Clay Sandstone Research Articles

Experimental Determination of the Rates of Precipitation of Authigenic Illite and Kaolinite in the Presence of Aqueous Oxalate and Comparison to the K/Ar Ages of Authigenic Illite in Reservoir Sandstones

AbstractThe importance of precipitation rate as an effective control on illite and kaolanite formation during diagenesis has been examined by measuring precipitation rates, from Al fluid concentration, in a Dickson fluid-sampling vessel at 160°-250° and 500 bars (50 MPa). These experiments are considered to be analogues of the precipitation of clays in sandstones from porewaters containing dissolved carboxylic acids, which have a transient stability and may influence aluminosilicate solubility. Precipitated illite had a lath-shaped morphology and its composition was consistent with authigenic illite in sandstones. Kaolinite formed tabular rather than vermicular shaped crystals. Kaolinite precipitation rate was two orders of magnitude faster than illite precipitation and was rate-limited by the decomposition of oxalate; kaolinite formation should be equilibrium-controlled at virtually all stages of burial. Extrapolation of illite precipitation rate to burial temperatures indicates that the first appearance of illite in a burial sequence may be kinetically controlled. A model of illite precipitation based on these experimental results has been used to predict the time required to precipitate illite during burial of a sandstone, taking into account temperature changes during burial. For northern North Sea examples, a predicted illitization threshold of -60°C occurring at 60–80 Ma corresponds to the observed initiation of authigenic illite precipitation. Times of around 2–5 Ma would be required to reach a 98% approach to equilibrium at this threshold. The main phase of illite precipitation in the northern North Sea basin is a later, hydrologically controlled event (30-50 Ma). Equilibrium would be approached in around 0.1 Ma during this phase, which is consistent with the narrow illite K/Ar age range (1-5 Ma) recorded for some sequences.

Pore-Size Distribution in Sandstones: GEOLOGIC NOTE

Pore size in reservoir rocks is measured either directly in thin sections under a petrographic microscope or, nominally, by injecting mercury under pressure into the rock sample. The frequency distribution for nominal pore-size data shows a log-normal pattern similar to that for grain size in sedimentary rocks. Therefore, the same transformed size unit (phi = -log2 X) traditionally used for grain size can be used for pore-size measurement. A routine was created which measured the entry mercury capillary pressure (Pcatm), converted this pressure to pore size in millimeters (Xmm = 0.007/Pcatm), transformed the size values into phi units, and displayed the data as a pore-size frequency distribution. For clean, well-sorted sandstone , the distribution tend to a bell-shaped normal curve. Sandstones free from gravel and clay tend to show bimodal pore-size distributions as the grain-size sorting gets poorer. In fine to medium sandstones, a pore-size mode of 5-6 phi is associated with intergranular voids. Authigenic kaolin develops a pore-size mode around 8 phi. Quartz cement tends to obliterate small pores. Carbonate cement does not show preference for pore size, blocking eventually all the pore space. Clay sandstones have a pore-size mode of 11-12 phi. Pores smaller than 2 µm (phi > 9) are considered to be micropores; they provide a large available surface and, accordingly, a high irreducible water saturation.

Experimental Simulation of Clay Precipitation within Reservoir Sandstones 1: Techniques and Examples

Experiments have been performed in which cores of clay-free sand, and clay-forming mineral phases placed beneath the cores, have been treated hydrothermally with various solutions. Using experimental conditions in the range 150-500 degrees C and 0.75-1.0 kbar (75-100 MPa) illite, illite-smectite, kaolinite, 7 A-chlorite and trioctahedral smectite have been precipitated in the sand pore space with morphologies similar to those of authigenic clay minerals in sandstone petroleum reservoirs. This study demonstrates that the mechanism, rate of clay precipitation, and controls on clay morphology and mineral stability can all be determined by experimental studies. These results have direct application to the interpretation of clay mineral paragenesis in reservoir sandstones. The study demonstrates: 1) authigenic clays in sandstones are direct precipitates from solution, and do not require a preexisting clay coating; 2) grain surface effects, such as abrasion, and detrital clay coatings do promote nucleation of clays; and 3) clay morphology is controlled by precipitation mechanism, specifically, the formation of metastable clays which are subsequently modified. The synthesis of authigenic-type illite and illite-smectite is by direct precipitation from solution following dissolution of mineral phases such as detrital illite or feldspar. Both platy and fibrous illite morphotypes are synthesized; platy illites are more readily formed. The compositions of synthetic illite and illite-smectite clays are controlled by temperature and a (sub K (super +) ) / a (sub H (super +) ) fluid composition ratio. Platy illites show subtle changes in morphology with compositional variation. Illite-smectite and illite with low K content forms a crenulated boxwork texture, while more K-rich illites approaching muscovite composition form larger, discrete plates. No correlation between illite composition and fibrous formation is recognized. Kaolinite and chlorites were also precipitated in experiments. Vermiform kaolinite appears to be a direct precipitate. Kaolinite morphology, however, is controlled by precipitation mechanism; isolated, discrete kaolinite plates are associated with early formation of a mixed-layered smectitic phase. Chlorite synthesis proceeds via the formation of metastable 7 A-chlorite and expandable phases, which have previously been recognized as possible precursors to 14 A chlorites in reservoir sandstones.

Study of the variations in permeability and cationic exchange kinetics during solution changes in clay sandstone

Study of the variations in permeability and cationic exchange kinetics during solution changes in clay sandstone

Evaluation of Some Petrophysical Parameters for the Lower Cretaceous Reservoir Rocks, Western Desert, Egypt

Abstract Water saturation in otherwise hydrocarbon-saturated sandstones is closely related to the sandstone's clay mineralogy. Laboratory analysis of 20 sandstone core samples containing kaolinite, illite and chlorite is used to provide the basis for deciding which petrophysical parameters are most suitable for evaluating clay mineral content and distribution. Among the measured petrophysical parameters, the Mounce potential gives the most realistic correlation with actual clay mineralogy. The existence of both siderite and hematite as cementing materials within the Kharita sandstone pore spaces, reduces the cation-exchange capacity of clay minerals by a clogging action. Consequently, the Mounce potential has been diminished. Correlations of the measured petrophysical properties have been elucidated to throw some light on both the Alam El-Buieb and Kharita reservoir rocks.

Mineralogical analysis and the evaluation of the petrophysical parameter Vshale for reservoir description

Water saturation in otherwise hydrocarbon-saturated sandstones is closely related to the sandstone's clay mineralogy. Laboratory analysis of the clay mineralogy of three sandstones, containing kaolinite, chlorite and illite, is used to provide the basis for deciding which logs are most suitable for evaluating clay mineral content and distribution ( V shale). The gamma log, which is held traditionally to be a reliable ‘shale’ log, proves to be a poor indicator of shaliness (i.e. clay mineral content) in all three examples. Microresistivity (MSFL) logs, and in some instances neutron porosity logs, give the most realistic correlations with the actual clay mineralogy. Standard values for clay mineral properties which are often used in the petrophysical characterization of sandstones are thought to be unreliable especially when not used in conjunction with textural data from petrographic studies.

Diagenesis of Cotton Valley Sandstones, Catahoula Creek Field, Southern Mississippi: ABSTRACT

Three conventional cores from the deep Cotton Valley clastics in south Mississippi were analyzed by thin-section petrography, scanning electron microscopy, x-ray diffraction, and cathodoluminescence. The ^dgr13C and ^dgr18O values were determined on carbonate cements in sandstones. The sandstones are predominantly subarkose, arkose, and quartzarenites, with minor amounts of quartz and feldspathic wackes. K-feldspar, muscovite, and plagioclase are relatively fresh. Authigenic clay in sandstones is almost exclusively chlorite that occurs as a pore-lining and pore-filling phase, most commonly associated with porous zones. Feldspar diagenesis includes overgrowths of both potassic feldspar and albite. Typically, fresh detrital K-feldspar grains have overg owths of K-feldspar that are partly dissolved, whereas plagioclase grains (with partly dissolved interiors) have unaltered albite overgrowths. Quartz overgrowth development is pervasive where not hindered by chlorite. Carbonates include intergranular calcite, dolomite, and iron-rich dolomite. The earliest phase is a pore-lining dolomite cement, followed by a later ferroan calcite. Ferroan dolomite (or ankerite) is associated with chlorite in relatively porous intervals. The ^dgr13C values of mixed carbonates in sandstones range from -0.8 to -4.4^pmil (PDB), which is typical of normal marine carbonates. The ^dgr18O values range from -5.0 to -12.3^pmil (PDB). Formation temperatures of approximately 115° -160°C are suggested by the oxygen isotope data from al ost pure late-stage ferroan dolomite. Petrographic evidence suggests that coarser grained sands were cemented early in the burial history by dolomite and calcite. These cements were later leached by formation waters, possibly related to the generation of organic acids and/or carbon dioxide released during kerogen maturation. Resultant secondary porosity was preserved by formation of bladed chlorite. The source of the iron, manganese, and magnesium necessary for chlorite formation was presumably the release of ions from smectite to illite conversion in shales, because ferromagnesian rock fragments in the sandstones are rare. The feldspar volume and stability and the ^dgr13C values of carbonate cements suggest the pore water was probably initially seawater and later brine that migrated upward from the Louann Salt Present brines contain 7,300 ppm of potassium and 51,113 ppm of sodium. Meteoric diagenesis of these rocks was likely minimal. These data suggest that original depositional porosities and permeabilities and carbonate cementation are significant controls in secondary porosity formation in the Cotton Valley sandstones. End_of_Article - Last_Page 1424------------

Origins of Clays Controlling Permeability in Tight Gas Sands

Summary The low permeabilities of Cretaceous and Paleocene tight gas sands in Wyoming and Colorado commonly are related to the presence of high-surface-area authigenic clays. These clays include chlorite, illite, and mixed-layer smectite/illite formed by regeneration of detrital clays or by diagenetic alteration of chemically unstable framework components such as volcanic rock fragments, some feldspars, and ferric oxides and hydroxides. The success of exploration and exploitation drilling programs in tight sands can be enhanced significantly through an understanding of the origins of these clays. Introduction Establishing a successful exploration and production strategy for tight gas sands requires an understanding of why these sandstones are tight. Not surprisingly, the low permeabilities of tight gas sands can be attributed directly to the relative abundance of very small pores. Small pore size can be related to one or more of these factors: fine grain size, low porosity, and the presence of dispersed clays. Siltstones are good examples of the first control. Low porosities most commonly are produced by extensive cementation and are the dominant factor reducing permeability locally in such units as the "J" sand of the Wattenberg field and Hosston formation of Louisiana and Mississippi. In most tight gas sands with porosities in the range of 8 to 15 %, low permeabilities can be attributed to the presence of dispersed clays. Some tight gas sands contain large pores, most of which represent dissolved framework grains, but have low permeabilities because these larger pores are interconnected only through very small pores. Sandstones in the Mesaverde group of western Colorado commonly exhibit this characteristic. This discussion focuses on tight gas sands in which clays are the major mechanism permeability reduction and emphasizes the origin of these clays and how an understanding of their origin can help in the evaluation of exploration and Production potential. Influence of Clays on Permeability Clays vary widely in the morphologies of the individual particles and in their aggregate morphology. These properties affect surface area and relative abundance of micropores (a micropore is defined arbitrarily as a pose less than 1 um in diameter). Kaolinite (upper left, Fig. 1) exhibits a compact book-type morphology and, in many sandstones, tends to occur as clusters present in only a fraction of the pores. Most chlorite (upper right, Fig. 1) displays cardhouse arrangements having significantly higher surface area and microporosity than kaolinite. Mixed-layer smectite/illite (lower left, Fig. 1) commonly develops as honeycomblike aggregates of curled flakes with stubby spines projecting from the edges. It has a surface area and microporosity intermediate between smectite, which has cellular morphology and is not shown since it is uncommon in tight gas sands, and illite, which develops as curled flakes from whose edges highly elongate ribbonlike spines project (lower right, Fig. 1). The morphologies just described directly affect reservoir quality. Fine-grained, well-sorted sandstones without clays exhibit porosity/permeability relationships exemplified by the upper trend in Fig. 2. Sandstones with minor amounts (2 to 10%) of kaolinite, chlorite, and illite follow trends that increasingly diverge from that for clay-free sandstones. Adequate data are not available with which to plot accurately a trend for smectite and mixed-layer smectite/illite, but it is estimated that smectite follows the trend for chlorite and that the trend for mixed-layer smectite/illite is transitional between the trends for chlorite and illite. Clay Types There are two types of clays in sandstones, detrital and authigenic. Detrital clays are introduced into a sandstone by physical processes at the time of deposition or by biogenic processes shortly after deposition. JPT P. 2871^

Catagenesis in shales and occurrence of authigenic clays in sandstones, North Sabine H-49 well, Canadian Arctic Islands

Authigenic clays in eight sandstone samples from the North Sabine H-49 well in the Sverdrup Basin, N.W.T., have been studied by X-ray diffraction and scanning electron microscopy. The following sequence of authigenic minerals was observed with increasing burial depth: quartz–kaolinite, illitic 2:1 layer silicates, and chlorite. It is suggested that the formation of authigenic clays in sandstones can be considered in the context and as a product of catagenesis of adjacent shales. Upon burial, water is released from the shales by compaction, clay dehydration, and clay destruction. Continuing compaction carries the products of clay transformation to the sandstones where they precipitate to form authigenic clays. The nature of the authigenic clay is directly related to the physico-chemical conditions existing at various depths.

Mixed-Layer Clays in Eocene Interlaminated Shales and Sandstones: ABSTRACT

Kaolinite, illite, and mixed-layer montmorillonite-illite are present in interlaminated sandstones and shales from the Eocene Wilcox Formation of the Texas Gulf Coast at depths of 2,000 to 4,000 m. Ratios of kaolinite to discrete illite are higher in the sandstones than in the interlaminated shales. Mixed-layer clays in the sandstones are 5 to 20% more expansible and less ordered than those in interlaminated shales. These mineralogic differences are interpreted to reflect a significant difference in the solution chemistry of pore waters in the sandstones from that of pore waters in the interlaminated shales. At the relatively shallow depth of 2,000 m, mixed-layer montmorillonite-illite from the shales is roughly 30% expansible; this figure is significantly less than the 7 % expansibility reported by J. Hower et al for Miocene age mixed-layer clays at similar depths. Assuming similar source and geothermal gradient for the two sets of samples, two explanations may account for these differences. The most plausible explanation is that the expansibility of the mixed-layer clay is controlled by the montmorillonite-to-illite transformation rate. Because the Eocene sediments have had an additional 25 m.y. to react, the montmorillonite-to-illite transformation is more complete in these samples. An alternative explanation is that the chemistry of pore waters in the Wilcox Formation is significantly different from that of pore waters in sediments studied by Hower. End_of_Article - Last_Page 469------------

Authigenic Clays in Sandstones: Recognition and Influence on Reservoir Properties and Paleoenvironmental Analysis

ABSTRACT Thin-section/scanning electron microscope studies of thousands of sandstones representing many ages. compositions, and depositional environments indicate authigenic clays are far more common than previously recognized. Authigenic clays occur as pore linings, pore fillings, pseudomorphous replacements, and fracture fillings. Sandstones also contain allogenic clays. Allogenic clays originate as terrigenous material (dispersed matrix, sand-sized floccules, sand- to cobble-sized mud or shale clasts) or are introduced subsequent to deposition as a result of bioturbation or infiltration. An authigenic origin can be established on the basis of clay composition, structure, morphology and distribution, and sandstone textural properties. However, no individual criterion is an infallible indicat r of authigenic origin. The most reliable criteria are a) delicacy of clay morphology, which precludes sedimentary transport, b) occurrence of the clay as pore linings absent only at grain contacts, and c) composition radically different than associated allogenic clays. Distinctions between authigenic and allogenic clays become difficult if the latter are recrystallized or if either is extensively deformed by burial or tectonism. Studies of authigenic clays utilizing a combination of thin-section, scanning electron microscope with nondispersive elemental analyzer, and x-ray diffraction show that each of the major clay groups exhibit a limited number of distinctive morphologies. Smectite occurs as highly wrinkled or honeycomb-like pore linings, the individual flakes of which are not resol able. Illite forms pore-lining overlapping flakes whose edges tend to curl away from the grain surface and from which highly elongate lath-like projections may extend. Mixed-layer smectite/illite morphologies resemble those of both smectite and illite. Chlorite occurs primarily as pore-lining pseudohexagonal flakes with a cardhouse, honeycomb, or rosette arrangement. Kaolinite and dickite most commonly form pore-filling books of stacked pseudohexagonal flakes. Occasionally, they form thin pore-lining sheets of overlapping pseudohexagonal flakes. Authigenic clays are a major control on reservoir quality. Permeability and water saturation are particularly sensitive to the relative abundance of clays. Interpretations of depositional environment and provenanco based on sandstone composition nd texture may be inaccurate if the presence of authigenic clay is overlooked.

Some sedimentary structures from a Weald Clay Sandstone at Warnham Brickworks, Horsham, Sussex

Sedimentary structures in a sandstone in the Weald Clay are described. They are groove casts, possibly formed by plant fragments being moved by water, air-heave structures, scour structures, load casts, syndepositional faults and folds, trace fossils and ripple structures. From their study it is concluded that the sandstone was deposited by the sudden inrush of sediment-laden water; and a comparison is suggested with the crevasse sands of modern deltas.

Geological Exploration Between Upper Jurua River, Brazil, and Middle Ucayali River, Peru

The northern Territory of Acre, in the Alto Jurua River area, Brazil, and the middle valley of the Ucayali River, Peru, have not been studied geologically heretofore. It is a sparsely inhabited region. Flat plains, swamps, and tropical jungle make precise geological work in a short time rather precarious. However, as this contribution emphasizes, stratigraphical and structural conditions in the northwestern Territory of Acre, particularly the Serra do Moa region, show possibilities of important petroleum accumulations and the area deserves detailed surveying. The sediments range in age from Lower Cretaceous to Pleistocene and are of continental and marine intermittent deposition, although poorly fossiliferous. No rocks of igneous origin were observed. Tertiary and Cretaceous formations of the Pachitea, Pongo de Manseriche, and Contamana areas, Peru, can be stratigraphically correlated with those of the Serra do Moa and upper Jurua areas in the northwestern Territory of Acre. The structural characteristics correspond with the physiographical features. The Serra do Moa Range, of Andean folding and apparently the last eastern spur of the Cordillera Oriental, is an anticline in partly Cretaceous beds, traversed by several tributaries of the upper Jurua. Although oil seeps were not observed in the Serra do Moa region, they have been found in the headwaters of the Ahuaya River, a tributary of the upper Cashiboya River. The seeps occurred in faults in hard, red clay sandstones of the red-bed formation; the exact horizon from which the petroleum generated is not known since many seeps occur in Tertiary or Cretaceous outcrops. In the Pachitea River district, seeps have been observed and drilling is planned for the near future by a private concern. The region of the upper Jurua geologically is a part of the potentially oil-bearing structural belt extending along the eastern foothills of the Andes, south through Bolivia into Argentina and north through eastern Peru into Colombia and Venezuela.