The Eagle Ford Fm in southwest Texas is a self-sourced oil and gas reservoir currently stimulated through hydraulic fracturing to produce economic quantities of hydrocarbons. Both early and burial diagenesis has had a major impact upon the rocks and their resulting reservoir quality. In this presentation we infer mineral reactions and precipitation that took place during diagenesis through a combination of mineralogical and petrographic analysis using XRD, optical and electron microscopy. We highlight how such an understanding can improve prediction of rock properties and reservoir quality.
Along with compaction and de-watering, we infer that bacterial sulphate reduction had a major impact during early diagenesis as it resulted in significant calcite cement precipitation. Calcite cements infill bioclasts and foraminifera chambers, thereby significantly reducing intra-granular porosity. Fine grained calcite cements the matrix and coccolith fragments that resulted in reduction in inter-granular porosity and its interlocking texture will likely lead to an increase in rock brittleness. Optical microscopy and cathodoluminescence (CL) highlight the extensive and invasive calcite precipitation that occurs within concretional features in the Eagle Ford Fm. Zonation within the calcite cements suggests evolution in pore water chemistry and this is interpreted to be caused by changes in microbial organic matter oxidation.
Foraminifera chambers are commonly infilled with kaolinite, as well as or instead of calcite. There is no clear petrographic evidence to suggest which came first, but based on the fact the foraminifera are not compacted we infer early diagenetic origin. Unlike the calcite infills, kaolinite infills preserve significant inter-crystalline porosity. Authigenic kaolinite is also present as multiple crystal grains within the matrix, and replacing 30–60µm detrital grains- which we infer to be feldspars.
During late burial, authigenic quartz cement commonly precipitated around detrital quartz grains and calcite cements, further reducing inter-particle and inter-crystalline porosity. This source of this silica may have been clay mineral reactions or biogenic silica dissolution. Chlorite is present in the form of 5~15µm wispy flakes in the most thermally mature samples. The precipitation of clay minerals during deeper burial leads likely to a decrease in rock brittleness and a further reduction in micro-porosity in the matrix.