Fractured hydrothermal dolomite reservoirs in the Devonian Dundee Formation of the central Michigan Basin

Abstract

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 commun...

Origins and Geochemistry of Oolitic Dolomite of the Feixianguan Formation from the Yudongzi Outcrop, Northwest Sichuan Basin, China

“…The δ 13 C values vary from +2.8‰ to +3.1‰ (PDB) [68], depending on the various invertebrates and seawater cements present during the different geological periods. The silty to fine oolitic dolomites in the platform-margin oolitic beach facies have negative δ 18 In order to determine the sedimentary environment of carbonate rocks, we take advantage of δ 13 C and δ 18 O values to calculate Z (salinity index), which is a good indicator of the discrimination between marine facies and continental facies of the carbonate rocks [69], given by the equation: Z = 2.048 × (δ 13 C + 50) + 0.498 × (δ 18 O + 50) (PDB) [69]. Values of Z > 120 indicate marine carbonates, while Z < 120, is freshwater carbonate rocks, and when Z = 120, an unfinalized carbonate.…”

“…Because burial temperatures, water-rock interaction, and atmospheric precipitation affect δ 18 O values, increasing temperature causes negatively skewed δ 18 O in rocks that would otherwise exhibit positively skewed δ 18 O affected by water-rock interaction and atmospheric precipitation (Figure 8) [68]. In high-temperature burial environments, the light isotope 16 O preferentially enters the dolomite lattice and the heavy isotope 18 O becomes enriched in the diagenetic fluid [70].…”

“…The tidal flat dolomite has noticeably higher δ 18 Ο values than the platform-margin oolitic beach dolomite (Figure 8), indicating that burial and temperature effects may have influenced the latter dolomite type. These rocks can be further divided into those representing the shallow burial stage, and those from the moderate to deep burial stage, which are distinguished by more negative δ 18 O values in the latter compared with the former. Because burial temperatures, water-rock interaction, and atmospheric precipitation affect δ 18 O values, increasing temperature causes negatively skewed δ 18 O in rocks that would otherwise exhibit positively skewed δ 18 O affected by water-rock interaction and atmospheric precipitation (Figure 8) [68].…”

“…We employed the experimental oxygen isotope calibration of dolomite from Horita (2014) [75] to calculate the values of δ 18 Owater, in which the dolomite-water equilibrium fractionation factor for a [38]; (c) Seepage reflux dolomites from this study; (d) Shallow burial dolomites from this study; (e) Middle-to-deep burial dolomite from this study; (f) All burial dolomite from this study; (g) Group of burial Feixianguan dolomites [38]; (h) All the dolomite samples from this study.…”

“…Then, the δ 18 OPDB of the carbonates could be converted to δ 18 OSMOW values, using the same conversion formula for SMOW and PDB [76]: δ 18 OSMOW = 1.03091δ 18 OPDB + 30.91. The above Equations (1) and (3) could be used to reverse the oxygen isotope composition (δ 18 Odolomitization fluid), on the basis of the oxygen isotopic composition of dolomite minerals (δ 18 Odolomite) and the precipitation temperature (T) corresponding to the homogenization temperatures of inclusions of dolomites.…”