Showing papers on "Ankerite published in 1988"

Cementation and burial history of a low-permeability quartzarenite, Lower Cretaceous Travis Peak Formation, East Texas

Abstract: Fine-grained quartzarenites (and subar-koses) in the Lower Cretaceous Travis Peak Formation were extensively modified during burial diagenesis. Timing of diagenetic events can be constrained by combining petrographic and geochemical data with data on subsidence and thermal history. The geothermal gradient in the East Texas study area is 38 °C/km now, but it may have been as high as 44 °C/km when the Travis Peak was deposited because of elevated heat flow caused by crustal stretching associated with rifting of the Gulf of Mexico. Illite rims and dolomite were the first authigenic minerals to precipitate in Travis Peak sandstones. Dolomite probably formed soon after deposition at about 25 °C from water with a δ 18 O composition near SMOW. Next, extensive quartz cement, averaging 17% of the rock volume in well-sorted sandstones, occluded much of the primary porosity. The average δ 18 O composition of the quartz overgrowths indicates that they precipitated from meteoric fluids at temperatures of between 55 and 75 °C, at depths of 1 to 1.5 km. Dissolution of orthoclase and albitization of plagioclase followed quartz cementation and occurred prior to mid-Cretaceous movement of the Sabine Uplift. Illite, chlorite, and ankerite precipitated after feldspar diagenesis. The ankerite may have precipitated over a range of temperatures from about 80 to 125 °C, from fluids with δ 18 O composition of about +2‰ (SMOW); +2‰ is the average present composition of Travis Peak water. Most diagenesis ended when oil migrated into the Travis Peak. Later de-asphalting of the oil by solution of gas filled much of the remaining porosity with reservoir bitumen in some zones near the top of the formation.

Strontium isotopic evidence for the origin of siderite, ankerite and magnesite mineralizations in the Eastern Alps

Abstract: Metasomatic magnesite, siderite and ankerite mineralizations within Paleozoic sediments of the Northern Greywacke zone and its metamorphic counterpart in the crystalline “basement” (Upper Austroalpine unit) were investigated with respect to their strontium isotopic composition. The results support a sedimentary (s.l.) model for the magnesites within Visean platform carbonates of the Veitsch nappe and an epigenetic hydrothermal model for the siderites and ankerites in Devonian platform carbonates of the Noric nappe and from Huttenberg. The Fe transporting fluids can be characterized either as magmatogene or, more probably, as metamorphogene. By increasing the stage of recrystallization and/or remobilization due to at least two later metamorphic overprints, Sr in the various ore minerals beame ± enriched in radiogenic 87Sr.

Differential scanning calorimetry of minerals of the dolomite―ferroan-dolomite―ankerite series in flowing carbon dioxide

Abstract: The dolomite-ferroan-dolomite-ankerite series of carbonate minerals has been investigated in flowing carbon dioxide using high-temperature DSC. Decomposition products were analysed by X-ray diffraction. The minerals studied included dolomite containing no iron, and members substituted by iron in the molar ratio range of 0.082 to 0.49. Complete resolution of the three main endothermic features was observed and enthalpy values AH, determined from each. The individual and total enthalpy values showed a linear dependence upon the Fe and Mg content of the members across the series. A decrease in the enthalpy of decomposition occurs with increasing Fe content. The effect of Fe substitution was readily observed and the estimated limit of detection is less than 1% FeO. Accuracy of the measurements was limited by the purity of the minerals themselves.

Potential for coal calorific value corrections, dependent on the carbonate mineral type present

Abstract: In addition to calcite, other carbonate minerals in coal all undergo endothermic reactions on heating, i.e. dolomite, ankerite, siderite, aragonite, magnesite, rhodochrosite, witherite and strontianite. Of these, when determined in air, siderite and rhodochrosite give small exothermic resultants, while the amount of Fe in ankerites affects the actual endothermic values obtained. The magnitude of these carbonate decomposition reactions can be shown by DTA to be different and will need to be allowed for individually in calorific value corrections of coals containing them.

Petrology and Depositional Environments of Lower Tuscaloosa Formation (Upper Cretaceous) Sandstones in the North Hustler and Thompson Field Areas, Southwest Mississippi

Abstract: The lithology, diagenesis, and depositional environments of the Dykes, Denkman, and McComb sands of the "Stringer Sand" Member of the Lower Tuscaloosa Formation in the North Hustler and Thompson field areas of Amite County, Mississippi, have been determined by detailed description of cores from nine wells and examination of 61 thin-sections. The cores were recovered from driller depths of about 11,100 ft in the updip Tuscaloosa trend, which is being actively explored for stratigraphically trapped oil. Most of the sandstone samples (65%) are quartz arenites, and the remainder are sublitharenites: both types consist dominantly of fine- to very fine-grained sand. The average sandstone consists of 67.3% framework grains, 14.8% matrix, 9.2% cement, and 8.7% pore space. Monocrystalline quartz is by far the most abundant framework grain (88.7%), with polycrystalline quartz (6.6%), potassium feldspar (0.3%), plagioclase (0.1%), chert (1.9%), igneous (mostly volcanic) rock fragments (0.5%), metamorphic rock fragments (0.2%), and sedimentary rock fragments (1.5%) also present. The matrix for the most part consists of pore-filling clays, and includes organic matter and secondary pyrite. Quartz and ferroan dolomite (ankerite) are the most common cements; high concentrations of siderite and calcite occur as cement within some thin layers, and siderite is also found commonly as small concretions. Porosity is mostly secondary, resulting from partial dissolution of framework grains and cement. Quartz overgrowths probably formed during an early stage of diagenesis, followed by deposition of calcite cement which corroded and embayed quartz and other framework grains. Apparently the calcite was later replaced by ankerite. Late-stage pore-filling clays include vermicular kaolinite and chlorite, the latter occurring as rims and in dissolution-enlarged pore space. The depositional environment of the Dykes and Denkman sands in all wells is interpreted to be fluvial; stacked point bars are obvious in one well. The McComb sand, best developed in a wildcat well between Thompson and McComb fields, is marine, based on the occurrence of glauconite and shell fragments. The presence of a basal scour surface on shale indicates that this sandstone may be the product of marine reworking of a transgressed distributary mouth bar.

Controls on reservoir quality in tight sandstones of the travis peak formation, east texas.

Abstract: Most sandstone in the gas-bearing Travis Peak formation has undergone compaction and extensive cementation during its burial history, resulting in low porosity and permeability. Diagenetically induced permeability variations are superimposed on original permeability variations produced by depositional processes. Sandstones range from fluvial-channel to shallow-marine, bay and tidal-flat deposits. Three types of sandstone that differ in their original permeability distribution and lateral continuity are distinguished. Travis Peak sandstones have undergone a complex series of diagenetic modifications. Precipitation of authigenic quartz, ankerite, dolomite, illite, and chlorite, and the introduction of reservoir bitumen, were the most important causes of occlusion of primary porosity and reduction of permeability. Porosimeter-measured porosity is the best predictor of permeability, and there is a statistically significant inverse correlation between total volume of cement and permeability. Permeability decreases with depth in the Travis Peak, suggesting that the diagenetic processes that caused extensive cementation and resultant low permeability throughout most of the formation did not operate as completely on sediments deposited near the top of the formation.

Relationship of hydrothermal alteration to structure and stratigraphy at the Coniaurum gold mine, northern Ontario

Abstract: The gold mineralized zones of the Coniaurum mine, Porcupine camp, northeastern Ontario, are on the eastern end of the northeast-trending Hollinger–Mclntyre ore system. The ore zones are quartz–ankerite (plus accessories) veins and vein systems and associated pyritic wall rock, hosted by a sequence of mafic volcanic rocks and discordant quartz–feldspar porphyry stocks of Archean age.A least altered facies and three alteration facies can be distinguished within the mafic volcanic rocks: a chlorite facies, an ankerite facies, and a vein envelope facies. The chlorite facies is widespread, overprints the least altered facies (i.e., chlorite replaces actinolite), and hosts barren and locally mineralized quartz veins bordered by vein envelope facies alteration. The ankerite facies is coextensive with subparallel shear zones, which crosscut me axial trace of the Coniaurum anticline, and hosts most of the mineralized vein systems. Addition mineralization occurs within graphitic sediments in the crest area of the C...