Ankerite

About: Ankerite is a research topic. Over the lifetime, 859 publications have been published within this topic receiving 23960 citations.

Will mercury impurities impact CO2 injectivity in deep sedimentary formations? II. Mineral dissolution and precipitation

Abstract: Numerical simulations of carbon dioxide (CO2) injection into a sandstone reservoir (∼2 km depth) were used to investigate the geochemical effects of trace amounts of mercury (Hg, 7 and 190 ppbV), with and without hydrogen sulfide (H2S, 200 ppm). Geochemical reaction-path modeling shows that cinnabar precipitates as soon as the Hg-bearing CO2 reacts with the formation. Mercury does not condense to liquid, and the net volume change from mineral dissolution and precipitation is found to be negligible. Two-dimensional radial reactive transport simulations of CO2 injection at a rate of 14.5 kg/s (∼0.5 Mt/y) into a 400-m-thick formation at 106°C and 215 bar, with varying amounts of Hg and H2S, show that porosity changes only by about ±0.05% absolute (i.e., new porosity% = initial porosity% ±0.05), and that Hg readily precipitates as cinnabar in a zone mostly coinciding with the single-phase CO2 plume. This essentially negligible porosity change is not expected to affect permeability and CO2 injectivity. The precipitation of minerals other than cinnabar dominates the evolution of porosity. Although the predicted porosity change is small, the dissolution and precipitation predicted for individual minerals is not negligible. The main reactions include the replacement of primarily Fe-chlorite by siderite, of calcite by dolomite, and of K-feldspar by muscovite. Chalcedony is also predicted to precipitate from the dissolution of feldspars. Except for some replacement of pyrite by ankerite when H2S is deficient, the cases with and without H2S show similar results. Experimental measurements are needed to decrease uncertainty in simulation results.

Mechanism of CO_2-Sandstone Interaction and Formative Authigenic Mineral Assemblage

Abstract: When CO2 is injected into aquifer sandstone by natural or manual mode,diagenetic fluid will become weak acidic fluid,resulting in the decomposition of unstable minerals such as carbonate and feldspar and the precipitation of new minerals in host sandstone as well as the variation of sandstone porosity and permeability and formation water geochemical characteristics Moreover,only a small amount of CO2 dissolves in water as a form of gas,while most of it is solidified in the host sandstone in forms of secondary minerals like calcite,dolomite and siderite,etc The typical authigenic mineral assemblages include the dawsonite ?a ferruginous carbonate ?a other carbonate and the ankerite + kaolinite + authigenic quartz ones,in which the former is a typical authigenic mineral assemblage to trace CO2 migration and accumulation This study on mechanism of CO2-sandstone interaction and formative authigenic mineral assemblage not only broadens the field of fluid-rock interaction in sedimentary basins,reveals the mass transportation between deep fluid and upper reservoir or between shallow source rock (or other rocks) and adjacent reservoir rocks,but also provides basic geologic information for the researches of sandstone reservoir quality evaluation,CO2 gas pool (or field) prediction and CO2 subsurface storage

Cimentação Carbonática em Reservatórios Siliciclásticos - O Papel da Dolomita -

Abstract: Carbonates are important diagenetic cements in siliciclastic rocks thus important to determine these rocks as hydrocarbon reservoirs. The cement is the material had chemically precipitated partial or totally pore filling, affecting rock values of porosity and permeability. The acknowledgment of diagenetic patterns those are associated to the carbonatic cement precipitation and their impacts in the reservoirs quality can decrease the risks of exploration and exploitation of new reservoirs. Therefore is necessary the knowledge of origin and processes of carbonate cement's precipitation. These cements have distribution patterns, mineralogy, textures and isotopic compositions which vary spatial and temporally, depending of perform conditions in each diagenetic environment. One of the most important diagenetic cement is dolomite and the dolomite's group is compound by dolomite and ankerite. These minerals can be differentiated by analytical techniques such as optical petrography, staining techniques, cathodoluminescence, scanning electron microscopy and isotopes. Besides that, dolomite cement shape in a reservoir can display different forms: rhombs, poikilotopic and saddle in a variety of dimensions, pore filling, replacing detrital carbonate grains, concretions, nodules or stratified layers. Primaries calcite and aragonite replaced can promote precipitation of dolomite through increase of temperature and by presence of Mg-being fluids. The main entrance conditions to form dolomitic cement are: (i) alkaline solutions from pre-existence rocks weathering or evaporitc environments; (ii) marine waters; (iii) clay alteration; (iv) CaCO3 polymorphs dissolution; (v) dissolution of bioclasts. An interesting example of dolomitic cementation is the Carmopolis Member of the Muribeca Formation, hydrocarbon reservoir of the Camorim Field (Sergipe-Alagoas Basin, northeastern Brazil).

Diagenetic differences in tight sandstone reservoirs in two delta fronts: an example from the Chang 4 and 5 members of the Yanchang Formation in the Longdong area, Ordos Basin, China

Abstract: With the intensification of oil and gas exploration, tight sandstone reservoirs have received an increasing amount of attention, particularly with regard to the genesis of tight reservoir rock. The Upper Triassic Yanchang Formation in the Longdong area of the Ordos Basin has developed a typical tight, oil-bearing, clastic reservoir (lithic arkose and feldspathic litharenite, grain size is mainly 0.1~0.3 mm in diameter). During the depositional period of the Chang 4 and 5 members, the two provenance systems of the southwest and northeast developed in the study area. In the southwest, sandstones in the lower part of distributary channels are coarser with fewer quartz overgrowth and ankerite and better reservoir quality (porosity about 12%, permeability about 1 mD). In the northeast, chlorite coating is thicker (> 4 vol%) in the underwater channel sandstones (porosity is about 14%, permeability is about 2 mD) than in the mouth bar sandstones. Sandstones in the upper part of distributary channels are finer with lower permeability (about 0.1 mD). Authigenic ankerite mainly appears around detrital dolomite as an overgrowth. The SiO2 in the quartz overgrowth most likely came from the transformation of smectite to illite and the dissolution of feldspar. In the northeast, only 2 vol% of chlorite rims significantly inhibited quartz overgrowth, but they probably blocked and delayed the dissolution of feldspars by acids. We present results here that show the diagenetic differences in sand bodies in delta fronts are influenced by sediment size, maturity, and the composition of framework grain; the materials that compose authigenic minerals mainly come from the alteration of sandstones. As a whole, the formation of tight reservoir rocks in the study area is closely related to sedimentary facies, composition of framework grain, cement type and content, and development of dissolution.

Mineral assemblages, fluid inclusions, pyrite trace elements, and s-o isotopes of gold ores from the cenozoic daping deposit, sw china: implications for the genesis of complex orogenic lode gold systems

Abstract: The Cenozoic Daping orogenic gold deposit, on the southeastern margin of the Tibetan Plateau, China, contains four lode types that contain a total of 55 t gold. Pyrite-chalcopyrite–dominated (VA) and galena-dominated polymetallic sulfide veins (VB) hosted by Neoproterozoic diorite are associated with quartz-sericite-chlorite ± epidote (± barite in VB veins) alteration. Pyrite-dominated (VC) and pyrite-pyrrhotite–dominated veins (VD) hosted by Silurian dolostone (intercalated with carbon-bearing argillaceous limestone in VD veins) are related to ankerite-siderite-quartz ± sericite alteration. All have free gold spatially and temporally associated with pyrite, chalcopyrite, galena, pyrrhotite, or quartz. Most VA and VB veins are controlled by steeply SW-dipping ductile-brittle shear zones with jigsaw wall-rock breccias in VB veins, whereas gently SW-dipping faults host VC and VD veins. There are some significant differences between different veins: (1) there were more acidic mineralization conditions for VA and VB veins relative to VC and VD veins, and more oxidized conditions for VB veins relative to other veins; (2) pyrite is rich in Co-Ni in VA and VB veins, compared to enrichment in As-Au for VC and VD veins; (3) sulfide δ34S values of 3.2 to 11.8‰ (median 8.2‰) for VA, VC, and VD veins contrast with −4.6 to +0.9‰ (median 0‰) for VB veins. The contrasting mineral parageneses, pH values, and pyrite geochemistry can be attributed to fluid-rock interaction as evidenced by replacements of amphibole by sericite in diorite and dolomite by ankerite and siderite in dolostone. The lower (~8‰) VB sulfide δ34S values and interpreted fluid oxidation relative to other veins, together with the presence of breccias and barite, can be explained by phase separation due to flash vaporization triggered by extreme hydrofracturing. The consistent NW-trending vein sets, similar median S-O isotope ratios of original ore fluids, and lack of multistage overlap of gold mineralization and alteration zones support a single-source fluid for gold mineralization at Daping. This study is diagnostic rather than just indicative in that it systematically and quantitatively portrays the mineralization diversity in an orogenic gold system formed by a single-source fluid regulated by the external fluid-rock interactions and internal hydrofracturing.