ROLE OF DOLOMITIZATION AND SEQUENCE STRATIGRAPHIC FRAMEWORK IN THE DEVELOPMENT AND EVOLUTION OF PORE SYSTEM ARCHITECTURE

Diagenetic modification of carbonates is in large part related to the primary depositional fabric, texture and composition of allochems, as well as the sequence stratigraphic framework, especially as related to sea level drawdowns. A core from the Devonian Richfield Formation in the Michigan Basin has been interpreted as having been deposited in an arid tidal flat setting. The core is characterized by a sequence stratigraphic framework composed of a hierarchy of shallowing upward cycles consisting of subtidal and intertidal mudstones-packstones, capped by microbial mats and evaporites. The highest frequency cycles are 1-3m thick with medium scale cycles 3-5m thick, both of which are stacked within an overall shallowing upward package of around 40 meters.

Varying degrees of dolomitization occur near the different frequency cycle caps, with dolomite interpreted as likely being related to evaporitic conditions and the evaporative pumping and/or evaporative reflux of Mg-rich brines. Scanning electron microscopy and optical microscopy of thin sections indicate that overall porosity and permeability is related in large part to the degree of dolomitization, which varies from well-developed sucrosic dolomites with high porosity and permeability to planar-s dolomites with low porosity and permeability. The degree of dolomitization may be related to longer residence time in a sabkha environment related to the frequency of Milankovitch-band sea level change, or possibly regional climatic changes with greater or lesser degrees of evaporation. Dolomitization and the resulting porosity and permeability are highest in original carbonate mudstones just below the cycle caps. Thin layers of oolitic grainstones show no evidence of dissolution porosity, suggesting that the ooids were originally calcitic and not aragonitic, which is consistent with likely Greenhouse conditions.