Is the noun marvel an abstract noun?

Yes, the noun 'marvel' is a singular, common, abstract noun; a Word for something that evokes surprise, admiration, or wonder; a Word for a concept. Example:

The Grand Canyon is a marvel. (The 'Canyon' is a concrete noun, a physical place, but that it is a marvel is an opinion, an idea, knowledge about its nature.)

The Word 'marvel' is also a verb.