"comprises mainly" is a correct and usable phrase in written English.
It is used to indicate that an object or group contains mostly or primarily certain components. For example, "The new mall comprises mainly luxury stores and restaurants."
Al-Sabʿatayn comprises mainly transverse and seif dunes.
Most young men migrate north, and the hamlet now comprises mainly women, children and the elderly.
The group comprises mainly snuff-taking, dreadlocked youths who champion old traditions like female genital cutting and oath-taking.
The extreme west of the peninsula comprises mainly lowlands, particularly around the harbours at Dingle, Ventry, and Smerwick.
Though the exhibition comprises mainly film works, one can't wander around and stop for a few minutes watching films as you please.
The alliance comprises mainly small, low-lying states in Africa, the Caribbean, Indian Ocean, Mediterranean, Pacific and South China Sea, including Belize, Cape Verde, the Maldives, Jamaica, Singapore and Papua New Guinea.
The interior comprises mainly open grassland roamed by large herds of cattle, waiting to be converted into the world's finest beef.
Class I of the Yonge Nicol system comprises mainly filter feeders; most members of classes II, IIIA, and IIIB are selective feeders.
Cal FC are managed (not "coached", he insists) by Fox Soccer's Eric Wynalda, and comprises mainly of players who for various reasons have so far fallen through the cracks of the professional game in the US.
His opening hour comprises mainly tales of married life, in which Bishop pretends to be gay to annoy his wife, complains about having to attend his children's births, and mimes the removal of a flat-chested woman's Wonderbra.
This strategy, Mr. Koda explained, is part of a larger ambition to expand the scope of the Costume Institute, whose 80,000-piece collection comprises mainly examples of European high style.
