The quality of reservoirs is determined by the flow, distribution, and storage of fluids within them. Tight reservoirs are usually characterized by low structure maturity, reduced composition maturity, low porosity, and relatively low permeability.
The quality of reservoirs is enhanced, preserved, or reduced through a process called diagenesis. This process changes partially or completely the fabric of the reservoir that will directly affect the levels of permeability and porosity.
Diagenesis according to Jackson (1997), has been defined as all chemical, physical, and biologic changes experienced by sediments following its first deposition, and through its lithification. In other words, it refers to the biological, chemical, and physical processes that sedimentary materials undergo after the occurrence of weathering and deposition. In controlling the quality of trapped carbonate reservoirs, diagenesis plays a rather significant role.
Carbonates are sedimentary rocks created by strata of carbon sediments broken down by aquatic organisms. Carbonate reservoirs are artificial bodies of water that hold large deposits of carbonate.
Diagenesis in carbonate reservoirs occurs after carbonate has been formed. It alters the rock structure, affects the minerals, porosity, and amount of resource deposits. Researchers have found that the diagenetic process starts from deposition which causes the burial of sediments and uplifts the topography of the surface.
Deposition is the concentration of sediments in a particular area. As deposition takes place, layers of sediment are stacked on top of each other. As they are stacked, the topography of rock rises — upliftment. In the process of layering upon each other, sediments are buried within the newly formed land’s crust (burial).
In carbonate reservoirs, diagenesis is grander near sediment areas. As this process takes its natural steps, essential diagenetic processes occur. They involve compaction, dissolution, cementation and leaching, dolomitization, porosity, and replacement.
After deposition takes place, layers of sediment grains lay on each other. The sediment particles below are buried by sediments 50-100 meters above. As they face internment, the weight of the above particles compacts the ones below. The sediment grains below are introduced to high pressures from the ones on top and are therefore squashed into rocks.
Cementation and Leaching
Cementation is the solidifying of compact sediment grains by the downward movement of minerals (leaching) by water through the pore spaces. After compaction, the water in carbonate reservoirs flows through the pores of buried and compact grains with minerals. Think of it as building blocks bind by cement. As water precipitates into the grains below, the minerals act as a binding material.
Cementation affects porosity and permeability of the reservoir’s rocks. Cementation also occurs differently depending on the type of water the reservoir carries. In freshwater, the water leaches calcite and dolomite calcite minerals. In marine settings cements consist of magnesium calcite and aragonite.
This is the course by which carbonate mineral dolomite is molded from the replacement of calcium ions with magnesium. Dolomitization is important in the diagenetic process. The carbonate minerals present in carbonate reservoirs are a product of a magnesium and calcium process.
The dolomites are formed by its conjunction with ancient tidal flats. The process, however not discerned fully, is thought to involve evaporation mechanisms in fresh and marine waters. Further, this process helps in reducing porosity and is said to create the right level of reservoir porosity that enhances the overall quality.
Porosity
Porosity is the volume of pore spaces in a rock. That is, the spaces left during cementation. Porosity affects the texture of the rock, shape, and sorting. Cementation, dissolution, and dolomitization affect porosity. Cementation reduces porosity, it improves the binding modification of the rock thereby making it less porous, while dissolution increases porosity and makes the rock more permeable. Dolomitization, on the other hand, reduces porosity in the reservoir. Conversely, the level of porosity indicates the incompleteness of cementation and excessive dissolution.
Dissolution is the reverse process of cementation. It involves the suspension of cemented minerals. This process is selective. Sometimes, magnesium calcites may escape dissolution while aragonitic shells might succumb.
When carbonate rocks are exposed to the external environment, the mineral they are composed of goes through chemical changes. Dissolution transpires amongst soluble materials such as carbonate, etc.
Once mineral deposits of rocks are dissolved, they concentrate in another place. The previous minerals present in the reservoir is then replaced by new minerals depending on the type of chemicals exchanged.
Diagenesis affects what resources or minerals can be attained in carbonate reservoirs. In some cases, oil
, gas and other mineral deposits deplete after diagenetic activities. Researchers now try to determine where the chemical and physical changes of diagenesis took place to explore carbonate reservoirs and determine its quality.
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