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.