Showing papers on "Ankerite published in 2009"

Impact of Diagenesis on Reservoir-Quality Evolution in Fluvial and Lacustrine-Deltaic Sandstones: Evidence from Jurassic and Triassic Sandstones from the Ordos Basin, China

Abstract: Linking diagenesis to depositional facies and sequence stratigraphy enables better prediction of spatial and temporal distribution of diagenetic alterations, and thus of evolution of reservoir quality in sandstones. This thesis demonstrates that employing this approach is possible because depositional facies and sequence stratigraphy can provide useful information on parameters controlling the near-surface diagenesis, such as changes in: (i) pore-water chemistry, (ii) residence time of sediments under certain geochemical conditions, (iii) detrital composition and proportion of extra- and intra-basinal grains, and (iv) types and amounts of organic matter.Evidence from four case studies enabled the development of conceptual models for distribution of diagenetic alterations and of their impact on evolution of reservoir quality in sandstones deposited in paralic, shallow marine and fluvio-lacustrine environments. Diagenetic alterations that have been constrained within the context of depositional facies and sequence stratigraphy include: (i) carbonate cement (microcrystalline and equant calcite spars dolomite over poikilotopic calcite), pyrite and glaucony are most abundant in progradational braid-delta fan sequences, particularly along the topsets (i.e. maxiumum flooding surface, MFS) and along parasequences boundaries in the deltaic facies of the early highstand systems tract HST, (ii) cementation by coarse spar calcite, dolomite, and the formation of moldic porosity by the dissolution of framework carbonate grains are most abundant in the aggradational fan deltas sequences, (iii) eogenetic kaolinitization of framework silicates is largely restricted to the fluvial and paralic HST sandstones, whereas telogenetic kaolinite may occur in the transgressive systems tract TST sandstones too, (iv) formation of goethite ooids in the TST sediments, (v) formation of glaucony, siderite spherules, and extensive grain-coatings, grain-replacing and ooidal berthierine, more in the TST than in the HST sediments, particularly below the transgressive surface TS and MFS, (vi) cementation by calcite with (δ18OV-PDB = -11.5‰ to -5.4‰) and Fe-dolomite/ankerite (δ18OV-PDB = -10.8‰ to -9.6‰) occurs in both TST and HST sandstones, (vii) syntaxial quartz overgrowths are most extensive in the HST sandstones owing to the presence of incomplete grain-coating berthierine/chlorite, (viii) greater amounts of micro-porosity in the TST sandstones than in the HST sandstones are related to the greater amounts of berthierine/chlorite in the former sandstones, and (ix) chlorite rims around quartz grains retarded the precipitation of quartz overgrowths, and hence prevented a greater loss of primary intergranular porosity in fluvio-lacustrine sandstones. Therefore, constraining the distribution of diagenetic alterations in the contexts of depositional facies and sequence stratigraphic context is a powerful approach to be used in hydrocarbon exploration.

Geological sequestration of carbon dioxide in Deccan basalts: preliminary laboratory study

Abstract: Laboratory experiments to probe the carbon dioxide sequestration potential of picritic basalts from the Deccan Basalt volcanic province, Maharashtra are reported. Growth of certain secondary carbonates is clearly seen over the surface of picritic basalts, reacted with water and carbon dioxide in its supercritical condition. The reactions were kept for about 5 months duration at a temperature and pressure of 100°C and 60 bar of CO 2 respectively. Handpicked soft grains of secondary carbonates were characterized using infrared spectroscopy and the observed signatures in two of the picritic basalt samples (IGP-40 and IGP-29) match well with ankerite (Fe-carbonate), with negligibly small amounts of released SiO 2 . While in another sample (IGP-36), the secondary carbonates have comparatively larger amounts of Si0 2 .

Organic and inorganic geochemistry of Ljubija siderite deposits, NW Bosnia and Herzegovina

Abstract: The Ljubija siderite deposits, hosted by a Carboniferous sedimentary complex within the Inner Dinarides, occur as stratabound replacement-type ore bodies in limestone blocks and as siderite-sulfides veins in shale. Three principal types of ore textures have been recognized including massive dark siderite and ankerite, siderite with zebra texture, and siderite veins. The ore and host rocks have been investigated by a combination of inorganic (major, trace, and rare earth element concen- trations), organic (characterization of hydrocarbons includ- ing biomarkers), and stable isotope geochemical methods (isotope ratios of carbonates, sulfides, sulfates, kerogen, and individual hydrocarbons). New results indicate a marine origin of the host carbonates and a hydrothermal- metasomatic origin of the Fe mineralization. The differ- ences in ore textures (e.g., massive siderite, zebra siderite) are attributed to physicochemical variations (e.g., changes in acidity, temperature, and/or salinity) of the mineralizing fluids and to the succession and intensity of replacement of host limestone. Vein siderite was formed by precipitation from hydrothermal fluids in the late stage of mineralization. The equilibrium fractionation of stable isotopes reveals higher formation temperatures for zebra siderites (around 245°C) then for siderite vein (around 185°C). Sulfur isotope ratios suggest Permian seawater or Permian evaporites as the main sulfur source. Fluid inclusion composition confirms a contribution of the Permian seawater to the mineralizing fluids and accord with a Permian mineralization age. Organic geochemistry data reflect mixing of hydrocarbons at the ore site and support the hydrothermal-metasomatic origin of the Ljubija iron deposits.

Carbonate spots: understanding the relationship to gold mineralization in Central Victoria, southeastern Australia

Abstract: Historically, carbonate spots have been identified as an indicator of gold mineralization throughout central Victoria, Australia. However, the exact timing relationships between the growth of carbonates, development of deformation fabrics, and the introduction of gold has only been determined in more recent times through isolated studies on individual gold deposits. Detailed examination of the evolution of hydrothermal alteration associated with the Magdala gold deposit at Stawell recognized the fact that there were at least two generations of carbonate growth, an early rounded ankerite phase that predated gold mineralization and a later euhedral siderite phase coincident with gold mineralization. This pattern of carbonate growth is repeated in the majority of significant gold deposits, including Bendigo and Ballarat, throughout central Victoria. Timing relationships within the carbonates suggest that a fluid was introduced along bedding planes and early deformation fabrics prior to the main upright folding events that significantly modified the original sedimentary basin. It is suggested that the early rounded carbonates may have formed as a result of anaerobic oxidation of methane, derived from the sediments and advected along normal growth faults within the sedimentary basin, through interaction with downward diffusing seawater sulfate. Although the growth of the early carbonates is not related to gold mineralization, the change in the speciation of the carbonate during the later carbonate event is critical and can be tracked using a simple geochemical index that can be used not only in areas of outcrop but also in conjunction with exploration undercover.

Origin of dolomite−ankerite cement in the Bravaisberget Formation (Middle Triassic) in Spitsbergen, Svalbard

Abstract: The organic carbon (OC)−rich, black shale succession of the Middle Triassic Bravaisberget Formation in Spitsbergen contains scattered dolomite−ankerite cement in coarser−grained beds and intervals. This cement shows growth−related compositional trend from non−ferroan dolomite (0-5 mol % FeCO3) through ferroan dolomite (5-10 mol % FeCO3) to ankerite (10-20 mol % FeCO3, up to 1.7 mol % MnCO3) that is manifested by zoned nature of composite carbonate crystals. The 13 C (−7.3‰ to −1.8‰ VPDB) and 18 O (−9.4‰ to −6.0‰ VPDB) values are typical for burial cements originated from mixed inor− ganic and organic carbonate sources. The dolomite−ankerite cement formed over a range of diagenetic and burial environments, from early post−sulphidic to early catagenic. It reflects evolution of intraformational, compaction−derived marine fluids that was affected by disso− lution of biogenic carbonate, clay mineral and iron oxide transformations, and thermal de− composition of organic carbon (decarboxylation of organic acids, kerogen breakdown). These processes operated during Late Triassic and post−Triassic burial history over a tem− perature range from approx. 40C to more than 100C, and contributed to the final stage of cementation of the primary pore space of siltstone and sandstone beds and intervals in the OC−rich succession. Key wor ds: Arctic, Svalbard, Middle Triassic, cementation, petrography, geochemistry, carbon and oxygen isotopes.

Data report: X-ray analyses of bulk sediment in IODP Holes U1320A and U1324B, northern Gulf of Mexico 1

Abstract: We present the results of 276 X-ray diffraction analyses of bulk sediment from Brazos-Trinity IV (Integrated Ocean Drilling Program [IODP] Hole U1320A) and Ursa (IODP Hole U1324B) Basins. The mineralogy at both locations is qualitatively very similar, with quartz and phyllosilicates dominating the assemblages and calcite, dolomite, K-feldspars, plagioclases, and halite present in varying amounts. Ankerite (iron-rich dolomite) was observed in only one sample. Although the composition of the sediment is relatively monotonous, trends in the percent occurrence of each fraction are observed and can be linked to changes in the lithostratigraphic units and core descriptions from Expedition 308. The data presented here will help establish a link between geomechanical behaviors of the sediments and matrix effects driven by mineral composition.

Geochemical characteristics and their geological implications of cherts from Bafangshan-Erlihe area in western Qinling orogen

Abstract: The Fengxian-Taibai area,Shanxi Province,is situated in the western section of the famous Qinling orogene,central China,with rich mineral resources.This study deals with chert from the transitional zone between the Gudaoling(D_2) and the Xinghongpu(D_3) formations of the Fengxian-Taibai area.The chert formation is the ore-hosting strata of the Bafangshan-Erlihe metallic deposit.The chert is mainly dark gray in color,hard and highly dense,with massive,lamellar and brecciated sedimentary structures.It was slightly metamorphosed(mainly recrystallized).Cryptomere-crystallitic quartz is the dominant mineral of the studied chert,with small amounts of ankerite,calcite,sericite,carbon and the likes.It is shown by geochemical analysis that the studied chert is higher than 80%in content of SiO_2,with the highest reaching 95.3%,but low in contents of TiO_2 and Al_2O_3.The value of Al/(Al+Fe+Mn) ranges in 0.13～0.64.Most of trace elements are much lower in content than the Clarke value,however,Ba, As,Sb,B,Ag and Hg are higher.Plotted in the diagram of V/Y～Ti/V and Ti～V,the samples of chert fall into the ocean basin area,from mid-oceanic ridge to the oceanic margin,with most in pelagic area and part near oceanic margin area.The content ofΣREE ranges in 3.28×10~(-6)～20.66×10~(-6).The samples of chert may be divided into two groups(H and S) based on their NASC-normalized REE distribution pattern.The group H is characterized by NASC-normalized REE distribution pattern inclining to the left,i.e.,richer in HREE relative to LREE,evidently indicating origin of hydrothermal sedimentation.The group S is slightly different from the group H,implying the contamination of non-hydrothermal sedimentation on hydrothermal one.The value of (La/Lu)_N is scattered in 0.36～1.99,characterized by the occurrence of two peaks in 0.36～0.70 and 1.00～1.99.The first peak represents the environment of the vents in the oceanic basins or deep fractures in mid-oceanic ridges,while the second one denotes the environment of far ocean basin or abysmal sea plain influenced by continental sources.The value of(La/Yb)_N ranges in 0.32～2.21, with most in the two peaks of 0.32～0.72 and 1.74～2.21.(La/Yb)_N has similar geological implications to(La/Lu)_N.The value of (La/Ce)_N varies in 1.04～3.28,with an average of 1.62 and most being higher than 1.00,indicating that the chert formed mainly in oceanic basin or abysmal sea plain,partly in the continental margin.It is concluded from the present study that the original geochemical characteristics of chert are well conserved despite of later reworking.

Diagenetic‐geochemical patterns and fluid evolution history of a lower jurassic petroleum source rock, middle atlas, morocco

Abstract: Pliensbachian to earliest Toarcian marls and argillaceous limestones exposed at the surface near Ait Moussa (Boulemane Province, Middle Atlas) include the only examples of effective petroleum source rocks so far known in the Moroccan Atlas rift basins. The outcrop interval includes hemipelagic, peri-Tethyan low latitude source rocks with Type II kerogen (total thickness of 2.5 m with mean TOC of around 3.2%). Early diagenetic, anoxic remineralisation of sedimentary organic matter resulted in hydraulic fractures, calcite cementation, a negative shift of carbon and oxygen stable isotopes relative to marine values (Δδ13C = -1.1; Δδ18O = -2.0), framboidal pyrite, and relative enrichment of the middle weight rare earth elements (REE). In combination, these attributes of early diagenesis may assist in the identification of other source rock intervals of similar age and setting. Progressive burial produced three generations of Fe-calcite cemented veins, followed by three generations of replacive dolomite and concluded by ankerite replacing dolomite. Compaction fluids initially caused a slight positive shift of δ13C values (Δδ13C =+0.4), a flattening of the REE distribution pattern and an increase in REE content, together related to the dewatering of clay. Dolomitizing fluids (dol-2 and dol-3) record a positive shift of δ13C values (Δδ13C =+0.9) suggesting the effects of methanogenesis or an uptake of heavy δ13C from underlying rock formations during fluid migration. Dol-3 is an Fe-bearing saddle dolomite that carries a positive Eu-anomaly (Eu/Eu*= 8.1) best explained by ascending hydrothermal fluids which are presumably of Middle Eocene age. A first migration of bitumen is recorded as fluorescent inclusions in dol-2 (Late Jurassic - Cretaceous), but bituminous fluids remained normally pressured until the establishment of inclined stylolites during Late Eocene tectonic compression. Comparative organic-geochemical analyses (GC, NMR of inclusions, non-expelled and expelled bitumen) indicate that thermal maturation advanced significantly after the onset of migration. Differences in terms of Pr/n-C17, Ph/n-C18 ratios and aromaticity corroborate the effects of differential expulsion. An exploration strategy should consider both secondary migration via opened tectonic stylolites in association with late-diagenetic fractures and a persisting tightness that then could have created an unconventional oil reservoir.

Lithification of Dawsonite-Bearing Sandstone in the Qingshankou Formation in the Qian'an Oil Field of the South Songliao Basin

Abstract: The Qingshankou Formation in the Qian'an oil field of the south Songliao basin contains abundant dawsonite-bearing sandstone.The clastic composition,cements,authigenic minerals,diagenetic paragenesis succession and fluid evolution are investigated through polarizer microscope,scanning electron microscope,alizarin red-staining,X-ray diffraction etc.The results shows that the sandstone is fine-grained and micro-grained,poorly to moderately sorted silt dawsonite-bearing arkoses and silty dawsonite-bearing lithic arkoses.The dominant cements and authigenic minerals include dawsonite,ankerite,calcite,secondary quartz oergrowths and authigenic clay mineral(kaolinite and illite).Part of dawsonite occurs as radial,bundle and plate,and some are very special as large massive condensates.The paragenetic sequence are in the order of calcite-kaolinite,secondary quartz overgrowths-illite-dawsonite and ankerite.The authigenic minerals formed before CO2 charging include multi-stages of calcite assemblages,kaolinite and secondary quartz overgrowths assemblages.While the main cements formed after CO2 charging are dawsonite and ankerite.Fluid evolution are in a sequence of alkaline-acidic-the transition from acid to alkaline fluids and/or transition from meta-alkalescence-alkaline fluids.

Origin of siderite mineralisation in Petrova and Trgovska Gora Mts., NW Dinarides

Abstract: The Petrova and Trgovska Gora Mts (Gora=Mountain) are Variscan basement units incorporated into the northwestern Dinarides during the Alpine orogeny They host numerous siderite-quartz-polysulphide, siderite-chalcopyrite, siderite-galena and barite veins, as well as stratabound hydrothermal-replacement ankerite bodies within carbonates in non-metamorphosed, flysch-like Permo-Carboniferous sequences The deposits have been mined for Cu, Pb, Ag and Fe ores since Medieval times Fluid inclusion studies of quartz from siderite-polysulphide-quartz and barite veins of both regions have shown the presence of primary aqueous NaCl−CaCl2±MgCl2−H2O±CO2 inclusions The quartz-sulphide stage of both regions show variable salinities; 27–262 wt% NaCl eq for the Trgovska Gora region and 34–234 wt% NaCl eq for the Petrova gora region, and similar homogenisation temperatures (100–230°C) Finally, barite is precipitated from low salinity-low temperature solutions (37–158 wt % NaCl equ and 115–145°C) P-t conditions estimated via isochore construction yield formation temperatures between 180–250°C for the quartz-sulphide stage and 160–180°C for the barite stage, using a maximum lithostatic pressure of 1 kbar (cc 3 km of overburden) The sulphur isotope composition of barite from both deposits indicates the involvement of Permian seawater in ore fluids This is supported by the elevated bromium content of the fluid inclusion leachates (120–660 ppm in quartz, 420–960 ppm in barite) with respect to the seawater, indicating evaporated seawater as the major portion of the ore-forming fluids Variable sulphur isotope compositions of galena, pyrite and chalcopyrite, between −32 and +27‰, are interpreted as a product of incomplete thermal reduction of the Permian marine sulphate mixed with organically- and pyrite-bound sulphur from the host sedimentary rocks Ore-forming fluids are interpreted as deep-circulating fluids derived primarily from evaporated Permian seawater and later modified by interaction with the Variscan basement rocks 40Ar/39Ar data of the detrital mica from the host rocks yielded the Variscan age overprinted by an Early Permian tectonothermal event dated at 266–274 Ma These ages are interpreted as those reflecting hydrothermal activity correlated with an incipient intracontinental rifting in the Tethyan domain Nevertheless, 75 Ma recorded at a fine-grained sericite sample from the alteration zone is interpreted as a result of later resetting of white mica during Campanian opening/closure of the Sava back arc in the neighbouring Sava suture zone (Ustaszewski et al 2008)

Origin of carbonate cements in Cretaceous sandstones from lower Benue Trough, Nigeria: Evidence from petrography and stable isotope composition

Abstract: Beds of authigenic carbonates were identified from three Cretaceous lithostratigraphic units of the Lower Benue Trough, Nigeria. Three carbonate lithologies were recognized by petrographic analysis in the study area. Carbonate-cemented sandstones are dominated by ferroan calcite cements with subordinate amount of siderite and dolomite/ankerite cements formed throughout the diagenetic history of the Asu River Group, Eze-Aku Group and Campano-Maastrichtian proto-Niger Delta sequences. δ18O for the cements range from -4.22 to -6.91‰(PDB) in Asu River Group; -5.23 to 12.66‰(PDB) in Eze-Aku and -4.45 to 6.89‰(PDB) in Campano- Maastrichtian proto-Niger Delta sandstones. δ13C values averaged -5.23‰ PDB for Asu River Group; -11.03‰ PDB and -5.88‰ PDB for Eze-Aku and Campano-Maastrichtian proto-Niger Delta sandstones. Petrography and geochemical data suggest that the mixing of meteoric and marine waters in the sediments caused dolomitization in Asu River group and Eze-Aku Group sediments while deep burial diagenesis under marine depositional environment precipitated calcite cements. Both authigenic and biogenic carbonate cements with low and high δ13C values exits in the sandstones analyzed. The low δ13C values of Asu River Group, Eze-Aku and Campano-Maastrichtian cements that range from 0 to 2.98 ‰ suggest that they are shallow water carbonates. Biogenic carbonate cements were also identified in some of the sandstones. The carbonate cements were formed under the influence of both meteoric- and marine diagenetic conditions which changed in time and space. The dolomitized rocks occur just below erosion surfaces on the continental sediments and on which shallow marine facies are developed.

Characteristics and genetic mechanism of carbonate cement in sandstone reservoirs of Yongjin area in central Junggar Basin

Abstract: Secondary pores and carbonate cements are quite well developed in sandstones near the unconformity between Jurassic and Cretaceous strata of Yongjin area in central Junggar Basin. Data obtained by thin-sections analysis, cathodoluminescence emission analysis and SEM/EDX show that carbonate cements seem to be the uppermost cements composed of ferroan sparry calcite and ankerite and are distributed mostly in the depth interval of 100 m below the unconformity, with their contents increasing with the depth. Carbon and oxygen isotope and elements determination in mudstone values indicate that atmospheric fresh water did not have a distinct effect on reservoir characters below the unconformity, but the development of carbonate cements and pores was closely related to the organic acid. With the action of the organic acid, early carbonates, feldspar and clay minerals provided materials for later carbonates, and the dissolved matters moved to the unconformity by acidic fluids and deposited again, which led to the superimposition of high contents of porosity and carbonate cements at the depth. As the ferroan sparry carbonate cements were formed later and the sandstone reservoir was on the whole not subjected to any other constructive diagenesis, the development of carbonate cements must have played an apparent controlling effect on reservoir characteristics.

Mineralogy, geochemistry and genesis of carbonate– silica serpentines (listwaenite) in north of nain ophiolite (west of central iran)

Abstract: The mantle sequence serpentinized peridotites of north of Nain have been converted to carbonate- silica assemblage known as listwaenite at various temperature and pH in response to CO2 and SiO2 bearing fluids. On the basis of carbonate and silica minerals content, three types of listwaenite have been recognized: carbonate, carbonate- silica and silica listwaenites. Generaly carbonate minerals are magnesite, dolomite, ferrite-dolomite and ankerite. Calcite and other carbonate minerals are rare. Silica minerals (including quartz, chalcedony and chert) accompanied by accessory Cr-spinel, chlorite, sulfide minerals also occur in listwaenites. Silica matrix in silica listwaenite containing high amounts of Hg, Ag and As elements and shows that t hese elements have transported by SiO2 bearing hydrothermal fluids as arsenide complexes.

Origin of Dolostone in Lucaogo Formaion of Middle Permian in Santanghu Basin

Abstract: Carbonate rock of Lucaogou Formation in Middle Permian is a major reservoir rock in Santanhu Basin and dolostone accounts for more than 50% in composition of the carbonate rock. There are two types of dolostone,one is the bedded dolostone and the other is the patch dolomite. In the bedded dolostone,the mineral dolomite has a good idiomorphism; on the contrary,in the patch dolostone,the idiomorphism of dolomite is not good. The patch dolostone is composed of ankerite with crassitude crystal replacing calcite. The dolostone has a Mg/Ca ratio of 1; a Sr content of 31×10-6-527×10-6,averaged by 217.8mg/kg; and a difference in δ18O between calcite and dolomite being less than 6. In addition,gypsum rock is not widely found in Lugcaogou Formation,with the salinity being 124.43. Based on the aforementioned idexes,we hold that the dolostone in Lucaogou Formation of Middle Permian in Santanghu basin is of the mixed water origin and have built the correspondent dolomitization model.

Method of producing ultrafine carbonate powders

Abstract: FIELD: chemistry. ^ SUBSTANCE: invention can be used in chemical engineering of inorganic substances and materials. The method of producing ultrafine carbonate powders involves carbonation of an aqueous suspension of powders of natural carbonates with particle size not greater than 100 mcm under excess carbon dioxide pressure with simultaneous mechanical agitation of the suspension. Siderite is used to obtain iron carbonate, magnesite is used to obtain magnesium carbonate, dolomite is used to obtain calcium-magnesium carbonate and ankerite is used to obtain calcium-iron-magnesium carbonate. The carbonation process is carried out at temperature between 4 and 50C and high carbon dioxide pressure ranging from 0.8 to 2.5 MPa without additional stabilisation of acidity of the medium. Further, carbon dioxide CO2 pressure is reduced to atmospheric pressure and ultrafine carbonate powders are precipitated from saturated solutions of unstable hydrocarbonates. ^ EFFECT: invention allows for producing ultrafine carbonates from mineral material using simple technology. ^ 3 ex

Ultrasteep composition gradients within ankerite grains from regionally metamorphosed marls

Abstract: Ankerite (Ank) grains frequently contain cores of almost pure dolomite (Dol) in the garnet zone and at lower grades of three areally extensive stratigraphic units from both Buchan and Barrovian terrains in northern New England, USA. Dol cores and Ank rims have Fe/(Fe+Mg) in the range 0.006-0.011 and 0.16-0.23, respectively. Ank grains with dolomite cores have many features closely analogous to K-feldspar rims around albite cores produced experimentally by incomplete hydrothermal reaction between albite and KCl solution [1]: (a) Ank-Dol contacts are irregular but sharp at the micron scale, (b) The orientations of the Ank and Dol crystal lattices are identical, (c) Fe/(Fe+Mg) of Ank is often slightly greater at the Ank-Dol contact than further away, (d) The Ank-Dol contact is often decorated with open pores. Based on these similarities, Ank appears to have replaced detrital Dol by a dissolution-reprecipitation mechanism during diagnesis or very low grade regional metamorphism.