Organic Acid Species Research Articles

PH buffering by metastable mineral-fluid equilibria and evolution of carbon dioxide fugacity during burial diagenesis

Numerous potential pH buffers including reactions among aqueous organic acid and carbonate species carbonate and silicate minerals are typically present during burial diagenesis. Buffering of pH in natural systems is a function of mass action, mass balance kinetic constraints. In most sedimentary basins, carbonate and silicate minerals are present in amounts sufficient to buffer pH the activities of aqueous species are consistent with metastable equilibrium among observed diagenetic minerals. These observations indicate that mass balance and kinetic constraints are relatively less important than mass action constraints measured by the buffer index, β, here defined as follows: β = −dξ dpH The buffer index ultimately dictates which buffer reaction controls pH under diagenetic conditions; buffer reactions with high β values are favoured over those with low values. Buffer indices for a number of potential diagenetic buffer reactions have been calculated by reaction path modeling. Heterogeneous equilibria among carbonate and silicate minerals and an aqueous phase have greater β values than those for homogeneous reactions among aqueous carbonate and organic acid species. This implies that pH, calcite dissolution ƒ CO 2 are strongly dependent on carbonate-silicate-fluid interactions during diagenesis. The role of carbonate-silicate reactions in controlling pH is tested by examining the evolution of CO 2 fugacities with temperature during burial diagenesis. Carbon dioxide fugacities have been calculated by reaction path modeling of diagenetic carbonate-silicate equilibria for sedimentary and geothermal systems. Calculated CO 2 fugacities are in general agreement with observed CO 2 fugacities. The combination of high buffer index, apparent metastable equilibrium between diagenetic minerals and waters the relatively accurate prediction of z. hfl; CO 2 trends with temperature suggest that carbonate-silicate reactions are important in determining the evolution of fluid compositions in sedimentary basins and influence the course of dissolution events in burial diagenesis.

Interactions between organic acids anions in formation waters and reservoir mineral phases

Formation waters from the Norwegian continental shelf have been analysed for short-chain organic acid anions. Acetic acid is the major component, and the observed levels are in the range 0.3–20 mM in uncontaminated formation water samples. Studies of individual reservoirs and formations over time and area show a quite uniform distribution of the acids within each unit for large volumes of fluid. The organic acid anion levels are generally low in reservoirs containing biodegraded oil. Statistical analysis has been used to find correlations between the organic and inorganic ion compositions and the reservoir conditions and shows a dominating negative correlation between the organic acid anion levels and the levels of inorganic ions, indicating that the main effect of the organic ions is to increase the stability of mineral phases. Laboratory experiments have also been performed and show similar effects for sandstone samples. Under oxidizing conditions the buffering capacity of the organic acid species is also important, as it prevents the development of extreme pH conditions caused by pyrite oxidation. In carbonate systems few systematic effects were observed. The results, both for the natural systems and the laboratory experiments, show that the presence of organic acids in the formation waters does not contribute to the development of secondary porosity in the reservoir rock. The observed relationships rather indicate decreasing concentration levels of inorganic ions in the formation waters with increasing levels of organic anions and thus an increase in mineral stability.

Simultaneous enzymatic determination of l(−) malic acid and l(+) lactic acid in wine by flow injection analysis

A simultaneous determination of two organic acid species in wine by a flow injection analysis method using enzyme-immobilized open tubular reactors is described. The design of the optimal geometry of the FIA system is especially important to avoid the matrix fluorescence. Reproducible results were obtained in the determination of l(−) malic acid and l(+) lactic acid using l-MDH and l-LDH, respectively. The assay reactions utilize the fluorimetric determination of a common product, NADH. The sample was dialyzed on line and injected into the FIA-system. The system responds linearly (log-log calibration graphs) to the injected sample (80 μl) in concentrations ranging from 0·1 m m to 5 m m and 0·05 m m to 3 m m for malic and lactic acid, respectively. The maximum sample throughput is 15 h −1. The results obtained with both proposed and HPLC methods are in good agreement. The l-MDH and l-LDH reactors were stable for six months when used daily at optimal pH.

Cementation-Dissolution Timing Relative to Hydrocarbon Emplacement, San Joaquin Basin, California: ABSTRACT

Calcite cements occur as thin bands (up to 1 m thick by 200 m long) in a Stevens turbiditic sandstone (upper Miocene) reservoir at North Coles Levee. These cements formed from evolved marine power water and show up to 10 mole % of mainly Fe + Mg. Stable isotopic analysis and estimated isotopic compositions of pore fluid indicate the cements formed from 40/sup 0/ to 80/sup 0/C and that delta/sup 13/C ratios range from +1.2 to -12.66. A burial history curve indicates the cements formed from 7.5 to 3.5 Ma. Dissolution of calcite, widespread dissolution of plagioclase (up to one-third total porosity), precipitation of kaolinite, and migration of hydrocarbon (500 million bbl) occurred after this time. Hence considerable mass transfer is implied over a relatively short period of time, and oil migration closely followed the dissolution event. Increased concentrations of acids during kerogen maturation as observed by Carothers and Kharaka in 1978 for organic acid species at 80/sup 0/ to 100/sup 0/C in present-day pore waters of the basin may have caused the dissolution process. Very recent, possible post-oil migration processes in the field include albitization of plagioclase and dolomite rhombs growing in crushed biotites. These stress-related processes may bemore » the indirect result of low fluid pressures (1000 psi at 8500 ft) from release of the reservoir gas cap in 1965. At present, calcite with relatively high magnesium contents (to 8 mole %) precipitate as scales in the tubing string, presumably a result of CO/sub 2/ degassing where pressure drop occurs in the flow line.« less

Determination of organic and inorganic acid species in the atmosphere and in rain-water by ion chromatography

Determination of organic and inorganic acid species in the atmosphere and in rain-water by ion chromatography