Approximately 4.7 billion t of original coal resources, ranging from lignite A to subbituminous C in rank, are estimated to be present in the Sonda coal field. These resources occur in 10 coal zones in the Bara Formation of Paleocene age. The Bara Formation does not out crop in the area covered by this report. Thin discontinuous coal beds also occur in the Sonhari Member of the Laki Formation, of Paleocene and Eocene age, but they are unimportant as a resource of the Sonda coal field.

The coal resource assessment was based on 56 exploratory drill holes that were completed in the Sonda field between April 1986 and February 1988. The Sonda coal field is split into two, roughly equal, areas by the southwestward flowing Indus River, a major barrier to the logistics of communications between the two halves. As a result the two halves, called the Sonda East and Sonda West areas, were evaluated at different times by slightlydifferent techniques; but, because the geology is consistent between the two areas, the results of both evaluations have been summarized in this report. The resource estimates for the Sonda East area, approximately 1,700 million t, were based on the thickest coal bed in each zone at each drill hole. This method gives a conservative estimate of the total amount of coal in the Sonda East area. The resource estimates for the Sonda West area, approximately 3,000 million t, were based on cumulative coal bed thicknesses within each coal zone, resulting in a more liberal estimate. In both cases, minimum parameters for qualifying coal were a thickness of 30 cm or greater and no more than 50% ash; partings thicker than 1 cm were excluded. The three most important coal zones in the Sonda field are the Inayatabad, the Middle Sonda and the Lower Sonda. Together, these three coal zones contain 50% of the total resources. Isopachs were constructed for the thickest coal beds in these three coal zones and indicate large variations in thickness over relatively small distances. Coal beds in the Sonda coal field were difficult to correlate because of poor core recovery in some intervals and abrupt lateral thinning and thickening. Most coal zones are separated by 5–10 m of interburden, although in some places the interburden between zones is over 100 m thick. More closely spaced drill holes should clarify and significantly improve coal zone correlations in the Bara Formation.

Coal resources in the Sonda coal field were calculated for three reliability categories; measured, indicated, and inferred. The most reliable estimates are those for the measured category. Measured coal resources are approximately 91 million t, or about 2% of the total resource; indicated resources are 681 million t, or about 14% of the total; and inferred resources, the least reliable resource category, are 3,931 million t, or 84% of the total resources. The distribution of resources by reliability category is due to the relatively wide spacing (approximately 5 km) between core holes.

Analyses of 90 coal samples, on an as-received basis, indicate average ash and sulfur contents of 13.7% and 3.6%, respectively, and a range in rank from lignite A to subbituminous C. Calorific values for these samples range from 6,000 to 8,000 Btu/lb (1 Btu = 1055J; 1 lb = 4536 kg).