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1 Introduction to the Role of Organic Acids in Geological Processes.- 2 Techniques and Problems in Sampling and Analyzing Formation Waters for Carboxylic Acids and Anions.- 3 Distribution and Occurrence of Organic Acids in Subsurface Waters.- 4 Organic Acids from Petroleum Source Rocks.- 5 Material Balance Considerations for the Generation of Secondary Porosity by Organic Acids and Carbonic Acid Derived from Kerogen, Denver Basin, Colorado, USA.- 6 Role of Soil Organic Acids in Mineral Weathering Processes.- 7 Chemistry and Mechanisms of Low-Temperature Dissolution of Silicates by Organic Acids.- 8 Comparison and Evaluation of Experimental Studies on Dissolution of Minerals by Organic Acids.- 9 Experimental Studies of Organic Acid Decomposition.- 10 Application of Thermodynamic Calculations to Geochemical Processes Involving Organic Acids.- 11 Metal Transport in Ore Fluids by Organic Ligand Complexation.- 12 Geochemical Models of Rock-Water Interactions in the Presence of Organic Acids.- 13 Organic Acids and Carbonate Stability, the Key to Predicting Positive Porosity Anomalies.- 14 How Important are Organic Acids in Generating Secondary Porosity in the Subsurface?.

This chapter is intended to provide sufficient information to allow one to read the following chapters in any order they prefer. The objective and need for this book are presented in the introductory section. Nomenclature for organic acids is presented on an elementary level for nongeochemists. A historical account is then given on the role of organic acids in geological processes, which is followed by a brief synopsis of each chapter. The final section explores the needs for future research in terms of natural system studies and laboratory experimental studies.

Data on source and reservoir rocks in the Wattenberg area of the Denver Basin were used to evaluate the hypothesis that carboxylic acids and carbon dioxide generated from the thermal maturation of organic matter create secondary porosity in sandstone reservoirs. For the Lower Cretaceous source rocks, information on the type of kerogen, richness, maturity, and thickness was collected for each formation. We used subsidence modeling to determine the temperature and burial history of the source rocks. For the reservoir and potential reservoir sandstones (Fox Hills, Larimer-Rocky Ridge, Terry, Hygiene, Codell, Dakota J, and Lyons) overlying the deepest part of the basin, the composition, total macroporosity, and secondary porosity were determined using 115 thin sections cut from cores from 18 wells. Data on the stratigraphic thickness needed to calculate volume (for 1 km(su2)) of sandstones came from geophysical logs. Porosity in nonreservoir sandstones throughout the stratigraphic column was evaluated using geophysical logs and thin sections of drill cuttings. These nonreservoir sandstones contain only trace amounts of porosity and were not included in the calculations. Also, three carbonate formations were examined in thin section, but disregarded for this study because of lack of macroporosity.

Pore aperture size estimated from mercury injection tests has been used to evaluate seals for traps and to explain the locations of stratigraphic hydrocarbon accumulations. However, mercury injection tests are expensive and therefore not abundant. This paper develops empirical equations for estimating certain pore aperture size parameters from routine core analysis. The relationship of porosity, uncorrected air permeability, and various parameters derived from mercury injection-capillary pressure curves was established using multiple regression on a database of 202 samples of sandstone from 14 formations that range in age from Ordovician to Tertiary. These sandstone formations vary in composition and texture. A series of empirically derived equations also permits the calculation of pore aperture radii corresponding to mercury saturation values that range from 10 to 75% in increments of five. This makes it possible to construct a calculated pore aperture radius distribution curve using porosity and permeability from core analysis.

ABSTRAcr Clay coats which may be continuous or discontinuous originate from soils as cutans from infiltration of clay in sand and sandstone and authigenically as newly formed or regenerated clay minerals Allogenic cutans and infiltration deposits have a laminar morphology whereas authigenic clay coats commonly have a radial morphology Thick well developed continuous clay coats regardless of origin may retard quartz cementation by masking the surface of detrital quartz grains and preventing the nucleation of quartz overgrowths Chlorite is the most effective of the clay minerals in preserving intergranular porosity and appears to be important in very deep sandstone reservoirs The most favorable amount of chlorite to preserve porosity is variable 4 to 7 volume percent for the Berea Sandstone and 5 to 13 volume percent for the Tuscaloosa Sandstone for example Smaller amounts of chlorite permit quartz to nucleate and destroy porosity and greater amounts result in porosity reduction by infill of pores Clay coats do not retard epitaxial cements e g carbonates and sulfates which may cover clay coats and occlude porosity Clay coats may occur in highly lithic e g 35 lithic material sandstones but are not important because physical com paction dominates diagenesis and destroys porosity Experimental growth of clay coats shows that clay flakes are flatly attached to detrital sand grains and curl upward to form a radial fibrous morphology This attached root zone may explain why clay coats are effective at blocking nucleation of quartz cement Ex perimental work also shows that mineralogy may provide an initial substrate control over the precipitation of clay coats by providing an in situ source of the cations needed to precipitate the clay Later the clay coats nucleate on other framework grains farther from the site of initial nucleation

Compaction experiments showed that the degree of physical compaction of a lithic sand is related to the amount and type of lithic material present. These experiments produced a product that is texturally indistinguishable from natural lithic sandstones. Generally, sedimentary lithic fragments are more ductile than metamorphic lithic fragments. Volcanic lithic fragments, when altered to phyllosilicate minerals by weathering or diagenesis, are extremely ductile. Mineralogy and intragranular microporosity influence ductility. Ratios based on weight percent of soft, ductile minerals (clay and micas) to hard, brittle minerals (quartz and feldspar) within the lithic fragments provides a good indication of ductility. A ratio greater than 1 indicates the lithic material will be h ghly ductile. Microporous lithic fragments are more ductile than dense lithic fragments. Physical compaction models were developed based on experimental data from well-sorted lithic sandstones for which the percentage of preserved porosities were functions of the effective stress, the type of lithic material, and the quantity of lithic material. These models are derived from laboratory sandstones with at least 25% lithic grains in the following categories: moderately ductile metamorphic lithic fragments, highly ductile shale lithic fragments, and extremely ductile altered volcanic lithic fragments. Application of the compaction models of the experimental compaction techniques to modern subsurface sand samples (or outcrop equivalents) can provide valuable estimates of the preserved porosities (i.e., reservoir potential) for lithic sandstones that may be exploratory objecti es in frontier basins. Examples using the models and experimental compaction techniques for exploration are given. Experimental data showed that precompaction cement and overpressure may be beneficial to porosity preservation. An early formed partial cement can retard compaction and preserve porosity by stabilizing the sand pack. A later cement does not have this beneficial effect. Overpressure, when developed early, retards compaction by reducing the effective stress. Of course, a late-developing overpressure is not effective at preserving porosity because compaction is an irreversible process.

Carboxylic acids in subsurface waters have been proposed as agents for dissolving feldspars and complexing aluminum to create secondary porosity in sandstones. Previously published experimental work indicated high aluminum mobility in the presence of carboxylic acid solutions. In order to further evaluate aluminum mobility, alkali feldspar dissolution experiments were run at 100 degrees C and 300 bars in the presence of mono- and dicarboxylic acids and their anions. Experimental results imply that under reservoir conditions, aluminum-organic anion complexes are insignificant for acetate and propionate and possibly significant for oxalate and malonate. Propionate appeared to inhibit alkali feldspar dissolution and, hence, may retard aluminum mobility. Dissolution of feldsp r in the presence of oxalic and acetic acid can be explained by enhanced dissolution kinetics and greater aluminum mobility under low-pH conditions. The general absence of such low-pH fluids in subsurface reservoirs makes this an unlikely mechanism for creating secondary porosity. Also, the thermal instability of oxalate and malonate limits their aluminum-complexing potential in reservoirs at temperatures above 100 degrees C.

Granitic pebble morphology was studied in the detritus of the Merced River, Sierra Nevada, California, where minimum distance of transport of granitic pebbles can be determined. Modern alluvium was sampled to determine: (1) percentage of granitic pebbles; (2) percentage of broken rounds; (3) roundness; (4) shape. Granitic pebbles downstream from a large cataract show bimodal distributions. A paucity of intermediate-sized clasts (8 to 32 mm), coupled with an abundance of broken rounds in the same size range, and a decrease in overall roundness occurs directly downstream from the high-energy environment of the cataract. Movement of the intermediate-sized pebbles through the cataract may be mainly by saltation, whereas larger pebbles move by rolling-sliding traction and smaller pebbles by suspension. The critical factor in pebble breakage is probably momentum, and in the cataract environment the saltating intermediate-sized clasts may attain the necessary velocity-mass product to break when they collide with boulders in the stream bed. Granitic pebble abundance, as determined by pebble counts, appears to reflect proximity to source rather than areal abundance of source rock in the drainage area. Comparison between granitic talus and stream pebbles shows equidimensional shape controlled by source-rock characteristics; no obvious evolution in shape results from transport, even where considerable breakage has occurred.

Authigenic chlorite coatings on detrital quartz grains influence the formation of overgrowths and preservation of porosity in the Spiro Sand, Arkoma Basin, Oklahoma. Thick continuous coatings preserve porosity because pressure solution is retarded and quartz overgrowths are nonexistent. If the coatings are sparse, thin, and discontinuous, the sandstone is nonporous because of pressure solution and quartz overgrowths. Intermediate stages between these two extremes also are present.

Thin-section/scanning electron microscope studies of thousands of sandstones representing many ages. compositions, and depositional environments indicate authigenic clays are far more common than previously recognized. Authigenic clays occur as pore linings, pore fillings, pseudomorphous replacements, and fracture fillings. Sandstones also contain allogenic clays. Allogenic clays originate as terrigenous material (dispersed matrix, sand-sized floccules, sand- to cobble-sized mud or shale clasts) or are introduced subsequent to deposition as a result of bioturbation or infiltration. An authigenic origin can be established on the basis of clay composition, structure, morphology and distribution, and sandstone textural properties. However, no individual criterion is an infallible indicat r of authigenic origin. The most reliable criteria are a) delicacy of clay morphology, which precludes sedimentary transport, b) occurrence of the clay as pore linings absent only at grain contacts, and c) composition radically different than associated allogenic clays. Distinctions between authigenic and allogenic clays become difficult if the latter are recrystallized or if either is extensively deformed by burial or tectonism. Studies of authigenic clays utilizing a combination of thin-section, scanning electron microscope with nondispersive elemental analyzer, and x-ray diffraction show that each of the major clay groups exhibit a limited number of distinctive morphologies. Smectite occurs as highly wrinkled or honeycomb-like pore linings, the individual flakes of which are not resol able. Illite forms pore-lining overlapping flakes whose edges tend to curl away from the grain surface and from which highly elongate lath-like projections may extend. Mixed-layer smectite/illite morphologies resemble those of both smectite and illite. Chlorite occurs primarily as pore-lining pseudohexagonal flakes with a cardhouse, honeycomb, or rosette arrangement. Kaolinite and dickite most commonly form pore-filling books of stacked pseudohexagonal flakes. Occasionally, they form thin pore-lining sheets of overlapping pseudohexagonal flakes. Authigenic clays are a major control on reservoir quality. Permeability and water saturation are particularly sensitive to the relative abundance of clays. Interpretations of depositional environment and provenanco based on sandstone composition nd texture may be inaccurate if the presence of authigenic clay is overlooked.

Running Duke field, Houston County, Texas, is on a small domal structure in the East Texas basin. The field currently consists of five gas wells producing from the Lower Cretaceous Rodessa limestone. Seven additional gas completions have been plugged. The Rodessa yields water-free gas production from zones with water saturations in excess of 60%. Porosity ranges from 7 to 14%. The depositional model depicted for the Rodessa in this field starts with the development of an active organic buildup. This was successively overlain by ooid-skeletal-peloid lime sands, lagoonal lime muds, and subsequently by anhydrite and shale. Six lithofacies, based on core study, were recognized for the Rodessa. Each facies has a uniform diagenetic history in the field, making porosity development predictable for each facies. The one grainstone facies forms the reservoir in the field and has two distinct diagenetic sequences; one consists of a marine isopachous cement followed by a later partial infilling of pore space by sparry calcite. The resulting intergranular porosity has a unimodal pore-size distribution. In the other sequence, a discontinuous meteoric phreatic cement is followed by sporadic patches of sparry cement. The resulting pore-size distribution is bimodal with intergranular macropores and intercrystalline micropores within peloids and micritized ooids. Water-free gas production is limited to the rock with the bimodal pore system: macropores hold movable gas and micropores hold bound water within grains. Where core data are not available, cross-plotting techniques using geophysical logs are suggested as methods for separating the productive from the nonproductive grainstone.

Ferro-magnesian calcite occurs as cement in terrigenous sandstones as old as Ordovician from North America and the West Indies. Based on microprobe analyses, ferro-magnesian calcite cements have 1 to 10 wt. % FeCO3 and 11 to 39 wt. % MgCO3. Cement of this nature previously has not been analytically documented. Because of the propensity of iron to substitute for magnesium in the calcite lattice, what appears to be high magnesian calcite, based on X-ray diffraction techniques, actually may be ferro-magnesian calcite in terms of chemical composition. Subsurface occurrences indicate that with depth, a transition may take place from low-magnesian calcite through ferro-magnesian calcite to dolomite.

A ternary pore geometry classification scheme for reservoir rocks places intergranular and intercrystalline porosity at a common pole. These porosity types tend to have well interconnected pores and usually are good reservoirs. Intragranular, moldic, and vuggy porosity are grouped at another pole of the classification triangle. Rocks with these porosity types usually have poorly interconnected pores and low permeability. Rocks with microporosity are grouped at the final pole of the triangle. These may be argillaceous sandstones, finely textured carbonates, diatomites,or tripolitic cherts. Water‐wet microporous rocks hold bound water. Rocks with significant amounts of microporosity and/or intragranular, moldic, and vuggy porosity need fractures, either natural or induced, to make an attractive reservoir. Fracture porosity may occur by itself or combined with any other porosity type. Reservoirs typically contain multiple pore types, although one type often predominates. Large scale cavernous features in carbonates occasionally contain oil.

This study discusses the influence that sedimentology, diagenesis, and basin subsidence history have on porosity and permeability of the Upper Cretaceous Terry sandstone of Spindle field in the Denver basin. The Terry sandstone consists of lithic subarkoses and feldspathic litharenites deposited in a marine bar complex. The reservoir facies, which was deposited in a marine bar crestal position, is a cross-bedded sandstone with a mixture of primary and secondary macropores. Approximately half of the pore volume is microporosity associated with clay minerals and lithic fragments. The nonreservoir facies was deposited on the bar-flank margin as lower-energy, alternating layers of mud and sand, which were extensively bioturbated to produce muddy sands. Ductile deformation of lithic fragments is a major factor in porosity loss for the reservoir facies. Chlorite and calcite are the most important mineral cements. Authigenic quartz, dolomite, ferroan calcite, kaolinite, feldspar, pyrite, and tourmaline are present in minor to trace amounts. Precipitation of these minerals is believed to be related to the evolution of marine water. Secondary porosity formed by dissolution of pore-filling and replacement calcite and framework feldspars. Sandstones with pervasive calcite cement have no secondary porosity developed, which suggests some permeability may be needed to develop secondary porosity. Plagioclase feldspars become more albitized with increasing depth. A temperature-depth subsidence profile predicts a maximum temperature of only 85°C, at least 15° lower than in Tertiary rocks, which suggests that kinetic considerations may also be important to albitization. Vitrinite reflectance (0.65%) and interbedded shales with ordered mixed-layered illite/smectite with up to 50% expandability also support the lower temperature regime. Calculations suggest that seawater can supply sufficient sodium to account for albitization. Albitization proceeded more rapidly in permeable sandstone, where the plagioclase was in direct contact with the pore fluids. Plagioclase in relatively impervious sandstones eventually became albitized, probably through diffusion.

Granulation caused by cataclasis reduces porosity and permeability in indurated quartz sandstones of the Simpson Group. Pore aperture size in granulated rock also is drastically decreased because breakage of grains and spalling of quartz overgrowths leads to a bimodal distribution of grain sizes: subangular to well-rounded, fine- to medium-grained quartz in an angular silt- and clay-sized quartz matrix. This mechanical reduction in pore aperture size creates the potential for fault-sealing traps. Granulation occurs along individual or anastomosing seams up to a few millimeters thick and is more pervasive near major faults. Undisturbed blocks, up to 30 cm across, of porous and permeable sandstone commonly are bounded in three dimensions by planar seams of tight granulated ock. This cellular nature influences the entrapment of hydrocarbons, as shown in asphalt quarries, and could affect the degree and type of diagenesis. Because of the abundance of granulated material associated with major faults, it appears possible that faults may serve as seals even when sandstone is faulted against sandstone. In friable sandstones studied, the clay coatings binding the grains are disrupted, fragmented, and then rearranged by grain rotation along insignifcant faults with about 15-cm throw, leading to a pronounced decrease in porosity and permeability as well as significant reduction in pore aperture size. The pore aperture size change associated with these friable sandstones in fault zones is not sufficient to constitute a seal.

The Foster sand represents local stream channels that cut into Morrowan strata while draining southeastward in the McAlester basin, Oklahoma. Four channel systems, ranging from 1 to 6 mi wide and up to 20 mi long, have been mapped. During the hiatus these streams brought detritus to the southern part of the basin where marine sedimentation was continuous and the Spiro sand was forming in a shore zone environmental complex that probably included beaches, bars, tidal flats, tidal channels, and lagoons. During a northward transgression the Spiro sand was deposited across the basin and locally overlies the Foster sand. The Foster sand is a fine-grained quartz sandstone with well-rounded, moderately sorted grains. The Spiro sand is most commonly supermature, but is submature or mature locally. This sandstone has subangular to well-rounded, poorly to very well-sorted (1.05 to 0.31 phi), very fine to medium grains (0.11 to 0.30 mm). Intergranular pore space in the Foster and Spiro sands has been reduced by pressure solution and formation of quartz overgrowths, carbonate cement, and chlorite cement. Chlorite is authigenic and apparently of two different ages. The chlorite coats the surface of detrital quartz and preserves intergranular porosity by retarding pressure solution and formation of overgrowths. The Spiro sand has been subdivided into eight facies with limestone, shale, and quartz sandstone end members. Analysis of these facies, primarily by isopach maps and grain size, indicates that dry gas production limits have been established, with the possible exception of reservoirs south of the Choctaw fault in the structurally complex frontal zone of the Ouachita Mountains. The Foster sand is a good dry gas reservoir, but has limited distribution.

Various types of pore casts, replicas of void space, useful for the study of pore geometry can be prepared from rocks impregnated with plastic. Conventional light microscopes are suitable for examination of most pore casts, however the scanning electron microscope is definitely superior and is essential for good photographic illustrations. Study of pore casts provides information on the distribution and shape of pores and interconnecting passageways that influence permeability and control the flow of fluids.

In untransported material from granitic rocks A-twin/C-twin and Untwinned/Twinned ratios vary inversely with grain size. These characteristics persist in transported detritus. Plagioclase twins tend to be destroyed during stream transport as shown by an increase in A-twin/C-twin and Untwinned/Twinned ratios. This is probably the result of breakage along composition and twin planes, and breakage of untwinned plagioclase into smaller grains. Because of these effects coarser grained plagioclase is preferred for provenance studies.

Examination of quartzose sandstones with the scanning electron microscope reveals details and complexities not discernible with a petrographic microscope. Incipient quartz overgrowths occur either as growths with poorly defined crystal facies that are loosely interconnected over the surface of the detrital nucleus or as isolated growths with well defined crystal faces. If sufficient void space is present these incipient overgrowths may enlarge to partly envelop the nucleus and form well defined crystal faces. This appears to take place in the following ways: (1) overlap and/or mergence of individual subunit overgrowths with the same crystallographic orientation unitl so tightly packed that an atom cannot distinguish between subunits and growth continues as a single crystal; (2) envelo ment of earlier multiple overgrowths by an outer shell so that the underlying complex nature of the overgrowth is masked. The crystal lattices of the overgrowth and nucleus are connected at isolated points. Much of the area between overgrowth and nucleus is void space, which forms the "dust" line visible in thin section. Secondary quartz probably eventually infills many of these voids, which destroys the "dust" line unless impurities are present also. Presolved surfaces between quartz grains vary from relatively smooth to rough with ridges, knobs, furrows, and pits. A pressure solution surface may have a series of ridges and furrows, commonly in a radial pattern, along the margin.

Large pores in carbonate rocks hold and transmit fluids, whereas associated micropores may hold irreducible water; i.e., water not available for flow. Analysis of borehole logs of microporous carbonate rocks can result in misleadingly high calculated water saturations and possibly bypassing of a potential oil or gas reservoir. Microporosity occurs in carbonate rocks that range from friable to well indurated. Intercrystalline micropores may be present in micrite and within ooids, pisolites, micrite intraclasts, pellets, and cryptocrystalline grains. Micropores in ancient rocks appear to result from (1) formation of micrite envelopes, (2) void space present after transition of aragonite to calcite, or (3) incipient weathering and dissolution. Aggrading neomorphism of micrite to microspar tends to destroy microporosity.

A technique utilizing a petrographic microscope with a flat stage for analysis of provenance has been developed. Procedure is to concentrate feldspar in the medium or coarser sand range by flotation techniques, then to separate this concentrate by means of heavy liquids into potash feldspar, albite, oligoclase-andesine, and calcic plagioclase. Thin sections of these plagioclase separations are examined for twin characteristics (C-twins, A-twins, and untwinned), and zoning characteristics (progressive, oscillatory, and unzoned). This leads to inferences about the presence of metamorphic, plutonic igneous, or volcanic-hypabyssal rocks in the source area. Only the predominant rock type usually can be detected. An exception to this is volcanic and hypabyssal rocks, which commonly yield pl gioclase with diagnostic properties.

Observations on oscillatory zoning, progressive zoning, and the lack of zoning in plagioclase feldspar of igneous and metamorphic rocks indicate a use in the interpretation of provenance of sedimentary rocks. Plagioclase in volcanic and hypabyssal rocks is characterized by oscillatory zoning, whereas this type of zoning is relatively rare in plutonic igneous and metamorphic rocks. The presence of zoned plagioclase, regardless of the type of zoning, is strongly indicative of an igneous rock.

Porosity and permeability changes in Pleistocene corals are controlled either directly or indirectly by marine and fresh-water diagenetic processes. These diagenetic events create as well as destroy pore space. The transition of aragonite to calcite and accompanying precipitation of sparry calcite in voids are the major factors influencing porosity and permeability of the corals in the vadose zone. Porosity decreases primarily because of the growth of sparry calcite in pores, whereas permeability increases because of changes in pore aperture size that result from a fabric change associated with recrystallization. Mercury-injection-capillary-pressure data indicate that calcitic corals have larger and better sorted pore apertures than do aragonitic corals.