# 3.3 Primary and Secondary Porosity

The porosity of earth materials originates during two phases: 1) during the deposition of sediments, lithification or cooling of crystalline rock; and 2) after deposition as the earth material is exposed to other conditions such as compaction, weathering, fracturing and/or metamorphism. As a result, earth materials can have porosities dominated by primary conditions during initial formations, secondary events after formation, or both.

# Primary Porosity

The porosity of an earth material during its original formation is referred to as primary porosity. A number of factors influence the porosity of earth materials. The primary porosity of granular material is affected by the shape and packing of grains, the distribution of grain sizes (sorting and uniformity) and the porosity of the particles themselves (Figure 10). The degree of cementation during lithification of sedimentary rocks will also affect the primary porosity.

Porosity is highest when materials are loosely packed and grains are uniform in size and shape. As a thought experiment, consider two identical rooms (e.g., same total volume), one full of glass spheres the size of soccer balls and one full of 1 cm diameter spherical glass marbles. The spheres in each room are arranged to form cubes (cubic packing) as illustrated in Figure 10a. Based on this information, what would be the porosity of each room? The answer is that the porosity of both rooms would be exactly the same because the uniformly sized spheres in a cubic arrangement produce the same volume of voids in both rooms. The porosity of both rooms would be 48%. This can be confirmed by determining the number of spheres of any given size within the room, calculating their volume, then dividing by the volume of the room. If instead, two wheel barrows full of marbles were dumped into the room full of soccer-ball-sized spheres the porosity would decrease as some of the large void space would be partially filled with solid marbles reducing the total pore space volume (Figure 10d). If the spheres of either size were packed in a rhombohedral arrangement, the porosity of both rooms would be 26%. A cubic packing is the loosest possible arrangement and rhombohedral packing is the tightest possible arrangement. Therefore, the minimum and maximum porosity for a material made of uniformly sized spheres is 26% and 48%, respectively.

When grains making up an earth material have an internal porosity, the material is said to have dual porosity (Figure 10e). In some cases, this internal porosity may or may not contribute to the value of effective porosity of the material.

After sediments are deposited, they often become lithified as time passes, that is, minerals precipitate from the groundwater, cementing the grains together. The cement fills some of the original pore volume, and so the cemented sediment will have less porosity than the sediment had before cementation occurred (Figure 10f).

If elongated particles such as the typical microscopic particles or platelets comprising clay are packed in cubic or rhombohedral arrangements, their shape would cause the porosity to be lower than for the spheres (Figure 10g, h). However, clay deposits often have high porosities because their microscopic mineral platelets are negativity charged, pushing the platelets apart and allowing water molecules to occupy the space between the grains. The small, uniformly sized, elongated particle shapes, coupled with their charged nature, results in a large volume of open space even though the spaces are small. As a consequence, porosities of clays can range from 40% to over 70% (Figure 10; Table 1 and Table 2).

# Secondary Porosity

Many of Earth’s rock materials have little to no primary porosity when formed. That is, there is little to no void space in the material when it is first formed. Examples of this include rocks formed: from cooling lava (igneous rocks); by precipitation of minerals during evaporation of water (e.g., salt and limestone); and those formed by the heating, folding and compression of any pre-existing rock (metamorphic rocks). However, porosity of such rocks can increase because of weathering, fracturing, and/or dissolving (Table 1 and Table 2.).

Secondary porosity is the additional porosity acquired after the original rock formation process (Figure 11). A rock that is fractured or weathered (including solutioned) after its initial formation has secondary porosity. Whether the porosity is primary or secondary, the combined properties are included in the effective porosity. In most cases, the development of secondary porosity increases the effective porosity of a porous material.

As an example of the magnitude with which fracturing can enhance porosity of a rock, assume that a well cemented sandstone has an effective porosity of 0.13 (13%). Then, after deposition and lithification, the sandstone is uplifted by Earth’s tectonic forces and eroded such that the upper 100 m is jointed. If a cube of the sandstone is 1 m3 (total volume) with 10 fully penetrating vertical joints (1 m long by 1 m wide) and each joint aperture is 100 microns wide (1 micron = 1 × 10-6 m), then the additional pore space provided by the fractures is 0.001 m3 (10 joints × (100 × 10-6 m) × 1m × 1m). Dividing the fracture pore space by the total sample volume results in the addition of 0.001 to the total sample effective porosity, 0.131. If fractures were the only source of effective porosity in this 1 m3 of rock, the effective porosity would be 0.001 (0.1%). Though fractures may not provide much water storage capacity, they may provide the only source of groundwater in some geologic environments. In addition to increasing porosity, fractures also act to enhance the transmission of water by creating fracture networks and joining pore spaces that were not originally connected.

The creation of secondary porosity by physical and chemical weathering of earth materials can also have a significant effect on limestones and dolostones as well as igneous and metamorphic rocks. For example, granites are igneous rocks that form with almost no primary porosity, yet some have weathering induced porosity of 5% to 25%.

Just as secondary porosity enhances the overall void space in earth materials, the porosity of both unconsolidated and consolidated materials decreases with time as younger sediments are deposited on top of older layers, burying and compressing them. The weight of the overlying material compresses the older layers to varying degrees depending on the nature of the material and the initial arrangement of the sediment grains. Generally, porosity is less in formations with greater burial depths (Figure 12), but the variable character of material with similar names (e.g., silt, clay) and the differences in the weight of the overlying sediments makes it nearly impossible to predict the value of porosity given the type of material and depth of burial as indicated by the large scatter on the graphs of Figure 12.

The examples shown in Figure 12 are for unconsoliated sediments that were sampled to depths of 200 to 300 m. Additional loss of porosity occurs when sediments and rocks are overlain by thicker packages of earth materials. For example, Helm (1982) found porosities decreased by 25 to 50% within the first 1000 to 2000 m of burial and continued to decrease at greater depths (up to 75% at 3000 m depths). Porosity data for sandstones, carbonates and shales are frequently reported in the oil and gas literature as these types of formations are often associated with energy reservoirs.