For example, assuming a single identifying column, PKID, then something like just:
SELECT T.Code1, Avg(T.Reserve) AS AvgRes
FROM tblTableName AS T
WHERE T.PKID In
(
SELECT TOP 150 T2.PKID FROM tblTableName T2 WHERE T2.Code1 = T.Code1 ORDER BY Code2 DESC
)
GROUP BY T.Code1
On...