This summary highlights selected information contained elsewhere in this prospectus. This summary may not contain all of the information that may be important to you. You should read carefully this entire prospectus, including “Risk Factors,” and the documents we incorporate by reference into this prospectus. We have provided definitions for the oil and natural gas terms used in this prospectus in the “Glossary of Oil and Natural Gas Terms” included in this prospectus.

      We are an independent oil and natural gas company engaged in exploitation, acquisition, exploration and production activities primarily in the Permian Basin, Rocky Mountains, Mid-Continent, Gulf Coast and Michigan regions of the United States.

      Since our inception in 1980, we have built a strong asset base and achieved steady growth through both property acquisitions and exploitation activities. During 2005, we have completed four separate acquisitions of producing properties for an aggregate purchase price of $897.7 million. The proved reserves of the acquired properties are estimated to be approximately 801.9 Bcfe as of the acquisition effective dates, representing an average cost of $1.12 per Mcfe of estimated proved reserves acquired. As of July 1, 2005 and on a pro forma basis for these acquisitions, our estimated proved reserves totaled 1,642.6 Bcfe, representing an 89.8% increase in our proved reserves since January 1, 2005. Our pro forma estimated September 2005 average daily production was 238.0 MMcfe/d, representing a 26.7% increase over our December 2004 average daily production and implying a pro forma average reserve life of approximately 18.9 years.

      The following table summarizes our pro forma estimated proved reserves and pre-tax PV10% value in our core areas and our pro forma standardized measure of discounted future net cash flows as of July 1, 2005, in each case giving effect to our acquisitions of the Postle properties and the North Ward Estes and ancillary properties, which closed on October 4, 2005, as if such acquisitions had occurred as of July 1, 2005, and our pro forma estimated September 2005 average daily production, giving effect to our acquisition of the North Ward Estes and ancillary properties. Pro forma September 2005 average daily production includes the actual production for the North Ward Estes and ancillary properties during September 2005, which was prior to our acquisition of these properties.

      We expect to continue to build on our successful acquisition track record and seek property acquisitions that complement our existing core properties. Additionally, we believe that our significant drilling inventory, combined with our operating experience and efficient cost structure, provide us with significant organic growth opportunities. We have budgeted approximately $180 million for development drilling capital expenditures in 2005. Through September 30, 2005, we have invested $114.6 million of our budgeted expenditures for the drilling of 171 gross (78.4 net) wells with 150 productive wells, representing an 88% success rate. Based on current availability and access to drilling rigs in our areas of operations, we anticipate significant drilling activity during the remainder of the year.

      In 2005, we acquired from Celero Energy, LP the operated interests in two producing oil and gas fields as well as positions in several other smaller fields, totaling 734.0 Bcfe of estimated proved reserves. On August 4, 2005, we acquired properties in the Postle Field, located in the Oklahoma Panhandle, and on October 4, 2005, we acquired properties in the North Ward Estes Field and certain other smaller fields, located in the Permian Basin.

      The effective date of both acquisitions was July 1, 2005. The total purchase price was approximately $802.2 million comprised of $343 million in cash paid at the closing of the Postle properties and $442 million in cash paid at the closing of the North Ward Estes properties along with 441,500 shares of our common stock. We funded the acquisition of the Postle properties through borrowings under the credit agreement of Whiting Oil and Gas Corporation, our wholly-owned subsidiary. We funded the acquisition of the North Ward Estes properties with the net proceeds from the private placement of our 7% Senior Subordinated Notes due 2014 and our common stock offering, both of which closed on October 4, 2005.

      Postle Field. The Postle Field, located in Texas County, Oklahoma, includes five producing units and one producing lease covering a total of approximately 25,600 gross acres (24,223 net) with working interests of 94% to 100%. Three of the units are currently under CO₂ enhanced recovery projects. There are currently 88 producing wells and 78 injection wells completed in the Morrow zone at 6,100 feet. The Postle properties produced at an estimated average net daily rate of 4,122 barrels of oil (including NGLs) and 1,025 Mcf of natural gas during the month of September 2005. In the Postle Field, the estimated proved reserves are 53% PDP, 4% PDNP and 43% PUD.

      The Postle Field was initially developed in the early 1960’s and unitized for waterflood in 1967. Enhanced recovery projects using CO₂ were initiated in 1995 and continue in three of the five units. We plan to expand the current CO₂ projects into the rest of the units. These expansion projects include the restoration of shut-in

      In connection with the acquisition of the Postle properties, we acquired 100% ownership of the Dry Trails Gas Plant located in the Postle field. This gas processing plant separates CO₂ gas from the produced wellhead mixture of hydrocarbon and CO₂ gas, so that the CO₂ gas can be reinjected into the producing formation. Plans are underway to increase the plant capacity from its current capacity of 43 MMcf/d to 83 MMcf/d by 2007 to support the expanded CO₂ injection projects.

      We also acquired a 60% interest in the 120 mile TransPetco operated CO₂ transportation pipeline serving the Postle Field, thereby assuring the delivery of CO₂ at a fair tariff. A long-term CO₂ purchase agreement was recently executed with a major integrated oil and gas company to provide the necessary CO₂ for the expansion planned in the field.

      North Ward Estes. The North Ward Estes Field includes six base leases with 100% working interest in 58,000 gross and net acres in Ward and Winkler Counties, Texas. There are currently approximately 580 producing wells and 180 injection wells. The Yates formation at 2,600 feet is the primary producing zone with additional production from other zones including the Queen at 3,000 feet. As part of this acquisition, we also acquired the rights to deeper horizons under 34,590 gross acres in the North Ward Estes Field. The North Ward Estes properties produced at an estimated average net daily rate of 4,185 barrels of oil (including NGLs) and 2,974 Mcf of natural gas during the month of September 2005. In the North Ward Estes Field, the estimated proved reserves are approximately 16% PDP, 17% PDNP and 67% PUD.

      The North Ward Estes Field was initially developed in the 1930’s and full scale waterflooding was initiated in 1955. A CO₂ enhanced recovery project was implemented in the core of the field in 1989, but was terminated in 1996 after a successful top lease by a third party. We plan to expand the waterflood operations in the field as well as reinitiate a CO₂ project in 2007.

      Included in the North Ward Estes acquisition were interests in certain other fields encompassing approximately 51,200 gross acres (33,000 net). These other fields include approximately 800 oil and gas wells within the Permian Basin of Texas and New Mexico. These properties produced at an estimated average net daily rate of 800 barrels of oil (including NGLs) and 1,898 Mcf of natural gas during the month of September 2005. Combining the North Ward Estes and other fields, the estimated proved reserves of 492.5 Bcfe are approximately 20% PDP, 16% PDNP and 64% PUD.

      Low Cost Acquisition in Core Operational Areas. Based on the purchase price of approximately $802.2 million and estimated proved reserves of 734.0 Bcfe as of the effective date of the acquisitions, we acquired the Celero properties for approximately $1.09 per Mcfe of estimated proved reserves. With the acquisition of the North Ward Estes properties, we added estimated proved reserves of 492.5 Bcfe to our Permian Basin region, making it our largest region comprising 46.5% of our pro forma total estimated proved reserves as of July 1, 2005, up from 32% as of January 1, 2005. Our addition of the Postle Field estimated proved reserves of 241.5 Bcfe increased our Mid-Continent region reserves to 17.3% of our pro forma total estimated proved reserves as of July 1, 2005, up from 3% as of January 1, 2005.

      Additional Near-Term Celero Property Development Opportunities. The aggregate estimated total proved reserves for the Celero properties are approximately 31% PDP, 12% PDNP and 57% PUD. An active development program is underway, and we expect to commit to capital expenditures of approximately $197 million from July 2005 through 2006. Total capital expenditures of approximately $533 million, including $166 million for CO₂ purchases, are estimated to be required for future development of the proved reserves. In total, we expect to spend approximately 80% of the $533 million of total development capital over the next 5¹/2 years. The addition of the future $533 million capital expenditures to the approximately $802.2 million acquisition price would yield an all-in acquisition and development cost of $1.82 per Mcfe.

      Integration Plan. We hired 47 of Celero’s field level employees, many of whom have extensive experience in the acquired fields. In addition, we assumed Celero’s Midland, Texas, office lease and hired 27 of Celero’s technical and support office staff. We expect that the acquired properties will continue to be operated and managed by the current personnel and the ongoing development activity to continue without

      Institutional Partnerships Interests. On June 23, 2005, we completed our acquisition of all of the limited partnership interests in three institutional partnerships managed by our wholly-owned subsidiary Whiting Programs, Inc. The purchase price was $30.5 million for estimated proved reserves of approximately 17.4 Bcfe as of the acquisition effective date, resulting in a cost of $1.75 per Mcfe of estimated proved reserves. The partnership properties are located in Louisiana, Texas, Arkansas, Oklahoma and Wyoming. The average daily production from the properties was 4.0 MMcfe/d as of the effective date of the acquisition. We funded the acquisition using cash on hand.

      Green River Basin. On March 31, 2005, we completed our acquisition of operated interests in five producing gas fields in the Green River Basin of Wyoming. The purchase price was $65.0 million for estimated proved reserves of approximately 50.5 Bcfe as of the acquisition effective date, resulting in a cost of $1.29 per Mcfe of estimated proved reserves. We operate approximately 95% of the average daily production from the properties, which was 6.3 MMcfe/d as of the effective date of the acquisition. We funded the acquisition through borrowings under our wholly-owned subsidiary Whiting Oil and Gas Corporation’s credit agreement.

      Our goal is to generate meaningful growth in both production and free cash flow by investing in oil and gas projects with attractive rates of return on capital employed. To date, we have achieved this goal largely through the acquisition of additional reserves in our core areas. Based on the extensive property base we have built, we now have several economically attractive opportunities to exploit and develop our oil and natural gas properties and explore our acreage positions for production growth and additional proved reserves. Specifically, we have focused, and plan to continue to focus, on the following:

      Developing and Exploiting Existing Properties. Our existing property base and our acquisitions over the past two years have provided us with significant low-risk opportunities for exploitation and development drilling. Including the Celero acquisitions, we currently have an identified drilling inventory of approximately 1,300 gross wells that we believe will add substantial production over the next five years. Our drilling inventory consists largely of the development of our proved undeveloped reserves for which we have spent significant time evaluating the costs and expected results.

      Additionally, we have several opportunities to apply enhanced recovery techniques that we expect will increase proved reserves and extend the productive lives of our mature fields. Once integrated, we anticipate meaningfully increasing production from the Celero properties through the use of secondary and tertiary recovery techniques, including water and CO₂ floods. Our existing expertise, as well as the expertise of the Celero field employees we expect to retain subsequent to the acquisition, should enable us to effectively implement these recovery techniques over the next several years.

      Growing Through Accretive Acquisitions. Since our initial public offering in November 2003, we have announced eleven acquisitions totaling 1.2 Tcfe of estimated total proved reserves. Our experienced team of management, engineering and geoscience professionals has developed and refined an acquisition program designed to increase reserves and complement our existing properties, including identifying and evaluating acquisition opportunities, negotiating and closing purchases, and managing acquired properties. As a result of our disciplined approach, we have achieved significant growth in our core areas at an average cost of $1.16 per Mcfe of proved reserves through these eleven acquisitions.

      Pursuing High-Return Organic Reserve Additions. Our strategy includes the allocation of 10% to 20% of our annual drilling budget to higher risk projects, including step-out development drilling, trend extensions and exploration, that we believe will add substantially to our proved reserves and cash flow. Although exploration

      Disciplined Financial Approach. Our goal is to remain financially strong, yet flexible, through the prudent management of our balance sheet and active management of commodity price volatility. We have historically funded our acquisitions and growth activity through a combination of equity and debt issuances, bank borrowings and internally generated cash flow, as appropriate, to maintain our strong financial position. To support cash flow generation on our existing properties and secure acquisition economics, we periodically enter into derivative contracts. Typically, we use no-cost collars to provide an attractive base commodity price level while maintaining the ability to benefit from improvements in commodity prices.

      We believe that our key competitive strengths lie in our balanced asset portfolio, our experienced management and technical team and our commitment to effective application of new technologies.

      Balanced, Long-Lived Asset Base. As of October 1, 2005 and pro forma for the North Ward Estes acquisition, we had interests in 8,583 productive wells across 1,050,540 gross (483,630 net) developed acres in our five core geographical areas. We believe this geographic mix of properties and organic drilling opportunities, combined with our continuing business strategy of acquiring and exploiting properties in these areas, presents us with multiple opportunities for success in executing our strategy because we are not dependent on any particular producing regions or geological formations. As a result of the Celero acquisitions, we have enhanced the production stability and reserve life of our developed reserves. Additionally, the Celero properties contain identifiable growth opportunities to significantly increase production in the near- and intermediate-term.

      Experienced Management Team. Our management team averages over 25 years of experience in the oil and natural gas industry. Our personnel have extensive experience in each of our core geographical areas and in all of our operational disciplines. In addition, each of our acquisition professionals has at least 20 years of experience in the evaluation, acquisition and operational assimilation of oil and natural gas properties.

      Commitment to Technology. In each of our core operating areas, we have accumulated detailed geologic and geophysical knowledge and have developed significant technical and operational expertise. In recent years, we have developed considerable expertise in conventional and 3-D seismic imaging and interpretation. Our technical team has access to approximately 1,294 square miles of 3-D seismic data, digital well logs and other subsurface information. This data is analyzed with state of the art geophysical and geological computer resources dedicated to the accurate and efficient characterization of the subsurface oil and gas reservoirs that comprise our asset base. Computer applications, such as the WellView^® software system, enable us to quickly generate reports and schematics on our wells. In addition, our information systems enable us to update our production databases through daily uploads from hand held computers in the field. This commitment to technology has increased the productivity and efficiency of our field operations development activities.

      We are engaged in drilling activities throughout our core regions. The following tables set forth the number of productive and non-productive wells we have drilled through September 30, 2005, the estimated number of remaining wells we expect to drill in 2005 and our estimated capital expenditures during 2005 both for our company including the Celero properties from their dates of acquisition and for the Celero properties separately from their dates of acquisition. The information should not be considered indicative of future

      North Ward Estes Field. An active workover and drilling program is underway with five shallow drilling rigs, 15 workover rigs and one intermediate depth rig active in the North Ward Estes Field. Capital expenditures of approximately $417 million are estimated to be required for future development of the North Ward Estes Field, including approximately $127 million for CO₂ purchases, which will be capitalized, and approximately $290 million for tangible and intangible workover and drilling costs.

      An active refracturing program in the Yates formation in the North Ward Estes Field is underway. The new stimulations have been successful in repairing wellbore damage and opening additional pay. Over 100 refracs have been performed during 2005 and they continue at a pace of approximately eight to ten per week. Development projects, including waterflood restoration, infill drilling and lateral extension of the Yates reservoir, are also underway. The waterflood restoration program includes reactivation of shut-in producing wells and injection wells as well as the drilling of new wells to complete waterflood patterns. Additional drilling plans include 10 acre infill wells and step-out wells extending the Eastern edge of the Yates reservoir. Approximately 60 wells have undergone workovers and about 50 new wells have been drilled during 2005.

      Development plans for future years include the reactivation and expansion of the CO₂ flood in the Yates formation, which was active in the field from 1989 thru 1996. The CO₂ development plans are scheduled to begin in 2007 following the restoration and expansion of the waterflood operation.

      The intermediate depth drilling rig is active in the North Ward Estes Field drilling deeper pays, primarily the Penn formation. Three Penn wells have been drilled in 2005, with two on production and the third completing. A fourth Penn well is currently drilling. Other deeper horizons to be tested with additional drilling target the Montoya and Ellenburger.

      Parkway Field. We own a non-operated position in the Parkway (Delaware) Unit located in Eddy County, New Mexico. Production is from three sands within the Brushy Canyon, a sub group of the Delaware. This field is under active waterflood, and the operator is converting the 5 spot flood to a 9 spot pattern. Six wells have been drilled during 2005 and additional drilling is scheduled later in 2005 or early 2006.

      Would Have Field. We have continued development of this field with a total of nine wells drilled during 2005 targeting the Clearfork — Would Have, Dillard and the Canyon Reef. We have purchased additional

      Keystone Field. Currently, two drilling rigs are drilling Wichita Albany test wells in the Keystone Field. We have plans to keep one to two rigs active in the field for the remainder of the year drilling Wichita Albany, Devonian and possibly Ellenburger objectives. We completed a 3-D seismic survey over this field in June 2005 and are using this information to refine these drilling targets and identify additional objectives in this multi-pay field.

      Parks Field. This field is located in Stephens County, Texas and produces from several reservoirs, with primary production from the Caddo Lime at a depth of approximately 3,200 feet. This reservoir in Parks Field was never waterflooded and our plans are to re-drill the wells and install a waterflood. During 2005, we have drilled a total of nine Caddo formation wells. We are in the process of completing these wells.

      Signal Peak Field. We have participated in the drilling of four Wolfcamp wells in the Signal Peak Field during 2005. Two of these wells have been completed, drilling operations on the remaining two wells have just finished and completion operations are underway. Additional drilling is scheduled later in 2005.

      In the Williston Basin of North Dakota and Montana, we are currently operating two rigs capable of drilling new wells. We have also signed a contract for a third rig, which is scheduled to be delivered in October 2005. In addition, we have been utilizing a smaller rig to drill horizontal casing exits and the horizontal sections on existing wells.

      Big Stick (Madison) Unit. During early summer 2005, a 3-D seismic survey was completed over the Big Stick Field. The objective of this survey was to help us better understand the unitized formation, the Madison, and to identify additional deeper drilling opportunities in the Duperow and Red River. In early 2004, the Egly 20-1 well was placed on production as a Red River gas well. In May 2005, the Egly achieved a cumulative production of over one Bcf of gas. Information from the 3-D seismic survey indicates we have additional Red River opportunities in the field.

      Nisku A Drilling Program. During 2005, we have drilled a total of eight casing exit and grassroot horizontal Nisku “A” wells. Currently, we have 14 Nisku wells on production and one is being completed.

      Siberia Ridge Field. In the Siberia Ridge Field, we currently have 21 approved permits. Drilling was initiated in early September 2005. We plan to drill seven wells by the end of 2005. A total of 44 potential infill locations exist on our leases in the Siberia Ridge Field.

      Hiawatha West Field. Early in 2005, three wells were drilled in the Hiawatha West Field. These wells could not be completed at that time due to lease restrictions regarding wildlife in the area. In July 2005, drilling operations resumed, and in mid-August completion operations were initiated. Currently, we have fracture stimulated five of the wells and we are drilling our seventh well. We plan to have drilled and completed a total of ten wells by the end of 2005.

      Postle Field. An active workover and drilling program is underway with two drilling rigs and six workover rigs currently active in the Postle Field. Approximately $111 million of capital expenditures are estimated to be required for future development of the Postle Field, including $39 million for CO₂ purchases, which will be capitalized, and $1 million related to the PDNP reserves, which includes returning wells to production and workovers. Development of the PUD reserves will require an estimated $93 million of capital expenditures, including approximately $22 million of CO₂ purchases. The workovers are targeted at restoring production in shut-in wells and improving production and injection volumes in active wells. New wells are being drilled to optimize patterns in the existing CO₂ projects as well as expand the waterflood and CO₂ floods into additional areas. Work has also commenced to expand the capacity of the Dry Trails Gas Plant to handle the increased volumes of wellhead CO₂ and hydrocarbon gas that will be associated with the expansion plans.

      The following is a brief summary of some of the terms of the new notes. For a more complete description of the terms of the new notes, see “Description of the New Notes” in this prospectus.

      The following summary historical financial information for the year ended December 31, 2004 has been derived from, and is qualified by reference to, our audited consolidated financial statements and related notes. The following summary historical financial information for the nine months ended September 30, 2005 has been derived from, and is qualified by reference to, our unaudited consolidated financial statements and related notes. This information is only a summary and you should read it in conjunction with our financial statements and related notes incorporated by reference in this prospectus. The unaudited interim period financial information, in our opinion, includes all adjustments, which are normal and recurring in nature, necessary for a fair presentation for the periods shown. Results for the nine months ended September 30, 2005 are not necessarily indicative of the results to be expected for the full fiscal year. Our historical financial information includes the results of our recent acquisitions beginning on the following dates: Green River Basin, March 31, 2005; Institutional Partnership Interests, June 23, 2005; and Postle properties, August 4, 2005.

      The following summary unaudited pro forma financial information for the year ended December 31, 2004 and the nine months ended September 30, 2005 has been derived from our unaudited pro forma financial statements and related notes included elsewhere in this prospectus. This information is only a summary and you should read it in conjunction with material contained in “Unaudited Pro Forma Financial Statements” in this prospectus and our and Celero’s historical financial statements and related notes incorporated by reference in this prospectus. This summary unaudited pro forma financial information gives effect to our acquisition of the Green River Basin properties (through March 31, 2005), our acquisition of the Institutional Partnership Interests (though June 23, 2005), our acquisition of the Postle properties (through August 4, 2005), our acquisition of the North Ward Estes properties, our private placement of the old notes, the common stock offering and the use of the net proceeds from the private placement and the common stock offering to pay the cash portion of the purchase price for the acquisition of the North Ward Estes properties and to repay a portion of the debt we incurred in connection with the acquisition of the Postle properties as if such transactions had occurred as of January 1, 2004.

      The following table presents a reconciliation of net income to EBITDA:

      The following tables present summary information regarding our estimated net proved oil and natural gas reserves as of December 31, 2004 and as of July 1, 2005, and our operating data for the year ended December 31, 2004 and the nine months ended September 30, 2005. All calculations of estimated net proved reserves have been made in accordance with the rules and regulations of the SEC and, except as otherwise indicated, give no effect to federal or state income taxes. Our historical operating data includes results from our recent acquisitions beginning on the following dates: Green River Basin, March 31, 2005; Institutional Partnership Interests, June 23, 2005; and Postle properties, August 4, 2005. Our historical reserve data as of July 1, 2005 includes reserves from the Green River Basin and Institutional Partnership Interests acquisitions. The summary pro forma reserve data below gives effect to our acquisition of the Postle properties, which closed on August 4, 2005, and our acquisition of the North Ward Estes properties, which closed on October 4, 2005, as if such acquisitions had occurred as of July 1, 2005. The summary pro forma operating data below gives effect to our acquisitions of the Postle properties (through August 4, 2005), the North Ward Estes properties, and our other recent acquisitions (through their respective acquisition dates), as if such acquisitions had occurred as of January 1, 2004.

      The following table summarizes our pro forma estimated proved reserves in our core areas as of July 1, 2005 and the pro forma future capital expenditures estimated to be required to develop these reserves, in each case giving effect to our acquisitions of the Postle properties and the North Ward Estes and ancillary properties, which closed on October 4, 2005, as if such acquisitions had occurred as of July 1, 2005.

      The following summary historical financial information as for the years ended December 31, 2002, 2003 and 2004 and as of December 31, 2002, 2003 and 2004 has been derived from, and is qualified by reference to, our audited consolidated financial statements and related notes. The following summary historical financial information for the nine months ended September 30, 2004 and 2005 and as of September 30, 2004 and 2005 has been derived from, and is qualified by reference to, our unaudited consolidated financial statements and related notes. This information is only a summary and you should read it in conjunction with our financial statements and related notes incorporated by reference in this prospectus. The unaudited interim period financial information, in our opinion, includes all adjustments, which are normal and recurring in nature, necessary for a fair presentation for the periods shown. Results for the nine months ended September 30, 2005 are not necessarily indicative of the results to be expected for the full fiscal year. Our historical financial information includes the results of our recent acquisitions beginning on the following dates: Green River Basin, March 31, 2005; Institutional Partnership Interests, June 23, 2005; and Postle properties, August 4, 2005. Our historical financial information does not include the results of our acquisition of the North Ward Estes properties, which closed on October 4, 2005.

      The following tables present summary information regarding our estimated net proved oil and natural gas reserves as of December 31, 2002, 2003 and 2004 and as of July 1, 2005 and our historical operating data for the years ended December 31, 2002, 2003 and 2004 and the nine months ended September 30, 2004 and 2005. All calculations of estimated net proved reserves have been made in accordance with the rules and regulations of the SEC and, except as otherwise indicated, give no effect to federal or state income taxes. Our historical operating data includes results from our recent acquisitions beginning on the following dates: Green River Basin, March 31, 2005; Institutional Partnership Interests, June 23, 2005; and Postle properties, August 4, 2005. Our historical reserve data as of July 1, 2005 includes reserves from the Green River Basin and Institutional Partnership Interests acquisitions. Our historical reserve data does not include the results of our acquisition of the Postle Properties, which closed on August 4, 2005, or our acquisition of the North Ward Estes properties, which closed on October 4, 2005.