Diagenesis of the Permian Ecca Group Sandstones and Mudstones in the Eastern Cape Province, South Africa - Implications

Diagenesis is the most important factor that affects reservoir rock property. Smectite, kaolinite and illite are the major clay minerals that act as pore lining rims and pore-filling cement. Calcite precipitates locally in pore spaces and partly replaced clay matrix, feldspar and quartz grains at or around their margins. Precipitation of cements and formation of pyrite and authigenic minerals occurred during the syn- and early-diagenetic stages. This regime was followed by lithification and compaction which brought about an increase in tightness of grain packing, loss of pore spaces and thinning of bedding thickness due to overloading sediments and selective dissolution of framework grains. Mineral overgrowths, mineral replacement, clay-mineral transformation, dissolution, deformation and pressure solution occurred during late diagenetic stage. After rocks were uplifted, weathered and unroofed by erosion, this resulted in additional grain fracturing, decementation and oxidation of iron-rich minerals. The rocks were subjected to moderate-intense mechanical and chemical compaction during its progressive burial. Intergranular pores, secondary dissolution and fractured pores are well developed in the Ecca Group. The presence of fractured and dissolution pores tend to enhance reservoir quality. However, the isolated nature of the pores makes them unfavourable producers of hydrocarbons, which at best would require stimulation.