In Osage County, Oklahoma, who owns the underground 'pore space' (the empty spaces in rock and soil that can be used for carbon storage or other purposes), the surface landowner or the Osage Nation's mineral estate?

Plain-English summary

"Pore space" is the empty space inside rock and soil underground, the cavities, voids, and interstitial spaces between solid material. It is not soil and not rock; it is the room between them. Pore space is increasingly valuable for two reasons: carbon capture and storage (CCS), which involves injecting CO2 into deep underground formations, and underground water and gas storage.

In most of Oklahoma, the question of who owns pore space is settled: it belongs to the surface estate by both common-law rule (the "American rule") and statute (60 O.S. § 6). But Osage County is different. Under the 1906 Osage Allotment Act, the surface estate was allotted to individual Osage Nation members, and "oil, gas, coal, or other minerals" was reserved to the United States in trust for the Osage Nation. The Osage Nation Congress passed Bill ONCA 25-21 in 2024, codified at 22 ONC § 2-104, claiming that pore space is and always has been part of the Osage mineral estate.

Senator Dave Rader asked the AG: who is right about pore space ownership in Osage County?

The opinion answered with two conclusions:

1. The Osage Allotment Act transferred pore space to the individual surface allottees. Pore space is not "oil, gas, coal, or other minerals." Under federal and state law, pore space is empty space, not a "substance" that can be removed from the soil for commercial use. The Osage Act broadly conveyed to allottees "the right to manage, control, and dispose of [their] lands the same as any citizen of the United States" and "the right to use and to lease said lands for farming, grazing, or any other purpose not otherwise specifically provided" in the Act. Pore space falls within that broad surface-estate grant. The Indian canon of construction (which counsels reading ambiguous federal Indian statutes in favor of tribes) does not change the result because the term "other minerals" unambiguously does not include empty space.

2. Oklahoma law, not Osage tribal law, governs current ownership. Once a state enters the Union, "title to the land is governed by state law" (Montana v. United States). Tribes lack authority to regulate ownership or alienation of fee land held by non-Indians (Plains Commerce Bank v. Long Family Land & Cattle Co.). The Osage Nation's 2024 code provision asserting pore space as part of the mineral estate cannot override Oklahoma's statutory and common-law rule that pore space belongs to the surface owner.

Two important caveats:

• The opinion only addresses the general rule. Specific pore space ownership is determined by individual deeds. Some pore space may have been separately conveyed by deed; some may have been transferred by adverse possession or easement.
• Oil, gas, and naturally occurring substances within the pore space remain part of the mineral estate. Only the empty space itself belongs to the surface owner.

The Attorney General himself recused from this opinion (footnote 10) and the First Assistant AG executed it instead.

What this means for you

If you own surface estate land in Osage County

Per this AG opinion, you presumptively own the pore space underneath your land. That includes the right to lease pore space for carbon capture and storage, water injection, gas storage, and similar uses, subject to:

• Whether your specific deed or chain of title conveyed pore space rights separately.
• Whether the pore space is currently occupied by oil or gas (which would belong to the mineral estate).
• Whether someone else has acquired pore space rights through prescription or easement.

Before signing a CCS lease or storage agreement, have your title examined. The general rule supports your ownership, but your specific deed may differ.

If you operate a carbon capture or underground storage business in Osage County

You can negotiate with surface owners for pore space leases under the AG's analysis. But two practical complications:

1. Tribal and federal litigation risk. The Osage Nation has codified its contrary position. Until federal courts (or the Tenth Circuit) directly address pore space ownership under the Osage Act, expect litigation challenging your project. Underwrite that risk in your project economics.

2. BIA and federal regulatory overlay. While the AG's opinion concludes pore space is surface-estate property, Bureau of Indian Affairs regulations governing the Osage mineral estate (25 C.F.R. § 211 et seq.) may still affect operations. Coordinate with BIA on any project that potentially interacts with mineral development.

The AG's opinion provides legal cover for the State of Oklahoma's position but does not bind federal courts or the Osage Nation.

If you advise the Osage Nation or the Osage Minerals Council

The AG's opinion is the State of Oklahoma's official position. It is binding on Oklahoma state and local officials. It does not bind federal courts or the Osage Nation.

The Osage Nation has paths to challenge or contest the position:
- Federal litigation seeking a declaration that pore space is part of the mineral estate. The opinion notes none has been filed yet on this specific issue.
- Congressional amendment to the Osage Allotment Act to expressly include pore space in the mineral estate.
- Direct negotiation with surface estate owners for pore space rights, treating the question as contested.

The opinion's analysis is well-supported by federal case law (Watt v. W. Nuclear, Millsap v. Andrus, Osage Wind) and the consensus state-law American rule (Indiana, Kentucky, Michigan, Montana, Nebraska, New York, North Dakota, Oklahoma, Pennsylvania, Texas, Utah, Wyoming). Reversing it in court would require either distinguishing those cases or showing the Indian canon should reach further than the AG concluded.

If you advise oil and gas companies operating in Osage County

The opinion clarifies that pore space is not part of the mineral estate, but oil and gas within the pore space remain mineral estate property. Your operations to extract oil and gas continue to be governed by the Osage mineral lease framework administered by the BIA. What changes: if you are using pore space for non-extraction purposes (e.g., CO2 enhanced oil recovery, gas storage), you may need separate authorization from the surface owner.

If you are a real estate attorney handling Osage County transactions

Pay close attention to pore space provisions in deeds going forward. Older deeds may not address pore space because the legal question was not as commercially significant. Consider:

• Including explicit pore space conveyance or reservation language in new deeds.
• Reviewing chains of title for any prior pore space conveyances.
• Advising clients on the live legal dispute (Osage Nation's contrary position) when material to the transaction.

If you are an Oklahoma legislator

The AG opinion is a complete legal answer to the question presented. Legislative action is not required. But if members want to provide additional certainty (e.g., explicit statutory pore space conveyance procedures, statutory recognition of CCS-specific issues, dispute resolution for Osage County), 60 O.S. § 6 is the natural amendment vehicle.

Common questions

Q: What is pore space?
A: 60 O.S. § 6(B)(1) defines pore space as "any interstitial space not occupied by soil or rock, within the solid material of the earth, and any cavity, hole, hollow or void space within the solid material of the earth." It is the empty space underground, used for carbon storage, water injection, gas storage, and similar purposes.

Q: Why is pore space ownership a hot topic right now?
A: Carbon capture and storage. CCS removes CO2 from emission sources and stores it deep underground in pore space. The economic value of pore space is increasing, and several states (and the Osage Nation) are now defining pore space ownership rules.

Q: What is the "American rule" on pore space?
A: Most U.S. states, including Oklahoma, follow the rule that pore space belongs to the surface estate owner. Alaska is the only state that follows the "English rule," under which pore space belongs to the mineral estate.

Q: What is the Osage Allotment Act?
A: The 1906 federal statute (Act of June 28, 1906, ch. 3572, 34 Stat. 539) that allotted the surface estate of Osage County to individual Osage Nation members and reserved the mineral estate ("oil, gas, coal, or other minerals") to the United States in trust for the tribe. The mineral estate was originally to transfer to surface owners after 25 years, but Congress extended the federal mineral reservation in perpetuity in 1964.

Q: What did the Osage Nation enact in 2024?
A: The Osage Nation Congress passed Bill ONCA 25-21, codified at 22 ONC § 2-104, asserting that "[p]ore space in all strata underlying the surface of lands and waters within the Osage Reservation is and always has been an integral part of the Osage Mineral Estate" and that conveyances purporting to sever pore space title are void. The AG's opinion concludes this tribal code provision cannot displace federal and Oklahoma property law.

Q: Does the Indian canon of construction help the Osage Nation here?
A: The opinion concludes no, because the canon "has force only where a statute is ambiguous" (El Paso Nat. Gas Co.) and the term "other minerals" unambiguously does not include empty pore space.

Q: What about oil and gas in the pore space?
A: Those remain part of the mineral estate. The Osage mineral lease framework continues to govern extraction. The AG opinion is about the empty space, not the oil and gas it contains.

Q: Is this opinion binding?
A: It is binding on State of Oklahoma officials. It is not binding on federal courts or on the Osage Nation. Federal courts have not yet directly addressed pore space ownership under the Osage Act.

Q: Why did the Attorney General recuse?
A: Footnote 10 explains that "the circumstances surrounding this Opinion could 'cause a reasonable person with knowledge of the relevant facts to question [the Attorney General's] impartiality.'" The First Assistant AG executed the opinion under 74 O.S. § 28. The footnote does not specify the exact reason for the recusal.

Background and statutory framework

The legal status of land in Osage County is the product of a series of nineteenth-century federal acts. In 1872, Congress allowed the Osage Nation to occupy what became Osage County. In 1890, Osage County became part of the new Oklahoma Territory. In 1907, Oklahoma achieved statehood, and the Osage reservation was incorporated into the new state as Osage County.

Twelve days after the Oklahoma Enabling Act, Congress enacted the Osage Allotment Act. The Act allotted the surface estate to Osage tribal members, with "oil, gas, coal, or other minerals" reserved as a federal trust asset for the benefit of the tribe. Surface allottees received "the right to manage, control, and dispose of [their] lands the same as any citizen of the United States," including "the right to use and to lease said lands for farming, grazing, or any other purpose not otherwise specifically provided" in the Act. As the Supreme Court summarized in Choteau v. Burnet, the allottee was "competent to hold and make any use (except to grant mining leases) of all his lands."

Federal and state law share the same operating framework on pore space. The Tenth Circuit in Millsap v. Andrus (1983) defined "other minerals" in the Osage Act context as "inorganic substances that can be removed from the soil and used for commercial purposes." The Supreme Court in Watt v. W. Nuclear, Inc. (1983) similarly defined statutory mineral reservations as "substances that are mineral in character (i.e., that are inorganic), that can be removed from the soil, that can be used for commercial purposes." Pore space is not a substance and cannot be removed from the soil; it is empty space.

Oklahoma codified the American rule in 2011: "[U]ntil title to the pore space or rights, interests or estates in the pore space are separately transferred, pore space is property of the person or persons holding title to the land surface above it." 60 O.S. § 6(B)(2). The "rights associated with . . . oil or gas" remain with the mineral estate.

The opinion's tribal-jurisdiction analysis hinges on Montana v. United States, Plains Commerce Bank, and Castro-Huerta. Those cases establish that tribal regulatory authority over non-Indian fee land is limited and does not extend to dictating land ownership rules. The Osage Nation Code's pore-space provision is, in the AG's view, an attempt to do exactly that.

Citations and references

Federal statutes:
- Osage Allotment Act, 34 Stat. 539 (June 28, 1906)
- Act of October 6, 1964, Pub. L. No. 88-632, 78 Stat. 1008 (extending federal mineral reservation)

Oklahoma statutes:
- 60 O.S. § 6 (Pore space ownership)

Tribal code:
- 22 ONC § 2-104 (Osage Nation pore space provision)

Federal regulations:
- 25 C.F.R. § 211.3 (BIA mineral definitions)

Cases:
- Millsap v. Andrus, 717 F.2d 1326 (10th Cir. 1983)
- United States v. Osage Wind, LLC, 871 F.3d 1078 (10th Cir. 2017)
- Watt v. Western Nuclear, Inc., 462 U.S. 36 (1983)
- Plains Commerce Bank v. Long Family Land & Cattle Co., 554 U.S. 316 (2008)
- Oklahoma v. Castro-Huerta, 597 U.S. 629 (2022)
- Choteau v. Burnet, 283 U.S. 691 (1931)
- McGirt v. Oklahoma, 591 U.S. 894 (2020)

Source

• Landing page: https://oklahoma.gov/oag/opinions/ag-opinions/2025/ag-opinion-2025-14.html
• Original PDF: https://oklahoma.gov/content/dam/ok/en/oag/opinions/ag-opinions/2025/AG%20Opinion%202025-14%202025%20OK%20AG%2014.pdf

Original opinion text

OFFICE OF THE ATTORNEY GENERAL
STATE OF OKLAHOMA
ATTORNEY GENERAL OPINION
2025-14

The Honorable Dave Rader
Oklahoma State Senate, District 39
2300 N. Lincoln Blvd., Room 233
Oklahoma City, OK 73105

October 31, 2025

Dear Senator Rader:

This office has received your request for an Attorney General Opinion in which you ask, in effect, the following questions:

1. Did the Osage Allotment Act of 1906, ch. 3572, 34 Stat. 539, transfer title to pore space to individual members of the Osage Nation as part of the surface estate, or is pore space part of the "oil, gas, coal, or other minerals" held in trust by the United States for the benefit of the Osage Nation?
2. Is title to pore space in Osage County currently governed by Oklahoma law or Osage tribal law?

I. SUMMARY

Pore space is "any interstitial space not occupied by soil or rock, within the solid material of the earth, and any cavity, hole, hollow or void space within the solid material of the earth." 60 O.S.2021, § 6(B)(1). Pore space can be used in carbon capture and storage operations, which "remove[] carbon dioxide (CO2) from emission sources and store[] it in [the] pore space far beneath the land's surface," to store water, and for other purposes. This Opinion considers whether the right to use and lease pore space belongs to the surface estate or the mineral estate after the pore space is depleted of any oil, gas, or other substances that are naturally present within the pore space.

When mineral rights are held in trust by the United States for the benefit of an Indian tribe, we must apply "the general rule that statutes passed for the benefit of dependent Indian tribes . . . are to be liberally construed [with] doubtful expressions being resolved in favor of the Indians." While conflicting state law will be preempted by federal law, federal courts typically look to state law to determine property issues. Under Oklahoma law, pore space "is [the] property of the person or persons holding title to the land surface above it." 60 O.S.2021, § 6(B)(2). But the surface estate's interest in pore space does not include "rights associated with . . . oil or gas."

With these principles in mind, you ask whether the Osage Allotment Act ("Osage Act") transferred ownership of pore space in Osage County from the United States to individual members of the Osage Nation. The answer is yes.

You also ask whether ownership of pore space in Osage County is now governed by Oklahoma law or by a recent act of the Osage Nation Congress that purports to make pore space part of the mineral estate. Compare 60 O.S.2021, § 6, with 22 ONC § 2-104 (2024). Oklahoma law governs the question of whether pore space is a part of the mineral or surface estate.

II. BACKGROUND

The analysis of these issues must start with the treaties between the Osage Nation and the United States, along with Congressional acts that divided land ownership into a mineral estate held in trust by the United States for the benefit of the tribe and a surface estate granted to individual Osage allottees. The Osage Act's mineral estate comprises "oil, gas, coal, or other minerals covered by the lands," while the surface estate holds the right to use the land for "any other purpose not otherwise specifically provided for" in that Act. As explained below, the broad grant of control to the surface owners presumptively includes the right to use or lease pore space, which Congress did not include in the Osage mineral estate.

Recently, the Osage Nation attempted to limit the broad rights granted to surface-estate owners under the Osage Act. Specifically, the Osage Minerals Council passed a Resolution on April 1, 2022, claiming that pore space is part of the mineral estate held by the United States in trust for the tribe. Then, at the request of the Minerals Council, the Osage Nation Congress enacted Bill Number ONCA 25-21, which asserts that "[p]ore space in all strata underlying the surface of lands and waters within the Osage Reservation is and always has been an integral part of the Osage Mineral Estate. Title to pore space cannot be and has not been severed from title to the Osage Mineral Estate." 22 ONC § 2-104 (2024).

This Opinion concludes that the Osage mineral estate does not include pore space. Federal caselaw, federal regulations, and state law all support the conclusion that the Osage Act transferred ownership of pore space from the United States to the individual members of the Osage Nation. Applying these same principles, Oklahoma law governs whether pore space is currently part of the mineral estate or the surface estate. Thus, title 22, section 2-104 of the Osage Nation Code does not divest surface owners in Osage County of the right to use and lease pore space.

III. DISCUSSION

A. The Osage Act Transferred Title to Pore Space from the United States to the Individual Allottees.

Federal courts have construed the "other minerals" being held in trust for the Osage Nation in a manner that does not encompass empty space. The Tenth Circuit defined "other minerals" as "inorganic substances that can be removed from the soil and used for commercial purposes." But pore space is not a substance at all, nor can it be removed from the soil; pore space is a "void." Thus, under the Tenth Circuit's interpretation of the Osage Act, empty pore space is not within the Act's reservation of "other minerals."

This conclusion is further supported by the Tenth Circuit's more recent decision in United States v. Osage Wind, LLC, which interpreted federal regulations that implement the Osage Act. In Osage Wind, the Tenth Circuit held that "in practice, owners of the surface estate retain virtually uninhibited use of their lands." A surface owner holding such expansive property rights does not improperly intrude on the mineral estate by "digging a hole in the ground, displacing rock and soil in the process," or otherwise "merely encountering or incidentally disrupting mineral materials." Thus, the right to use or lease empty pore space falls within the surface-estate owners' right to virtually uninhibited use of their land. Any oil or naturally occurring gas in that pore space, however, belongs to the mineral estate, unless separately conveyed.

Oklahoma law also recognizes that pore space presumptively belongs to the owner of the surface estate. The Oklahoma Supreme Court has held in a closely analogous context (saltwater disposal wells) that the surface owner is entitled to lease underground storage rights. This rule is built on the "American rule," which recognizes that "in the absence of language in the severing deed dictating a different construction . . . the cavern is owned by surface owners." The Oklahoma Legislature recognized surface owners' common-law right to pore space by statutory amendment in 2011. See 60 O.S.2011, § 6.

Finally, because the term "other minerals" unambiguously does not include pore space, the Indian canon of construction does not permit a different conclusion. Under the Indian canon, "statutes passed for the benefit of dependent Indian tribes . . . are to be liberally construed [with] doubtful expression being resolved in favor of the Indians." But the Indian canon "has force only where a statute is ambiguous," and "is not . . . 'a license to disregard clear expression of . . . congressional intent.'"

Based on the foregoing authorities, the Osage Act transferred the right to use and lease pore space in Osage County to individual allottees. Pore space in Osage County presumptively belongs to the owner of the surface estate. But, to be clear, owners of the surface estate, or their predecessors in interest, may have relinquished their rights to use and lease pore space by deed or by operation of general property-law rules, including adverse possession or easement by prescription.

B. Present-Day Transfers of Title to Real Property in Osage County Are Controlled by Oklahoma Law, Not Osage Tribal Law.

The Osage Nation Code asserts that pore space "is and always has been an integral part of the Osage Mineral Estate" and that "[a]ny instrument or arrangement that purports to sever title to pore space from title to the Osage Mineral Estate is void." But Oklahoma law provides that "until title to the pore space or rights, interests or estates in the pore space are separately transferred, pore space is property of the person or persons holding title to the land surface above it." As previously noted, neither state nor tribal law directly controls the disposition of property under the Osage Act; that is a matter of federal law. As for transfers of title subsequent to the Osage Act, Oklahoma law controls.

In Montana v. United States, the Supreme Court held that, as a general rule, "the inherent sovereign powers of an Indian tribe do not extend to the activities of nonmembers of the tribe." But the Court recognized two exceptions to that general rule. Yet, even with those exceptions, the Court was careful to note that "[a]fter a State enters the Union, title to the land is governed by state law."

Almost thirty years after Montana, the Supreme Court took up the question of "whether land ownership and sale are 'activities'" that are subject to tribal regulation under the first Montana exception. In Plains Commerce Bank, the Court left no doubt that tribal regulation of ownership or alienation of nonmember fee land is "beyond the tribe's sovereign power[]" and "runs the risk of subjecting nonmembers to tribal regulatory authority without commensurate consent." The Court held that the exceptions "do[] not permit Indian tribes to regulate the sale [or ownership] of non-Indian fee land."

Oklahoma law presumptively controls who owns pore space pursuant to transfers of title made after the Osage Act. Osage tribal law, including title 22, section 2-104 of the Osage Nation Code, does not control title to pore space because tribes lack authority to regulate ownership or alienation of land held by non-Indians. Thus, Osage law cannot alter a non-Indian's title to pore space on fee land, or to otherwise restrict or set aside conveyances authorized by Oklahoma law. Pore space presumptively belongs to the owner of the surface estate unless separately conveyed.

It is, therefore, the official Opinion of the Attorney General that:

1. The Osage Allotment Act of 1906 transferred title to pore space in Osage County from the United States to individual allottees as part of the surface estate. Accordingly, pore space is presumptively a part of the surface estate.

2. Pore space ownership in Osage County is controlled by federal and Oklahoma law, not Osage tribal law.

AMIE N. ELY
FIRST ASSISTANT ATTORNEY GENERAL
[Attorney General recused; First Assistant executes pursuant to 74 O.S. § 28]

SAMUEL BLACK
ASSISTANT SOLICITOR GENERAL