Evaluating Convection and Compaction-Flow Styles of Burial Dolomitization in the Devonian Nisku Formation Pinnacle Reef Reservoirs, Alberta, Canada, Using Integrated Basin and Reactive Transport Models
The Frasnian Nisku Formation pinnacle reef reservoirs are a series of isolated buildups, c.1-2 km2 diameter by 100 m thick, which developed up to 20 km basinwards of the shelf margin. Petrography, isotope, trace element and fluid inclusion data, as well as stratigraphic relationships, burial history and tectonics, indicate pervasive dolomitization of the Nisku buildups occurred at 500-1500 m depth and 50-80 oC from seawater modified by water-rock interaction with surrounding shale. We evaluated two alternative hydrogeologic models of dolomitization proposed based on this integrated dataset: 1) Geothermal convection of seawater and 2) Compaction-driven flow. The numerical model Basin2 was used to simulate decompaction, thermal history and fluid flow evolution, and then to design and condition TOUGHREACT reactive transport models of dolomitization.
During burial, counter-rotating free convection cells develop in the more permeable Nisku buildups and interact with larger but slower convection cells that extend to the overlying Mississippian ocean. Flow rates in the buildups are up to 50 cm/yr during shallow burial but decrease to 5 cm/yr after deposition of the Mississippian. Dolomitization rapidly depletes Mg2+ from fluids in the buildups whereupon the delivery of additional Mg2+ is severely limited by slower convective flow through the low permeability overburden. Our simulations suggest that, convective flow of seawater did occur during burial, but could not have pervasively dolomitized the Nisku buildups.
The majority of fluid expelled in response to compaction from the underlying and surrounding shales flows vertically upwards, but lateral focusing of flow does occur within a 10 km radius of a buildup. Lateral flow velocities reach 1.2 cm/yr during early burial but temperatures are significantly less than the minimum interpreted dolomitization temperature. When the buildups enter the interpreted dolomite temperature window flow rates are significantly reduced and most pore fluid in the shale adjacent to the reef has previously been expelled. Using the most optimistic assumptions, Mg2+ mass balance calculations suggest that flow-driven compaction may explain the pervasive dolomitization of the Nisku buildups, a result we aim to substantiate with future Reactive Transport Models. Numerical models simulating basin history, fluid flow and chemical reactions can thus provide a robust method to evaluate proposed conceptual models of dolomitization.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009