Gregg, Jay M.¹
(1) University of Missouri-Rolla, Rolla, MO

ABSTRACT: Mississippi Valley-Type Mineralization and the Development of Dolomite Petroleum Reservoirs

Saline fluids, at temperatures from 60 to >250 degrees C, have affected almost every sedimentary basin in the world. These fluids commonly precipitate base metal sulphides ranging in volume from trace amounts to large economic ore deposits (Mississippi Valley-type or MVT deposits). The primary host of MVT deposits are platform dolomites. These deposits almost always display evidence of hydrocarbon emplacement. Sources and pathways of fluids associated with MVT mineralization are the subject of much recent debate and research.
Platform carbonates affected by MVT mineralization may undergo massive geochemical and textural alteration. This alteration includes: dolomitization of limestone, recrystallization of pre-existing dolomite, precipitation of large volumes of open space filling cements, and dissolution and brecciation of host rocks. These changes affect a number of reservoir properties. Dolomitization of limestone and/or recrystallization of pre-existing dolomite, at elevated temperature, commonly results in coarse crystal sizes and development of nonplanar texture which decreases permeability due to increased pore throat tortuosity. Open space filling dolomite, calcite, and quartz cementation results in dramatic reduction of micro and mesoporosity. Carbonate dissolution may result in a large scale increase of macroporosity due to the much larger flux of fluid through macropores as opposed to meso and micropores.
Exposure to mineralizing fluids may substantially alter porosity and permeability distribution, and thus the petrophysical properties of a carbonate. The resulting reservoir may have little resemblance to its unaltered precursor. Understanding the late diagenetic history of a dolomite is critical as this ultimately will effect its development strategy and production history.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.