Distribution of Clays and Shales in the Reservoir
The manner in which clays are distributed throughout the reservoir plays a key role in the approach that should be used for evaluating the reservoir as a conventional resource. A reservoir sandstone can be fitted to any of the four broad lithological distribution models (Figure 1):
- Clean sandstone essentially has no significant clay content.
- Dispersed clay/shale grains are interspersed throughout the sand, either as a coating on the sand grains, or by filling the pore spaces between the sand grains.
- Laminar clay/shale has thin layers of clay between layers of sand.
- Structural clay/shale clay grains, shale interclasts or nodules are in the formation matrix.
Clean Sandstones
Clean sandstones are made up of relatively pure sand. They contain essentially no clay minerals or shales and consist solely of sand grains. These sandstones were likely to have been deposited as a result of a single energy level flow regime. The basic Archie equation would be suitable for the log analysis of clean sandstones.
Dispersed Clay
Dispersed clays generally occur as a pore-filling component of the rock and have a variety of crystal sizes and shapes. They generate a broad spectrum of adverse effects on fluid flow and affect the fluid saturation properties, without necessarily having much effect on the total pore volume of the rock.
Overgrowths and Distinct Particles
Two key criteria used to define and contrast the types of dispersed clay within a sandstone formation are the clay crystal structure and its location. Dispersed clays are distributed throughout the sand in one of two distinct forms:
- As clay overgrowths that adhere to, or coat, the surface of the sand grains. Chlorites are a good example.
- As distinct particles of clay that fill some portion of the interstices between the sand grains. The smaller size of the clay particles allows them to line, or fill, the pore throats between the comparatively larger sand grains.
Three Common Forms of Dispersed Clay
The three most common forms of dispersed clays found in sandstone reservoirs are pore-filling, pore-lining, and pore-bridging.
Pore-Filling Clays
Pore-filling clays are the most common mode of occurrence for kaolinite, which typically develops as pseudo hexagonal, platy crystals that attach loosely to the pore walls, or occupy intergranular pores (Figure 2).
The crystal platelets may be stacked face to face, forming long crystal aggregates, or in the shape of booklets. Kaolinite crystals of either single or stacked platelet morphology are characteristically scattered patchily throughout the pore system. These kaolinite crystals are usually neither well attached to each other nor to the pore walls and can be easily mobilized by fast flowing pore fluids. Kaolinite crystals that extensively fill pores have a random arrangement with respect to one another and affect the rock’s petrophysical properties, primarily by reducing the intergranular pore volume, and by behaving as migrating fines within the pore system.
Pore-Lining Clays
Pore-lining clays attach to the walls of pores and form a relatively continuous, thin mineral coating (Figure 3).
Crystals attached perpendicularly to the pore wall surface are usually intergrown to form a continuous clay layer that contains abundant micropore space, with pore diameters of less than 2μ. Illite, chlorite and smectite typically occur as pore linings.
Pore-Bridging Clays
Pore-bridging clays are similar to pore-lining clays, except that they not only line pore walls, but also extend far into, or completely across, a pore throat to create a bridging effect (Figure 4).
These clays exhibit an extensive development of intertwined plates and fibers that produce an intricate network with abundant microporosity and a tortuous fluid flow pathway. Smectite, chlorite and illite all display this morphology, with illite being the most commonly found in the subsurface.
Sandstone intervals that contain dispersed clays are usually deposited under a single flow regime. The dispersed clays are subsequently formed within the sandstone as a result of authigenesis, or through post-depositional bioturbation or diagenesis. It is not uncommon for these clays to develop as a result of precipitation, or by the alteration of pre-existing silicate minerals.
Dispersed clays tend to increase the log-calculated water saturation while significantly reducing the resistivity, porosity, and permeability of the sandstone. When the dispersed clay content exceeds 30% of a sandstone’s pore space, it usually begins to impact the hydrocarbon production rates.
When dispersed clays form a coating on the sand grains, an increase in the irreducible water saturation takes place, along with a substantial reduction in the well log resistivity. Wells completed in shaly sandstones containing dispersed clays can produce hydrocarbons with no water cut because of their high irreducible water saturation.
When dispersed clays fill the pores between the sand grains, they take up space that would normally serve as a channel for fluid movement between the pores. The wettability of such clays tends to be greater than that of the surrounding quartz grains. The sum total of these factors is a reduction in both the porosity and the permeability, along with an increase in the water saturation.
Different Perspectives on the Role of Clays and Shales
Asquith (1990) noted that any log analysis in pay sandstones that contain dispersed clays must consider the authigenic origin of this clay. Asquith thought that most dispersed clay is diagenetically formed in place after the deposition of the sandstone. Because they form under different conditions than those which formed the adjacent shale beds, these dispersed clays may differ in composition and, more importantly, may exhibit different resistivities from that of the adjacent shales. Such a difference between the resistivity of the dispersed clay in the reservoir and the resistivity of the adjacent shale is especially critical in cases where the resistivity of the adjacent shale is greater than the resistivity of the shaly sandstones. Any log analysis through such zones should, therefore, use an equation that does not require a reference resistivity, termed Rsh, taken from the adjacent shales.
Thomas and Stieber (1975) built an interpretation model that assumes that “within the interval being investigated, there is no change in shale type and the shale mixed in the sandstone is mineralogically the same as the pure shale sections above and below the sandstone.” In their view, this similarity stems from the fact that both sandstone and shale facies “are derived from the same source material, carried by the same river and emptied into the same basin. The differentiation between sandstones and shales begins as the particles settle at differing rates according to their size and transport energy, and not mineral type.”
Thus, Thomas and Stieber feel that “the porosity destroying material introduced into a sandstone stratum will be of the same composition as the shales above and below the sandstone stratum. Of course, this will not be true for the diagenetic alteration of feldspars into clay within the sandstone stratum.”
Juhasz (1986) describes the significant impact that dispersed shale can have on hydrocarbon production:
“A certain amount of dispersed, pore-filling shale has a far more detrimental effect on a volumetric basis on the permeability of the sandstone than the same amount of shale concentrated into shale laminae between clean sandstone laminae. The permeability of a 33% porosity clean sandstone, for instance, would be reduced to practically zero if its pore space is filled with shale (that is, Vsh-33%), but it would retain two-thirds of its permeability if this shale is present in laminations only.”
Laminar Clays and Shales
Laminar clays and shales are distributed in a reservoir as relatively thin layers of allogenic clay or shale that have been deposited between otherwise clean layers of sandstone (Figure 5).
Each lamination of compacted clay, mudstone or siltstone is a distinct layer, and can vary in thickness as well as in the proportion of sand, silt, and clay. The overall sandstone and laminated clay interval reflects multiple cycles of deposition under a dual flow regime characterized by fluctuations in the depositional energy levels. This depositional environment requires higher energy for the deposition of the sand grains than are required for the lighter shale constituents of muds, clay minerals and silts.
Conventional resource reservoirs containing laminated shales alternate between layers of reservoir quality sandstones and thinner layers of shales and clays that have very low effective porosity. Such shale laminations do not significantly affect the resistivity, porosity or permeability of the adjacent sandstone streaks themselves, and a shale fraction of up to 60% can sometimes be commercial. Many standard logging tools lack the vertical resolution to differentiate between individual thin beds of sandstone and shale when the laminations are fine. This lack of vertical resolution causes many standard logging tools to average their readings over such alternating sequences of sandstone and shale.
Though laminated shale layers are often thinner than the adjacent sandstone layers, the clay constituents contribute a disproportionate change in the well log resistivity and porosity for their thickness. Petrophysical and reservoir properties between each layer may vary because of the changing proportions of clays within each lamination.
Asquith (1990) reasoned that, because of their detrital origin, shale laminations between sandstones normally have the same clays and water content as the adjacent thick shale beds. This similarity leads to his assumption that the resistivities of the laminated shale will be similar to those of the adjacent thick shales. Therefore, it is safe to use log analysis equations that require clay resistivities, and in such equations, the resistivity of adjacent shales is used to represent that of the shale in the shaly sandstone.
The problem of finely layered sands and shales is fairly common worldwide, including in Nigeria and Indonesia. In the Gulf Coast region of the United States, laminar shales have been found in about half of the low resistivity pay zones.
Structural Clays and Shales
Structural clays and shales consist of shale nodules or lithified clay fragments that have been intermixed with grains of sand to form part of the sandstone rock matrix. Unlike dispersed clays, whose grain size is so small that they occupy the interstices between framework grains, the structural clays have a grain size that is as large as the sand grains, therefore placing the structural clay fragments into the framework of the matrix. Figure 6 shows clasts of structural shale intermixed with grains of sand.
Structural clays or shales have three main origins:
- As reworked fragments of lithified shale that have been deposited simultaneously with sand grains of a comparable size
- As nodules that replace selected grains through diagenesis, such as when feldspar is transformed into clay
- As nodules introduced through bioturbation
Sandstone intervals that contain structural shales are deposited under a single flow regime, with the shale fragments being deposited simultaneously along with the sand grains. Because of their size, the structural shales act as framework grains and do not significantly alter the reservoir properties by clogging the interstitial spaces between grains. Structural shale does not usually occur in sufficient quantities to affect the reservoir quality.
When evaluating reservoirs that contain structural shales, the interpretation approach must account for the way in which the clay grains will affect the log responses, as opposed to just trying to evaluate the response through homogeneous sandstone.
According to Visser et al. (1988), structural and laminated shales produce similar log responses. These detrital clays can be compared to the adjacent thick shale beds for resistivity, thus enabling shaly sandstone equations that require a shale resistivity value to be used.
