Is it obligatory to abstain from oral sex with one's wife, or is it permissible given that it is included in His saying, the Almighty: "Your women are a tilth for you"? And does not being averse to this practice indicate a base nature?

The fundamental principle regarding enjoyment between spouses is permissibility, except for what has been explicitly prohibited by a text. What is mentioned in the question falls under general permissibility, as there is no text prohibiting it, alongside general texts that encourage noble character. It must be noted that it is not permissible to directly touch the private parts in this manner if there is impurity, for fear of swallowing it. It is better to refrain from such matters altogether.