Porosity Reduction In Sandstones Research Articles

Compaction control on diagenesis and reservoir quality development in red bed sandstones: a case study of Permian Rotliegend sandstones

Authigenic minerals formed during diagenesis in conjunction with compaction by burial have long been known to lead to porosity-loss of sandstones, and a subsequent deterioration in reservoir quality. The diagenetic impact on reservoir quality and permeability heterogeneity measured horizontal and vertical to bedding was characterized in three fluvio-eolian Lower Permian Rotliegend outcrops from the Flechtingen High, the northern Hesse Basin (both Germany) and the Vale of Eden (UK) using point-counting, polarized light-microscopy, helium pycnometry and permeability measurements. Results show significant porosity (10 to 35%) and permeability (0.01 to 10,000 mD) ranges largely independent of depositional environment. The major control on reservoir quality in Cornberg Sandstones are dolomite and siderite cementation in conjunction with illitization and illite and kaolinite cementation, leading together with quartz cementation to a mostly cemented IGV and poorest reservoir quality (avg. horizontal permeability: 0.96 mD). Flechtingen Sandstones are most intensely compacted due to the lack of significant early diagenetic cement phases and continuous illitic grain-to-grain coatings, which inhibited intense quartz cementation but enhanced chemical compaction at quartz grain contacts, resulting in intermediate reservoir quality (avg. horizontal permeability: 34.9 mD). Penrith Sandstones lack significant authigenic phases besides quartz due to carbonate dissolution during uplift. They show the least amount of detrital feldspars and clay minerals, leading to no major reservoir quality reduction by burial diagenetic clay mineral alterations, resulting in the highest reservoir quality (avg. horizontal permeability: 5900 mD). Additional results highlight higher horizontal to vertical permeability ratios kh/kv in less homogeneous sandstones of < 10 mD of 10, and in more homogenous, higher permeable sandstones > 1000 mD of 1. Although detrital and authigenic sample compositions vary throughout the studied areas, the general effect of grain coatings coverages on syntaxial cement inhibition and chemical compaction can be delineated. This study increases the understanding of porosity reduction in sandstones, as it confirms the necessity to differentiate between the illitic grain-to-grain coatings and illitic grain-to-IGV coatings. As a result, the enhancing effect of illite on chemical compaction on quartz grain-grain boundaries can be better constrained, as well as the effect of grain coatings on quartz cementation. This is relevant for reservoir quality and risk assessment in hydrocarbon and geothermal plays as well as in storage.

Whether the diagram of intergranular volume versus cementation volume is normative? —Take “depositional and compositional controls on diagenesis of the mixed siliciclastic-volcaniclastic sandstones: A case study of the Lower Cretaceous in Erennaoer Sag, Erlian Basin, NE China” for example

The diagrams of intergranular volume versus cementation volume brought up by Houseknecht (1987) and Ehrenberg (1989), are extensively used around the globe to assess the relative significance of compaction and cementation to porosity reduction in sandstones. However, Houseknecht (1987) overlooked the fact that the bulk sediment volume is dynamically compressed during burial, which directly leads to deviation in evaluating original porosity loss. The present discussion is based on the theoretical and contrastive analysis. These analyses indicate the compactional porosity loss calculated by Ehrenberg's (1989) method is always greater than Houseknecht's (1987) method with a maximum difference of 12.7%. In contrast, the cementation porosity loss using Ehrenberg's (1989) method is always less than Houseknecht's (1987). Besides, the demonstration based on the data of Wei et al. (2020) also supports Ehrenberg (1989). Therefore, the method proposed by Ehrenberg (1989) should be highly recommended to be granted as a reference far and wide.

A physical model for porosity reduction in sandstones

The experimental elastic moduli‐porosity trends for clean sandstones can be described by the modified upper Hashin‐Shtrikman (MUHS) bound. One geometrical (but not necessarily geological) realization is: as porosity decreases, the number of the pores stays the same and each pore shrinks while maintaining its shape. This concept of uniform porosity reduction implies that permeability is proportional to the effective porosity squared, and that formation factor is proportional to the inverse of the effective porosity. The effective porosity here refers to the part of the pore‐space that dominates fluid flow. The proposed relations for permeability and formation factor agree well with the experimentally observed values. These laws are different from the often used forms of the Kozeny‐Carman equation and Archie’s law, where permeability is proportional to the total porosity cubed and formation factor is proportional to the inverse of the total porosity squared, respectively. We suggest that the uniform porosity reduction concept be used in consolidated rocks with porosities below 0.3. The transition from high‐porosity unconsolidated sands to consolidated sandstones can be described by the cementation theory: the MUHS moduli‐porosity curves connect with those predicted by the cementation theory at the porosity of about 0.3. This scheme is not appropriate for modeling other porosity reduction mechanisms such as glass bead sintering because, during sintering, the pores do not maintain their shapes, rather they gradually evolve to rounder, stiffer pores.

A model for porosity reduction in quartzite reservoirs by quartz cementation

Quartz cementation is an important process of porosity reduction in sandstones. A mathematical model is presented for predicting the distribution and amount of quartz cement in a sandstone reservoir as a function of time and space. The approach combines kinetics and transport mechanism and incorporates the following assumptions: (1) no compaction during cementation, (2) transport of dissolved silica by advection, (3) surface-controlled growth, (4) sands simplified to a framework of spheres, and (5) isothermal system. The model predicts that low flow rates, small grain size, and higher temperatures favor limited volume of quartz cement because the pores are rapidly obstructed. Pervasive cementations are predicted in the presence of coarse-grain sands, high fluid flows, and low temperatures. Substantial volumes of quartz may occur within 250,000 years under conditions of shallow burial ( T = 20° C, u = 10 m y ). During conditions of deeper burial and higher temperatures ( T = 80°C), quartz cement should form during geologically rapid events rather than during the entire history of burial.

Porosity Reduction in Sandstones Induced by Mechanical Compaction: Cathodoluminescence Evaluation

Cathodoluminescence (CL) petrography has been instrumental in documenting the importance of chemical compaction (pressure solution) to porosity reduction in sandstones. Comparatively little work has been focused on the role of mechanical compaction, although this process is probably more pervasive. Brittle failure of quartz grains has been largely overlooked as a compaction mechanism because fractures are commonly healed by quartz cement that is not visible unless CL petrography is utilized. Grain boundary extension fractures form at grain contacts in response to overburden stress and are not related to tectonic stress. They result in extension of detrital quartz fragments into intergranular space, causing reduction of bulk volume, intergranular porosity, and total effective porosity. Samples of quartzose sandstones from sedimentary basins of diverse geologic history have been examined using CL petrography. This survey documents significant reduction of intergranular volumes by mechanical compaction to values as low as 20% in sandstones that have undergone little or no chemical compaction. Factors favoring the incidence of mechanical compaction induced by brittle failure include rapid burial, high temperatures, coarse grain size, and the absence of diagenetically early cements. This reconnaissance work suggests that mechanical compaction associated with brittle failure of quartz grains is more important than commonlymore » thought, and emphasizes the need for CL petrography to be integrated into routine analysis of sandstone reservoir quality.« less

Event Cementation in Sandstones

Precipitation of quartz overgrowths on sand grains is a common cause of porosity reduction in sandstones. The amount of quartz overgrowths (cement) often increases with depth so that the process of cementation is generally considered to be a direct response to burial and to proceed throughout the burial history of a sandstone. The authors show that some Mesozoic and Cenozoic sandstones are cemented during events 1-10 m.y. in length, periods one to two orders of magnitude less than their burial histories. Burial alone is unlikely to be the cause of quartz cementation. This new knowledge should markedly change our perception of porosity evolution in sandstones.

Simulation of porosity reduction in random structures

We describe an algorithm for computing the motion of a solid-liquid interface in 2D, which is applicable to geological pressure solution or to pressure sintering. The backward motion (toward the solid) of the interface is due to dissolution of the solid, and the forward motion (away from the solid) is due to the inverse process of reprecipitation. The interface velocity is assumed proportional to the difference between the solubility of the solid and the concentration of the solution. The former is dependent upon stress (the phenomenon of “pressure solution”), so our algorithm must also keep track of the stress. We use a Lagrangian grid, with constant-stress periodic boundary conditions. The method has been applied to porosity reduction in sandstone. It is applicable to other interface-following problems, such as freezing, if the motion is slow enough that heat transport is not rate-limiting.

Assessing the Relative Importance of Compaction Processes and Cementation to Reduction of Porosity in Sandstones: Discussion; Compaction and Porosity Evolution of Pliocene Sandstones, Ventura Basin, California: DISCUSSION

Assessing the Relative Importance of Compaction Processes and Cementation to Reduction of Porosity in Sandstones: Discussion; Compaction and Porosity Evolution of Pliocene Sandstones, Ventura Basin, California: DISCUSSION

Assessing the Relative Importance of Compaction Processes and Cementation to Reduction of Porosity in Sandstones: DISCUSSION

Assessing the Relative Importance of Compaction Processes and Cementation to Reduction of Porosity in Sandstones: DISCUSSION

Assessing the Relative Importance of Compaction Processes and Cementation to Reduction of Porosity in Sandstones: REPLY

Assessing the Relative Importance of Compaction Processes and Cementation to Reduction of Porosity in Sandstones: REPLY

Assessing the relative importance of compaction processes and cementation to reduction of porosity in sandstones : Houseknecht, D W Bull Am Assoc Engng GeolV71, N6, June 1987, P633–642

Assessing the relative importance of compaction processes and cementation to reduction of porosity in sandstones : Houseknecht, D W Bull Am Assoc Engng GeolV71, N6, June 1987, P633–642

Assessing the relative importance of compaction processes and cementation to reduction of porosity in sandstones : Houseknecht, D W Bull Am Assoc Engng GeolV71, N6, June 1987, P633–642

Assessing the relative importance of compaction processes and cementation to reduction of porosity in sandstones : Houseknecht, D W Bull Am Assoc Engng GeolV71, N6, June 1987, P633–642

Porosity Reduction in Sandstone by Quartz Overgrowth: DISCUSSION

A discussion of porosity reduction in petroleum reservoir sandstones due to quartz overgrowth is presented. The short communication deals with an article previously published on this subject.

Porosity Reduction in Sandstone by Quartz Overgrowth: DISCUSSION

A discussion of porosity reduction in petroleum reservoir sandstones due to quartz overgrowth is presented. The short communication deals with an article previously published on this subject.

Porosity Reduction in Sandstone by Quartz Overgrowth: REPLY

In "Porosity Reduction in Sandstone by Quartz Overgrowth," by Frederic Leder and Won C. Park (AAPG Bulletin, v. 70, no. 11, November 1986, p. 1713-1728), the fluid velocities reported are low by about 35%. However, because of a cancellation of errors, the results published for porosities are correct. The following derivation should be substituted for the published text, beginning with equation 11 (p. 1716): EQUATION (11) where ^ngr = fluid viscosity and ^nablaP = pressure gradient. EQUATION (12) where EQUATION (13) Equation 14 is correct as published. EQUATION (15) Neglecting ^dgrvv ^dgrvy and ^dgrv2P ^dgrvy2 , EQUATION (16) Rearranging and integrating gives equation 17 as published. The left side of equation 18 is correct as published; the right side, however, should be: EQUATION (18) and equation 19 becomes EQUATION (19) Equations 20-22 are correct. EQUATION (23) EQUATION (24) Considering fluid velocities, equation 40 should be EQUATION (40) The subscript zero is required since ^Thgr is a function of time. Equation 42 should be EQUATION (42) and EQUATION (43) Using equation 39, [EQUATION] Therefore, substituting equation 24 into equation 19, [EQUATION] and [EQUATION] By equation 40, [EQUATION] This is the correct expression for use in calculating porosity. Equation 40, as it was published, substituted as above into the published equations 19 and 24, will produce the same equation. This fortuitous result preserves the value of the calculations; however, the derivation presented in this erratum is correct and should be used. End_of_Article - Last_Page 483------------

Porosity Reduction in Sandstone by Quartz Overgrowth: ERRATUM

In Porosity Reduction in Sandstone by Quartz Overgrowth, by Frederic Leder and Won C. Park (AAPG Bulletin, v. 70, no. 11, November 1986, p. 1713-1728), the fluid velocities reported are low by about 35%. However, because of a cancellation of errors, the results published for porosities are correct. The following derivation should be substituted for the published text, beginning with equation 11 (p. 1716): EQUATION (11) where ^ngr = fluid viscosity and ^nablaP = pressure gradient. EQUATION (12) where EQUATION (13) Equation 14 is correct as published. EQUATION (15) Neglecting ^dgrvv ^dgrvy and ^dgrv2P ^dgrvy2 , EQUATION (16) Rearranging and integrating gives equation 17 as published. The left side of equation 18 is correct as published; the right side, however, should be: EQUATION (18) and equation 19 becomes EQUATION (19) Equations 20-22 are correct. EQUATION (23) EQUATION (24) Considering fluid velocities, equation 40 should be EQUATION (40) The subscript zero is required since ^Thgr is a function of time. Equation 42 should be EQUATION (42) and EQUATION (43) Using equation 39, [EQUATION] Therefore, substituting equation 24 into equation 19, [EQUATION] and [EQUATION] By equation 40, [EQUATION] This is the correct expression for use in calculating porosity. Equation 40, as it was published, substituted as above into the published equations 19 and 24, will produce the same equation. This fortuitous result preserves the value of the calculations; however, the derivation presented in this erratum is correct and should be used. End_of_Article - Last_Page 483------------

Assessing the Relative Importance of Compaction Processes and Cementation to Reduction of Porosity in Sandstones

At the depositional surface, well-sorted sand has approximately 40% porosity. During burial diagenesis, that porosity is reduced by mechanical compaction, intergranular pressure solution, and cementation. Mechanical compaction and intergranular pressure solution can both be considered compactional processes because they irreversibly reduce the intergranular volume of sand. In contrast, cementation occludes, but does not reduce, intergranular volume. The relative importance of compactional processes and cementation to porosity reduction can be quantified using a graph of intergranular volume vs. cement. This diagram can be used to evaluate which diagenetic processes have been most influential to intergranular porosity reduction and to determine why some sandstones retain better reservoir quality than others. The diagram can also be used to reconstruct pathways taken by sandstones during burial diagenesis. Results of applying this technique to data from the Nugget Sandstone and Bromide sandstone (Simpson Group) indicate that mechanical compaction and intergranular pressure solution were much more important than cementation in determining ultimate porosity. Moreover, the best porosity is preserved in samples that have undergone the least intergranular pressure solution. These conclusions emphasize the importance of integrating an evaluation of these compactional processes into analyses of reservoir sandstones and into models of burial diagenesis.

Experimental Assessment of Porosity Reduction Through Ductile Grain Deformation in Sandstones: ABSTRACT

The effect of ductile grain deformation on porosity in sandstones has been recognized for some time, but studies to date have been either qualitative or theoretical in nature. To test the quantitative significance of ductile grain deformation on porosity, a series of sands containing variable percentages (5-50%) of ductile grains mixed with equal-sized quartz were manufactured. These mixtures were compressed in a biaxial compression system at pressures from 4000 to 20,000 psi simulating burial to depths of up to 20,000 ft. Results indicate a strong negative correlation between porosity and depth of burial in sands with similar ductile grain content. At pressure less than 4000 psi porosity, reduction was largely the result of packing readjustment and grain rotation. At pressures greater than 4000 psi, however, there was progressive deformation of ductile components and a concurrent decrease in porosity. Porosity loss range from 1.2 to 2.1% per 1000 psi. A strong negative correlation also exists between porosity and ductile grain content. Porosity values of sands compressed to 10,000 psi ranged from 24% in samples with 20% ductile grains to 9% in samples with 50% ductile grains. A roughly linear trend is apparent, with porosity loss ranging from 0.6 to 1.9% formore » each additional 5% of ductile grains. Experiments with various grain sizes and additional ductile components indicate these variables also influence porosity reduction, although not as greatly as pressure or ductile grain content. These studies indicate ductile grain content can be an important control on porosity reduction in sandstones, particularly those which have been deeply buried. They also suggest there is a definable relationship between ductile grain content, depth of burial, and ultimate porosity. This relationship may be useful in predicting reservoir quality in deeply buried sandstones.« less

Porosity Reduction in Sandstone by Quartz Overgrowth

A mathematical model for predicting porosity histories due to quartz overgrowth in quartz-rich sandstone reservoirs has been developed. The model neglects secondary dissolution porosity and other diagenetic processes such as compaction and precipitation of carbonate and clay minerals. Nevertheless, the model is straightforward and easy to use to calculate porosity. The calculated porosity corresponds to measured porosity of simple quartz-cemented sandstones in 27 wells in North and South America. The model is based on fluid flow through initially porous, unlithified sands in which the fluid phase is saturated with Si(OH)4 and is always in equilibrium with quartz. As the continuously circulating fluid migrates updip toward the basin edge, it cools, and quartz precipitates into the pore spaces causing a loss of porosity. Basinal fluid velocities may be calculated assuming (1) thermal convection and (2) hydrostatic pressure due to recharge at the edge of the basin. Porosity diagrams relating porosity to geothermal gradient, burial rate, and depth of burial are compared to thermal maturation models of source rocks, fluid flow history, and grain-size distribution. The rate of porosity reduction depends on the following variables in decreasing order of significance: buria rate, age, initial porosity, basin size (dip angle), fluid dynamics, initial permeability, and geothermal gradient.

Effect of Non-Hydrostatic Stress on Chemical Processes During Diagenesis: ABSTRACT

Non-hydrostatic stress facilitates porosity reduction in sandstones at elevated temperatures. Experimental results indicate that chemical equilibrium between fluids and solids in fluid-saturated, porous rocks will not be attained under conditions of directed stress. In rocks in which pore fluid pressures are less than lithostatic (Pf < PL), downward directed stress at grain contacts can produce an inhomogeneous distribution of mineral solubilities. High strain at grain contacts causes higher solubilities of the solids and leads to dissolution (pressure solution), whereas growth may occur at solid/water interfaces with low surface strain. The driving free energy for coupled dissolution and growth reactions under non-hydrostatic loading is proportio al to PL - Pf. Consequently, the magnitude of the End_Page 917------------------------------ departure from solid/fluid equilibrium during diagenesis will increase with depth of burial if the pore fluid pressure is hydrostatic. When mineral/fluid equilibrium is not reached, the mineral assemblage developed may be controlled by kinetic rather than thermodynamic factors. Solid phases not stable at the prevailing temperature and fluid pressure may form if high local solubilities create supersaturation in the bulk fluid with respect to many solids. The growth of minerals which decreases the total supersaturation rapidly is favored. Hence, fast-growing less-stable phases (for example, clays, zeolites, and aragonite) may form or persist at the expense of more stable but more slowly forming phases (i.e., illite, feldspars, and calcite). Consequently, the influence of both kinetic factors and bulk phase equilibrium should be considered in evaluating the genesis of mineral assemblages formed during diagenesis and bur al metamorphism. End_of_Article - Last_Page 918------------