Diagenesis and Reservoir Properties of the Permian Ecca Group Sandstones and Mudstones in the Eastern Cape Province, South Africa: Implications for the Shale Gas Potential in the Karoo Basin

Abstract

Diagenesis is one of the most important factors that affect reservoir rock property. Despite there are many previous studies on the stratigraphy, sedimentology and general geology of the Ecca Group in the Main Karoo Basin of South Africa, there is still lack of knowledge on the diagenesis of the potentially feasible or economically viable sandstones and mudrocks of the Ecca Group. This study provide an account of the diagenesis and reservoir properties of sandstones and mudstones from the Ecca Group. Twenty-five diagenetic textures and structures are identified and grouped into three stages that include early diagenesis, burial diagenesis and uplift-related diagenesis. Clay minerals are the most common cementing materials in the sandstones. Smectite, kaolinite and illite are the major clay minerals that act as pore-filling matrix and pore lining rim-cement. Calcite precipitates locally in the pore spaces and partially or completely 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 early diagenetic stage. This process 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 sediment overloading and selective dissolution of framework grains. Mineral overgrowths, mineral replacement, clay-mineral transformation, dissolution, deformation and pressure solution occurred during burial diagenetic stage. After rocks were uplifted, weathered and unroofed by erosion, this resulted in decementation and oxidation of iron-rich minerals, and saussuritization of feldspar 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 rocks. The presence of fractured and dissolution pores tend to enhance reservoir quality. However, the isolated nature of the pore linkage makes them unfavourable producers of hydrocarbons, which at best would require stimulation. The understanding of the space and time distribution of diagenetic processes in these rocks will allow the development of predictive models of their reservoir quality, which may contribute to the reduction of risks involved in the hydrocarbon exploration.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018