Three-dimensional model of dolomitization patterns in the Salina Group A-1 Carbonate and A-2 Carbonate units, Sombra Township, Lambton County, southern Ontario

Dolomitization of carbonate rocks is a subject of considerable interest due to association with oil and gas reservoirs and Mississippi Valley Type ore deposits. Conceptual two-dimensional models of dolomitization are common in the literature, however numeric models supported by high quality data are rare to nonexistent. This paper presents three-dimensional (3-D) dolomitization patterns in the Salina Group A-1 Carbonate Unit and A-2 Carbonate Unit located in Sombra Township, Lambton County. The source data consists of percent dolomite measurements collected from 9727 drill cutting samples, stained with alizarin red, from 409 petroleum wells. Numerical interpolants of the percentage of dolomite versus limestone in the two formations are developed within the boundaries of lithostratigraphic formation layers derived from a 3-D geologic model of southern Ontario, published as GSC Open File 8795 (Carter et al. 2021b). The model was developed using Leapfrog© Works software with a 400 m grid resolution. Results show that increased proportions of dolomite vs limestone in both formations are spatially associated with the flanks and crests of pinnacles in the underlying Lockport Group carbonates, over which the B Salt has been dissolved, and the downthrown side of the Dawn Fault and Becher faults. In the A-1 Carbonate there is an increase in dolomite content over a minority of incipient reefs in the Lockport, and in the A-2 Carbonate Unit there is a gradational increase in dolomite content upwards from a basal limestone to 100% dolomite. The cross-cutting relationships of dolomite occurrence in the A-1 Carbonate on the flanks and crests of some pinnacles support a post-depositional burial diagenesis mechanism, consistent with previous interpretations. The pathway for the dolomitizing fluid was laterally through porous and permeable regional paleokarst in the underlying Lockport Group, uppermost Goat Island and Guelph formations, and upwards through the porous reefal carbonates of the pinnacles. Association of dolomitization haloes with dissolution features in halite of the overlying B Salt Unit further suggest that the dolomitizing fluids were also responsible for salt dissolution. The preferential association of dolomite with the Dawn and Becher faults suggest that movement of the dolomitizing fluid was also fault controlled. This project demonstrates the feasibility and merit of assignment and interpolation of attribute values constrained by lithostratigraphic layers in the regional 3-D geologic model of southern Ontario. Spatial associations of dolomite with other geological features are more clearly resolved than in a 2-D study.