Porosity Differentiation Within Interbedded Limestone-Dolostone Reservoirs
Interbedded limestones and dolostones have very different porosity frequency distributions in three large datasets of shallow-water carbonate platform strata: the Madison Formation (Mississippian, Montana), the Finnmark Platform (Pennsylvanian-lower Permian, offshore north Norway), and the Asmari Formation (Oligocene-Miocene, southwest Iran). In each case, the limestones have strongly positively skewed distributions (most values in the range 0-4%), whereas dolostones have more normal (Gaussian) distributions (most values around 8-15% and relatively few values <5%). The tight limestones form laterally extensive flow barriers that divide the reservoirs into thin stratigraphic compartments. Understanding the diagenetic processes controlling this porosity differentiation is therefore fundamental to building models of fluid displacement.
The tight limestones display both parasequence and sequence scales of clustering. As reported earlier for the Madison Formation, there is strong porosity partitioning in all three datasets between transgressive and regressive hemicycles. However, both the polarity of this partitioning (whether tops or bases of cycles are porous) and the facies involved vary depending on position within both platform paleogeography and lower-order sequence tracts.
The tight limestones of each dataset correspond with both: (1) muddy facies, commonly rich in fine siliciclastics, that have lost porosity by early matrix compaction and cementation, and (2) grainy facies that have lost porosity by calcite cementation. Geochemical data (Finnmark Platform) and comparison with datasets from shallow-buried carbonate platforms (Bahamas Transect and Marion Plateau; showing no comparable differences between limestone and dolostone porosity distributions) suggests that reservoir compartmentalization by tight limestone barriers may be a burial diagenetic process that is activated by a temperature threshold.
New Approaches in Carbonate Stratigraphy and Reservoir Modeling
2004 AAPG International Conference and Exhibition Technical Program