Chief Justice ROVIRA
delivered the Opinion of the Court.
The following question of law was certified to this court by the United States District Court for the District of Colorado in accordance with C.A.R. 21.1:
Under Colorado law, is the owner of an overriding royalty interest in gas production required to bear a proportionate share of post-production costs, such as processing, transportation, and compression, when the assignment creating the overriding royalty interest is silent as to how post-production costs are to be borne?
The district court provided three examples of post-production costs but left the term “post-production costs” undefined. We recognize that each of the activities posited in the certified question occurs throughout the gas production process. Gas may require processing to remove impurities for market[654]*654ing, and once marketable may be further processed into additional component products. Transportation is required when gas is moved from the wellhead to a central location to prepare it for transmission and consumption, commonly referred to as gathering. See John C. Jacobs, Problems Incident to the Marketing of Gas, 5 Inst, on Oil & Gas L. & Tax’n 271, 273 (1954). If no market for the gas exists near the wellhead, transportation may be required to move the gas to a distant market.1 Compression may be required to create sufficient pressure for the gas to enter a purchaser’s pipeline,2 or compression may occur to transform the gas into additional products.3 The parties understand the nature of the gas production, and agree that there exists a point in the production process when an overriding royalty owner may become obligated to bear a proportionate share of costs. They do not agree when proportionate allocation should occur. Because we cannot anticipate every conceivable type of post-production cost, and whether it occurs before or after a marketable product is obtained, we consider the certified question as if it were posed without the examples.
In addition, our consideration of the certified question is based on our understanding that “the assignment creating the overriding royalty interest” is indeed silent with respect to the allocation of post-production costs. The district court posed the certified question in. general terms, and we answer to provide guidance when an assignment does not address the allocation of post-production costs. Had the district court wanted this court to consider the assignment from the Garmans to Lee A. Adams, who eventually assigned the leases to Conoco, we believe the question would have been framed to elicit a more specific response. However, the briefs submitted by the parties and amici curiae focused on the general principles of oil and gas law relating to the allocation of post-production costs. Other than a limited request by the Garmans to consider the language of the assignment in this case, they also assumed that the “response to the question should be a statement of the general principles of Colorado law applicable to the issue.” While the Garmans asked us to “address the application of the legal principles announced to the undisputed facts of this case” we decline to do so.4 We believe we can respond appropriately to the district court on the law in Colorado without considering the specific assignment terms.
With this background in mind, limiting our response to those post-production costs undertaken to convert raw gas into a marketable product, and relying on the basic proposition that every oil and gas lease contains an implied covenant to market, we answer the certified question in the negative. We now turn to the facts which provide a foundation for the certified question and our answer.
I
During the years 1951 through 1953, M.B. and B.K. Garman acquired eight federal oil [655]*655and gas leases covering approximately 10,742 acres situated in Rio Blanco County, Colorado (the Leases). Through a series of assignments the Leases were transferred to Cono-co, Inc. (Conoco) subject to a reserved 4.00% overriding royalty interest now owned in equal shares by James P. Garman, Robert D. Garman and Mark Bruce Garman (collectively Garmans).5
The Leases are located in Dragon Trail Unit (Unit) and continue in full force and effect by the production of gas. Conoco operates both the Unit and the Dragon Trail Processing Plant (Plant).6 The Plant is located outside of both the Lease and the Unit boundaries. From the wellhead, gas enters a gathering line for transportation to the Plant. At the Plant, gas from the Unit is processed into three separate products: (1) residue gas; (2) propane; and (3) a combined stream of butane and natural gasoline (the “butane-gasoline stream”). The gross proceeds from the sale of the individual products are greater than the revenues which would have been obtained from the sale of the raw, unprocessed gas at the wellhead. Plant operations are typical of processing operations performed to enhance the value of gas. The parties have not stipulated as to the reasonableness of the processing costs.
Historically, Conoco has deducted the cost of certain post-production operations from the overriding royalty payments due to the Garmans.7 From January, 1987 until April, 1993, the Garmans’ proportionate share of post-production costs was $459,511 on overriding royalty payments totaling approximately $2.2 million. In 1993, the Garmans filed an action in federal court requesting declaratory relief to determine the parties’ rights under the original 1956 assignment creating the overriding royalty interest, and an accounting to determine whether post-production charges for the previous six years were properly assessed against their overriding royalty interest.
The Garmans argue post-production costs incurred to convert raw gas into a marketable product should not be charged against nonworking interest owners. Accordingly, they object to Conoco’s deduction, of the costs necessary to make gas from the Leases marketable. The Garmans concede that costs incurred after the gas is made marketable, which actually enhance the value of the gas, should be borne proportionately by all parties benefitted by the operations.8 [656]*656They argue, however, that no evidence exists to show Conoco’s operations increase the actual royalty amount paid to the Garmans. Finally, they argue under the doctrine of expressio unius est exclusio alterius that the assignment creating their overriding royalty prohibits Conoco from assessing post-production costs against their royalty.9
Conoco argues that all post-production costs incurred after gas is severed from the ground and reduced to possession should be borne proportionately by royalty, overriding royalty and working interest owners. Cono-co asserts severance occurs at the wellhead and that all expenses incurred after severance improve or enhance the value of the gas from its natural, unprocessed state. Thus, it claims that royalty and overriding royalty interest owners who benefit from these operations ought to share in the cost of all post-production operations.
II
A
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Chief Justice ROVIRA
delivered the Opinion of the Court.
The following question of law was certified to this court by the United States District Court for the District of Colorado in accordance with C.A.R. 21.1:
Under Colorado law, is the owner of an overriding royalty interest in gas production required to bear a proportionate share of post-production costs, such as processing, transportation, and compression, when the assignment creating the overriding royalty interest is silent as to how post-production costs are to be borne?
The district court provided three examples of post-production costs but left the term “post-production costs” undefined. We recognize that each of the activities posited in the certified question occurs throughout the gas production process. Gas may require processing to remove impurities for market[654]*654ing, and once marketable may be further processed into additional component products. Transportation is required when gas is moved from the wellhead to a central location to prepare it for transmission and consumption, commonly referred to as gathering. See John C. Jacobs, Problems Incident to the Marketing of Gas, 5 Inst, on Oil & Gas L. & Tax’n 271, 273 (1954). If no market for the gas exists near the wellhead, transportation may be required to move the gas to a distant market.1 Compression may be required to create sufficient pressure for the gas to enter a purchaser’s pipeline,2 or compression may occur to transform the gas into additional products.3 The parties understand the nature of the gas production, and agree that there exists a point in the production process when an overriding royalty owner may become obligated to bear a proportionate share of costs. They do not agree when proportionate allocation should occur. Because we cannot anticipate every conceivable type of post-production cost, and whether it occurs before or after a marketable product is obtained, we consider the certified question as if it were posed without the examples.
In addition, our consideration of the certified question is based on our understanding that “the assignment creating the overriding royalty interest” is indeed silent with respect to the allocation of post-production costs. The district court posed the certified question in. general terms, and we answer to provide guidance when an assignment does not address the allocation of post-production costs. Had the district court wanted this court to consider the assignment from the Garmans to Lee A. Adams, who eventually assigned the leases to Conoco, we believe the question would have been framed to elicit a more specific response. However, the briefs submitted by the parties and amici curiae focused on the general principles of oil and gas law relating to the allocation of post-production costs. Other than a limited request by the Garmans to consider the language of the assignment in this case, they also assumed that the “response to the question should be a statement of the general principles of Colorado law applicable to the issue.” While the Garmans asked us to “address the application of the legal principles announced to the undisputed facts of this case” we decline to do so.4 We believe we can respond appropriately to the district court on the law in Colorado without considering the specific assignment terms.
With this background in mind, limiting our response to those post-production costs undertaken to convert raw gas into a marketable product, and relying on the basic proposition that every oil and gas lease contains an implied covenant to market, we answer the certified question in the negative. We now turn to the facts which provide a foundation for the certified question and our answer.
I
During the years 1951 through 1953, M.B. and B.K. Garman acquired eight federal oil [655]*655and gas leases covering approximately 10,742 acres situated in Rio Blanco County, Colorado (the Leases). Through a series of assignments the Leases were transferred to Cono-co, Inc. (Conoco) subject to a reserved 4.00% overriding royalty interest now owned in equal shares by James P. Garman, Robert D. Garman and Mark Bruce Garman (collectively Garmans).5
The Leases are located in Dragon Trail Unit (Unit) and continue in full force and effect by the production of gas. Conoco operates both the Unit and the Dragon Trail Processing Plant (Plant).6 The Plant is located outside of both the Lease and the Unit boundaries. From the wellhead, gas enters a gathering line for transportation to the Plant. At the Plant, gas from the Unit is processed into three separate products: (1) residue gas; (2) propane; and (3) a combined stream of butane and natural gasoline (the “butane-gasoline stream”). The gross proceeds from the sale of the individual products are greater than the revenues which would have been obtained from the sale of the raw, unprocessed gas at the wellhead. Plant operations are typical of processing operations performed to enhance the value of gas. The parties have not stipulated as to the reasonableness of the processing costs.
Historically, Conoco has deducted the cost of certain post-production operations from the overriding royalty payments due to the Garmans.7 From January, 1987 until April, 1993, the Garmans’ proportionate share of post-production costs was $459,511 on overriding royalty payments totaling approximately $2.2 million. In 1993, the Garmans filed an action in federal court requesting declaratory relief to determine the parties’ rights under the original 1956 assignment creating the overriding royalty interest, and an accounting to determine whether post-production charges for the previous six years were properly assessed against their overriding royalty interest.
The Garmans argue post-production costs incurred to convert raw gas into a marketable product should not be charged against nonworking interest owners. Accordingly, they object to Conoco’s deduction, of the costs necessary to make gas from the Leases marketable. The Garmans concede that costs incurred after the gas is made marketable, which actually enhance the value of the gas, should be borne proportionately by all parties benefitted by the operations.8 [656]*656They argue, however, that no evidence exists to show Conoco’s operations increase the actual royalty amount paid to the Garmans. Finally, they argue under the doctrine of expressio unius est exclusio alterius that the assignment creating their overriding royalty prohibits Conoco from assessing post-production costs against their royalty.9
Conoco argues that all post-production costs incurred after gas is severed from the ground and reduced to possession should be borne proportionately by royalty, overriding royalty and working interest owners. Cono-co asserts severance occurs at the wellhead and that all expenses incurred after severance improve or enhance the value of the gas from its natural, unprocessed state. Thus, it claims that royalty and overriding royalty interest owners who benefit from these operations ought to share in the cost of all post-production operations.
II
A
“The fundamental purpose of an oil and gas lease is to provide for the exploration, development, production and operation of the property for the mutual benefit of the lessor and lessee.” Davis v. Cramer, 808 P.2d 358, 360 (Colo.1991). The lessor relinquishes its right to the mineral estate in exchange for a smaller non-risk and non-cost bearing royalty interest10 in any minerals discovered. See Wood v. TXO Production Co., 854 P.2d 880, 882-83 (Okla.1992) (“The lessor, who generally owns the minerals, grants an oil and gas lease, retaining a smaller interest, in exchange for the risk-bearing working interest receiving the larger share of the proceeds.”). Similar to a royalty, an overriding royalty is an interest in oil and gas produced at the surface, free of expense of production, generally assessed in addition to the usual mineral owner’s royalty. See, e.g., 8 Williams & Meyers at 859; see also Hagood v. Heckers, 182 Colo. 337, 347, 513 P.2d 208, 214 (1973). While the lease agreement creates the royalty obligation, overriding royalty interests are typically reserved or created by separate agreement.11 See 2 Williams & Meyers § 418. Though their contractual origins may differ, both royalty and overriding royalty interests are non-risk and non-cost bearing interests. See id. § 418.1 (“An overriding royalty is, first and foremost, a royalty interest.... it is an interest in oil and gas produced at the surface, [657]*657free of the expense of production.”). Naturally, the contracting parties are free to allocate the costs of compression, transportation and processing in their agreements. E.g., Magnetic Copy Serv. v. Seismic Specialist, Inc., 805 P.2d 1161, 1163 (Colo.App.1990). Often, however, these agreements fail to apportion expenses that may be incurred after the discovery of oil or gas.12
Though a lease is entered into for the mutual benefit of the parties, not all parties participate equally in lease development decisions. Royalty and overriding royalty interest owners (nonworking interest owners) defer to the risk-bearing parties (working interest owners) to decide where and when to drill, the formations to be tested and ultimately whether to complete a well and establish production.13 Here, Conoco objects to the Garmans’ desire to get a “free-ride” on certain costs incurred after the gas is brought to the surface. We believe, however, that the relationship between the parties specifically provides for a “free-ride” on costs incurred to establish marketable production..
B
No consensus exists regarding the allocation of expenses incurred after the discovery of gas. See 3 Kuntz § 40.5.14 Two lines of cases have developed in the oil producing states based upon differing views of when production is established and a royalty interest accrues. Texas and Louisiana have adopted the rule that nonoperating interests must bear their proportionate share of costs incurred after gas is severed at the wellhead. See, e.g., Dancinger Oil & Refineries v. Hamill Drilling Co., 171 S.W.2d 321 (Tex.1943); Martin v. Glass, 571 F.Supp. 1406, 1415 [658]*658(N.D.Tex.1983) (“Under the law of Texas, gas is ‘produced’ when it is severed from the land at the wellhead.”), aff'd 736 F.2d 1524 (5th Cir.1984); see also Merritt v. Southwestern Elec. Power Co., 499 So.2d 210 (La.Ct.App.1986) (under Louisiana’s reconstruction approach royalty payments are calculated by deducting costs incurred after gas reaches the wellhead). Conoco argues this interpretation best reflects the obligations of the parties.
In Kansas and Oklahoma a contrary rule has developed based on an operator’s implied duty to market gas produced under an oil and gas lease. Wood v. TXO Production Corp., 854 P.2d 880, 882 (Okla.1992) (“[T]he implied duty to market means a duty to get the product to the place of sale in marketable form.”); Gilmore v. Superior Oil Company, 192 Kan. 388, 388 P.2d 602, 606 (1964) (“Kansas has always recognized the duty of the lessee under an oil and gas lease not only to flnd if there is oil and gas but to use reasonable diligence in finding a market for the product.”).15 Wyoming has codified the marketability approach.16 The Federal government also requires that a lessee “place gas in marketable condition at no cost to the Federal Government....” 30 C.F.R. § 206.153© (1993).17
Arkansas and North Dakota have reached similar conclusions when considering lease royalty clauses which are silent as to allocation of post-production costs. A lease which provides for the lessor to receive “proceeds at the well for all gas” means gross proceeds when the lease is silent as to how post-production costs must be borne. Hanna Oil & Gas Co. v. Taylor, 297 Ark. 80, 759 S.W.2d 563, 565 (1988); see also West v. Alpar Resources, Inc., 298 N.W.2d 484, 491 (N.D.1980) (when the lease does not state otherwise lessors are entitled to royalty payments based on percentage of total proceeds received by the lessee, without deduction for costs).
The Garmans and Amici Curiae18 advance three separate theories to support their claim [659]*659that post-production costs are the sole responsibility of the lessee. The Garmans rely on the implied covenant to market which they assert imposes all costs associated with producing a marketable product on the lessee. The National Association of Royalty Owners urges us to consider the typical oil and gas lease habendum clause19 which generally requires production of oil or gas in paying quantities in order to extend the lessee’s interest. Finally, a group of southwestern Colorado landowners argue production activities do not cease until the operator obtains a marketable product that can be delivered to a pipeline.20 While the parties have approached the question from slightly different angles, the common thread connecting these theories is the existence of an obligation upon the lessee, as the party charged with lease development, to complete all operations necessary to market the gas produced from the leasehold. This obligation is captured in what we have previously identified as the implied covenant to market.21
In Colorado we have recognized four implied covenants in oil and gas leases: to drill; to develop after discovery of oil and gas in paying quantities; to operate diligently and prudently; and to protect leased premises against drainage. Davis v. Cramer, 808 P.2d 358 (Colo.1991); Mountain States Oil v. Sandoval, 109 Colo. 401, 125 P.2d 964 (1942); see also Gillette v. Pepper Tank Co., 694 P.2d 369 (Colo.App.1984). “Embodied in the duty to operate diligently and prudently is the implied covenant to market.” Davis, 808 P.2d at 358. In Davis we explained the covenant obligates the lessee to engage in marketing efforts which “would be reasonably expected of all operators of ordinary prudence, having regard to the interests of both lessor and lessee.” Id. at 363 (quoting Gillette, 694 P.2d at 372).
Conoco argues that the implied covenant to market exists separately from the allocation of marketing costs. We disagree. Implied lease covenants related to operations typically impose a duty on the oil and gas lessee. See, e.g., 5 Kuntz §§ 57.1 to 62.5.22 Accordingly, the lessee bears the costs of ensuring compliance with these promises. Cf. Warfield Natural Gas Co. v. Allen, 261 Ky. 840, 88 S.W.2d 989, 991 (1935) (the lessee has the exclusive right to produce gas and find a market, and pays the expenses of doing both as consideration for its seven-eighths of production). The purpose of an oil and gas lease could hardly be effected if the implied covenant to drill obligated the lessor to pay for his proportionate share of drilling costs. In our view the implied covenant to market obligates the lessee to incur those post-production costs necessary to place gas in a condition acceptable for market. Overriding royalty interest owners are not obligated to share in these costs.23
[660]*660Allocating these costs to the lessee is also traceable to the basic difference between cost bearing interests and royalty and overriding-royalty interest owners. Normally, paying parties have the right to discuss proposed procedures and expenditures and ultimately have the right to disagree with the course of conduct selected by the operator. Under the terms of a standard operating agreement nonoperating working interest owners have the right to go “non-consent” on an operation and be subject to an agreed upon penalty. See A.A.P.L. Form 610-1989 Model Form Operating Agreement Art. Vl.b.ii. This right checks an operator’s unbridled ability to incur costs without full consideration of their economic effect. No such right exists for nonworking interest owners.
We find Conoco’s argument that industry practice allows proportionate allocation of post-production costs unpersuasive. Before one can be bound by industry custom “he must know of it or it must be so universal and well-established that he is presumed to have knowledge of its existence.” Pittman v. Larson Distrib. Co., 724 P.2d 1379, 1384-85 (Colo.App.1986). Further, the parties must have contracted with reference to the custom. Id. Custom and industry practice may be an appropriate consideration when Conoco deals with other oil exploration companies. Cf. Pletchas v. Von Poppenheim, 148 Colo. 127, 130, 365 P.2d 261, 263 (Colo.1961) (Parties engaged in same occupation are presumed to have knowledge of business usage). Often, however, executing an oil and gas lease, or assigning a federal lease won under the previously existing federal lottery system is the extent of a party’s contact with the oil industry. See Piney Woods County Life Sch. v. Shell Oil Co., 726 F.2d 225, 240 (5th Cir.1984) (“[Shell’s] allegation of ‘custom’ is self-serving. The payment of royalties is controlled by lessees, and lessors have no ready means of ascertaining current market value other than to take lessees’ word for it.”), aff'd in part on remand, 905 F.2d 840 (5th Cir.1990). Conoco cannot invoke industry custom to limit the rights of royalty and overriding royalty owners unsophisticated in the intricacies of mineral development.24 While we acknowledge that parties who reserve or create overriding royalty interests may be familiar with the oil and gas industry, no cogent argument exists to treat these nonworking interest owners differently from other royalty owners.25
We do not here impose an additional duty on the lessee or expand the duty previously recognized; the duty to market, always undertaken in good faith, may be limited when compliance would be uneconomical or unreasonable. Davis, 808 P.2d at 362; Davis v. Cramer, 837 P.2d 218, 222 (Colo.App.1992). The marketing obligation does not depend on whether production would be more economic if nonworking interest owners were obligated to share in these post-production costs.
C
Our answer is limited to those post-production costs required to transform raw gas into a marketable product.26 As we explained at the outset, many different types [661]*661of expenses may be involved in the conversion process. Upon obtaining a marketable product, any additional costs incurred to enhance the value of the marketable gas, such as those costs conceded by the Garmans, may be charged against nonworking interest owners.27 To the extent that certain processing costs enhance the value of an already marketable product the burden should be placed upon the lessee to show such costs are reasonable, and that actual royalty revenues increase in proportion with the costs assessed against the nonworking interest.28 We are not, however, called upon today to consider the reasonableness of Conoco’s expenses incurred to process, transport or compress already marketable gas.
For the above reasons our answer to the certified question is that, absent an assignment provision to the contrary, overriding royalty interest owners are not obligated to bear any share of post-production expenses, such as compressing, transporting and processing, undertaken to transform raw gas produced at the surface into a marketable product.
ERICKSON, J., specially concurs.
VOLLACK, J., joins in the special concurrence.