Plankton

Key Takeaways

• Marine phytoplankton are the dominant source of Type II kerogen, the organic matter type that generates oil and condensate during thermal maturation: the cell membranes, storage lipids (triglycerides, wax esters), and photosynthetic pigments (chlorophylls, carotenoids) of marine algae, dinoflagellates, and cyanobacteria are rich in hydrogen-carbon bonded structures (aliphatic chains and cyclic terpenoids) that upon burial and heating (60-180 degrees Celsius over geological timescales) break down by thermolysis into the liquid hydrocarbons of crude oil; the hydrogen index (HI) of Type II kerogen, measured by Rock-Eval pyrolysis on source rock samples, typically ranges from 400-700 milligrams of hydrocarbon generated per gram of total organic carbon (mg HC/g TOC), compared to 100-200 for humic (Type III) kerogen from terrestrial plant material; the high HI of marine algal kerogen reflects the abundance of aliphatic (oil-prone) molecular structures relative to aromatic or oxidized structures that characterize terrestrial lignin and cellulose; the world's major oil provinces, including the Middle East, the North Sea, the Gulf of Mexico, and offshore West Africa, source their oil predominantly from Type II kerogen derived from marine planktonic organisms deposited in Cretaceous, Jurassic, and Triassic epicontinental seaways where anoxic bottom conditions preserved the organic matter now generating oil at depth.
• Biomarker geochemistry uses the molecular fossils preserved from ancient planktonic organisms to trace crude oil back to its source rock and reconstruct depositional environment: pristane (a C19 isoprenoid) and phytane (a C20 isoprenoid) are degradation products of the phytol side chain of chlorophyll-a, which is present in all photosynthetic organisms including marine phytoplankton, and the pristane-to-phytane ratio (Pr/Ph) in crude oil is a proxy for the redox conditions of the source rock depositional environment (Pr/Ph greater than 3 indicates oxic deposition typical of organic-poor shales; Pr/Ph less than 1 indicates anoxic deposition typical of euxinic basins where H2S was present, such as the Black Sea analog environments that produced the Permian Zechstein and the Triassic Kupferschiefer source facies); sterane biomarkers (C27, C28, C29 regular steranes) derived from the membrane sterols of eukaryotic algae and higher organisms reflect the relative abundance of different plankton communities in the source rock, with high C27 steranes indicating marine dinoflagellate-dominated communities and high C29 steranes indicating freshwater or terrestrial plant-influenced environments; tricyclic terpane biomarkers (from bacterial hopanoids and algal tetraterpenoids) provide additional source rock environment and maturity information that complements the sterane and isoprenoid signatures.
• Lacustrine plankton, particularly the freshwater alga Botryococcus braunii and the prasinophyte algae of the Tasmanites group, produce Type I kerogen with exceptional oil-generating potential (HI 700-1,200 mg HC/g TOC) in non-marine source rocks that are the source of significant oil reserves in rift basin plays worldwide: Botryococcus produces large quantities of long-chain alkadienes and triterpenoids (botryococcanes) that are structurally distinct from marine phytoplankton-derived biomarkers and are a characteristic signature of lacustrine-sourced crude oils; the Cretaceous Kimeridgian lacustrine source rocks of the South Atlantic (which sourced the Campos and Santos Basin oils offshore Brazil), the Eocene Green River Shale lacustrine source rocks (which generated the oil in Uinta Basin fields in Utah), and the Permian Lucaogou Formation lacustrine source rocks (which generated the Mahu tight oil in the Junggar Basin of China) all derive their oil-generating potential from lacustrine phytoplankton deposited under the anoxic bottom conditions that are periodically established in stratified, chemically or thermally stratified lake basins; the lacustrine play concept has expanded significantly since the 2000s as geochemical tools improved the ability to identify lacustrine-sourced oils and trace them to their source kitchens in frontier rift basins.
• Palynology, the study of organic microfossils including fossilized plankton, uses the preserved organic walls of marine dinoflagellate cysts (dinocysts), acritarchs (organic-walled microfossils of uncertain affinity, probably representing algal cysts), and green alga Tasmanites as biostratigraphic markers for dating and correlating reservoir and source rock formations: dinoflagellate cysts have species-specific stratigraphic ranges (the time intervals during which each species existed before extinction or evolutionary change) that have been calibrated against the geological time scale from thousands of well sections worldwide, allowing palynologists to assign a geological age to a formation from the assemblage of dinocyst species present in cuttings or core samples; in exploration wells where other biostratigraphic markers (foraminifera, nannofossils) are absent or poorly preserved (in highly oxidizing conditions where calcareous microfossils dissolve, or in terrigenous sediments where foraminifera are rare), organic-walled dinocyst and acritarch palynology may be the only available biostratigraphic tool for well-to-well correlation and for placing reservoir intervals within the sequence stratigraphic framework of the basin; the presence of specific dinocyst assemblages in reservoir intervals also provides information about the water depth and salinity of the depositional environment at the time of deposition, which constrains the paleogeographic reconstruction used to predict reservoir connectivity and sand quality variation across the field.
• Total organic carbon (TOC) content and the ratio of algal to terrestrial organic matter in a potential source rock are direct reflections of the planktonic productivity of the ancient water body and the preservation efficiency of the depositional environment: source rocks with TOC greater than 2% and HI greater than 300 mg HC/g TOC (good to excellent oil-prone source rocks) typically accumulated under conditions where planktonic productivity was high (driven by upwelling nutrient supply or restricted basin chemistry that concentrated nutrients) and where preservation was excellent (anoxic or euxinic bottom waters that minimized aerobic decomposition); the Black Sea, the Cariaco Basin, and the anoxic portions of the Holocene Baltic Sea are modern analogs for the depositional environments that produce the richest petroleum source rocks, with TOC values of 5-20% in the deepest, most anoxic basins; the Oceanic Anoxic Events (OAEs) of the Cretaceous (OAE1a, OAE2, and others), during which global ocean bottom waters became anoxic for millions of years due to a combination of high planktonic productivity and sluggish ocean circulation, produced source rock intervals of exceptional richness (the Cenomanian-Turonian black shales of the North Atlantic and the Tethys) that are the source of major Cretaceous oil provinces including the Middle East and offshore West Africa.