Discriminating cool-water from warm-water carbonates and their diagenetic environments using element geochemistry: the Oligocene Tikorangi Formation (Taranaki Basin) and the dolomite effect

Fields portrayed within bivariate element plots have been used to distinguish between carbonates formed in warm- (tropical) water and cool- (temperate) water depositional settings. Here, element concentrations (Ca, Mg, Sr, Na, Fe, and Mn) have been determined for the carbonate fraction of bulk samples from the late Oligocene Tikorangi Formation, a subsurface, mixed dolomite-calcite, cool-water limestone sequence in Taranaki Basin, New Zealand. While the occurrence of dolomite is rare in New Zealand Cenozoic carbonates, and in cool-water carbonates more generally, the dolomite in the Tikorangi carbonates is shown to have a dramatic effect on the "traditional" positioning of cool-water limestone fields within bivariate element plots. Rare undolomitised, wholly calcitic carbonate samples in the Tikorangi Formation have the following average composition: Mg 2800 ppm; Ca 319 100 ppm; Na 800 ppm; Fe 6300 ppm; Sr 2400 ppm; and Mn 300 ppm. Tikorangi Formation dolomite-rich samples (>15% dolomite) have average values of: Mg 53 400 ppm; Ca 290 400 ppm; Na 4700 ppm; Fe 28 100 ppm; Sr 5400 ppm; and Mn 500 ppm. Element-element plots for dolomite-bearing samples show elevated Mg, Na, and Sr values compared with most other low-Mg calcite New Zealand Cenozoic limestones. The increased trace element contents are directly attributable to the trace element-enriched nature of the burial-derived dolomites, termed here the "dolomite effect". Fe levels in the Tikorangi Formation carbonates far exceed both modern and ancient cool-water and warm-water analogues, while Sr values are also higher than those in modern Tasmanian cool-water carbonates, and approach modern Bahaman warm-water carbonate values. Trace element data used in conjunction with more traditional petrographic data have aided in the diagenetic interpretation of the carbonate-dominated Tikorangi sequence. The geochemical results have been particularly useful for providing more definitive evidence for deep burial dolomitisation of the deposits under the influence of marine-modified pore fluids

Distribution of diagenetic alterations in siliciclastic shoreface deposits within a sequence stratigraphic framework: Evidence from the Upper Jurassic, Boulonnais, NW France

The distribution of diagenetic alterations in Upper Jurassic, siliciclastic shoreface sediments from NW France has been linked to the sequence stratigraphic framework. Calcite cement in mudrocks and sandstones of the transgressive (TST) and lower part of the highstand (HST) systems tracts is microcrystalline and occurs as continuously cemented layers and stratabound concretions. The average delta(18)O(V-PDB) (-2.6%omicron) and Sr-87/Sr-86 (0.7078) compositions of microcrystalline calcite indicate precipitation from largely marine pore waters. >Calcite cement in sandstones of the forced regressive wedge (FRWST) and lowstand (LST) systems tracts is poikilotopic and occurs mainly as stratabound concretions. Complete dissolution of the carbonate grains and concomitant precipitation of poikilotopic calcite cement with low average delta(18)O (-5.3%omicron) and radiogenie Sr-isotope (0.70882) signatures suggest incursion of meteoric waters into sandstones during relative sea-level lowstand. The poorly lithified sandstones interbedded with sandstones cemented by poikilotopic calcite concretions display evidence of diagenesis under episodes of and to semiarid paleoclimate, including: (i) partial cementation by opal, chalcedony, gypsum, and minor vadose calcite cement, (ii) mechanically infiltrated clays and Fe-oxides, and (iii) secondary porosity owing to partial dissolution of carbonate grains. The integration of diagenesis into sequence stratigraphy allows better elucidation and prediction of the spatial and temporal distribution of diagenetic alterations and related reservoir-quality modifications in shoreface sediment

Diagenesis of a limestone reservoir (Lower Cretaceous), Abu Dhabi, United Arab Emirates: Comparison between the anticline crest and flanks

Petrographic, stable-isotope and fluid-inclusion analyses were conducted on a Lower Cretaceous limestone reservoir, onshore Abu Dhabi, United Arab Emirates in order to compare the diagenetic processes and products in the oil zone (i.e. crest) versus water zone (i.e. flanks) of a giant oilfield anticline. The near-seafloor, shallow and intermediate burial (< 1 km) diagenetic processes across the anticline include micritization of allochems, mechanical compaction, cementation by calcite (rim, syntaxial overgrowths and equant spar) and rhombic dolomite, peloids dissolution, partial dolomitization, and incipient stylolitization. Diagenetic processes during tectonic compression of the foreland basin in Late Cretaceous and concomitant oil migration were mediated by basinal brines. These processes, which are more extensive in the flanks than the crest, include cementation by calcite, subordinate saddle dolomite, and minor dickite, fluorite, and sphalerite. Additional diagenetic processes subsequent to the main tectonic compression phase and oil migration and emplacement in the crest have occurred during deep burial (i.e. the flanks; present-day depth 2.5–3.5 km), and include extensive stylolitization and cementation by blocky calcite. These processes account for the poorer reservoir quality of limestones in the water zone in the flanks than the oil-saturated limestones in the crest. This paper demonstrates that variations in the role of diagenesis on distribution and evolution of reservoir quality across anticlinal structures of oilfields can be better understood in the light of: (i) the timing of generation, migration and emplacement/saturation of oil, and (ii) burial-tectonic evolution of the basin and related geochemical evolution of formation waters and flux of basinal fluids

Quartz and fe-dolomite cements record shifts in formation-water chemistry and hydrocarbon migration in Devonian shoreface sandstones, Ghadamis Basin, Libya

Cementation by quartz overgrowths and subsequently by carbonates is a very common global paragenetic phenomenon in sandstones of many basins that is poorly constrained in the literature. Integrated petrography, electron microprobe, stable isotopes, fluid inclusion microthermometry, and Raman spectroscopy have helped unravel the diagenetic conditions, fluid flow, and hydrocarbon migration during the development of quartz overgrowths and later Fe-dolomite cement in the Devonian shoreface sandstones (oil and gas reservoirs), Ghadamis Basin, NW Libya. The high homogenization temperatures of Fe-dolomite (Th ¼ 119 to 1408C) together with its low d18OVPDB values (–17.6% to –13.2%) and saddle-crystal morphology suggest the flux of hot basinal brines. The fluid-inclusion microthermometry shows a shift from NaCl-dominated brines during quartz cementation (16.0 to 17.3 wt.% NaCl eq.) to NaCl and CaCl2-dominated brines during Fe-dolomite cementation (15.6 to 18.4 wt.% NaCl eq.). The latter brines probably circulated through overlying Mesozoic carbonate and evaporite strata before descending deep into the basin. The similarity in Th ranges for Fe-dolomite and quartz overgrowths, despite the shift in formation-water chemistry suggests precipitation under similar geothermal conditions. The presence of oil-filled inclusions in quartz overgrowths suggests that cementation occurred during oil migration, whereas the presence of methane in inclusions in the Fe-dolomite cement suggests precipitation during gas migration. This study shows that fluid inclusion microthermometry and Raman spectroscopy of the widespread paragenetic sequence of quartz overgrowths followed by carbonate cement are crucial to unravel changes in formation-water chemistry and fluid (including hydrocarbon) migration in sedimentary basins. Copyright © 2018, SEPM (Society for Sedimentary Geology)

Determination of Nonvolatile Organic Carbon in Aquifer Solids After Carbonate Removal by Sulfurous Acid

The precise determination of total organic carbon (TOC) in aquifer materials is important in the estimation of retardation coefficients for organic compound transport. Commonly used methods for TOC determination after carbonate removal by acidification include: weight loss on ignition of solids at 550° C, acid-dichromate oxidation, and combustion at 800–950° C. Since carbonate carbon frequently is more abundant than organic carbon all TOC methods depend on efficient and complete carbonate removal prior to the TOC quantification step. A method for the determination of TOC in aquifer solids involving carbonate removal by 0.73 M sulfurous acid and subsequent combustion at 800° C was tested on solids from three aquifers and on mineral standards. The carbon quantification by combustion and infrared detection of CO 2 was accurate at solid organic carbon contents between 30 and 10,000 ΜgC/g. The acid treatment added small amounts of carbon to the sample, and siderite removal was incomplete. For samples without siderite or dolomite, the accuracy of TOC determinations was enhanced by grinding the solids. Measurement of very low TOC contents (50–100 /ΜgC/g) requires supplementary testing of grinding and acid treatment effects.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65974/1/j.1745-6584.1997.tb00053.x.pd