Dolomitization of the Aguathuna Carbonates in Western Newfoundland, Canada: Implications for a Potential Hydrocarbon Reservoir

Azmy, Karem¹, Denis Lavoie², Ian Knight³ (1) Memorial University of Newfoundland, St. John's, NF (2) Geological Survey of Canada, Quebec City, QC (3) Geological Survey of Canada, St. John's, NF

The upper part of the St. George Group of western Newfoundland consists of the lower Catoche (~ 200m thick) and the upper Aguathuna (~ 100m thick) formations, which are carbonate deposits of a shallow marine platform. Sedimentation was paused by a major subaerial exposure (St. George Unconformity), which is believed to have likely developed a significant pore system in the underlying carbonates. However, the sequence has been affected during burial by multiple phases of dolomitization that caused differential changes in the rock porosity. The Aguathuna dolomites are classified, based on petrography, into three main generations ranging in crystal size between ~ 4µm (almost micritic) and 2mm and occurring as both replacements and cements. The occurrence of fabric retentive dolomicrites implies that dolomitization likely started at early stages of diagenesis. Dolomitization seems to be pervasive in the upper part of the formation so that it significantly occluded the pores. On the contrary, some intervals in the lower part exhibit enhancement in their porosity through the development of intercrystalline pores. The development of lower permeable layers overlain by an impermeable (seal) cap suggests a possible potential diagenetic hydrocarbon trap. Unlike sabkha deposits, the Aguathuna dolomites do not have evaporite interlayers. Furthermore, their low Sr contents (~ 86 ppm) and the δ¹⁸O of earlier dolomites (-3.3 to -6.9‰ VPDB) are also difficult to reconcile with an origin from concentrated brines. The Sr/Ca molar ratios (0.0032 to 0.0116), calculated for the dolomitizing fluid, suggest a contribution from a non-marine, possibly meteoric, water component.