Secondary Intercrystalline Research Articles

Geothermal carbonate reservoirs and their sustainability: what can natural hydrothermal systems tell us?

Minerals that make up carbonate rocks (i.e. calcite) have very high chemical reactivity, allowing for various types of porosity-altering deformation processes such as dissolution and recrystallization (e.g. dolomitization, silicification), that produce secondary porosity. Even though water enriched with CO2 may quickly equilibrate by dissolving calcite, carbonate is also very dynamic, and cemented veins may still act as planes of weakness compared to the host rock allowing fluids to open up and then seal these fractures again. Therefore, natural carbonate rocks often show not only the numerous deformation stages overprinting one another, but also the same veins opening up again and again forming crack-seal structures.Natural fossil hydrothermal systems were investigated within outcrops at Nowa Wioska, Czatkowice, Pomorzan, Trzebionka, Orzel Bialy mines, Dziura cave and Hucianski Klin crest in the south of Poland to evaluate the influence of hydrothermal activities on the rock pore evolution. We present in more detail the Triassic carbonate sequence near Nowa Wioska village in the Silesian-Kraków region, where hydrothermal water had been flowing through an open fault zone, forming various hydrothermal products: secondary intercrystalline pores, vugs, breccias, metal ore and mineral precipitates. The impact of these products on the permeability of the geothermal system was analyzed in comparison with the adjacent host rock unaffected by tectonic and hydrothermal processes. We observed that at least three successive episodes of fracturing occurred during the Tertiary demonstrating that fluids were introduced episodically in the Triassic rocks. Fracture- and vug-filling calcite, saddle dolomite, pyrite, sphalerite, galena, quartz and clays were preferentially observed on the downthrown side of the fault that underwent a more intense tectonic deformation that had a decisive influence on the migration of mineralizing hydrothermal solutions. Even though mineral and metal-ore precipitates, sediments and solution / collapse breccias fill most of these karsts and fractures, the porosity and permeability of the hydrothermally altered rocks remain quite high compared to the host rock. Moreover, secondary dissolution forming vugs increase this porosity and permeability even further. The studied examples suggest that once geothermal system has been active, it stays as a preferential zone for the following deformation events, making it sustainable for tens of thousands or even millions of years.

Pore structure and fractal characteristics of Ordovician Majiagou carbonate reservoirs in Ordos Basin, China

Fractal analysis was performed on carbonate core plug samples from the Ordovician Majiagou carbonate reservoirs in the Ordos Basin using mercury intrusion capillary pressure (MICP), nuclear magnetic resonance (NMR), and the x-ray computed tomography (CT) measurements to improve our understanding of the pore structure characteristics. The relationships between pore structure parameters and the fractal dimensions were investigated. The pore systems are dominated by secondary intercrystalline pores and enlarged dissolution pores as well as microfractures. The fractal curves from MICP analysis break into two segments at the Swanson’s parameter. The small pore-throat systems can be described by the fractal theory, whereas pores connected by relatively large throats (greater than the pore-throat radius at the Pittman’s hyperbola’s apex) are not cylindrical in shape, cannot be described by a capillary tube model, and tend to have apparent fractal dimensions larger than 3.0. The fact that the entirety of the capillary curve cannot be fit by a single fractal dimension implies that there are multiple pore systems present with different fractal dimensions. The CT analysis shows that the pores are dispersed in the three-dimensional spaces mainly with elliptical shapes. The NMR measurements are sensitive to pore-body size and MICP probes pore-throat dimensions, the latter being complementary to the pore-body–size distribution. None of the CT, MICP, and NMR techniques provide “right” or “wrong” answers to the pore-throat systems, but they probe different aspects of the pore systems. This study assumes the pore shapes to be spherical in general, and then the fractal dimension is calculated from the NMR transverse relaxation time (T2) spectrum. The fractal dimensions of all the samples are calculated, and the accuracy of the fractal model is verified by the high regression coefficients. Almost all the pore systems can be described by fractal theory, and the fractal dimensions are strongly correlated with the T2 values separating the immovable fluid and the free fluid. Microfractures may bias T2 toward larger values, making it hard to derive fractal dimensions from NMR measurements. The coexistence of small pores (pore radius 50 μm) results in a heterogeneous pore distribution and a high fractal dimension. Reservoir quality increases with the complexity degree of the microscopic pore structure. Conversely, samples that are dominated by small pore systems tend to have a lower fractal dimension and a less complex pore structure.

Steel KODUR 460MC under Creep Condition

This paper analyses a failure of the micro alloyed steel (Re= 500MPa) at a temperature range 400 - 500 oC in the tensile test conditions and in the creep conditions. The failure character in condition tensile test was changeless and the fraction was transcrystalline ductile. The structure near a fraction showed a considerable deformation texture (reduction of area = 76%). The contraction and the fraction stress considerable was decreased by the increase of the time up to the fraction and the by the increase of temperature in the creep conditions. There were formed secondary intercrystalline cracks except the central crack in the area fraction. Intercrystalline cracks were started step by step to be connected each other. The fraction was started to obtain a mixed status alike and the intercrystalline (creep) fraction part was started to increase.

Role of Anhydrite Diagenesis in the Creation of Porosity in Middle Permian Dolostone Reservoirs: ABSTRACT

In middle Permian dolostone reservoirs, the most abundant pore type is secondary intercrystalline porosity. Generally ranking second in abundance, but first in many instances, are pores created by dissolution of anhydrite that occurred in the Permian. Much anhydrite was emplaced diagenetically during and following early dolomitization, as nodules, blocky porphyroblasts, and cement, and as combination cement and replacement. Generally, anhydritization was followed by an influx of a low-salinity fluid, which dissolved anhydrite - creating tertiary porosity - or altered it to gypsum, hemihydrate, silica, calcite, or fluorite. Dissolution of anhydrite nodules and porphyroblasts creates distinctive molds with angular and stair-step outlines. Dissolution of anhydrite, which was emplaced as cement and replacement, enhances the original primary and secondary pores by the increment that was replaced. This dissolution also produces voids with angular and stair-step outlines. Replacive anhydrite tends to nucleate within organic-rich components, such as burrows, stromatolites, skeletal grains, ooids, and peloids. Many dolomite crystals have cloudy organic-rich centers, which represent the original replacive crystals, and clear rims of epitaxial dolomite cement. Anhydrite preferentially nucleates within the organic-rich cores, and subsequent dissolution produces hollowed dolomite crystals or intracrystalline porosity. Intracrystalline pores constitute the predominant pore types within many intervals. Anhydrite commonlymore » replaces skeletal grains or ooids, and the replacement of such anhydrite produces a mold that replicates the original shell (biomold). It is possible to determine how many biomolds actually record dissolution of replacive anhydrite. The best Permian reservoirs contain intercrystalline porosity in conjunction with pores created by dissolution of anhydrite and grains.« less

Deposition, Diagenesis, and Porosity Relationships in the Glorieta Formation, Keystone (Holt) Field, Winkler County, Texas: ABSTRACT

Production of hydrocarbons from the Chevron 7C H. E. Lovett well, Keystone (Holt) field, is from the upper part of the Glorieta formation (Leonardian). The field is located near the western margin of the Central Basin platform (Permian basin) on a present-day structural high. The 116-ft (35.4-m) core contains at least 7 cycles of deposition, which consist, upward from the base, of progradational subtidal, intertidal and supratidal deposits. Supratidal deposits predominantly consist of dolostones with fenestral cavities; sabkha deposits are not represented. Scattered nodules of nonevaporitic anhydrite have been emplaced within subtidally deposited carbonates after dolomitization. Intrabiopelgrapestone grainstones, oointrabiopelgrainstones, intrabiopelpackstones and wackestones, and intrapelpackstones and wackestones are the predominant lithofacies. Dolostone is the predominant lithology. The cored interval was exposed subaerially several times, and episodes of freshwater diagenesis were interspersed with influxes of dolomitizing and anhydritizing fluids. Most dolostone intervals record the following five stages of diagenesis: (1) early dolomitization; (2) emplacement of nonevaporite anhydrite as cement, replacement or a combination of both; (3) dissolution of anhydrite; (4) precipitation of dolomite cement; and (5) emplacement of second generation anhydrite as cement and replacement. Some intervals contain additional stages of diagenesis including precipitation of the clay mineral dickite and calcite as cements. The core contains many highly porous dolostone intervals, and 9 distinct pore types are preserved. These include primary intergranular, fenestral, and intrabiotic voids; secondary intercrystalline pores; hollow micrite envelopes; biomolds, oomolds and fractures; and tertiary anhydrite molds. The most abundantly represented pores are secondary intercrystalline and tertiary anhydrite molds. End_of_Article - Last_Page 482------------

Porosity Styles of the Midale Field in Williston Basin of Southeastern Saskatchewan: ABSTRACT

The Midale oil field, southeastern Saskatchewan, lies on the northeastern flank of the Williston basin. The reservoir is in limestones, dolomites, and evaporites (mainly anhydrite) of the Midale Beds (Mississippian), that were deposited during a predominantly regressive episode on a shallow shelf. The Midale Beds are divided into a lower, middle, and upper zone. Many of the characteristic pore types in these zones, as observed in thin section and under SEM, can be related to both original depositional environment and postdepositional diagenetic modifications. The dominant styles of porosity in the fine-grained argillaceous limestones of the lower zone are secondary intraparticle, moldic, or microvuggy. These fabrics result from the preferential dissolution of cement or very fine shell debris. A lack of pore interconnections precludes these sediments from being effective reservoir rocks. The most significant pore type in the middle zone is secondary intercrystalline porosity within fine-grained dolomite. This fabric is the result of solution of calcite or aragonite from between rhombs after incomplete dolomitization. A crinoidal grainstone with pervasive early diagenetic syntaxial rim cement forms a tight trap in the middle zone. The upper zone consists of fractured calcareous microcrystalline dolomite. The main pore type is a non-fabric selective system of oblique to vertical microfractures, which may be associated with regional uplift or local salt solution. The presence of dolomite rhombs on fracture surfaces indicates that dolomitization was relatively late, postdating fracturing. The microfractures in the upper zone counteract the porosity-occluding effects of stylolitization and secondary anhydritization. End_of_Article - Last_Page 587------------

Cyclic Hydrologic and Diagenetic Events in San Andres Formation: Geologic Implications: ABSTRACT

In the San Andreas Formation in Roosevelt, Curry, Quay, and DeBaca Counties, New Mexico, cyclic and diagenetic events reflect rhythmic fluctuations in sea level, climate, and sedimentation. Depositional cycles in the San Andres record: (1) rapid transgressions which accompanied eustatic rises in sea level; (2) seaward progradations of subtidal, intertidal, and supratidal (sabkha) facies during stillstands of sea; (3) subaerial exposure during lowstands of sea level; and (4) continuous subsidence of the area. Effects of fluctuation in sea level, climate, and sedimentation were transmitted to San Andres carbonates through changes in hydrology which produced distinctive diagenetic overprints. The following cyclic hydrological events are recorded in many San Andres intervals: (1) dolomitizing fluids dolomitized initially unstable carbonates; (2) hypersaline brines emplaced anhydrite as partial to nearly complete replacement of carbonate (anhydritization) and as cement; and (3) low-salinity ground waters dissolved much anhydrite, brought about replacement of anhydrite by gypsum, hemihydrate, calcite, and silica and also caused dedolomitization. Many San Andres carbonate intervals bear evidence of having been subjected to overprints of two cyclic fluid invasion sequences. Nearly all primary and secondary porosity in dolostone has been occluded by dolomite, anhydrite, and calcite cements. In limestones (exclusive of dedolostones) nearly all primary and secondary porosity has been occluded by calcite and anhydrite cements. Many San Andres limestones were formed by paramorphic replacement of dolomite by calcite (dedolomitization). Secondary intercrystalline and moldic porosity in the original dolostone have been preserved after dedolomitization. Diagenesis and porosity relations within the San Andres reflect the relative flux of dolomitizing, anhydritizing, and low-salinity fluids within a given interval. Tertiary porosity is extensively preserved in intervals in which the low-salinity stage of a hydrologic cycle has not had its overprint superseded by dolomitizing or anhydritizing fluids of a subsequent cycle. The most highly porous intervals contain both secondary intercrystalline and anhdrite moldic porosity. End_of_Article - Last_Page 763------------

Deposition and Diagenesis of Horquilla Carbonates, Big Hatchet Peak Section, Pedregosa Basin: ABSTRACT

In the Big Hatchet Peak section the Pennsylvanian-Permian (Morrowan to Wolfcampian) Horquilla Formation is 3,230 ft (985 m) thick and consists of shallow shelf carbonates which accumulated along the outer shelf margin of the Alamo Hueco basin. The lower Horquilla (Morrowan to Desmoinesian) is 1,363 End_Page 762------------------------------ ft (415 m) thick and consists largely of micrite and fossiliferous limestones with abundant chert in many beds. Depositional cycles, consisting of grainstones, packstones, and wackestones, occur throughout the interval. The upper Horquilla (Desmoinesian to Wolfcampian) is 1,867 ft (569 m) thick and consists of limestones and dolostones; chert is less abundant than in the lower Horquilla. Phylloid algal biostromes are present in some upper Horquilla limestone intervals. Six laterally extensive dolostone units range in thickness from 48 to 154 ft (15 to 47 m). In both upper and lower Horquilla limestones many grainstones were subjected to intertidal and subtidal cementation within marine environments. Micritization of ooids and skeletal grains, which also developed in the marine environment, is abundant in most Horquilla limestones. Nearly all Horquilla limestone intervals bear two or more of the following evidences of having stabilized within fresh-water diagenetic environments: (1) selective dissolution of aragonitic shells and formation of hollow micrite envelopes; (2) precipitation of meteoric vadose and phreatic calcite cements; (3) recrystallization of original lime mud matrix; (4) replacement by silica which first nucleated within ooids, shells, and peloids and then invaded the surrounding matrix to form nodules or discontinuous layers. At one stage most Horquilla limestones contained large volumes of primary and/or secondary porosity, all or virtually all of which has been occluded by subsequent precipitation of marine or meteoric cements. All upper Horquilla dolostones were formed by neomorphic dolomitization of originally unstable carbonate sediments. At one stage considerable secondary intercrystalline and moldic porosity existed in most dolostone intervals; much of this was subsequently obliterated by epitaxial dolomite cements on original rhombs and by coarse recrystallization. Gravitational calcite cements partly filled many of the larger voids, and outer tips of many of these microstalactites subsequently became paramorphically dolomitized. In the upper Horquilla, anhydrite porphyroblasts and nodules were emplaced in many limestone and dolostone intervals by hypersaline groundwaters to form tertiary (third order) stairstep molds, many of which are still open in dolostones. The only preserved porosity is in dolostones and these may represent potential hydrocarbon reservoirs in the subsurface. End_of_Article - Last_Page 763------------

Dolostone Reservoirs in Horquilla Formation (Pennsylvanian-Permian), Big Hatchet Peak Section, Hidalgo County, New Mexico: ABSTRACT

Dolostone reservoirs with net thickness of 484 ft (148 m) in upper Horquilla, are exposed in the Big Hatchet Peak section of southwestern New Mexico and constitute the best petroleum objective demonstrated to date in the Pedregosa basin. This shallow-marine carbonate section includes uppermost Paradise Formation (Chesterian) and 3,230 ft (985 m) of Horquilla (Morrowan-Wolfcampian). The lower member of the Horquilla consists of limestone and chert; a disconformity at the top formed during the Desmoinesian. The upper member is at least 1,867 ft (569 m) thick; limestones contain phylloid-algal biostromes and ancient solution cavities; alternating laterally extensive dolostones are 54 to 148 ft (16.5 to 45 m) thick and exhibit much vuggy porosity. Petrographic evidence demonstrates that the limestones were stabilized within freshwater diagenetic environments, that they contained much primary or secondary porosity, and that practically all of it was occluded by marine or freshwater cements. Dolostones also contained much secondary intercrystalline and moldic porosity that was partly occluded by epitaxial cements and coarse recrystallization of initial neomorphic rhombs. Anhydrite porphyroblasts were emplaced by hypersaline waters and were dissolved later by fresh ground waters to form molds with distinctive stairstep outlines; this tertiary (third stage) porosity in dolostones was partly filled by gravitational cements, and some microstalactite tips were dolomitized paramorphically. Dark Horquilla limestones are rated as fair petroleum-source units. Land-derived kerogens indicate sources of gas; alteration indices of 3 to 3+ indicate moderate thermal history. Dolostones in the upper member are rated as fair to good reservoir units. Matrix porosity and permeability are poor but should improve farther south along the shelf edge and on the slope into Alamo Hueco basin. End_of_Article - Last_Page 571------------

Porosity Examples from IAB Field, Coke County, Texas: ABSTRACT

IAB field produces oil from an Upper Pennsylvanian (Cisco-Canyon) carbonate buildup situated on the western edge of the Eastern shelf. The reservoir is approximately 6 mi (9 km) long, 2 mi (3 km) wide, and 300 to 1,000 ft (100 to 300 m) thick. The upper surface of this buildup is irregular with several pinnacles, probably caused by subaerial erosion that occurred during drops in sea level. From examination of cores, 10 lithofacies have been identified and divided according to texture, sediment types, fossils, and mineralogic composition. Facies recognized are subtidal normal-marine deposits, ranging from relatively quiet-water skeletal wackestones to relatively high-energy, shallow-water oolitic grainstones. Porosity consists of both primary and secondary types and is controlled by depositional fabric, dolomitization, and freshwater diagenesis. Five main porosity types are (1) primary interparticle and (2) primary intraparticle porosity, both in grain carbonate rocks, (3) secondary leached grain and moldic porosity in grain carbonate rocks, created during a drop in sea level, (4) secondary vug porosity in muddy or very fine-grained carbonate rocks, also created during a drop in sea level, and (5) secondary intercrystalline and vug porosity in dolomites. Muddy carbonate beds had low initial effective porosities because of poor sorting, whereas grain-rich carbonate beds had high initial effective porosities owing to good sorting. Following deposition, sea level probably dropped, and the buildup was exposed to freshwater vadose and phreatic diagenesis. Solution of various grains and simultaneous precipitation of equant calcite cements produced secondary leached porosity and reduced primary interparticle and intraparticle porosity. Porosity preserved during this period of subaerial exposure and freshwater diagenesis was probably later reduced during deeper burial by additional cementation and compaction. End_of_Article - Last_Page 457------------

Facies, Diagenesis, and Porosity Relations of Buda, Georgetown, McKnight, and West Nueces Carbonate Rocks of Maverick Basin: ABSTRACT

Through much of Dimmit and northern Webb Counties, the Buda and Georgetown limestones are remarkably homogeneous; both consist of very dense algal calcisphere, Globigerina, Inoceramus, and echinoderm wackestones and packstones; calcispheres constitute the predominant biotal component in both. In central and western Dimmit County dolomitization produced secondary intercrystalline porosity in several Georgetown intervals; these voids are now filled with solid hydrocarbon. In this area gas is produced in the Georgetown from tertiary voids which were formed when fresh groundwaters dissolved replacement anhydrite after hydrocarbons had accumulated in secondary intercrystalline voids. The Buda has no reservoir potential in this area. Westward from eastern Dimmit County, the McKnight and West Nueces change facies from oobiograinstones and packstones to biopelgrapestone grainstones and packstones to biopelwackestones in western Dimmit County. The McKnight exhibits well-developed depositional and diagenetic cycles. These cycles record interaction of the following: (1) eustatic fluctuations in sea level, (2) regional progradation of supratidal, intertidal, and subtidal facies during stillstands of sea level, (3) changes in climate from arid to semiarid or subhumid, (4) continuous subsidence. Consequently, the McKnight has been subjected to highly complex multicycle diageneses that include freshwater diagenesis, dolomitization, anhydritization, silicification, and dedolomitization. Anhydrite layers of the upper and low r anhydrites were formed by replacement of carbonates. Secondary intercrystalline porosity in dolostone layers has been filled by what is now solid hydrocarbon which accumulated at shallow depths. Gas production in the McKnight, throughout the area, is from tertiary anhydrite molds which were created after solid hydrocarbons had accumulated in secondary voids. Much dickite cement also is present in secondary voids in the McKnight. The West Nueces apparently contains no anhydrite, but tertiary anhydrite molds were abundantly formed and then largely filled by carbonate cements, as were primary and secondary voids. Reservoir potential of the West Nueces probably has not been properly evaluated. Because mechanisms of anhydrite emplacement are so poorly understood, the distribution of porosity, formed by its dissolution, is unpredictable. End_of_Article - Last_Page 1604------------