We've seen "Black Oil" on The X-Files and when we saw Braniac this season on Smallville, he shape-shifted and moved in a black oil form. Greencapt is spot on ... don't overcomplicate the issue. Throw in the technology being developed from some alien source (say, materials brought back from John Jameson's moon walk referred to in Spidey 2) and you've got it.

Or even simpler, just a meteorite hitting NYC and Eddie Brock being at ground zero. No mad-scientist plot needed.

Edit: Or Spidey showing up at the scene, getting "taken over" by the symbiote ... and that's all. No deep alien plot, but still respectful to the otherworldly origins of Venom.