Ronald C. Surdam

Organic-Inorganic Interactions and Sandstone Diagenesis

The maturation of organic material in hydrocarbon source rocks and inorganic diagenetic reactions in reservoir sandstones are a natural consequence when a prism of sedimentary rocks is buried. We can predict the distribution of porosity and permeability enhancement in potential hydrocarbon reservoirs by integrating the reaction processes characterizing the progressive diagenesis of a reservoir/source rock system. A variety of observations suggests that the organic solvents needed to increase aluminosilicate and carbonate solubilities in sandstones can be generated either by thermal or oxidative cracking of carbonylic or phenolic groups from kerogen in adjacent source rocks. For example, nuclear magnetic resonance (NMR) spectra of kerogen show that peripheral carbonylic and phenolic groups are released from the kerogen molecule before liquid hydrocarbons are generated. Experimental data indicate these water-soluble organic species can significantly affect the stability of both carbonates and aluminosilicates. Water-soluble organic acid anions (carboxylic) have been observed in oil-field waters in concentrations up to 10,000 ppm, and they commonly dominate the alkalinity in the fluid phase from 80° to 120°C. We can model the integration of organic and inorganic diagenetic reactions by constructing a series of potential reaction pathways with increasing temperature for a system that includes aluminosilicates, carbonates, organic chelate species (carboxylic and phenolic), and CO2. The important chemical divides in these diagenetic flow diagrams are dependent on temperature, the nature of the pH buffer (carbonate species or organic acid anion species), and the relationship between organic acid anions and PCO2 (P = partial pressure). Forward predictive capabilities result when this general diagenetic model is placed in a time-temperature framework. The detailed organic and inorganic geochemistry and the general thermal scenario used in the time-temperature ana ysis must be basin specific. Casting the diagenetic history of a sandstone into this type of process-oriented model helps us move from a descriptive mode to a predictive mode of analysis. Two types of information result: (1) general optimum conditions for porosity and permeability enhancement in sandstones are delineated and (2) specifically, the degree and potential for porosity and permeability enhancement are determined. Predictive models have been developed for several tectonic settings, including rift or pull-apart basins and intermontane or Laramide basins. From these reconstructions, we can forward-predict the porosity-enhancing potential of a diagenetic system based on an understanding of the reaction process in a time-temperature framework.

Difunctional carboxylic acid anions in oilfield waters

Abstract Recent models of porosity enhancement during sandstone diagenesis have called upon the metal complexing ability of difunctional carboxylic acid anions in subsurface waters to explain aluminosilicate dissolution. Although carboxylic acid anions have been known to exist in oilfield waters since the turn of the century, until now the existence of significant concentrations of difunctional carboxylic acid anions has not been documented. Data from this study show that difunctional carboxylic acid anions can exist in concentrations up to 2640 ppm, and can account for nearly 100% of the organic acid anions in some oilfield waters. Formation water samples with exceptionally high concentrations of difunctional carboxylic acid anions are found in reservoirs which are at maximum levels of thermal exposure, and which are presently in the 80–100°C thermal window. Plagioclase dissolution experiments performed with natural oilfield waters and artificial solutions indicate that waters with high difunctional acid anion concentrations are capable, by organo-metallic complexation, of being “apparently oversaturated” with respect to total aluminum concentrations compared to the inorganic solubility of kaolinite by several orders of magnitude. Dissolution experiments simulating a specific geologic environment (Stevens Sandstone, southern San Joaquin Basin, California; using natural oilfield waters and Stevens Sandstone core samples), produced plagioclase and calcite dissolution textures similar to those noted in well cores from the Stevens Sandstone, as well as raising total aluminum concentrations in these experimental solutions several orders of magnitude over the solubility of kaolinite. Bactericidal preservation of oilfield waters is essential for meaningful analyses of organic acid anions; bacterial degradation of waters can cause dramatic changes in both concentration and distribution of carboxylic acid anions, especially the difunctional acid anions. In addition, elevated temperatures must be avoided (i.e. gas chromatography with FID), for many of the carboxylic acid anions are unstable at temperatures above 100°C. Some oilfied waters have a much higher concentration of difunctinal carboxylic acids anions than was previously thought, and hence a greater capacity to dissolve aluminosilicate framework grains and transport aluminum.

Lacustrine sedimentation during the culminating phase of Eocene Lake Gosiute, Wyoming (Green River Formation)

During deposition of the Laney Member of the Green River Formation, Eocene Lake Gosiute evolved from a saline, alkaline lake to a freshwater lake. This evolution reflects the change from a closed-basin hydrologic regime to an open-basin hydrologic regime; sedimentation in the Lake Gosiute system was strongly influenced by the relationship between evaporation and inflow of waters into the basin and, during deposition of the upper Laney lacustrine beds, to the outflow of lake water into the Piceance Creek basin. Stratification sequences, sedimentary structures, and mineralogy of lithofacies in the Laney Member provide insights into the evolution of the lake and into the competing factors that determined the type of sediment accumulated in the lake and fringing environments. Carbonate sedimentation was strongly influenced by lacustrine transgressions and regressions across a very low topographic gradient. Terrigenous rocks mark progradation of beach and deltaic shorelines during times of greater precipitation when more terrigenous detritus was transported to the lake. Hydrochemistry of Lake Gosiute during the deposition of the Laney Member was controlled largely by surface and spring water. Calcite was precipitated in the lake as a result of mixing of calcium-rich inflow and saline-alkaline lake waters. Dolomite formed as a result of periodic flooding and drying of the fringing carbonate mud flat, where carbonate muds were saturated with saline-alkaline lake waters and underwent evaporative pumping. Some surface waters reaching the lake contained high concentrations of salts dissolved from efflorescent crusts generated by capillary draw of connate waters in alluvial and mud-flat sediments.

Mineral reactions in the sedimentary deposits of the Lake Magadi region, Kenya

The authigenic minerals, principally zeolites, in the Pleistocene to Holocene consolidated and unconsolidated sediments of the Magadi basin in the Eastern Rift Valley of Kenya have been investigated. Samples were available from outcrops as well as drill cores. The following reactions can be documented: (1) trachytic glass + H 2 O → erionite, (2) trachytic glass + Na-rich brine → Na-Al-Si gel, (3) erionite 4- Na + → analcime + K + + SiO 2 + H 2 O, (4) Na-Al-Si gel → analcime + H 2 O, (5) calcite + F-rich brine → fluorite + CO 3 −− , (6) calcite + Na-rich brine → gaylussite, and (7) magadiite → quartz + Na + + H 2 O. Erionite is the most common zeolite present, but minor amounts of chabazite, clinoptilolite, mordenite, and phillipsite were also recognized. Erionite can form directly from trachytic glass by the addition of H 2 O only. It is characteristic of the Magadi basin because of the low content of alkaline earths in the volcanic glasses and in the solutions interacting with them. Analcime is common in outcrops of the High Magadi and Oloronga Beds. It forms from erionite by a reaction probably initiated by a lowering of the silica activity, which results from the transformation of magadiite to chert. Analcime in the drill-core samples grew at the expense of a Na-Al-Si gel. This gel forms at the lake shore and is washed into the lake during flooding conditions. Fluorite is common in the core samples and can be explained by reaction of the fluoride-rich brines with calcium in the sediments, principally detrital calcite. Authigenic albite and potassium feldspar were not recognized, probably for reasons of reaction kinetics. The presence of authigenic minerals can be accounted for by considering the chemical compositions of the starting materials, mainly volcanic glasses, and the brines they come in contact with. Lake Magadi represents a unique opportunity for studies of diagenesis because authigenic minerals are forming there at the present time and because the evolution of its waters is well known.

Depositional Environment of the Green River Formation of Wyoming: A Preliminary Report

A new model is proposed for the depositional environment of the Green River Formation of Wyoming, which is based on a combination of lake and playa environments. We propose that oil shales and trona beds accumulated in shallow lakes which were fringed by large playa flats. In these playa flats alkaline brines evolved through evaporation and precipitation of calcium carbonate and protodolomite in the capillary zone near the ground-water table. Dolomitic mudstones, marlstones, and calcareous and siliciclastic sandstones were the products of occasional floods on the playa; chief evidence consists of the assemblage of sedimentary structures found in these rocks, indicating frequent exposure to air, strong shallow-water currents, transport, and resedimentation. The playa-lake model leads to a more consistent picture of the hydrology, brine evolution, mineral formation, and sedimentation in the Green River basins than the previously accepted stratified lake concept.

Green River Formation, Wyoming: A Playa-Lake Complex

Recent observations in the Green River Formation suggest that ancient “Lake Gosiute” was a playa-lake complex (Eugster and Surdam, 1973). In this paper, the new playa-lake model is tested in a basin-wide study of surface and subsurface observations. The rocks deposited in and around “Lake Gosiute” can be divided into three distinct facies: (1) marginal silt and sand, (2) carbonate mud flat, and (3) lacustrine. Each lithologic facies has a characteristic carbonate mineral assemblage. The marginal facies is characterized by calcite concretions and calcareous cements. The mud-flat facies is characterized by calcite and (or) dolomite. The lacustrine facies is characterized either by trona (sodium carbonate) or by oil shale (either calcitic or dolomitic). The regional distribution pattern of lithologic facies and mineral zones in the Green River Formation of Wyoming is identical with that of modern playa-lake complexes. Moreover, in the Tipton Shale Member, once supposed to have been deposited in a large, deep, open, fresh-water lake (Bradley, 1963), there is strong evidence demonstrating large fluctuations in the position of the shoreline and progressive increases in salinity and alkalinity of the lake water. By mapping the regional distribution and types of lateral changes characterizing individual stromatolite units, the fluctuations in shoreline position can be quantified. The vertical distribution of fossils and ooliths in the Green River Formation allows an evaluation of water chemistry. In addition, the assemblage of sedimentary structures in the Tipton Shale Member is compatible only with a sedimentologic model characterized by shallow-water deposition and frequent subaerial exposure. Thus, the deep-water stratified-lake model is untenable not only for the Wilkins Peak Member but also for the Tipton Shale Member of the Green River Formation. In contrast, the playa-lake model is consistent with the physical, chemical, and paleontologic aspects of the Green River Formation of Wyoming.

Alkalinity and formation of zeolites in saline alkaline lakes.

The solubility of rhyolitic glass increases with increasing alkalinity, whereas the ratio of silicon to aluminum decreases with increasing alkalinity. The strong correlation observed between alkalinity and zeolite mineralogy in saline, alkaline lakes is thought to be a function of this relationship between pH and the Si/Al ratio. It is suggested that this function is a result of the reaction between silicic glass and alkaline solution whereby (i) a gel forms, whose Si/Al ratio is controlled by the Si/Al ratio of the solution, and (ii) a zeolite forms from the gel, whose Si/Al ratio is, in turn, controlled by the composition of the gel.

Carboxylic acid anions in formation waters, San Joaquin Basin and Louisiana Gulf Coast, U.S.A.—Implications for clastic diagenesis

Abstract Carboxylic acid anions (CAA) in formation waters are of interest to studies of clastic diagenesis because of their ability to buffer formation water Eh and pH (and thus substantially contribute to controls on carbonate mineral stability), and their ability to complex and transport Al and Si from the site of aluminosilicate mineral dissolution during diagenesis. Carboxylic acid anions are also extremely important to the aqueous geochemistry of Ca, Fe, Mn, Pb and Zn. Some formation waters from sedimentary basins contain high concentrations of CAAs. The analyses of 20 formation waters from the San Joaquin Basin, California and 20 formation waters from the Louisiana Gulf Coast Basin presented in this study, show concentrations as high as 8100 ppm monofunctional and 370 ppm difunctional CCA; by comparison, previously reported analyses indicate monofunctional CAA occir in concentrations up to 10,000 ppm and difunctional CAA may occur in concentrations up to 2610 ppm. Analyses of drilling muds and scale soaps presented in this study show that few if any difunctional CAA in the study area can be attributed to contamination from these sources. Additionally, aqueous extracts of crude oils contain both mono- and difunctional acid anions, as do the aqueous and petroleum phases of hydrous pyrolysates. Previously unreported dissolution experiments, equilibrium computer simulations, and hydrous pyrolysis experiments support those already published and suggest that CAA are generated during thermal maturation of kerogen and expelled from the shale along oil-wet microfractures. Upon entering the water-wet sandstone pore, the hydrophilic CAA partition into the aqueous phase. Organic-inorganic reactions may occur which from CAA-metal complexes. Because the complexes are hydrophobic, they partition in the petroleum phase, where present. Carboxylic acid anions are of great importance to clastic diagenesis over the temperature range in which they dominate fluid alkalinity; certainly, no other viable mechanism has been advanced which adequately explains the observed aluminosilicate mineral dissolution with subsequent mass transfer of A1, as well as the carbonate mineral diagenetic successions observed in sand-shale systems world-wide. The utility of modeling these observations of organic-inorganic diagenesis is limited only by the ability to model the concentration and distribution of CAA through space and time.

Organic-inorganic reactions during progressive burial: key to porosity and permeability enhancement and preservation

The development of porosity and permeability enhancement and preservation or both in many sandstones is a function of aluminosilicate or carbonate mineral stability. The dissolution of aluminosilicate minerals and subsequent porosity and permeability enhancement is a problem of aluminium mobility. Our experimental data demonstrate that it is possible to increase the mobility of aluminium significantly and to transport it as an organic complex in organic acid solutions. These organic acids, primarily carboxylic and phenolic acids, also have the ability to dissolve carbonate grains and cements. W. W. Carothers & Y. K. Kharaka (G. cosmochim. Acta 44, 323—332 (1980)) have shown that concentrations of carboxylic acid anions range up to 5000/106 (by mass) over a temperature range of 80-100 °C in some oil-field formation waters. Our experiments show that acetic acid solutions at the same concentration and over the same temperature range can increase the solubility of aluminium by one order of magnitude, whereas oxalic acid solutions increase the solubility by three orders of magnitude. Phenols such as hydroquinone and catechol are almost as effective as oxalate. The textural relations observed in the experiments are identical to those in sandstones containing porosity enhancement as a result of aluminosilicate dissolution. A natural consequence of the burial of sedimentary prisms is the maturation of organic material. Nuclear magnetic resonance (n.m.r.) studies have shown that carbonyl and phenol groups are removed from the kerogen molecule before the generation of hydrocarbons. Thermal degradation or the action of mineral oxidants, or both (the reduction of Fe3+ released from clay diagenesis and the reduction of polysulphides are two examples) may be the mechanisms by which the peripheral groups containing the phenols and carboxylic acids are released from the kerogen molecule. The experiments suggest that the enhancement of porosity in a sandstone as the result of aluminosilicate or carbonate dissolution is the natural consequence of the interaction of organic and inorganic reactions during progressive diagenesis. Because of the time and temperature dependence of these reactions, the actual reaction sequences will be sensitive to the thermal and stratigraphic history of the source reservoir sediments.

Redox Reactions Involving Hydrocarbons and Mineral Oxidants: A Mechanism for Significant Porosity Enhancement in Sandstones

Hydrocarbon invasion into a sandstone containing mineral oxidants and carbonate or sulfate intergranular cements may result in redox reactions and significantly enhanced porosity. For years, geologists have noted that when hydrocarbons invade red sandstones, significant bleaching (i.e., iron reduction) takes place. The reactions responsible for the color distribution in the red (oxidized) and white (reduced) zones are reactions of iron oxides (± sulfate) with hydrocarbons. The iron oxides (± sulfate) oxidize the hydrocarbons (reductant) to oxygenated organic compounds; the Fe2O3 (oxidant) is reduced by hydrocarbons to pyrite (± chlorite). Commonly, the red sandstones are tight due to carbonate and sulfate cements, whereas the white zones within them are more porous. These redox reactions are of three types: [EQUATION] or [EQUATION] or [EQUATION] The produced organic acids are available to dissolve carbonate cements via the reaction [Equation]. Volumetric calculations demonstrate that if a hematite-stained sandstone (1.5% Fe2O3) is invaded by a fluid containing a 50/50 mixture of water and hydrocarbons, and redox reactions result, enough organic acid and consequent carbonate dissolution could occur to generate 8-14% additional porosity. More subtle redox reactions involving hydrocarbons and mineral oxidants have the potential to significantly enhance porosity in any sandstone. These redox reactions may explain why hydrocarbon accumulations appear to have created porosity in some cases.