Edward D. Pittman

Relationship of porosity and permeability to various parameters derived from mercury injection-capillary pressure curves for sandstone

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.

Authigenic clays in sandstones; recognition and influence on reservoir properties and paleoenvironmental analysis

ABSTRACT 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.

Compaction of lithic sands: Experimental results and applications

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.

Effect of fault-related granulation on porosity and permeability of quartz sandstones, Simpson Group (Ordovician), Oklahoma

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.

Microporosity in Carbonate Rocks: GEOLOGICAL NOTES

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.

Secondary porosity revisited; the chemistry of feldspar dissolution by carboxylic acids and anions

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.

Relationship between chlorite coatings on quartz grains and porosity, Spiro Sand, Oklahoma

ABSTRACT 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.

Use of Pore Casts and Scanning Electron Microscope to Study Pore Geometry

ABSTRACT 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.

Plagioclase Feldspar As An Indicator Of Provenance In Sedimentary Rocks

ABSTRACT 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.

Destruction of plagioclase twins by stream transport

ABSTRACT 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.