网友断句有误。句子成分分析如下：

New tax incentives 主语 for oil firms taking on very complex projects定语1, combined with a high oil price定语2, have encouraged 谓语a flurry of investment into old fields and those once thought too costly to exploit 宾语. 

说明：

1 for 介词短语为定语修饰主语。其中taking on...现在分词短语为定语修饰oil firms。

2 combined with...为过去分词短语。有二种理解：可以理解为主语的非限制性定语，也可以理解为方式状语修饰谓语动词。

3 主语为复数，谓语动词has encouraged为错误，应改为复数have encouraged。

4 into old fields and those once thought too costly to exploit为介词短语作定语修饰investment, 所以既不是状语，也不是宾补。