Sandstone diagenesis

A B S T R A C T Diagenesis exerts a strong control on the quality and hetero-geneity of most clastic reservoirs. Variations in the distribution of diagenetic alterations usually accentuate the variations in depositional porosity and permeability. Linking the types and distribution of diagenetic processes to the depositional facies and sequence-stratigraphic framework of clastic successions provides a powerful tool to predict the distribution of diage-netic alterations controlling quality and heterogeneity. The het-erogeneity patterns of sandstone reservoirs, which determine the volumes, flow rates, and recovery of hydrocarbons, are controlled by geometry and internal structures of sand bodies, grain size, sorting, degree of bioturbation, provenance, and by the types, volumes, and distribution of diagenetic alterations. Variations in the pathways of diagenetic evolution are linked to (1) depositional facies, hence pore-water chemistry, depositional porosity and permeability, types and amounts of intrabasinal grains, and extent of bioturbation; (2) detrital sand composition; (3) rate of deposition (controlling residence time of sediments at specific near-surface, geochemical conditions); and (4) burial thermal history of the basin. The amounts and types of intra-basinal grains are also controlled by changes in the relative sea level and, therefore, can be predicted in the context of sequence stratigraphy, particularly in paralic and shallow marine environments. Changes in the relative sea level exert significant control on the types and extent of near-surface shallow burial

Siliciclastic sediments owe their origin mainly to land sources. New discriminant-function-based major-element diagrams for the tectonic discrimination of siliciclastic sediments from three main tectonic settings: island or continental arc, continental rift, and collision, have been constructed for the tectonic discrimination of high-silica [(SiO 2 ) adj = 63%-95%] and low-silica rocks [(SiO 2 ) adj = 35%-63%], where (SiO 2 ) adj refers to the SiO 2 value obtained after volatile-free adjustment of the ten major-elements to 100 wt.%. These diagrams are based on worldwide examples of Neogene-Quaternary siliciclastic sediments from known tectonic settings, log e -ratio transformation of ten major-elements with SiO 2 as the common denominator, and linear discriminant analysis of the log e -transformed ratio data. The success rates of these diagrams as judged from the original data varied from 84.5% to 93.6%. These diagrams were successfully tested on Neogene to Quaternary rocks not included in the original database. These discriminant diagrams were also successfully applied on older high-silica and lowsilica sandstones and shales of Paleoarchean-Ediacaran age. Finally, these diagrams were shown to be useful against chemical changes related to analytical errors, weathering, recycling and post-depositional processes.

We present the results of a multidisciplinary study on the relationships between the structural archi- tecture of an extensional fault system in poorly lithified shallow marine sands and the associated pattern of diagenetic carbonate concretions. Based on their shape, spatial distribution, cement texture and chemistry, and isotopic signature, carbonate concretions are grouped into (1) tabular (encompassing also nodular and lens- shaped) concretions developed within the fault zones, and (2) elongate and coalescent strata-bound concre- tions formed adjacent and mostly parallel to fault zones. Cement chemistry and morphology, as well as stable carbon–oxygen isotopic signatures, indicate that tabular concretions formed during early diagenesis in the vadose mixing marine–meteoric zone, possibly during coseismic rupture propagation and, in the interseismic periods, as a result of slow capillary suction along the low-permeability fault zones. On the other hand, elon- gate concretions formed after regional uplift and during telodiagenesis by precipitation from meteoric, phreatic water flowing parallel to the fault zones, which acted as a hydraulic barrier. The concretion patterns record the evolution of fluid flow orientation and the changing chemistry of fault-zone fluids, which were fundamentally driven by fault system propagation and resulting fault architecture during two major diagenetic stages.

The pore space fractal dimension was measured using a model of a porous body based on the Menger sponge and mercury porosimetry data for selected samples of the reservoir sandstones of the Weglowka oil field (SE Poland) in a pore-throat diameter range between 91 and 0.0090 μm. Based on the digital analyses of the two-dimensional images of thin sections impregnated with blue-dyed epoxy and taken under an optical microscope as well as the images of thin sections taken under a cold field emission gun scanning electron microscope (FEGSM) in the backscattered electron image mode, the current paper tries to quantify the pore space of sandstones by using the box counting method. The results derived from analysis of the pore-throat diameter distribution by mercury porosimetry revealed the multifractal structure of the pore space of sandstone in two separated ranges of the pore-throat size considerably lesser than the pore-throat diameters (10–50 μm) corresponding to threshold pressures. This means that only the pore throats connecting wider parts of the pore network (pores) exhibit the fractal structure. The assumption that the fractal dimension monitoring the distribution of the pore-space volume within the smallest pore-throat diameters characterizes the overall pore-throat network in the rock sample provides a device to set apart the distribution of the pore-throat volume from the distribution of the pore volume. On the other hand, the fractal dimensions derived from the image analysis of thin sections describe the pore space as a whole.

Los Paramoudras de Jaizkibel son de gran interés ya que se desarrollan en arenisca de edad Eoceno, una litología inusual para su formación. Tanto la concreción del Paramoudra, como el tubo que constituye el ichnofósil, están compuestos de granos de cuarzo, y lo que los diferencia (a simple vista y al microscopio) es la coloración del cemento que los aglutina, el cual se encuentra en pequeñas cantidades. El análisis mineralógico normal y por difracción de rayos X (DRX) no permite discriminar la composición de estos cementos, debido a su pequeño tamaño de grano y potente señal difractada por los granos mayores de cuarzo. En este trabajo se utiliza la espectroscopía Raman para determinar la composición de los tubos y cementos de Paramoudras a través de una sección continua, ilustrada mediante macro y micro fotografías. La composición de los tubos es compleja. Se discuten sus características e ichnología.

The late-Permian succession of the Upper-Indus Basin is represented by carbonate dominated Zaluch Group, which consists of Amb, Wargal and Chhidru Formations. These formations accumulated on the southwestern shelf of the Paleo-Tethys Ocean, north of hydrocarbon-producing Permian strata of the Arabian Peninsula. The reservoir properties of mixed clasticcarbonate Chhidru Formation are evaluated based on Petrography, Scanning Electron Microscopy (SEM), Energy Dispersive Xrays-Spectroscopy (EDX) and X-ray Diffraction (XRD) techniques. The diagenetic features, ranging from marine (isopachous fibrous calcite, micrite), through meteoric (blocky calcite-I, neomorphism and dissolution) to burial (poikilotopic cement, blocky calcite-II-III, fractures, fracture-filling and stylolites) are recognized. Major porosity types included fracture and moldic, while inter-and intra-particle porosities also existed. The visual porosity from 1.5-7.14% with an average of 5.15% was recognized. The sandstone facies (CMF-4) had the highest average porosity of 10.7%, while siliciclastic grainstone microfacies (CMF-3) showed average porosity of 5.3%. The siliciclastic mudstone microfacies (CMF-1) and siliciclastic wacke-packestone microfacies (CMF-2) showed the lowest porosity of 4.8% and 5.0%, respectively. Diagenetic processes like cementation, neomorphism, stylolitization and compaction have reduced the primary porosities. However, processes of dissolution and fracturing have produced secondary porosity. On average, the Chhidru Formation in Nammal Gorge (Salt Range) showed promising and at Gula Khel Gorge (Trans-Indus) lowest porosity.

Se presentan los resultados obtenidos con microscopía electrónica de barrido (SEM) y microanálisis EDX, sobre una muestra de Paramoudra en arenisca de edad Eoceno (Formación Jaizkibel). La morfología de las tres partes de la muestra (concreción, tubo y cordón central) es semejante. La trama mayoritaria es de granos de cuarzo (con pequeños fragmentos detríticos de otras rocas) unidos por un cemento silíceo, con cantidades menores de carbonato de calcio, y trazas de otros elementos (aluminio, potasio, sodio, magnesio, hierro, cloro). El microanálisis EDX muestra un enriquecimiento en cloro y aluminio de la parte interna del tubo, en relación a la concreción externa.

Los Paramoudras de Jaizkibel son de gran interés ya que se desarrollan en una litología inusual para su formación (flysch de arenisca de edad Eoceno). Tanto la concreción del Paramoudra, como el tubo que constituye el ichnofósil y su cordón central, están compuestos de granos de cuarzo, y lo que los hace diferenciables (a simple vista y al microscopio) es la coloración del cemento que los aglutina, el cual se encuentra en pequeñas cantidades. El análisis mineralógico normal y por difracción de rayos X (DRX) no permite discriminar la composición de estos cementos, debido a su poca cantidad, pequeño tamaño de grano y potente señal difractada por los granos mayores de cuarzo, por todo lo cual se hace necesario recurrir a técnicas analíticas más finas. En este trabajo se utiliza la espectroscopía Raman y la espectroscopía por fotoelectrones de rayos X (XPS) para determinar la composición química y mineralógica de los Paramoudras. En los cementos fueron hallados óxido de silicio en forma de cuarzo, carbonato de hierro en forma de siderita, y carbonato de calcio en forma de calcita y aragonito. La composición de los tubos es más compleja y plantea diversos interrogantes. Se discuten diversos aspectos sobre su origen, características e ichnología.

Clay coatings have been widely accepted by many workers as an explanation for preserving high porosity in deeply buried sandstones, but few workers have realized that similar effects can be produced by microcrystalline quartz coatings. This phenomenon can be expected only under special circumstances, but in such cases it can have profound consequences for exploration.

Previous studies of jointing in sedimentary rocks have shown that joint-network architecture is controlled by mechanical stratigraphy, which is described by (1) the thickness and rigidity of stratigraphic units and (2) the nature of interfaces between beds. Using joint pattern to infer rigidity contrast between beds is complicated by the possibility that material properties of the beds may change between fracturing events. Observation of both an early bed-contained joint set and a differently oriented late throughgoing joint set in the same beds of growth strata in the Oliana anticline in the Spanish Pyrenees may reflect changing mechanical properties (i.e., via diagenesis) during the folding process. Using a Schmidt hammer, we assess the rigidity contrast between the individual units and show that the present-day contrast is not great enough to terminate joints at interfaces. This result is consistent with an interpretation that the late-stage throughgoing joints formed in strata with conditions similar to those of the present day and that the early bed-contained joints formed when the rigidity contrast between beds was significantly greater than that of the present day. For example, differential diagenesis rates between layers of differing grain size would produce temporally changing mechanical stratigraphy. We propose that changes in mechanical stratigraphy may have previously been unrecognized in other stratigraphic sections and that these changes affect joint-network architecture and subsequent fluid-flow pathways.

The fluvial Triassic reservoir subarkoses and arkoses (2409·5–2519·45 m) of the El Borma oilfield, southern Tunisia, were subjected to cementation by haematite, anatase, infiltrated clays, kaolinite and K-feldspar at shallow burial depths from meteoric waters. Subsequently, basinal brines controlled the diagenetic evolution of the sandstones and resulted initially in the precipitation of quartz overgrowths, magnesian siderite, minor ferroan magnesite and anhydrite. The enrichment of siderite in 12C isotope (δ13CPDB= - 14·5 to - 9‰) results from derivation of carbon from the thermal decarboxylation of organic matter. During further burial, the precipitation of dickite and pervasive transformation of kaolinite into dickite occurred, followed by the formation of microcrystalline K-feldspar and quartz, chlorite and illite, prior to the emplacement of oil. Present day formation waters are Na-Ca-Cl brines evolved by the evaporation of seawater and water/mineral interaction and are in equilibrium with the deep burial (≤ 3·1 km) minerals. These waters are suggested to be derived from the underlying Silurian and Devonian dolomitic mudstones.

Fraktalna struktura systemu porowego ska³ opiera siê na modelu g¹bki Mengera (Turcotte, 1992), narzêdziem s³u¿¹cym do obliczenia wymiarów fraktalnych ich przestrzeni porowej zaoe s¹ rozk³ady oerednic porów otrzymane w testach porozymetrycznych (Angulo i in., 1992; Such, 1998). W próbkach piaskowców dolnokredowych, nale¿¹cych do ró¿nych litofacji oraz klas zbiornikowych, wyró¿nionych z uwagi na wielkooeae wspó³czynnika porowatooeci efektywnej, wyznaczono wartooeci wymiarów fraktalnych i zakres wielkooeci kapilar, w których zaznacza siê struktura fraktalna. Ze wzglêdu na zaburzenia pomiarów porozymetrycznych pod wp³ywem efektu brzegowego nie okreoelano wymiaru fraktalnego w próbkach o porowatooeci efektywnej mniejszej od 8% (Such, 2002). W ka¿dej próbce, w której wykonano testy porozymetryczne, przeprowadzono równie¿ badania szlifów mikroskopowych impregnowanych nie-biesk¹ ¿ywic¹. Dokonano oceny wielkooeci strat porowatooeci wskutek kompakcji (wskaŸnik COPL -compaction porosity less) i cementacji (indeks CEPL -cementation porosity less), które wyznaczono w stosunku do pierwotnej porowatooeci wed³ug zaleceñ Ehrenberga (1995).

Porous sandstones are important reservoirs for geofluids. Interaction therein between deformation and cementation during diagenesis is critical since both processes can strongly reduce rock porosity and permeability, deteriorating reservoir quality. Deformation bands and fault-related diagenetic bodies, here called “structural and diagenetic heterogeneities”, affect fluid flow at a range of scales and potentially lead to reservoir compartmentalization, influencing flow buffering and sealing during the production of geofluids. We present two field-based studies from Loiano (northern Apennines, Italy) and Bollène (Provence, France) that elucidate the structural control exerted by deformation bands on fluid flow and diagenesis recorded by calcite nodules associated with the bands. We relied on careful in situ observations through geo-photography, string mapping, and unmanned aerial vehicle (UAV) photography integrated with optical, scanning electron and cathodoluminescence microscopy, and stable isotope (δ13C and δ18O) analysis of nodules cement. In both case studies, one or more sets of deformation bands precede and control selective cement precipitation. Cement texture, cathodoluminescence patterns, and their isotopic composition suggest precipitation from meteoric fluids. In Loiano, deformation bands acted as low-permeability baffles to fluid flow and promoted selective cement precipitation. In Bollène, clusters of deformation bands restricted fluid flow and focused diagenesis to parallel-to-band compartments. Our work shows that deformation bands control flow patterns within a porous sandstone reservoir and this, in turn, affects how diagenetic heterogeneities are distributed within the porous rocks. This information is invaluable to assess the uncertainties in reservoir petrophysical properties, especially where structural and diagenetic heterogeneities are below seismic resolution.

Individual quartz overgrowths in siltstone of the late Cambrian Eau Claire Formation (Fm.) are systematically zoned in oxygen isotope ratio (δ 18 O). In situ analysis of δ 18 O was performed with 3 and 15 μm beam spots by secondary ion mass spectrometer (SIMS) on detrital quartz grains and quartz overgrowths. These results from thin lenses within impermeable mudstones reflect samples that were sealed from basin-wide fluid flow and compliment previous studies of more permeable sandstones. Individual grains of detrital quartz (DQ) are homogeneous in δ 18 O. The average δ 18 O values in fine-grained detrital quartz in mudstones and siltstones and in coarser-grained quartz in the Eau Claire Fm., Mt. Simon and St. Peter Sandstones (Ss.) are essentially identical at δ 18 O = 10‰ VSMOW, suggesting that detrital quartz is dominantly igneous in origin. The δ 18 O values of overgrowth quartz (OQ) of buried samples from the Illinois Basin are higher and quartz overgrowths are systematically zoned outward from the detrital cores. These gradients are similar to those from the underlying Mt. Simon Ss., and are best explained by increasing temperatures during burial. Pressure solution is evident in thin section and may have supplied significant silica for overgrowths. In contrast to the deeply buried samples from the Illinois Basin, quartz overgrowths in samples from the Wisconsin Arch are homogeneous and higher in δ 18 O. Those overgrowths are interpreted as quartz cements formed in a near-surface environment (b 40 °C), which is consistent with geological evidence that these rocks were only shallowly buried (b 500 m). Based on these δ 18 O(OQ) results and the modeled thermal history during burial of the basin, the earliest-formed quartz overgrowths were produced at low temperature from low δ 18 O(water) around 450 Ma. The δ 18 O values in traverses of single overgrowths decrease by up to 9.1‰, showing continued cementation with increased burial, pressure solution, and heating until ~ 250 Ma. In traverses of the outermost zone of some overgrowths, oxygen isotope values become constant or increase slightly, possibly due to clay mineral dehydration reactions or later fluid infiltration. We present a new cementation and basin evolution model, in which the δ 18 O of cement correlates to the age of formation and the late overgrowths formed between 270 and 250 Ma, during and/or after the migration of brines that formed the Pb–Zn deposits of the Upper Mississippi Valley District (270 Ma). Cementation around 270 Ma would have reduced permeability, possibly ending the flow of ore forming brines.

A study of the Middle Jurassic Rannoch Formation in the Gullfaks Field shows that there is no relationship between the content and distribution of kaolinite and location relative to the late Cimmerian unconformity. From petrographic data most of the kaolinite is interpreted to be detrital, and only traces of authigenic kaolinite are observed. Mass-balance calculations are used to provide guidelines as to the likelihood of kaolinite being formed and preserved in sandstones under the unconformity. The result of the calculations shows that the propagation rate of the dissolution front in the sandstones was probably on average slower than the erosion rates during the formation of the late Cimmerian unconformity. Hence, transformation of significant amounts of feldspar and mica to kaolinite probably did not take place within the sandstones which at present underlie the unconformity in the Gullfaks Field. Periods of exposure during formation of unconformities may thus be less effective causes of kaolinitization in sandstones than has been assumed.

Depositional microfacies is an important influencing factor in the development of continental hydrocarbon-bearing reservoirs. However, the diagenetic responses to the depositional microfacies and their implications for reservoir quality variation remain enigmatic, which limits detailed reservoir description and prediction of the location of
high-quality reservoirs. The tight continental reservoirs of the Yanchang Formation in the Ordos Basin were targeted with the aim of clarifying the differences in diagenetic, petrophysical and pore-structural characteristics of tight reservoirs within different microfacies. A multi-methodological approach, involving petrographic assessments, logging evaluations, physical property experiments, digital SEM imaging analyses, 3D micro-CT imaging analysis, X-ray diffraction mineral identification and mercury intrusion experiments, was utilized to identify eight delta-front-lacustrine microfacies in this study. The results show that different microfacies reservoirs vary in compaction, dissolution, and clay mineral cementation. Interdistributary bays, shallow lacustrine mudstones, and semi-deep lacustrine mudstones that are rich in plastic clay and mica are those most likely to be affected by physical and chemical compaction. Reservoirs affected by meteoric water percolation and hydrocarbon generation, such as distributary channels, mouth bars and sheet sand reservoirs, are characterized by well-developed dissolution pores. Interbedded sand-mud deposits and lacustrine tidal incursions lead to strong ferrocalcite cementation into beach bar sandstones. The occurrence of authigenic chlorite and illite cements displays prominent microfacies zonation; kaolinitic and siliceous cementation show less variation than other diagenetic processes in different microfacies reservoirs. Finally, this study proposes two evaluation parameters: a pore connectivity index, E, and a pore structure evaluation parameter, A, to assist in clarification of the differences in reservoir quality resulting from the range of depositional microfacies. This provides guidelines for detailed reservoir description and grading evaluation.

The lowermost Carboniferous rocks in the Cockburnspath area of east Berwickshire (southern Scotland) are interpreted as coastal floodplain sediments. A lower mudstone-dominated unit is composed of silty mudstones and shales with subordinate sandstones and argillaceous ferroan dolomites (cementstones). These are interpreted as distal floodplain sediments with periodic crevasse-splay deposition. The dark grey colour of the mudrocks suggests deposition in reducing conditions, probably in floodplain lakes. Most of the cementstones are concretionary, some with septarian cracks, suggesting an early diagenetic origin. An immature palaeosol suggests periodic pedogenesis under improved drainage. A synsedimentary erosion surface indicates incision of a valley into the floodplain, presumably in response to base-level lowering.

The fluvial Triassic reservoir subarkoses and arkoses (2409·5–2519·45 m) of the El Borma oilfield, southern Tunisia, were subjected to cementation by haematite, anatase, infiltrated clays, kaolinite and K-feldspar at shallow burial depths from meteoric waters. Subsequently, basinal brines controlled the diagenetic evolution of the sandstones and resulted initially in the precipitation of quartz overgrowths, magnesian siderite, minor ferroan magnesite and anhydrite. The enrichment of siderite in 12C isotope (δ13CPDB= - 14·5 to - 9‰) results from derivation of carbon from the thermal decarboxylation of organic matter. During further burial, the precipitation of dickite and pervasive transformation of kaolinite into dickite occurred, followed by the formation of microcrystalline K-feldspar and quartz, chlorite and illite, prior to the emplacement of oil. Present day formation waters are Na-Ca-Cl brines evolved by the evaporation of seawater and water/mineral interaction and are in equilibrium with the deep burial (≤ 3·1 km) minerals. These waters are suggested to be derived from the underlying Silurian and Devonian dolomitic mudstones.

The sedimentological analysis of several profiles, registered within the eastern part of the frontal margin of the Silesian nappe, reveal the following lithofacies of the subaqueous gravity flows: conglomeratic sandstones (SC), sandstones (S), sandstone–mudstone couplets (SM), mudstone–sandstone couplets (MS) and mudstones (M). Apparent density and water absorptivity at atmospheric pressure were determined in samples of sandstones and subordinate gaizes belonging to the lithofacies SC, S, SM and MS. The selected samples were the subjects of further observations. Blue–epoxy–impregnated thin sections were examined in transmitted light and cathodoluminescence. The mercury injection versus capillary pressure curves, the coefficients of permeability and effective porosity were also determined. The graphical grain–size parameters, the grain–to–grain contacts and the grain packing indices were establised. The mineral composition of framework grains and cements as well the contents of rigid and ductile components were assessed with the use of modal analysis of thin sections. The porosity loss due to compaction (COPL) as well due to cementation (CEPL) were evaluated in reference to original sandstone porosity (OP) according to recommendations by Ehrenberg (1995).
The fractal dimensions were determined from the cumulative curves of pore size distribution obtained from the mercury injection tests on the assumption the power dependence between intrusion volume and capillary pressure (Angulo et al., 1992). Only samples with the coefficients of effective porosity greater than 8% were taken into consideration (Such, 2002). The results show that the pore space of sandstones reveal the multifractal structure in the range of pore diameters much smaller than the threshold diameter. It determines that only capillaries which constitute the channels connecting the chambers (pores) display the fractal structure. The evelopment of pore space of the Early Cretaceous Weglowka sandstones is similar (Stanczak, 2009) as well the pore space of the Istebna and Ciezkowice sandstones (Such, 2002). It was established that the pore space of the Lgota sandstones is characterized by two fractal dimensions, the first D1 = 2.784–2.984 for the least diameters of capillaries 0.609–0.009 μm and the second D2 = 1.542–2.974 in the range of capillaries 17.269–0.045 μm. It was found that the larger the fractal dimension D1  the higher coefficient of effective porosity as well the higher the threshold diameter and the average diameter of effective size of the throats. There is the increase of both fractal dimensions D1  and D2  with the decreasing content of ductile grains. On the other hand the decrease of COPL is related to the increase of D1 . The same relationship was found for the Weglowka sandstones (Stanczak, 2009). The procedure of determination of the fractal dimension D1  allows to obtain the distribution of the channels volume and the pores volume which constitute the pore space of rock. It is the essential tool of distinguishing the reservoir classes of rocks according to the parameter D1.

Angulo R.F., Alvarado V., Gonzales H., 1992 – Fractal dimensions from mercury intrusion capillary tests. Society of Petroleum Engineers, 23695–MS.

Ehrenberg S.N., 1995 – Measuring sandstone compaction from modal analyses of thin sections: how to do it and what the results mean. Journal of Sedimentary Research, A65: 369–379.

Stanczak G., 2009 – Variability of fractal structure of pore space of the Early Cretaceous Weglowka sandstone (the Subsilesian unit, the Outer Carpathians). Przeglad Geologiczny, 57, 4: 318.

Such P., 2002 – Investigations of a pore space of reservoir rocks with the use of the fractal approach. Prace Instytutu Górnictwa Naftowego i Gazownictwa, 115: 3–28.

Se describe la composición química y mineralógica de envolturas y tubos de Paramoudras en arenisca de la Formación Jaizkibel (de edad Eoceno) en base a espectroscopía Raman. Las estructuras tubulares atraviesan gangas de naturaleza arcillo-arenosa que delimitan los Paramoudras, e incluso prosiguen en el sustrato de arenisca adyacente, variando su composición. En los cementos de las envolturas y tubos han sido identificados cuarzo, calcita, aragonito, anhidrita, aluminita, anatasa (dióxido de titanio), magnetita (óxido de hierro II-III), silicatos de magnesio, y compuestos del azufre en forma de sulfatos y sulfuros. Se comenta la ocurrencia de los distintos minerales, sus características e ichnología.