UPDATE 1-Sirius eyes oil assets worth $500mln, shares rise

* Expects to make announcement on Nigerian oil field shortly

* Shares gain 15.8 pct (Adds detail)

LONDON Feb 16 (Reuters) - Oil and gas explorer Sirius Petroleum (SRSP.L) said it is working on a deal that could give it the opportunity to acquire oil and gas assets worth at least $500 million by 2012, sending its shares up 15.8 percent.

Sirius said on Tuesday it had entered into an agreement with South Africa High-Tech Energy Consultancy (SAHT), which aims to introduce Sirius to oil and gas assets worth over $500 million by year-end 2011.

A spokesman for Sirius said the location and ownership of the assets was not being disclosed at the current time.

The company, whose main strategy is looking for oil and gas opportunities in Nigeria, also said it expected to make an announcement shortly regarding the acquisition of its first marginal oil field opportunity in Nigeria.

SAHT will be able to subscribe for 80 million Sirius shares, at a price of 7.125 pence a share, should a deal to acquire the assets go ahead.

In December, Sirius had 520 million shares outstanding.

Shares in Sirius Petroleum were up 15.8 percent at 8.25 pence at 1555 GMT, valuing the company at 43 million pounds ($67.45 million).

"The agreement with SAHT introduces an additional and significant potential opportunity for Sirius to acquire a portfolio of oil and gas assets," said Sirius director Toby Haywood in a statement.

Haywood sits on the board of Africa-focused oil and gas explorer Afren (AFRE.L) which has established itself as an prominent independent oil company in Nigeria.

SAHT is owned by Vikrom Koompirochana, a former Thai diplomat, and Wong Fan Woon, a former executive of Total Access Communications DTAC.BK. (Reporting by Sarah Young, editing by Will Waterman) ($1=.6375 Pound)