Burial Diagenesis and Creation of Deep Secondary Microporosity in Limestones: Criteria for Documentation
Jeffrey J. Dravis
Secondary microporosity occurs in many Mesozoic and younger deeply buried limestone reservoirs, and in some it predominates. Examples include the Jurassic of the U.S. Gulf Coast and Europe, Cretaceous of the Middle East, and Tertiary of southeast Asia.
Documenting microporosity created during burial diagenesis and discerning it from that promoted by near-surface freshwater diagenesis have important implications for subsurface porosity prediction, brine evolution, and basin hydrology.
When considered collectively, the following criteria support documentation of deep-burial secondary microporosity: (1) lack of physical evidence for subaerial exposure; (2) microporosity distribution which is areally extensive but confined only to grainstone or low-mud packstone facies; (3) presence of pervasive grain-to-grain pressure solution and a corresponding absence of precompaction freshwater cements; (4) occurrence of associated calcite cements whose epiflourescence, admixture with hydrocarbons, or geochemical signatures negate a freshwater origin but are consistent with their precipitation under burial conditions; (5) preservation of abundant microporosity adjacent to stylolites or microstylolites; and (6) development of identical microporosities in stable calcitic grains, a henomenon uncommon in fresh water. Hydrogen sulfide and/or carbon dioxide associated with limestone reservoirs may also reflect substantial deep-burial dissolution.
Documentation of secondary microporosity development during burial diagenesis offers renewed optimism when prospecting for limestone sequences previously considered too deeply buried to possess adequate reservoir quality. At the same time, it provides a case history framework which is required for properly evaluating brine chemistry evolution and fluid migration in the subsurface.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.