pore-water chemistry Pore waters fill the interstices of sediments and sedimentary rocks. Their chemical composition reflects both the origin of the waters and also the way in which the original composition has been modified by diagenetic (post-depositional) reactions with rock minerals and organic matter.
Scientists aboard the
RRS Challenger first revealed in 1879 that the composition of pore waters can change very quickly after sediments are deposited. Improved sampling techniques, inspired partly by the Deep Sea Drilling Project (1968–83), had subsequently led to a much greater understanding of the key controls on the chemistry of the pore waters within shallow-buried sediments. Recently deposited sediments are unstable mixtures of terrestrial weathering products and organic matter. Within millimetres of the sediment–water interface, microbes start to exploit the system's inherent chemical energy by sequentially catalysing reactions between organic matter and oxidized phases such as oxygen, nitrate, manganese oxides, iron oxides, and sulphate. The composition of pore waters is changed by the reactions, losing oxygen, nitrate, and sulphate, and gaining iron, manganese, ammonium, bicarbonate, and hydrogen sulphide. Continued metabolism of residual organic matter by methanogenic bacteria results in the production of biogenic methane which, under favourable circumstances, can accumulate in commercial quantities. Pore waters also sensitively record the occurrence of other reactions, such as the dissolution, precipitation, and recrystallization of phosphates, carbonates, and sulphides, during early diagenesis.
The composition of waters deeper within sedimentary basins is known primarily through the drilling activities of petroleum companies. Salinities vary from almost zero to 300 000 milligrams per litre (mg l
−1) and are strongly influenced by patterns of fluid flow and proximity to subsurface evaporite deposits. However, the occurrence of highly saline subsurface brines was known substantially prior to the onset of petroleum exploration; historically, brines were a valuable source of salt and are known to have been exploited from neolithic times in Europe and China. Present-day commercial interest focuses on the problems that can occur when pore waters (often known within the industry as formation waters) within petroleum reservoirs mix with waters which are injected into the reservoir to maintain pressure during production. Chemical incompatibility between the fluids leads to the precipitation of mineral scales, which can severely impede oil production.
Chloride, sodium, and calcium are the most common ions in pore waters within deeply buried sediments, with lesser amounts of most elements. These major elements are largely derived from sea water trapped with the sediment at deposition and by the dissolution of buried evaporites. Fluid mixing and reactions with clay and carbonate minerals exert important, additional controls on the composition of pore waters. Ratios of the major cations such as sodium, potassium, magnesium, and calcium are strongly influenced by chemical reactions between waters and rock minerals, as is pH. More detailed analyses—for example, the oxygen and hydrogen isotopic composition of the water and its complement of noble gases—give further insights into the origin and age of the water, and its history of interaction with associated rock minerals. The pore water is thus an integral part of a sedi-ment or sedimentary rock, revealing much about its post-depositional history.
Andrew C. Aplin
