Abstract: Porosity Loss by Deep-Burial Thermochemical Sulphate Reduction (TSR) In Upper Devonian Leduc Dolostones, Southern Rimbey-Meadowbrook Reef Trend, Alberta

Eric Mountjoy, Eva Drivet

Upper Devonian dolostone (Leduc) reservoirs of the southern Rimbey-Meadowbrook reef trend show a progressive overall decrease in porosity below depths of about 2,300 m from near 10% to 2% as a result of pore-filling by late-stage minerals. The amounts of dolomite, calcite, sulphur, and metallic sulphides tend to increase downdip to depths greater than 3500 m. This pattern, associated with increasing H₂S downdip in pooled gases and light ^dgr¹³C values of the calcites, suggests that these minerals are related to thermochemical sulphate reduction (TSR). Anhydrite, the last and most abundant and ubiquitous pore-filling phase, also increases downdip from about 4 to 7% of the total porosity. Anhydrite was either formed during the later phases of TSR, or may have been recipitated from formation fluids due to cooling during Tertiary uplift.

Extensive subsurface solution took place before the above late-stage pore-filling phases. Dissolution caused brecciation of early replacement dolomite and later coarse-crystalline dolomite cements. This dissolution and brecciation may be related to early phases of TSR, but other mechanisms which generated acids earlier during diagenesis of organic matter or transformation of clay minerals could also have been responsible. The above processes have had a profound effect on reservoir character between burial depths of 2,300 to 4,000 m.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994