12) with slight enrichment in Light Rare Earth Element (LREEs) and depletion in Heavy Rare Earth Element HREEs except in sample BH1 which shows gradual increase from LREE to HREE (La-Lu). Absence of negative Ce anomaly and the slight positive Ce anomaly in the oolithic and pisolithic ironstone samples is attributed to enrichment of Ce relative to other REE3+ through mobilization of Ce as Ce3+ under reducing condition during early diagenesis and re-precipitation at the oxidized front while the LREE may be attributed to contribution of enriched particulate during chemical precipitation and recrystallization of iron and during kaolinitization. Also, the absence of Ce fractionation suggests that detrital sedimentation dominates over a hydrothermal contribution. Similarity, the positive Eu anomaly suggests that the samples are sourced from continental source. The absence of significant correlations or weak correlations between the Ce/Ce* and REE, and between Ce/Ce* and Eu/Eu*, suggests a slight effect of diagenetic processes on the REE concentrations. The lower Y/Ho ratios rules out marine input and points towards the influence of terrigenous materials. Thus, the combinations of LREE depletion along with low Y/Ho ratios below those of seawater suggest records of continental signatures which are significantly masked by detrital components. With all the above inferences, Fe oxides and oxyhydroxides and phosphates minerals which have a high affinity for REE in diagenetic fluid along with kaolinite are the possible sources of the REE contamination. Thus, the change in REE abundance results from changes in various diagenetic or progressive alterations of REE contents with increasing geological time, scavenging of REE on iron oxides and oxyhroxides and affinity of the REE to Kaolinite. It also reflects the existence of REE-rich minerals such as monazite and apatite. Thus, the above evidence from the REE studies have refuted marine or hydrothermal sources for these deposits and points towards detrital sources from fluvial input into the basin as material for the formation of the precursor for the ironstone.

Diagenetic changes on bone histology of Quaternary mammals from a tropical cave deposit in southeastern Brazil

Karst caves are suitable environments for the accumulation and preservation of fossils. Cave deposits are often complex and the environmental conditions within cave sites result from intricate interactions between various biological, physical and chemical factors. However, it is not fully understood how the complexity of the environmental conditions of caves influences bone diagenesis. The study of the initial stages of bone diagenesis depends to a large extent on understanding the changes in the bone histology. To contribute to this issue, we examine a set of postmortem changes affecting the bone histology of Quaternary mammals that accumulated naturally in Locus 2, a pitfall site in Cuvieri Cave, located in the tropical region of Brazil. Our analyses show that bones deposited in caves may be subject to a peculiar set of environmental conditions that in tropical regions may prevent the preservation of bone histological structure. The effect of diagenetic processes on the bones differs depending on the taphonomic stage of the bone and the diagenetic alterations appear to have influenced each other. The deposition of bioclasts following the entrapment of individuals in Locus 2 favours the proliferation of bacteria on bones and appears to be important in directing the diagenetic alteration. The hydrological regime of the cave, that is recharge with potential phases of higher humidity, also is important in directing the diagenetic alteration and further decreased the preservation potential of the bone microstructure. The formation of macroscopic and microscopic cracks related to bone weathering in caves shows that the taphonomic processes peculiar to these environments are poorly understood, highlighting the need for more research to be conducted on cave taphonomy.

Unraveling the reservoir heterogeneity of the tight gas sandstones using the porosity conditioned facies modeling in the Whicher Range field, Perth Basin, Western Australia

Tight sandstones of the late Permian Willespie Formation constitute an important reservoir rock in the Whicher Range gas field of the Perth Basin. The sandstones under the effect of sedimentary conditions and diagenesis show some degree of heterogeneity reflecting in reservoir properties and production history. The Willespie Formation consists of fine to coarse-grained and gravelly feldspathic sandstones intercalated with shale, siltstone and coal, deposited in a meandering river system. Different diagenetic processes including compaction, cementation (authigenic clays, calcite and siliceous) and dissolution have severely affected the pore system properties of the reservoir sandstones, as they are considered as tight sandstones. In this study, three-dimensional modeling of reservoir sandstones has been performed using stochastic modeling algorithms for facies and porosity properties. A preliminary facies analysis of the main reservoir rocks based on core and well logs data provided the basis for reservoir zonation and modeling. Regarding the close relationship between acoustic impedance with depositional/diagenetic characteristics of reservoir facies and their porosity, this seismic attribute was used as a secondary parameter in porosity modeling. The results indicate a close relationship between sedimentary characteristics and reservoir properties. Based on the extracted models, most of the porous zones are related to the clean and coarse sandstones of the fluvial channels accumulating in the upper parts of the reservoir. In fact, initial sedimentary characteristics have the main impact on the distribution of reservoir zones, their thickness and continuity in the field and controlling large-scale reservoir heterogeneity which has been enhanced by the effect of diagenetic processes on the pore system properties and controlling the internal reservoir heterogeneity in next stages. Distinctive variability in reservoir properties towards the upper reservoir units and also among different wells can be considered for optimizing exploration and development targets of the field.

Effect of diagenesis on the petrophysical properties of the Miocene rocks at the Qattara Depression, north Western Desert, Egypt

This study focuses on the effect of diagenetic processes on the petrophysical characteristics of the Miocene rocks exposed east of the Qattara Depression, north Western Desert, Egypt. Several techniques were applied on the collected rock samples in order to determine their mineralogic composition and the diagenetic processes they have undergone. The petrophysical analyses are conducted on horizontal plugs representing the Miocene Formations in the study area. They mainly include porosity, permeability, and density. Petrographic analysis revealed that the Moghra Formation is composed mainly of quartzarenites with few shale and limestone intercalations. The Marmarica Formation, on the other hand, is composed mainly of sandy dolomicrite, sandy biodolomicrite, and sandy biomicrite facies. The main diagenetic processes encountered are neomorphism, dolomitization, dissolution, cementation, compaction, and replacement. Values of porosity, permeability, grain, and bulk densities for the studied plugs derived from the Moghra Formation range from 14.7 to 27.8 %, from 0.01 to 35.39 mD, from 2.51 to 2.79, and from 2.01 to 2.32 g/cm3, respectively, while they range from 2.9 to 40.8 %, from 0.002 to 14739.15 mD, from 2.67 to 2.8 g/cm3, and from 1.62 to 2.65 g/cm3 for the samples of Marmarica Formation. Both petrographical and petrophysical studies revealed primary and secondary origins of the sample porosity and permeability and that the studied sandstones can be considered as good hydrocarbon reservoirs. In addition, the studied carbonate rocks are characterized by high effective porosity and permeability due to the secondary enhancement through the dissolution of fossils and other components implying that their corresponding subsurface occurrences represent good reservoir rocks.

Possibility of Huntite presence in the Triassic limestones of Opole Silesia

Streszczenie W niniejszym artykule przedstawiono wyniki badań próbek skał węglanowych pobranych z utworów triasowych, w obszarze Śląska Opolskiego. Przedmiotem badań była identyfikacja huntytu - fazy węglanowej, o podwyższonej, w porównaniu z kalcytem magnezowym i dolomitem, zawartości magnezu. Huntyt powstaje zwykle wskutek procesów hydrotermalnych, wietrzenia dolomitu lub w wyniku przeobrażenia, w warunkach wysokich temperatur, kalcytu magnezowego. W skałach osadowych występuje w utworach strefy wadycznej (aeracji). W celu stwierdzenia obecności huntytu, wybrane próbki skał poddano analizom FTIR, rentgenowskiej oraz badaniom w mikroobszarach. Minerał ten zidentyfikowano w skałach wapienia muszlowego, w wapieniach warstw gogolińskich, górażdżańskich, dziewkowickich i karchowickich. W przypadku analizy FTIR, o obecności huntytu świadczy występowanie w widmach absorpcyjnych, w próbkach 1 i 4 pasma 1555 cm-1, w próbce 3 - pasma 1562 cm-1, a w próbce 2 - pasma 1572 cm-1. W dyfra kto gra mach rentgenowskich wszystkich badanych próbek, fazę węglanową uznaną za huntyt zidentyfikowano na podstawie linii dy|frakcyjnej, o najwyższej intensywności i wartości dhkl - 2,83 A. Badania w mikroobszarach pozwoliły na zidentyfikowanie huntytu w wapieniach dziewkowickich oraz karchowickich. Oznaczone w punktach mikroobszarów badanych próbek, zawartości magnezu, wahają się od 14,01% Mg (23,23% MgO) do 16,18% Mg (26,83% MgO). Wartości te przekraczają znacznie wartość stechiometryczną, typową dla dolomitu (21,86% MgO, 13,12% Mg). Oznaczona w mikroobszarach ilość magnezu w huntycie jest jednak niższa od wartości stechiometrycznej dla tej fazy węglanowej, która wynosi 20,65% Mg (34,25% MgO). Prawdopodobnie, wskutek procesów diagenezy (de- dolomityzacja ?) mogło dojść do usunięcia części jonów magnezu z kryształów huntytu. Analizując możliwości tworzenia się huntytu, fazy węglanowej o podwyższonej zawartości magnezu, w więk- szym stopniu niż Mg-kalcyt czy dolomit, można podejrzewać, że minerał ten powstał w obszarach zbiornika germańskiego, w których procesy diagenezy zachodziły przy udziale wód strefy wadycznej.

Effects of diagenetic processes on seismic velocity anisotropy in near-surface sandstone and carbonate rocks

The effects of diagenetic processes on velocity anisotropy in Ferron sandstone and Ellenburger carbonate core samples are investigated using petrophysical/petrological and P-wave velocity ( V p) data at room pressure and ambient (air dry) saturation. The V p data are measured in both vertical ( V p v) and horizontal ( V p h) directions (parallel and perpendicular to the axes of the cores, respectively), using a 125-kHz sonic probe. The petrophysical/petrological properties for the Ferron sandstone are bulk porosity, fluid permeability and volume fractions of quartz, clay, feldspar and calcite; for the Ellenburger carbonate, they are bulk porosity, vertical permeability, horizontal permeability, maximum permeability, grain density and bulk density. In the Ferron sandstone samples, the V p h values are higher (∼8.5%) than the V p v values, which is attributed to laminated bedding or alignment of grain orientation. Correlations between V p anisotropy and minerals that represent diagenetic processes are clearly demonstrated in the Ferron sandstone. Anisotropy decreases with decreasing feldspar, increasing calcite and increasing clay. This correlation suggests that calcite cementation and pore-filling kaolinite reduce velocity anisotropy of the Ferron sandstone by reduction of the net grain alignment or preferred pore orientations. The heterogeneity of the Ellenburger carbonate caused by multiple karsting and burials does not provide observable correlations between the velocity anisotropy and petrophysical/petrographic properties. In the Ellenburger carbonate, V p v values are mostly higher (∼7.9%) than the V p h values and are likely to be related to near-vertically aligned regional fractures. The orientation and amount of velocity anisotropy and of permeability is scale dependent when fractures are present. Seismic anisotropy is closely related to subsurface lithology, rock fabric and texture, and to its diagenetic history. Consequently, analysis of seismic anisotropy provides an important tool for reservoir characterization and environmental and engineering applications.

Effects of diagenetic processes on seismic velocity anisotropy in near-surface sandstone and carbonate rocks

The effects of diagenetic processes on velocity anisotropy in Ferron sandstone and Ellenburger carbonate core samples are investigated using petrophysical/petrological and P-wave velocity ( V p ) data at room pressure and ambient (air dry) saturation. The V p data are measured in both vertical ( V p v ) and horizontal ( V p h ) directions (parallel and perpendicular to the axes of the cores, respectively), using a 125-kHz sonic probe. The petrophysical/petrological properties for the Ferron sandstone are bulk porosity, fluid permeability and volume fractions of quartz, clay, feldspar and calcite ; for the Ellenburger carbonate, they are bulk porosity, vertical permeability, horizontal permeability, maximum permeability, grain density and bulk density. In the Ferron sandstone samples, the V p h values are higher (∼8.5%) than the V p v values, which is attributed to laminated bedding or alignment of grain orientation. Correlations between V p anisotropy and minerals that represent diagenetic processes are clearly demonstrated in the Ferron sandstone. Anisotropy decreases with decreasing feldspar, increasing calcite and increasing clay. This correlation suggests that calcite cementation and pore-filling kaolinite reduce velocity anisotropy of the Ferron sandstone by reduction of the net grain alignment or preferred pore orientations. The heterogeneity of the Ellenburger carbonate caused by multiple karsting and burials does not provide observable correlations between the velocity anisotropy and petrophysical/petrographic properties. In the Ellenburger carbonate, V p v values are mostly higher (∼7.9%) than the V p h values and are likely to be related to near-vertically aligned regional fractures. The orientation and amount of velocity anisotropy and of permeability is scale dependent when fractures are present. Seismic anisotropy is closely related to subsurface lithology , rock fabric and texture, and to its diagenetic history. Consequently, analysis of seismic anisotropy provides an important tool for reservoir characterization and environmental and engineering applications.

The Effect of Diagenetic Processes on the Radiochronology of Soft Sediments Using210Pb and137Cs

Core samples taken from North Lake, a brackish lake located in south Louisiana, were used to compare radiochronological methods for soft sediments. The radionuclides137Cs and210Pb were used to determine the ages of the core segments. The similarities in the vertical profiles of native lead and the radiolead suggested that diagenesis must be considered before accurate dates can be obtained. Correcting for the effects of diagenesis led to improved correlations with the137Cs dates and sedimentation rates. Sedimentation rates based on the corrected210Pb dates were 0.234 g/cm2year compared to the value for137Cs of 0.210 g/cm2year.

A Distributed Reactivity Model for Sorption by Soils and Sediments. 3. Effects of Diagenetic Processes on Sorption Energetics

Isosteric heats of phenanthrene sorption were calculated from isotherms measured at different temperatures on three natural soil materials having organic carbon fractions subjected to varying degrees of diagenetic alteration. The isotherms were highly nonlinear, and the sorption process became less exothermic with increased solid-phase loading. These observations are consistent with a classical surface adsorption reaction. However, isosteric heat values calculated using a pure supercooled solute standard state (-16.3 to +25.5 kJ/mol) were less exothermic than expected for a surface adsorption process and least exothermic for the sorbent having the highest sorption capacity

Studies on bedded cherts

Radiolarian cherts from two localities in North-western Italy have been studied by optical and scanning electron microscopy, X-ray diffraction and X-ray fluorescence analysis. The quality of preservation of the radiolarians decreased from dark red to pale cherts. The effect of diagenetic processes (solution, reprecipitation and recrystallization) was least in red cherts where contained hematitic and clay material inhibited the movement of pore solutions, and greatest in the pale cherts resulting in the destruction of the original skeletal material. It is suggested that these radiolarian cherts were produced from an accumulation of siliceous radiolarian skeletons subsequently altered diagenetically with consequent loss of skeletal structure.