Diagenesis and Geochemical Evaluation of the Siliciclastic Permian Ecca Group Sandstones: New Insight Into Hydrocarbon Potential in the Main Karoo Basin

Abstract

The Ecca Group in the southern Main Karoo Basin is of particular interest for hydrocarbon exploration in South Africa. The sandstone of the Ecca Group is of Permian age and deposited in marine environment. To date, there is lack of geochemical and diagenetic study on the sandstones. This study aims to provide new insight on the geochemical and diagenetic evolutionary processes of the Ecca Group focusing on the siliciclastic rocks. The diagenetic processes of the Ecca Group can be categorised into early , shallow burial, deep burial and uplifted diagenesis. Six types of cements were found in the sandstones, which are dominantly smectite and quartz cements, with fewer amounts of kaolinite, hematite, calcite and feldspar cements. In the early diagenetic stage, clay and quartz cementation, and initial compaction largely reduced the porosity of the sediments; authigenic quartz, pyrite and smectite/illite clays were formed within the sediments. In the shallow burial diagenetic stage, quartz and feldspar overgrowths started to form; point and long grain contact, more compaction and mineral recrystallization were developed; clay mineral conversion from smectite/kaolinite to illite and then sericite were created. In the deep burial diagenetic stage, recrystallization of clay and quartz minerals, concave-convex and suture grain contact, pressure-solution and stylolite, grain deformation and fracturing, and calcite replacement and albitization occurred. In the uplift stage, dissolution pore and cracks, chloritization, decementation, caliche evaporation, oxidation and weathering of Fe-Mn rich minerals and secondary load-relieving crack and fracture were created. Primary intergranular pores, secondary dissolution and fractured pores are well developed in the sandstones. The primary porosity is relatively low in the Ecca Group due to clay matrix and quartz cementation, but the secondary porosity is moderate due to dissolution, tectonic fractures and joints, and unroofed load-relieving weathering, which enhanced reservoir quality. Rock-eval pyrolysis shows that the sandstones have TOC ranging from 0.11 to 7.35 wt. %. The van Krevelen diagram shows that the sandstones are of Type II and mixed Type II-III kerogen. Tmax values range from 318 to 601, whereas the vitrinite reflectance values range from 2.22 to 3.93 %. Based on the geochemical data, it can be inferred that source rocks are immature to over-matured and have potential of producing gas in present-day.

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