Sasol Form 20-F for the year ended 30 June 2021 - Book - Page 55
and computer-derived geological models, using the
Minex 6 edition of the GEOVIA/ MINEX software.
Reserves and composite qualities are computed using
established and recognised geo-statistical techniques.
Directional drilling. Directional drilling from
surface to in-seam has been successfully applied for
several years. A circular area with a radius of
approximately 1,4 km of coal deposit can be covered by
this method from one drill site. The main objective of
this approach is to locate dolerite dykes and
transgressive dolerite sills, as well as faults with
displacements larger than the coal seam thickness.
General stratigraphy
The principal coal horizon, the Number 4
Lower Coal Seam, provides some 91,67% (2020—
91,67%) of the total proved and probable reserves. The
Number 4 Lower Coal Seam is one of six coal horizons
occurring in the Vryheid Formation of the Karoo
Supergroup, a permo-carboniferous aged, primarily
sedimentary sequence. The coal seams are numbered
from the oldest to the youngest.
Horizontal drilling. This technique is applied
to all operational underground mines and supplies
short-term (minimum three months) exploration
coverage per mining section. No core is usually
recovered, although core recovery is possible, if
required. The main objective is to locate dolerite dykes
and transgressive sills intersecting the coal mining
horizon, by drilling horizontal holes in the coal seam
from a mined out area. A drilling reach of up to 1 km is
possible, although the average length is usually 800 m
in undisturbed coal.
The Number 4 Lower Coal Seam is a
bituminous hard coal, characterised by the following
borehole statistics:
Aeromagnetic surveys. Many explorations are
usually aero-magnetically surveyed before the focused
exploration is initiated. The main objective is to locate
magnetic dolerite sills and dykes, as well as large-scale
fault zones.
Geophysical wireline surveys of directional
boreholes. Geophysical surveys are routinely conducted
in the completed directional drilled boreholes. This
results in the availability of detailed information
leading to increased confidence of the surface
directional drilling results.
the depth to the base of the seam ranges
from 40 m to 241 m with an average
depth of 135 m below the surface
topography. All the current mining done
on this seam is underground;
the floor of the seam dips gently from
north to south at approximately
0,5 degrees;
the thickness of the seam varies in a range
up to 10 m with a weighted average
thickness of 3,7 m. In general, thinner
coal is found to the south and thicker coal
to the west adjacent to the Pre-Karoo
basement highs;
the inherent ash content (air dried basis) is
an average 26,46%, which is in line with
the coal qualities supplied during the past
30 years to Secunda operations;
the volatile matter content is tightly
clustered around a mean of 22,9% (air
dried); and
the total sulphur content (air dried), which
primarily consists of mineral sulphur in
the form of pyrite and minor amounts of
organic sulphur, averages 1,01% of the
total mass of the coal.
Secunda operations
The coal supplied to Secunda operations is the
raw coal mined from the four mines supplying Secunda
operations exclusively and the secondary product from
the export beneficiation plant.
We have carried out extensive geological
exploration in the coal resource areas, and undertake
additional exploration to update and refine the
geological models. This allows for accurate forecasting
of geological conditions and coal qualities, and also
effective planning and utilisation of coal reserves.
Computation and storage of geological information
We store geological information in the
acQuire database. We conduct regular data validation
and quality checking through several in-house methods.
Data modelling is conducted by manual interpretation
54