Although Google has its servers spread across the whole world, it would be quite hard to say where the search engine's bots mostly originate from. What I suggest would be to block the IP ranges but add an exclusion clause that matches against the User-Agent
for search bots like:
SetEnvIfNoCase User-Agent (googlebot|bingbot|yahoo!\sslurp) is_search_bot
<Directory /docroot>
Order Deny,Allow
Deny from 3.0.0.0/8
Deny from 4.0.0.0/25
Deny from 4.0.0.128/26
Deny from 4.0.0.192/28
Deny from 4.0.0.208/29
Allow from env=is_search_bot
</Directory>