Changes

===Kerogen Type I===

===Kerogen Type I===

Kerogen type I is predominantly composed of the most hydrogen-rich organic matter preserved in the rock record. Often the organic matter is structureless (amorphous) alginite and, when immature, fluoresces golden yellow in ultraviolet (UV) light. A large proportion of type I kerogen can be thermally converted to petroleum and therefore is rarely recognizable in thermally mature or postmature rocks. Sometimes in thermally immature rocks, morphologically distinct alginite is structurally or chemically assignable to specific algal or bacterial genera. These organic-walled microfossils have high H/C values because they formed hydrocarbons biologically. Some examples of pure assemblages with type I kerogen properties include the following: (1) the lacustrine alga Botryococcus braunii, which sometimes retains its diagnostic cup-and-stalk colonial morphology and/or its unique chemical compound, botryococcane (Moldowan and Seifert, 1980); (2) Tasmanites spp., which are low-salinity, cool water, marine algal phyto-plankton with unique physical features (Prauss and Reigel, 1989); and (3) the Ordovician marine organic-walled colonial microfossil Gloeocapsomorpha prisca, with its diagnostic physical appearance and unique chemical signature (Reed et al., 1986). Where kerogen type I is widespread, it is mapped as organic facies A. It usually forms in stratified water columns of lakes, estuaries, and lagoons.

Kerogen type I is predominantly composed of the most hydrogen-rich organic matter preserved in the rock record. Often the organic matter is structureless (amorphous) alginite and, when immature, fluoresces golden yellow in ultraviolet (UV) light. A large proportion of type I kerogen can be thermally converted to petroleum and therefore is rarely recognizable in thermally mature or postmature rocks. Sometimes in thermally immature rocks, morphologically distinct alginite is structurally or chemically assignable to specific algal or bacterial genera. These organic-walled microfossils have high H/C values because they formed hydrocarbons biologically. Some examples of pure assemblages with type I kerogen properties include the following: (1) the lacustrine alga Botryococcus braunii, which sometimes retains its diagnostic cup-and-stalk colonial morphology and/or its unique chemical compound, botryococcane;<ref>Moldowan, J. M., and W. K. Seifert, 1980, First discovery of botryococcane in petroleum: Chemical Communications, v. 34, p. 912-914.</ref> (2) Tasmanites spp., which are low-salinity, cool water, marine algal phyto-plankton with unique physical features (Prauss and Reigel, 1989); and (3) the Ordovician marine organic-walled colonial microfossil Gloeocapsomorpha prisca, with its diagnostic physical appearance and unique chemical signature (Reed et al., 1986). Where kerogen type I is widespread, it is mapped as organic facies A. It usually forms in stratified water columns of lakes, estuaries, and lagoons.

Kerogen type I is concentrated in condensed sections where detrital sediment transport is low and primarily pelagic. Condensed sections occur in offshore facies of transgressive systems tracts in marine and lacustrine settings. Although this extension of terminology from marine to lacustrine environments may be unfamiliar at first, lacustrine rocks are formed by the same dynamic processes that form marine rocks (i.e., sediment supply, climate, tectonics, and subsidence), although changes in lake levels often reflect local changes in runoff, evaporation, and sediment basin filling rather than the global and relative sea level changes postulated for marine sediments (Haq et al., 1988).

Kerogen type I is concentrated in condensed sections where detrital sediment transport is low and primarily pelagic. Condensed sections occur in offshore facies of transgressive systems tracts in marine and lacustrine settings. Although this extension of terminology from marine to lacustrine environments may be unfamiliar at first, lacustrine rocks are formed by the same dynamic processes that form marine rocks (i.e., sediment supply, climate, tectonics, and subsidence), although changes in lake levels often reflect local changes in runoff, evaporation, and sediment basin filling rather than the global and relative sea level changes postulated for marine sediments (Haq et al., 1988).