(Corrects first name of analyst in paragraph 9)
By Tom Bergin
LONDON, April 8 (Reuters) - Royal Dutch Shell Plc's agreed $70 billion takeover of rival BG Group couldtrigger pre-emption rights in key oil and gas fields that woulderode the potential benefits of the deal for the Anglo-Dutch oilgiant.
Shell said a main driver of its bid for BG Group was thegas-focussed British group's position in Brazil. Two exploration blocks, named BM-S-9 and BM-S-11, account foralmost all the value of BG's Brazil assets.
But BG said in its annual report, published last week, that:
"In certain specific circumstances, it is possible that BGGroup's partners in BM-S-9 (Petrobras and Repsol Sinopec Brasil)have a right of first refusal to acquire BG Group's interest ..in the event of a change of control of BG Group plc".
BG and Shell declined to comment on the BM-S-9 rights.
When Shell Chief Executive Ben van Beurden was asked aboutchange of control provisions on an analyst call on Wednesday, hesaid these could trigger pre-emption rights in relation to BG'sstake in Karachaganak, a field in Kazakhstan.
He did not mention BM-S-9, which contains the Sapinhoa andLapa fields.
Analysts said that failing to secure BG's stake inKarachaganak might not affect the key strategic drivers of thedeal. However, not buying BM-S-9 would be a big loss.
"Brazil is central to the acquisition," said Neill MortonMorton, oil analyst at Investec.
"Sapinhoa is pretty important. It's one that Shell islooking to get its hands on," he added.
Block BM-S-9 is estimated to contain billions of barrels ofoil which can be extracted at moderate costs. Analysts atBernstein estimated last month that Sapinhoa could be worth $6.5billion, while Morton said the Reading-based group's stake inthe block could be worth $10 billion.
Tom Ellacott, head of the corporate analysis team atresearch group Wood Mackenzie, said a failure to secure BM-S-9probably wouldn't be a deal-breaker but would likely force arethinking of the terms of the deal.
Analysts said Petrobras and Repsol may not be keen toexercise their right to buy BG's stake in BM-S-9, due to a lackof funds or strategic impetus, but that Chinese state-controlledSinopec could mount a bid.
"Sinopec has a pretty reasonable position in Brazil and theywould probably be keen to increase that further," Ellacott said.
Repsol declined comment. Petrobras and Sinopec were notavailable for comment. (Reporting by Tom Bergin; Editing by Elaine Hardcastle)