Cal. Code Regs. tit. 14, § 1981.1 – Exploratory Well Requirements (No Production Casing).

(a) Base of fresh waters--a minimum of 30 meters (about 100 feet) of cement straddling the interface or transition zone whether behind casing or uncased.
(b) Shoe plug (all casing, including conductor pipe)--straddle with 30 meters (about 100 feet) of cement.
(c) Where the well has been drilled with air, a bridge plug shall be placed at the shoe of the surface casing and the bridge plug shall be capped with at least 60 meters (about 200 feet) of cement.
(d) Surface plug--15 meters (about 50 feet) minimum. May be either neat cement or concrete mix.