Diagenesis of Volcanogenic Sandstones
Ronald C. Surdam, Laura J. Crossey
Volcanogenic sandstones are commonly considered to have little or no potential as hydrocarbon reservoirs because numerous highly reactive components (e.g., glass and unstable mineral phases) are present in these sandstones. With progressive burial, volcanogenic sandstones undergo a relatively predictable series of diagenetic reactions. Typically, this reaction pathway is characterized by early hydration and carbonization reactions resulting in the formation of clays, zeolites, and carbonates. Later reactions are characterized by dehydration of earlier formed zeolites and albitization of plagioclase. These dehydration reactions result in the formation of laumontite and analcime. Most of the steps in this reaction pathway result in porosity reduction.
Previous work on organic-inorganic interactions demonstrated that the porosity of these volcanogenic sandstones may be enhanced in the subsurface. During diagenesis of buried organic matter, oxygen-bearing functional groups (e.g., carboxylic acids and phenols) are released prior to hydrocarbon generation. These organic acids and phenols can destabilize aluminosilicate and carbonate minerals, thereby enhancing porosity.
Whether this porosity enhancement occurs depends on the quantity of organic material in the stratigraphic section and the configuration of remnant porosity. Either paucity of source rocks or complete occlusion of porosity by early diagenetic phases will eliminate the process of porosity enhancement in volcanogenic sandstones. The successful prediction of regions of enhanced porosity in volcanogenic terrains will depend on a detailed understanding of the timing of organic and inorganic reactions during the burial history.
Rapid sedimentation rates (dilution of organic material) and the presence of unstable phases characterizing many volcanogenic terrains make volcanogenic sandstones high-risk exploration targets. Whereas in a less reactive diagenetic system, any one of several optimizing conditions may result in formation of a quality hydrocarbon reservoir, the marginal nature of volcanogenic sandstones requires the operation of additional optimizing conditions to create a viable exploration target.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.