Dolomitization and Late Secondary Porosity Development in Nisku Reefs (Late Devonian) of Alberta: ABSTRACT

Dolomitization and associated calcite dissolution are important controls on reservoir quality in coral reefs of the Nisku Formation. Average porosities of 13 to 15% and permeabilities of 3 darcys are recorded in the fully dolomitized reefs, while lower average porosities of 3 to 5% and permeabilities of 350 milli-darcys are recorded in partly dolomitized reefs. The close correlation between dolomites and high porosity and permeability is best understood by examining the paragenetic sequence and spatial distribution of the dolomites. Dolomitization occurred over a long period of burial and resulted in the formation of two major types of dolomites that are volumetrically significant. The first is characterized by matrix-selective, gray, cloudy, 20 to 150-µ crystals that grade from scattered subhedral and euhedral rhombs to interlocking crystalline mosaics. Matrix dolomites are slightly calcitic (51 to 53 mole % Ca), have a low iron content (less than 0.04 oxide wt. %), show a pronounced fabric selectivity toward micrite, and are abundant in all of the reefs. The second major type of dolomite is characterized by pervasive, brown, cloudy, 60 to 300-µ crystals that occur in the flanks of structurally updip reefs and throughout the downdip reefs. The pervasive dolomite exhibits similar chemical characteristic to the matrix dolomite and in some places replaces it. Concentration of dolomite crystals and dissolution of individual rombs along stylolites indicate that the matrix dolomite initially formed at shallow depths. As dolomitization progressed, dolomite recrystallization and cementation along with extensive calcite dissolution resulted in dramatic increases in porosity and permeability. Calcite dissolution continued after dolomitization ceased, but much of the dissolution is coeval with dolomitization. Evidence for the coeval relationship includes a complete gradation from partly dolomitized corals with dissolution of parts of the calcite skeleton to totally dolomitized rock with biomoldic porosity. Dolomite overgrowths, some of which are enriched in iron (up to 1.5 oxide wt. %), formed during progressive burial. Dolomite cements commonly e tend into tension fractures that displace and are displaced by stylolites, indicating formation at greater depths than the precursor crystals. Matrix and pervasive dolomites exhibit values for 18O from -2.5 to -6.0^pmil PDB and 13C from +2.5 to +6.0^pmil PDB. The negative shift of 18O values from those of modern dolomites formed at shallow depths may be the result of formation at elevated burial temperatures. Pervasive dolomites are the result of recrystallization of the matrix dolomite and possibly primary dolomitization of the reef-flank facies. Isotopic data support a strongly rock-buffered system. Late dolomites and secondary porosity in the Nisku reef trend increase in abundance down structural dip, toward the southwest. Thus, much of the dolomite and late porosity is post-Devonian and probably formed during the late Paleozoic and early Mesozoic when the regional structure tilted to the southwest. Although all Nisku reefs form stratigraphic traps, late diagenetic overprints in some significantly enhanced their reservoir quality. End_of_Article - Last_Page 412------------