Open source well information viewer
Your best bet is probably OpendTect.

It's not quite what you asked for. OpendTect is a desktop application and does not use HTML/JavaScript for display. It is dual licensed; the free version is licensed under the GPL. It will read wells and seismic no problem, but I'm not sure about FMI in the free version.

Agile started G3.js, a JavaScript library based on D3.js for logs and seismic, but we didn't get very far. (I'm the founder of Agile; I'd welcome a reason to start that project up again.) Agile also has welly, a Python library for well logs and lithology.

There are lists of other open source tools for geophysics and geology on Wikipedia.