A large coalfield exists in the Polda Basin, west of Lock, on central Eyre Peninsula. One Jurassic coal deposit in the Polda Formation and 2 discrete Tertiary deposits have been recognized, containing reserves of over 300 million tonnes of Jurassic coal and 180 million tonnes of Tertiary coal. The Tertiary deposits are poorly defined, and the potential exists for large increases in reserves. The Jurassic coal is relatively low in rank and is comparable with Leigh Creek coal in terms of calorific value, sodium and sulphur contents, but its ash content is higher and more variable. This deposit occurs in the Polda Formation, a Middle to Late Jurassic sedimentary unit of coal, clay and unconsolidated sandy and gravelly sediments. The Tertiary coal is lower in rank than the Jurassic, and has higher moisture, sodium and sulphur contents and a lower calorific value, properities comparable with those of coals of the northern Saint Vincent Basin. Aquifers of variable transmissivity containing highly saline groundwater exist above and below the Jurassic coal sequence. Immediately to the west is the Polda Freshwater Basin. The possible effect of any mine dewatering activity on this resource is unknown. In this Envelope appraisal drilling details, location plans, interpretive lithology logs and coal analytical results are presented, along with geological, hydrogeological, geotechnical, mining and economic assessments.