The control of hydrothermal dolomitization and associated fracturing on porosity and permeability of reservoir facies of the Upper Devonian Jean Marie Member (Redknife Formation) in the July Lake area of northeastern British Columbia

Abstract

Abstract The Jean Marie Member of the Redknife Formation in the July Lake area of northeastern British Columbia consists of mainly limestones and partially dolomitized limestones, with variable dolomite content and porosity. Dolomitization occurred from the flow of hydrothermal brines up through fault conduits, resulting in the formation of patchy metre-thick dolostones at the top of the Jean Marie, close to the faults, and the partial dolomitization of limestones away from the faults. Most pores in the Jean Marie Member are from the secondary dissolution of calcitic components and vary in size from micropores to larger molds, as well as some even larger cm-scale vugs in the few dolostones. Petrographic relationships constrain the interval of dissolution of calcite to that following a phase of non-ferroan equant mosaic calcite cement but preceding a phase of saddle dolomite cement, either prior to, concurrent with or after matrix replacement dolomitization. Furthermore, the selective occurrence of molds (or near-molds) of stromatoporoids and corals and that of cm-scale vugs to either rocks with a high degree of matrix replacement dolomite or to pure dolostones most logically suggests that dissolution of calcite is concurrent with dolomitization. If leaching of calcite was either prior or subsequent to dolomitization, these pore types should also be present in limestones and partially dolomitized limestones with low dolomite content. In addition, there is a general relationship that the degree of calcite dissolution, and hence porosity occurrence, is greater within intervals that have a high-matrix dolomite content or in limestones that occur adjacent to pure dolomites. Dissolution of calcite is therefore interpreted to be part of the dolomitization process. The results of a regional statistical analysis of dolomite content and porosity from full-diameter core analysis data provides more compelling evidence that dissolution of calcite is part of the dolomitization process. In this analysis, porosity was normalized to the limestone (calcite) fraction of the rock. Mean values of limestone-fraction porosity were then computed for each 0.05 dolomite fraction (or 5% fraction) of the rock and plotted against the values of the dolomite fraction. This resulted in a remarkably constrained linear trend, with limestone-fraction porosity increasing from 4.5% for samples with no dolomite content to approximately 12% for samples with 50% dolomite content. The results of this analysis demonstrate that dissolution of calcite starts at a very low dolomite content and increases in a linear trend with increasing dolomite content up to at least 50%. Matrix permeabilities measured from Jean Marie reservoir facies are generally less than one millidarcy. Centimetre-scale, non-planar fractures provide the permeability for commercial gas deliverability rates. They are of tectonic origin and are interpreted to have formed during the fault-controlled hydrothermal phase, in latest Devonian (?) to early Mississippian time.