Establishing the Relationship between Fracture-Related Dolomite and Primary Rock Fabric on the Distribution of Reservoirs in the Michigan Basin

This topical report covers the year 2 of the subject 3-year grant, evaluating the relationship between fracture-related dolomite and dolomite constrained by primary rock fabric in the 3 most prolific reservoir intervals in the Michigan Basin (Ordovician Trenton-Black River Formations; Silurian Niagara Group; and the Devonian Dundee Formation). The characterization of select dolomite reservoirs has been the major focus of our efforts in Phase II/Year 2. Fields have been prioritized based upon the availability of rock data for interpretation of depositional environments, fracture density and distribution as well as thin section, geochemical, and petrophysical analyses. Structural mapping and log analysis in the Dundee (Devonian) and Trenton/Black River (Ordovician) suggest a close spatial relationship among gross dolomite distribution and regional-scale, wrench fault related NW-SE and NE-SW structural trends. A high temperature origin for much of the dolomite in the 3 studied intervals (based upon initial fluid inclusion homogenization temperatures and stable isotopic analyses,) coupled with persistent association of this dolomite in reservoirs coincident with wrench fault-related features, is strong evidence for these reservoirs being influenced by hydrothermal dolomitization. For the Niagaran (Silurian), a comprehensive high resolution sequence stratigraphic framework has been developed for a pinnacle reef in the northern reef trend where we had 100% core coverage throughout the reef section. Major findings to date are that facies types, when analyzed at a detailed level, have direct links to reservoir porosity and permeability in these dolomites. This pattern is consistent with our original hypothesis of primary facies control on dolomitization and resulting reservoir quality at some level. The identification of distinct and predictable vertical stacking patterns within a hierarchical sequence and cycle framework provides a high degree of confidence at this point that results will be exportable throughout the basin. Ten petrophysically significant facies have been described in the northern reef trend, providing significantly more resolution than the standard 4-6 that are used most often in the basin (e.g. Gill, 1977). Initial petrophysical characterization (sonic velocity analysis under confining pressures) shows a clear pattern that is dependent upon facies and resulting pore architecture. Primary facies is a key factor in the ultimate diagenetic modification of the rock and the resulting pore architecture. Facies with good porosity and permeability clearly show relatively slow velocity values as would be expected, and low porosity and permeability samples exhibit fast sonic velocity values, again as expected. What is significant is that some facies that have high porosity values, either measured directly or from wireline logs, also have very fast sonic velocity values. This is due to these facies having a pore architecture characterized by more localized pores (vugs, molds or fractures) that are not in communication.

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