这里用have 没错
结构是 have so high an oil content.有很高的油含量
你是不清楚so在这里的用法
so+adj+a/an+n.=such a/an adj+n.
She has such a big problem that I dare not talk to her.
She has so big a problem that I dare not talk to her.
我们平常都用惯such了,所以一时被误导觉得中心词是high 谓语要用are而已.
实际是content,指algae 有很高的油含量.换成such就更清楚了：
Some types of large of algae have such an high oil content that scientists do...