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Caprock and Bedrock

Caprock and Bedrock are two distinct types of sediment, with different properties, that require different dredging methods and equipment.

Different industries define caprock and bedrock differently

Neither caprock nor bedrock are really rock, but various types of sediment.To dredge properly, it is imperative to define what the caprock and bedrock properties are. Also it is important to be aware that the definitions of caprock and bedrock in the oil and gas industry differ from those in the dredging industry. The dredging industry distinguishes between caprock and bedrock as outlined below.

Caprock

Caprock is the hard, impermeable surface layer above relatively soft and sometimes unconsolidated sediments. It is formed by the zonal concentration of aluminous, calcareous, silicious elements drawn to the surface by capillarity, i.e., capillary action where the uptake of water occurs by itself. Read more
Caprock has been formed during periods when the sea level was low, for instance, during the Ice Ages. It is formed as groundwater creeps to the surface of sandy layers by capillarity and then evaporates when it reaches the surface. The minerals that were present in the evaporated water stay behind and cement the sand grains at the surface together.This forms a solid crust on top of the sandy layer. When the crust has been formed, the capillarity eventually stops because all the pores have been closed by the cementation process, and then the cementing process stops. For this reason, caprock layers are often relatively thin (30 – 100 cm) and the sandy layer underneath the caprock remains as a granular soil. When the sea level rises again, a new sandy layer forms. This process can occur several times, so it is possible that at certain locations multiple sequences of caprock layers with sandy layers in between may be present. Caprock in the Middle East can better be defined as a calcareous layer (calcarenite, calcilutite etc.) because the term caprock is a description of the way it has been formed and not about the type of rock. In that sense, caprock is not a “type of rock”. In the dredging industry, defining it in a borehole log with the following annotation might be more informative: 0 – 75 cm Calcarenite (Caprock).

Bedrock

Bedrock is the rock that remains solid and relatively unweathered beneath the superficial layers of soil and/or weathered rock. When one digs through the caprock layers, eventually one reaches unweathered rock, which is thicker than the caprock layer. Bedrock is usually found at great depths, but bedrock can also start at the seabed. No granular layer will be found underneath the bedrock. In the field of construction and dredging, this is the bottom-most layer. In the Middle East this layer is often Limestone. Again, as with caprock, bedrock is not a type of rock. The preferred annotation for bedrock would be: 300 cm – till end of borehole: Limestone (Bedrock). Sometimes caprock is not present, but other times, as stated above, there can be multiple layers of caprock. In most parts of the world, both caprock and bedrock are present and are often encountered in one borehole. Caprock always occurs above the bedrock.