LONDON, June 18 - UK-based oil explorer Tullow Oil Plc said on Monday it had made a significant oil discovery offshore Ghana.

Tullow said in a statement that an exploration well hit a large hydrocarbon column. Industry sources said its size suggested the field is now likely to contain at least 300 million-400 million barrels of oil.

The structure drilled extends into an adjacent exploration block in which Tullow has a 49.95 percent interest, suggesting material reserves could also be present there.

Dealers said Tullow shares were likely to rise when the market opened.

"The discovery of oil in the Mahogany well represents a major event for the Republic of Ghana and for Tullow. Based on evidence to date, ultimate reserves are likely to be materially in excess of previous estimates," Chief Executive Aidan Heavey said in a statement.