Low oil prices are forcing drillers to dramatically curtail risky exploration, especially in frontier regions such as Africa. The reality of crude’s slump fully hit the continent in May as operating rigs halved in a month, the biggest slide since records started in 1975, according to Baker Hughes data. Some massive oil and gas discoveries have been made in Africa this century — from Ghana to Mozambique — but the prospects of similar ones in the future look bleak as companies such as Tullow Oil Plc and Sasol Ltd. are slashing operations.