Riverstone Energy said it has invested up to $100m (£64.9m) in US-based oil and gas investment group Carrier Energy Partners II LLC.The FTSE 250 group has invested $67m via its Riverstone Global Energy and Power Fund VI, with the remaining $33m coming from Riverstone Energy Ltd.Riverstone and Carrier formed Carrier Energy Partners I in 2013 and, since then, the Texas-based group has participated in two joint ventures targeting development of oil and gas assets in the Delaware Basin and in a legacy conventional field located in South Texas."We are excited to grow our relationship with the Carrier team as we execute on this non-operated oil and gas acquisition approach in today's environment," said Riverstone Holdings partner Robert Tichio.