The Highveld Coalfield in the Mpumalanga Province of South Africa is situated on the northern part of the main Karoo Basin. The Karoo Supergroup can be divided into the Dwyka, Ecca, Beaufort, Stormberg and Drakensberg Groups.

The Highveld Coalfield is underlain by a thin sequence of Dwyka and Middle Ecca strata lying on an undulating floor composed of felsites, granites, and diabase associated with the Bushveld Complex.

The Vryheid Formation of the Ecca Group, where the coal occurs, conformably overlies the tillite with minor conglomerates, sandstones, shales and coal. In the Highveld (and Witbank) coal fields, the glacial deposits of the Dwyka Group are directly overlain by rocks of the Vryheid formation, as the Pietermaritzburg formation is absent in these coal fields. The overlying Volksrust Formation is also either not developed or completely eroded away in the central Mpumalanga coal fields.

Pre-Karoo dolerites have greatly intruded the geological sequence of the Highveld Coalfield. There are two main dolerite sills in the coalfield, with more extensive thinner sills 1.5 m to 15 m thick. Where a dolerite sill has intruded, the seam is faulted, resulting in a vertical throw ranging from 6 to 25 m.

A major dolerite sill of up to 14 m is generally situated below the No. 2 seam in the Pit 4, 23 and 23 extension areas.

The basin depth is approximately 300 m, with the coal zone approximately 70 m thick. Five major coal seams occur in the Highveld, numbered from the bottom upwards, No. 1, 2, 3, 4, and 5. The coal seams are generally described as follows:
- No.1 Seam is thin and mainly discontinuous
- No.2 Seam is approximately 1.5-4 m
- No.3 Seam is thin and discontinuous
- No.4 Seam is 1-12 m, laterally continuous
- No.4 Upper Seam is 1-4 m thick and is separated from No.4 Lower Seam which is 4-12 m thick by sandstone
- No.4A Seam occurs above the No.4 Upper Seam
- No.5 Seam is 1-2 m thin.

The Kriel Colliery reserves are situated on the northern margin of the Highveld coalfield. The majority of the coal mined at Kriel Colliery is from the No.4 Seam. The total thickness of the No.4 Seam in the Kriel coal field is generally between 4 and 5 m thick, where the top seam attains a thickness of 1.5 to 2 m. The No 4 seam is regionally overlain by a thick interlaminated shale-sandstone/siltstone and underlain by a typical white Ecca sandstone layer. Glauconitic sandstone, indicative of transgressive marine periods, is present above the Nos. 4 and 5 coal seams, forming useful stratigraphic markers.

Proven coal reserves in the Kriel Coalfield are the No 2, 4 and 5 seams, but only the No 4 seam is economically extracted. The coal is ranked as low-grade bituminous steam coal and the depth of the coal seam ranges from 6 m to 85 m below the ground. All coal historically and currently produced is dedicated to the Kriel Power station.

Kriel’s underground operation is mined a combination of continuous mining and conventional mechanised bord and pillar mining. The opencast operation are used strip mining, dragline, truck and shovel methods.

Board and pillar mining is typically used for flat or gently dipping bedded ores or coal seams. Once the coal seam has been accessed, the ore is mined utilising a regular grid of mining tunnels and involves progressively excavating panels into the coal seam whilst leaving behind pillars of coal to support the mine. The coal is then removed in a regular pattern while the rooms are mined out through the tunnels by drill and blast methods or a Continuous Miner and deposited onto a conveyor. The open chambers remaining following coal removal is referred to as a stope. The remaining coal contained within the pillars may be won at closure by a retreating system, starting at the farthest point from the stope access, allowing the roof to collapse and fill in the stope. This allows a greater recovery as less ore is left behind in pillars.