We have a Custom ASP.Net web service hosted on the same server as SharePoint but
running in a separate web application under a different application pool
account.
This web service is using SharePoint object model to access some site objects.
(We have to use object model that’s why we are hosting the web service on same
SP server)

We are running into permission issues while creating SPSite objects in web
service. It looks like the application pool account should have db_owner rights
on Content DB of SharePoint site. Although some documentation says  it should
have read access only, but we have verified that this not the case.

Our requirement is to have a separate web application and a separate application
pool account for the web service and grant minimum rights on SharePoint content
db.

Can anyone guide me what should be the minimum permission settings.