The Pennsylvania Supreme Court has reaffirmed and applied the doctrine of estoppel by deed in the case of Shedden v. Anadarko E. & P. Co., L.P, 136 A.3d 485 (Pa. 2016) and distinguished it from the doctrine of equitable estoppel. Equitable estoppel "recognizes that an informal promise implied by one's words, deeds or representations which leads another to rely justifiably thereon to his own injury or detriment, may be enforced in equity" while "[i]n contrast, the doctrine of estoppel by deed precludes one who conveys an interest in land that he does not own, but subsequently acquires the title thereto, from denying the validity of the first conveyance."
In this case, an owner leased oil and gas rights to a 62-acre parcel while actually owning only 50% of them. When the owner later acquired the other 50% of the oil and gas rights, the doctrine of estoppel by deed folded those newly acquired rights into the previous lease, immediately granting the lessee the rights to 100% of the oil and gas rights.
The owner had argued that lessee had not relied on the promise to lease 100% of the rights because it reduced its payments upon learning that the land owner only owned 50% of the oil and gas rights. The Pennsylvania Supreme Court noted that this fact was irrelevant because detrimental reliance is not required under the doctrine of estoppel by deed. The point of the doctrine is not to protect the promisee who relies on a promise but to prevent the promisor from taking back a promise already made once the promisor becomes able to fulfill it.