Manuscript Am.B. 340, owned by the Staatsbibliothek Berlin, contains some 45 organ pieces, presumably copied, adapted and/or composed by Wilhelm Karges. The model or original of this Fantasia in G is not known to me. Karges made a composition out of it, that consists of two halves with some scales across the keyboard to join the two halves.
Each half is build on a motif that appears several times in all the voices of the piece. In the second half the motif appears most of the times in stretto in well written counterpoint. The style is not unlike that of Froberger. It does however not appear in the edition of Froberger organ works I posses. That volume however is rather old and might not be complete. If someoneknows on what original this composition might be based, I'd very much like to hear it.
Score available here: http://partitura.org/index.php/wilhelm-karges-fantasia-in-g