LU, FENGHU, and IHSAN S. AL-AASM, Univ. of Windsor, Windsor, Ontario

ABSTRACT: Multistage Dolomitization of Mississippian Turner Valley Formation, Quirk Creek Field, Alberta, Canada

The Mississippian Turner Valley carbonates of the Quirk Creek field, Alberta, are gas-producing, invariably dolomitized rocks. Reservoir porosity is mostly represented by secondary, intercrystalline and moldic porosity. Multistage dolomitization in the Turner Valley Formation occurred at different geological times. Four types of dolomite have been identified: microdolomite, patchy dolomite, pervasive matrix dolomite and coarse dolomite. Microdolomite (4-10 micrometers) is dense, and locally occurs only in sabkha lithofacies. It is characterized by its high Sr, and Na concentrations and relatively heavy (isotope){18}O values (0 to -2.22 o/oo PDB). These parameters suggest sabkha evaporative dolomitization. However, its stoichiometry, overgrowth rims and variable values of {87}Sr/{86} S (0.70773 to 0.70874) may reflect recrystallization during late dolomitization. Patchy dolomite (30 to 150 micrometers) floats between skeletal grains and is distributed along dissolution seams and early stylolites ((isotope){18}O = -0.79 to -4.29 o/oo PDB). This type of dolomite is interpreted to have formed during early chemical compaction and also underwent modification during late dolomitization events. Pervasive matrix dolomite (30 to 300 micrometers) is the most abundant and porous type of dolomite. It has modified or obliterated most earlier diagenetic fabrics. Because the pervasive matrix dolomite has the lowest trace element concentrations (e.g. Sr, Na), slightly radiogenic Sr isotopes (0.70834 to 0.70848), slightly depleted (isotope){18}O (-1.11 to -4.81 o/oo PDB), and commonly coexists with secondary anhydrite, a mixing zone associated with a regional groundwater system beneath the Mississippian/Pennsylvanian unconformity is envisaged here for its formation. The diagenetic fluids resulted from flushing the over lying evaporites by meteoric water and mixing it with seawater. These fluids could have been the source of secondary anhydrite as well as pervasive matrix dolomite. Coarse dolomite (0.5 to 2 mm) consists of euhedral rhombs, replaces matrix dolomite, and crosscuts late stylolites. Coarse dolomite has high Fe, Mn concentrations, radiogenic Sr (0.70836 to 0.70875) and the lowest (isotope){18}O (-2.12 to -6.47 o/oo PDB), suggesting later formation in a deeper burial environment.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.