Calcite Fillings Research Articles

High-Temperature Diagenesis in Shallow Chalk Reservoir, Skjold Oil Field, Danish North Sea: Evidence from Fluid Inclusions and Oxygen Isotopes

Samples of calcite-filled fractures from the chalk reservoir of the Skjold oil field were studied by fluorescence microscopy, cathodoluminescence microscopy, fluid inclusion microthermometry, and oxygen and carbon stable isotope and trace element analyses. Two generations of calcite infilling are tentatively related to two major phases of faulting caused by salt movements. Owing to recrystallization of the first generation of infillings, the chemical, isotopic, and fluid inclusion data only apply to the later generation. Results of the fluid inclusion studies suggest a precipitation temperature of the infilling calcite significantly above what it should be, applying today's geothermal gradient (52°C/km or 2.85°F/100 ft) to the past. This higher temperature indic tes that hot water ascending from deeper strata may have flushed the reservoir, apparently during a late Miocene to Pliocene faulting phase when ring faults guided the water to the reservoir. This feature may be typical of most chalk reservoirs in the Danish Central graben where diapirs have penetrated the chalk.

Stable isotopes of fissure-filling calcite from Finnsjön, Uppland, Sweden

δ 18O and δ 13C was analysed for calcite fissure fillings from different depths, down to 533 m, in Svecokarelian rocks of east Sweden. A significant correlation with depth is noticed for δ 18O. There also are differences between sealed and open fissure fillings in both δ 18O and δ 13C. Calcite fillings which originated in high- and low-temperature fluids have been found within single fissures. These calcites have different textures. One population of calcite has δ 18O values of −5.7 to −9.0‰ and has been precipitated by water enriched in δ 18O relative to that now present in the bedrock. Another population of calcite generally has δ 18O values of −17.0 to −19.5‰ and δ 13C values of −2.5 to −5.5‰; these calcites have been precipitated under hydrothermal conditions. A third calcite population ( δ 18 O = −9.1 to −16.9‰ ) precipitated from differentwaters at different temperatures that include present conditions. Calcites which could have been precipitated during present conditions mostly originate from open fissures.

High Fluid-Inclusion Homogenization Temperatures in Carbonates of Lower Ordovician Beekmantown Group in Northern Appalachian Basin: ABSTRACT

Data from analysis of fluid inclusions in carbonates of the northern Appalachian basin indicate higher paleotemperatures and greater depths of burial than have been inferred for the rocks of this region. Preliminary research has revealed fluid homogenization temperatures averaging 96°C (205°F) for the formation of saddle dolomite, 114°C-170°C (237°F-338°F) for calcite vein fillings, and 290°C (554°F) for calcite cements in samples from the Mohawk and Champlain valleys of New York state. The calcite-filled veins sampled in the Champlain valley of eastern New York display higher average homogenization temperatures than similar veins from the Mohawk valley of central New York. This difference may reflect a higher post-Early Ordovician paleogeothermal gradient operative in eastern New York. Drusy calcite cements in samples from central New York are interpreted as precipitates from saline brines having temperatures between 267°C (512°F) and 302°C (576°F). These temperatures support conodont alteration data obtained by others for the rocks of this area. Using a geothermal gradient of 25°C/km (72°F/mi), a former depth of burial in excess of 9 km (5.6 mi) is implied. Seismic and gravity data do not show evidence of the presence of post-Early Ordovician shallow plutons. Therefore, it appears unlikely that precipitation at the high temperatures measured resulted from magma-derived hot meteoric fluids. End_of_Article - Last_Page 1930------------

Dedolomitization, Dolomitization, and Chertification in Fort Payne Formation: Relative Timing and Mechanism: ABSTRACT

Samples of the Fort Payne Formation (Lower Mississippian) of central Tennessee typically contain 50% or more chert, with the bulk of the balance consisting of replacive dolomite. Low-iron calcite and ferroan calcite are common and minor ankerite is also present. The relative sequence of diagenetic replacement was established by cross-cutting relations as: low-iron calcite replaced by chert replaced by dolomite, introduction of ankerite, and finally, replacement by ferroan calcite of both chert and dolomite (dedolomitization). Thus dedolomitization was the last diagenetic phase. Complete replacement, rim replacement, and replacement of cores of dolomite rhombs by ferroan calcite was observed. Ferroan calcite fills veins, vugs, and intergranular pores and replaces sponge sp cules. Dedolomite occurs in both surface and subsurface samples, and there is no evidence for an unconformity within or adjacent to the Fort Payne, suggesting that the dedolomite is not related to exhumation and weathering. Minor mineralogy and sedimentary structures suggest a subtidal shelf, quiet-water environment of deposition. Stratigraphic relations suggest shallow burial. Dedolomitization of the Fort Payne occurred after lithification, probably during shallow burial, when ferrous iron was derived from indigenous minerals. End_of_Article - Last_Page 501------------

Porosity Evolution of Upper Miocene Reefs, Almeria Province, Southern Spain

Sea cliffs 40 km east of Almeria, southeastern Spain, expose upper Miocene reefs and patch reefs of the Plomo formation. These reefs are formed of scleractinian corals, calcareous algae, and mollusks. The reef cores are as much as 65 m thick and several hundred meters wide. Fore-reef talus beds extend 1,300 m across and are 40 m thick. The reefs and reef breccias are composed of calcitic dolomite. They lie on volcanic rocks that have a K-Ar date of 11.5 m.y. and in turn are overlain by the upper Miocene Vicar Formation. In the reef cores and fore-reef breccia beds, porosity is both primary and postdepositional. Primary porosity is of three types: (a) boring clam holes in the scleractinian coral heads, cemented reef rocks, and breccias; (b) intraparticle porosity within the corals, Halimeda plates, and vermetid worm tubes; and (c) interparticle porosity between bioclastic fragments and in the reef breccia. Postdepositional moldic porosity was formed by the solution of aragonitic material such as molluscan and coral fragments. The Plomo reef carbonate rocks have high porosity and permeability, and retain a great amount of depositional porosity. Pores range in size from a few micrometers to 30 cm. The extensive intercrystalline porosity and high permeability resulted from dolomitization of micritic matrix. Dolomite rhombs are between 10 and 30 µ across. More moldic porosity was formed by the dissolution of the calcite bioclasts. Some porosity reduction has occurred by incomplete and partial sparry calcite infilling of interparticular, moldic, and intercrystalline voids. The high porosity and permeability of these reefs make them important targets for petroleum exploration in the western Mediterranean off southern Spain. In these offshore areas in the subsurface the volcanic ridge and the Plomo reef complex are locally onlapped or overlapped by 350 m or more of Miocene(?) and Pliocene fine-grained sedimentary rocks. The possibility exists that the buried Plomo reef deposits may form traps for oil and gas in the offshore areas southwest of the type locality. Stratigraphic traps also may occur where the Neogene sequence above the Plomo reef complex onlaps the volcanic ridge.

Submarine Diagenesis (Aragonite Dissolution, Cementation by Calcite, and Dolomitization) in Ordovician Galena Group, Upper Mississippi Valley: ABSTRACT

The Galena Group is a fossiliferous, dominantly carbonate unit about 85 m thick, which was deposited in a broad epeiric sea mostly below wave base. Submarine dissolution of aragonite and cementation by calcite appear to have proceeded simultaneously in Galena sediments. Commonly, the enclosing sediment (mostly carbonate mud) lithified before shell aragonite dissolved, resulting in moldic voids. Most of these voids were later cemented with block calcite spar; some were filled by bioturbation. Some burrow fills became lithified but the enclosing matrix remained soft. Where this occurred, sparry replacements of aragonitic bioclasts now exist only within these burrow fills; lithification preserved bioclast outlines. Where scouring later exhumed lithified burrow fills, they produced topographic highs on scoured surfaces or intraclasts with meniscus-fill fabric. Hardgrounds very commonly occur in most exposures of the Galena; many are bored. Sparry calcite fills cracks in some hardgrounds, and is transected by borings or by the overlying bed. Spar-filled voids suggestive of former aragonitic clasts are preserved in the upper centimeters beneath hardgrounds in some strata which otherwise lack these fossils. Fine to very fine crystalline dolomite fills burrows extending downward from many hardgrounds. Individual dolomite crystals are abraded at scoured surfaces and at margins of intraclasts. These features suggest that dolomitization occurred on the seafloor. End_of_Article - Last_Page 697------------

Carbonate Mud Mounds from Lower Ordovician Wah Wah Limestone, Ibex Area, Western Millard County, Utah: ABSTRACT

Four carbonate lenses in the upper Wah Wah Limestone, western Utah, were core drilled. Thin sections and peels show that the lenses are mud mounds consisting primarily of micrite matrix, with some bioclastic debris, and minor (< 1%) intraclasts, pellets, iron minerals, and calcite fillings. Bioclastic debris consists of sponges, echinoderms, shell fragments (brachiopods and trilobites), and an encrusting problematic organism. Porosity is less than 1%; dolomitization averages less than 10%. The lenses are extremely burrowed. These fragments were transported to the buildups by currents which varied from fairly low energy to moderately high energy. The absence of frame-building organisms in growth position indicates that these buildups are mud mounds rather than reefs. Fo mation of the mud mounds could have resulted from sediment trapping by some organism, from sediment heaping by currents, or by a combination of both processes. The mud mounds are similar to those of other geologic periods. The mud mounds, although similar to reefs in external appearance, could have significance in that they serve as examples of what reefs are not. They also demonstrate that not all reeflike carbonate bodies offer potential for petroleum. Analysis of the interiors of carbonate lenses is required to distinguish reefs from mud mounds. End_of_Article - Last_Page 555------------

Fascicular-optic Calcite: A Replacement of Bundled Acicular Carbonate Cements

ABSTRACT A previously unreported, coarsely crystalline calcite fills pores and is composed of crystals that possess inclusion-patterns and lattice-curvatures mimetic after bundled acicular carbonate cements. This calcite fabric, named 'fascicular-optic' because of the characteristic divergent optic-axis pattern within each crystal, is possibly widely distributed in certain limestones. It has been found within both shallow-marine and deep-water (pelagic) limestones. The replacement mechanism responsible for generating fascicular-optic calcite is believed to involve the lateral joining together (coalescence) of the former acicular crystals, either directly if the original cement had a low-Mg calcite mineralogy, or by coalescence of acicular aragonite or high Mg-calcite crystals, followed by subsequent stabilization (calcitization or magnesium loss).

Processes of Conversion of Aragonite to Calcite with Examples from the Cretaceous of Texas

ABSTRACT Many invertebrate skeletons and skeletal fragments in modern carbonate sediments are aragonitic, but, when found in carbonate rocks, they are nearly always calcitic. Two processes of conversion of this aragonite to calcite have been suggested: 1. a process of in situ conversion, and 2. a solution-deposition process in which the aragonite is dissolved and replaced by drusy calcite. This basic process can be further subdivided into four types depending on the relative timing of this solution of aragonitic skeletons and skeletal fragments, lithification of carbonate mud matrix, and deposition of the drusy calcite. If solution of the aragonitic allochems occurs before lithification of the carbonate mud matrix, the resulting void will collapse. Solution-deposition may occur after li hification of the carbonate mud matrix but before any drusy calcite has formed in the originally existing voids within fossils, resulting in fossil replacement without preservation of the outlines of internal cavities. If drusy calcite has started to form in original voids within fossils before the solution conversion process begins, replacements retaining outlines of internal cavities will result. If solution of the aragonite occurs before lithification of the carbonate mud and after drusy calcite infilling of original voids, only internal molds are formed. Originally aragonitic fossils from well cores from the lower Cretaceous of Texas give evidence of all but one of these processes. Important variables in determining which conversion process occurs are original shell structure, fluid hemistry, grain size, and time. The process of conversion has an important bearing on the development of carbonate petroleum reservoirs.

Basalt from a Deep Well in Lawrence County, Indiana

An amygdaloidal, ophitic, and slightly porphyritic basalt occurs beneath Cambrian or Precambrian sedimentary rocks in a deep well in Lawrence County, Indiana. The rock has been highly altered and contains more than 35 per cent chlorite. Many of the plagioclase $$(An_{56})$$ microlites and small orthopyroxene crystals have been chloritized, but clinopyroxene, which typically is found as phenocrysts, is unaltered. Plagioclase $$(An_{46-62})$$ phenocrysts have been altered to muscovite, and large orthopyroxene crystals have been replaced by biotite. The alteration of chlorite has produced pumpellyite. Quartz, calcite, dolomite, analcite, potash feldspar, chlorite, and pumpellyite fill veins and cavities. Sparse magnetite and secondary pyrite are present, and red iron oxides are abundant in the upper part of the basalt. The basalt from Lawrence County is very similar texturally, mineralogically, and chemically to the Precambrian Keweenawan flows of Michigan and to a Precambrian augite andesite microporphyry f...