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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 10-Q

For the quarterly period ended September 30, 2011

OR

For the transition period from                    to                    

Commission File No. 333-113340

MEWBOURNE ENERGY PARTNERS 05-A, L.P.

Registrant’s Telephone Number, including area code: (903) 561-2900

Not Applicable

(Former name, former address and former fiscal year, if changed since last report)

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 of 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days.    Yes  x    No   ¨

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files).    Yes  x    No   ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company in Rule 12b-2 of the Exchange Act. (Check one):

Indicate by check mark if the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act).    Yes  ¨    No  x

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MEWBOURNE ENERGY PARTNERS 05-A, L.P.

INDEX

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MEWBOURNE ENERGY PARTNERS 05-A, L.P.

Part 1 - Financial Information

Item 1. Financial Statements

CONDENSED BALANCE SHEETS

September 30, 2011 and December 31, 2010

The accompanying notes are an integral part of the financial statements.

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MEWBOURNE ENERGY PARTNERS 05-A, L.P.

CONDENSED STATEMENTS OF OPERATIONS

For the three months ended September 30, 2011 and 2010 and

the nine months ended September 30, 2011 and 2010

(Unaudited)

The accompanying notes are an integral part of the financial statements.

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MEWBOURNE ENERGY PARTNERS 05-A, L.P.

CONDENSED STATEMENTS OF CASH FLOWS

For the nine months ended September 30, 2011 and 2010

(Unaudited)

The accompanying notes are an integral part of the financial statements.

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MEWBOURNE ENERGY PARTNERS 05-A, L.P.

CONDENSED STATEMENT OF CHANGES IN PARTNERS’ CAPITAL

For the nine months ended September 30, 2011

(Unaudited)

The accompanying notes are an integral part of the financial statements.

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MEWBOURNE ENERGY PARTNERS 05-A, L.P.

NOTES TO CONDENSED FINANCIAL STATEMENTS

(Unaudited)

1. Description of Business

Mewbourne Energy Partners 05-A, L.P. (the “Registrant” or the “Partnership”), a Delaware limited partnership, is engaged primarily in oil and gas development and production in Texas, Oklahoma, and New Mexico, and was organized on February 14, 2005. The offering of limited and general partnership interests began June 1, 2005 as a part of an offering registered under the name Mewbourne Energy Partners 04-05 Drilling Program, (the “Program”), and concluded July 29, 2005, with total investor contributions of $30,000,000 originally being sold to 1,128 subscribers of which $26,844,000 were sold to 998 subscribers as general partner interests and $3,156,000 were sold to 130 subscribers as limited partner interests. During 2007, all general partner equity interests were converted to limited partner equity interests. In accordance with the laws of the State of Delaware, Mewbourne Development Corporation (“MD”), a Delaware Corporation, has been appointed as the Partnership’s managing general partner. MD has no significant equity interest in the Partnership.

2. Summary of Significant Accounting Policies

Reference is hereby made to the Registrant’s Annual Report on Form 10-K for 2010, which contains a summary of significant accounting policies followed by the Partnership in the preparation of its financial statements. These policies are also followed in preparing the quarterly report included herein.

In the opinion of management, the accompanying unaudited financial statements contain all adjustments of a normal recurring nature necessary to present fairly our financial position, results of operations, cash flows and partners’ capital for the periods presented. The results of operations for the interim periods are not necessarily indicative of the final results expected for the full year.

3. Accounting for Oil and Gas Producing Activities

The Partnership follows the full-cost method of accounting for its oil and gas activities. Under the full-cost method, all productive and non-productive costs incurred in the acquisition, exploration and development of oil and gas properties are capitalized. Depreciation, depletion and amortization of oil and gas properties subject to amortization is computed on the units-of-production method based on the proved reserves underlying the oil and gas properties. At September 30, 2011 and 2010, all capitalized costs were subject to amortization. Proceeds from the sale or other disposition of properties are credited to the full cost pool. Gains and losses on the sale or other disposition of properties are not recognized unless such adjustments would significantly alter the relationship between capitalized costs and the proved oil and gas reserves. Capitalized costs are subject to a quarterly ceiling test that limits such costs to the aggregate of the present value of future net cash flows of proved reserves and the lower of cost or fair value of unproved properties. There were no cost ceiling write-downs for the three or nine months ended September 30, 2011 or 2010.

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4. Asset Retirement Obligations

The Partnership has recognized an estimated liability for future plugging and abandonment costs. The estimated liability is based on historical experience and estimated well life. The liability is discounted using the credit-adjusted risk-free rate. Revisions to the liability could occur due to changes in well ownership interests or well plugging and abandonment costs, or if federal or state regulators enact new well restoration requirements. The Partnership recognizes accretion expense in connection with the discounted liability over the remaining life of the well.

A reconciliation of the Partnership’s liability for well plugging and abandonment costs for the nine months ended September 30, 2011 and the year ended December 31, 2010 is as follows:

5. Related Party Transactions

In accordance with the laws of the State of Delaware, Mewbourne Development Corporation (“MD”), a Delaware Corporation, has been appointed as the Partnership’s managing general partner. MD has no significant equity interest in the Partnership. Mewbourne Oil Company (“MOC”) is operator of oil and gas properties owned by the Partnership. Mewbourne Holdings, Inc. is the parent of both MD and MOC. Substantially all transactions are with MD and MOC.

In the ordinary course of business, MOC will incur certain costs that will be passed on to owners of the well for which the costs were incurred. The Partnership will receive their portion of these costs based upon their ownership in each well incurring the costs. These costs are referred to as operator charges and are standard and customary in the oil and gas industry. Operator charges include recovery of gas marketing costs, fixed rate overhead, supervision, pumping, and equipment furnished by the operator, some of which will be included in the full cost pool pursuant to Rule 4-10(c)(2) of Regulation S-X. Services and operator charges are billed in accordance with the program and partnership agreements.

In general, during any particular calendar year the total amount of administrative expenses allocated to the Partnership by MOC shall not exceed the greater of (a) 3.5% of the Partnership’s gross revenue from the sale of oil and natural gas production during each year (calculated without any deduction for operating costs or other costs and expenses) or (b) the sum of $50,000 plus .25% of the capital contributions of limited and general partners.

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The Partnership participates in oil and gas activities through the Program. The Partnership and MD are the parties to the Program, and the costs and revenues are allocated between them as follows:

Item 2. Management’s Discussion and Analysis of Financial Condition and Results of Operations

Liquidity and Capital Resources

Mewbourne Energy Partners 05-A, L.P. was formed February 14, 2005. The offering of limited and general partnership interests began June 1, 2005 and concluded July 29, 2005, with total investor contributions of $30,000,000. During 2007, all general partner equity interests were converted to limited partner equity interests.

Future capital requirements and operations will be conducted with available funds generated from oil and gas activities. No bank borrowing is anticipated. The Partnership had net working capital of $411,892 at September 30, 2011.

During the nine months ended September 30, 2011, the Partnership made cash distributions to the investor partners in the amount of $1,475,387 as compared to $2,677,926 for the nine months ended September 30, 2010. The Partnership expects that cash distributions will continue during 2011 as additional oil and gas revenues are sufficient to produce cash flows from operations.

The sale of crude oil and natural gas produced by the Partnership will be affected by a number of factors that are beyond the Partnership’s control. These factors include the price of crude oil and natural gas, the fluctuating supply of and demand for these products, competitive fuels, refining, transportation, extensive federal and state regulations governing the production and sale of crude oil and natural gas, and other competitive conditions. It is impossible to predict with any certainty the future effect of these factors on the Partnership.

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Results of Operations

For the three months ended September 30, 2011 as compared to the three months ended September 30, 2010:

Oil and gas revenues. As shown in the above table, total oil and gas sales decreased by $186,454 a 22.0% decline, for the three months ended September 30, 2011 as compared to the three months ended September 30, 2010.

Of this decline, $60,747 and $277,168 were due to decreases in the volumes of oil and gas sold, respectively. The volumes sold decreased by 712 barrels (bbls) and 56,165 thousand cubic feet (mcf) for the three months ended September 30, 2011 as compared to the three months ended September 30, 2010. The decreases in volumes of oil and gas sold were primarily due to normal declines in production, which in some wells were substantial.

These decreases were partially offset by increases of $45,934 and $105,527 due to increases in the average prices of oil and gas sold, respectively. Average prices rose to $85.32 from $67.31 per bbl and to $4.93 from $4.27 per mcf for the three months ended September 30, 2011 as compared to the three months ended September 30, 2010.

Lease operations. Lease operating expense during the three month period ended September 30, 2011 decreased to $92,604 from $102,721 for the three months ended September 30, 2010 due to fewer well repairs and workovers.

Production taxes. Production taxes during the three month period ended September 30, 2011 decreased to $47,669 from $60,259 for the three month period ended September 30, 2010 due to lower overall oil and gas revenue.

Administrative and general expense. Administrative and general expense decreased to $33,491 for the three month period ended September 30, 2011 from $49,026 for the three month period ended September 30, 2010 due to decreased administrative expenses allocable to the Partnership and lower general expenses for reporting costs.

Depreciation, depletion and amortization. Depreciation, depletion and amortization for the three month period ended September 30, 2011 decreased to $104,089 from $156,280 for the three month period ended September 30, 2010 due to the decreased production volumes for the three month period ended September 30, 2011.

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Results of Operations

For the nine months ended September 30, 2011 as compared to the nine months ended September 30, 2010:

Oil and gas revenues. As shown in the above table, total oil and gas sales decreased by $1,255,435, a 38.1% decline, for the nine months ended September 30, 2011 as compared to the nine months ended September 30, 2010.

Of this decline, $240,695 and $1,078,454 were due to decreases in the volumes of oil and gas sold, respectively. The volumes sold decreased by 2,647 bbls and 220,696 mcf for the nine months ended September 30, 2011 as compared to the nine months ended September 30, 2010. The decreases in volumes of oil and gas sold were primarily due to normal declines in production, which in some wells were substantial.

Also included in this decline was $77,620 due to a decrease in the average price of gas sold. The average price fell to $4.89 from $5.03 per mcf for the nine months ended September 30, 2011 as compared to the nine months ended September 30, 2010.

These declines were partially offset by an increase of $141,334 due to an increase in the average price of oil sold. The average price rose to $90.93 from $73.56 per bbl for the nine months ended September 30, 2011 as compared to the nine months ended September 30, 2010.

Lease operations. Lease operating expense during the nine month period ended September 30, 2011 decreased to $313,001 from $320,097 for the nine months ended September 30, 2010 due to fewer well repairs and workovers.

Production taxes. Production taxes during the nine month period ended September 30, 2011 decreased to $143,192 from $246,291 for the nine month period ended September 30, 2010 due to lower overall oil and gas revenue.

Administrative and general expense. Administrative and general expense decreased to $106,929 for the nine month period ended September 30, 2011 from $155,434 for the nine month period ended September 30, 2010 due to decreased administrative expenses allocable to the Partnership and lower general expenses for reporting costs.

Depreciation, depletion and amortization. Depreciation, depletion and amortization for the nine month period ended September 30, 2011 decreased to $316,865 from $531,519 for the nine month period ended September 30, 2010 due to the decreased production volumes for the nine month period ended September 30, 2011.

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Item 3. Quantitative and Qualitative Disclosures about Market Risk

1. Interest Rate Risk

The Partnership Agreement allows borrowings from banks or other financial sources of up to 20% of the total capital contributions to the Partnership without investor approval. Should the Partnership elect to borrow monies for additional development activity on Partnership properties, it will be subject to the interest rate risk inherent in borrowing activities. Changes in interest rates could significantly affect the Partnership’s results of operations and the amount of net cash flow available for partner distributions. Also, to the extent that changes in interest rates affect general economic conditions, the Partnership will be affected by such changes.

2. Commodity Price Risk

The Partnership does not expect to engage in commodity futures trading or hedging activities or enter into derivative financial instrument transactions for trading or other speculative purposes. The Partnership currently expects to sell a significant amount of its production from successful oil and gas wells on a month-to-month basis at market prices. Accordingly, the Partnership is at risk for the volatility in commodity prices inherent in the oil and gas industry, and the level of commodity prices will have a significant impact on the Partnership’s results of operations. For the nine months ended September 30, 2011, a 10% change in the price received for oil and gas production would have had an approximate $204,000 impact on revenue.

3. Exchange Rate Risk

The Partnership currently has no income from foreign sources or operations in foreign countries that would subject it to currency exchange rate risk. The Partnership does not currently expect to purchase any prospects located outside of either the United States or United States coastal waters in the Gulf of Mexico.

Item 4. Disclosure Controls and Procedures

MD maintains a system of controls and procedures designed to provide reasonable assurance as to the reliability of the financial statements and other disclosures included in this report, as well as to safeguard assets from unauthorized use or disposition. MD’s Chief Executive Officer and Chief Financial Officer have evaluated the effectiveness of the design and operation of its disclosure controls and procedures with the assistance and participation of other members of management. Based upon that evaluation, MD’s Chief Executive Officer and Chief Financial Officer concluded that its disclosure controls and procedures are effective for gathering, analyzing and disclosing the information the Partnership is required to disclose in the reports it files under the Securities Exchange Act of 1934 within the time periods specified in the SEC’s rules and forms. Since MD’s December 31, 2010 annual report on internal control over financial reporting, and for the quarter ended September 30, 2011, there have been no changes in MD’s internal controls or in other factors which have materially affected, or are reasonably likely to materially affect, the internal controls over financial reporting.

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Part II – Other Information

Item 1. Legal Proceedings

From time to time, the Registrant may be a party to certain legal actions and claims arising in the ordinary course of business. While the outcome of these events cannot be predicted with certainty, the Partnership does not expect these matters to have a material effect on its financial position or results of operations.

Item 6. Exhibits and Reports on Form 8-K

    None.

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SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the Partnership has duly caused this report to be signed on its behalf by the undersigned thereto duly authorized.

Date: November 14, 2011

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INDEX TO EXHIBITS

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