Exhibit 99.1

MOR-1 UNITED STATES BANKRUPTCY COURT case name: ATP Oil & Gas Corporation petition date: 8/17/2012 CASE NUMBER: 12-36187 DISTRICT OF TEXAS: Southern proposed plan date: To be determined division: Houston MONTHLY OPERATING REPORT SUMMARY FOR SEPTEMBER 2012 MONTH 18/17/12 to 8/31/12 I 9/30/20121 I I I REVENUES (MOR-6) $ 17,962,067 $ 37,184,014 INCOME BEFORE INT; DEPREC./TAX (MOR-6) 1,618,169 6,864,948 NET INCOME (LOSS) (MOR-6) (32,699,588) (4,623,978) PA YMENTS TO INSIDERS (MOR-9) 75,859 152,426 I PAYMENTS TO PROFESSIONALS (MOR-9) 620,479 572,818 TOTAL DISBURSEMENTS (MOR-8) $ 386,317,124 j $ 68,035,477 j ***The original of this document must be filed with the United States Bankruptcy Court and a copy must be sent to the United States Trustee***             CIRCLE ONE REQUIRED INSURANCE MAINTAINED Are all accounts receivable being collected within terms? Yes ( No > AS OF SIGNATURE DATE EXP. Are all post-petition liabilities, including taxes, being paid within terms? (T O Wo . DATE Have any pre-petition liabilities been paid? \* ) No In accordance with the authority that has been granted to ATP under court orders, certain prepetition liabilities relating to payroll, employee expense reimbursements, independent contractor fees, utility fees, capital expenditures for our Clipper project, and fees due on our bank accounts CASUALTY YESP<)NO() 06-30-13 If so, describe have been paid. LIABILITY YES(X)NO() 10-31-12 Are all funds received being deposited into DIP bank accounts? (*Yes) Wo *”*™ ™”*>‘*”““ ™”“1’X. VEHICLE YES (X) NO ( ) 10-31-12 Were any assets disposed of outside the normal course of business? Yes (No) WORKER’S YES(X)NO() 06-30-13 If so, describe OTHER YES(X)NO() 06-01-13 Are all U.S. Trustee Quarterly Fee Payments current? QW) No What is the status of your Plan of Reorganization? We have not yet commenced negotiations with our stakeholders. A TTORNEY NAME: Charles S. Kelley, Esq. / certify under penalty of perjury that the following complete firm name: Mayer Brown, LLP Monthly Operating Report (MOR), consisting of MOR-1 through ADDRESS: 700 Louisiana St. MOR-9 plus attachments, is true and correct. Suite 3400 CITY, STATE, ZIP: Houston, TX 77002 SIGNED Chief Accounting Officer TELEPHONE/FAX: (713) 238-3000 (ORIGINAL SIGNATURE) Keith R. Godwin October 22, 2012 MOR-1(PRINT NAME OF SIGNATORY) DATE Revised 07/01/98

case NAME: ATP Oil & Gas Corporation CASE NUMBER: 12-36187 COMPARATIVE BALANCE SHEETS ASSETS FILING DATE’ MONTH MONTH MONTH MONTH MONTH MONTH 8/17/2012 8/31/2012 9/30/2012 CURRENT ASSETS Cash $ 6,119,734 $ 47,144,872 $ 54,512,686 Restricted Cash 4,840,908 14,841,112 82,549,173 Accounts Receivable, Net 48,962,845 48,202,892 51,860,095 Accounts Receivable, Affiliates 699,367,083 705,099,121 710,056,471 Prepaid Expenses and Other Current Assets 18,938,661 19,533,602 14,207,138 TOTAL CURRENT ASSETS 778,229,231 834,821,599 913,185,563 Oil & Gas Properties 2,914,007,759 2,939,625,568 2,988,835,104 Less Accumulated DD&A and Impairment 1,622,583,195 1,636,941,332 1,650,846,528 OIL & GAS PROPERTIES, NET 1,291,424,564 1,302,684,236 1,337,988,576 OTHER ASSETS Deferred Tax Assets—— Investments in Affiliates 892,154,554 888,614,697 869,098,552 Deferred Financing Costs—net 2,075,258 2,171,506 1,970,434 Furniture & Fixtures, net 482,093 463,201 440,959 Other Non current Receivable 12,269,065 12,273,726 12,278,238 TOTAL ASSETS $ 2,976,634,765 “$ 3,041,028,965 $ 3,134,962,322 * Per Schedules and Statement of Affairs MOR-2 Revised 07/01/98

case name: ATP Oil & Gas Corporation CASE NUMBER: 12-36187 COMPARATIVE BALANCE SHEETS LIABILITIES & OWNERS’ filingdate* MONTH MONTH MONTH MONTH MONTH MONTH EQUITY 8/17/2012 8/31/2012 9/30/2012 LIABILITIES post-petition LiABn.rnES(MOR-4) $ $ 501,051,920 $ 631,908,259 PRE-PETITION LIABILITIES Secured obligations “ 2,014,806,960 1,650,999,715 1,648,605,960 Priority unsecured obligations 11,287,816 11,287,816 10,759,598 Unsecured obligations 251,241,600 251,241,600 198,576,609 Other pre-petition obligations TOTAL CLAIMABLE PRE-PETITION LIABILITIES 2,277,336,376 1,913,529,131 1,857,942,167 Asset Retirement Obligation(1) 149,468,22T 150,114,665 149,970,805 Derivative Liability 1,962,381—- Other Long-term obligations(2) 544,832,478 510,150,491 536,415,044 TOTAL LIABILITIES 2,973,599,463 3,074,846,207 3,176,236,275 OWNERS’ EQUITY (DEFICIT) preferred stock 277,406,044 277,406,044 277,406,044 common stock 52,660 52,660 52,660 treasury stock (911,729) (911,729) (911,729) additional paid-in capital 536,339,243 536,976,475 537,384,047 8/1-8/17 translation adjustment p) 1,363,677 RETAINED EARNINGS: Filing Date (811,214,591) (811,214,593) (811,214,593) RETAINED EARNINGS: Post Filing Date (32,699,588) (37,323,566) Equity in earnings of ATPIP and ATP Titan: Post Filing Date—(3,426,511) (6,666,816) TOTAL OWNERS’ EQUITY (NET DEFICIT) 3,035,302 (33,817,242) (41,273,953) TOTAL LIABILITIES & OWNERS’ EQUITY $ 2,976,634,765 $ 3,041,028,965 $ 3,134,962,322 * Per Schedules and Statement of Affairs MOR-3 Revised 07/01/98 ARO represents GAAP present value estimate of future cash flows. ARO is subject to certain potential bonding requirements which have not been determined as of the date of this filing. Other long-term obligations include term NPI and ORRI interests which are accounted for as long-term obligations under GAAP because they are granted on proved properties where production is reasonably assured. Amounts associated with these other long-term obligations are not payable until production occurs. Our August 17 through August 31,2012 income was estimated using an allocation of 15/31 days for certain line items. Our balance sheet line items represent the August 17 balances when possible; otherwise amounts presented represent July 31 balances. Because of this, an equity translation adjustment is required.

case name: ATP Oil & Gas Corporation CASE NUMBER: 12-36187 SCHEDULE OF POST-PETITION LIABILITIES MONTH MONTH MONTH MONTH MONTH MONTH 8/31/2012 9/30/2012 TRADE ACCOUNTS PAYABLE $ 11,682,314 $ 9,262,572 ACCOUNTS PAYABLE—AFFILIATES 5,762,204 TAX PAYABLE Federal Payroll Taxes : : State Payroll Taxes—- Ad Valorem Taxes—- Other Taxes : : TOTAL TAXES PAYABLE—- SECURED DEBT POST-PETITION 414,344,609 524,877,029 ACCRUED INTEREST PAYABLE 583,204 4,625,503 ACCRUED PROFESSIONAL FEES* 746,512 440,553 OTHER ACCRUED LIABILITIES Payroll accruals—- Other accruals 55,145,058 77,503,779 Otherpayables 18,550,223 9,436,619 TOTAL POST-PETITION LIABILITIES (MOR-3) $ 501,051,920 $ 631,908,259 *Payment requires Court Approval MOR-4 Revised07/01/98

case name: ATP Oil & Gas Corporation CASE NUMBER: 12-36187 AGING OF POST-PETITION LIABILITIES MONTH 9/30/2012 DAYS I TOTAL I TRADE I FEDERAL I STATE I AD VALOREM, I ACCOUNTS TAXES TAXES OTHER TAXES 0-30 $ 4,819,432 $ 4,819,432 $ $ $—31-60 4,443,140 4,443,140—— 61-90———- 91+———- TOTAL $ 9,262,572 $ 9,262,572 $—I $—I $—I AGING OF ACCOUNTS RECEIVABLE DAYS TOTAL Revenue Joint Interest(1) Other(2) 0-30DAYS $ 44,923,054 $ 40,813,90?’ $ 4,037,506 ‘$ 71,641 31-60 days 3,156,677 2,420,432 677,225 59,020 61-90 days 641,855 2,484 546,397 92,974 91+days 3,138,509 (351,347) 2,703,161 786,695 TOTAL $ 51,860,095 $ 42,885,476 $ 7,964,289 $ 1,010,330 MOR-5 Revised 07/01/98 (1) 91+ day balance Includes $2,013,375 of amounts being withheld over Wl% dispute and $415,789 of amounts regarding Black Elk/Nippon lawsuit (2) 91+ day balance includes $317,135 due from Bluewater Industries and $469,560 due from Israel Tax Authority for VAT taxes

case name: ATP Oil & Gas Corporation CASE NUMBER: 12-36187 STATEMENT OF INCOME (LOSS) MONTH MONTH MONTH MONTH MONTH MONTH FILING TO 8/17/12 to 8/31/12 9/30/2012 DATE revenues (MOR-i) $ 17,962,067 $ 37,184,014 $ 55,146,081 gross profit ‘ 17,962,067 37,184,014 55,146,081 operating expenses: Lease Operating Expenses 3,596,446 7,405,660 11,002,106 Processing fees, related party 3,456,290 6,590,000 10,046,290 DD&A /Impairment 6,958,244 13,927,439 20,885,683 Accretion 508,076 1,059,370 1,567,446 Abandonment(Gain)/Loss 32,108 130,910 163,018 Compensation Expense 778,853 1,165,292 1,944,145 Professional Fees 882,462 (35,889) 846,573 Other General & Administrative 131,419 76,284 207,703 total operating expenses 16,343,898 30,319,066 46,662,964 income (loss) from operations (mor-1) 1,618,169 6,864,948 8,483,117 interest income 2,033,953 4,504,888 6,538,841 INTEREST (EXPENSE),NET (2,191,113) (13,303,416) (15,494,529) DERIVATIVES GAIN/(LOSS)—(30,879) (30,879) debt extinguishment gain/(loss) (6,937,100)—(6,937,100) total int, der.& other items (7,094,260) (8,829,407) (15,923,667) net income (loss) before reorganization] (5,476,091) (1,964,459) (7,440,550) reorganization (expenses): US Trustee fees (30,000) : (30,000) Professional fees (1,037,305) (2,659,519) (3,696,824) Debt valuation adjustments——_ Claims adjustments (26,156,192)—(26,156,192) Interest income—- REORGANIZATION EXPENSES, NET: (27,223,497) (2,659,519) ‘ ‘ (29,883,016) net income (loss) (mor-i) $ (32,699,588) $ (4,623,978) $ (37,323,566) Accrual Accounting Required, Otherwise Footnote with Explanation. MOR-6 Revised 07/01/98

case name: ATP Oil & Gas Corporation CASE NUMBER: 12-36187 CASH RECEIPTS AND MONTH MONTH MONTH MONTH MONTH MONTH FILING TO DISBURSEMENTS 8/17/12 to 8/31/12 9/30/2012 DATE 1. CASH-BEGINNING OF MONTH $ 6,119,734 $ 47,144,872 $ 6,119,734 RECEIPTS; COLLECTION OF NET REVENUE ACCOUNTS RECEIVABLE 21,003,439 34,319,343 55,322,782 COLLECTION OF JOINT INTEREST ACCOUNTS RECEIVABLE—681,206 681,206 OTHER OPERATING RECEIPTS (See Attached)—310,803 310,803 FINANCING RECEIPTS (See Attached) 406,338,823 40,091,939 446,430,762 total receipts” 427,342,262 75,403,291 502,745,553 Withdrawal) Contribution by Individual Debtor MFR-2* disbursements; term-limited orri 940,089 2,991,609 3,931,698 term-limited npi 12,645,837 12,645,837 derivative settlements paid (received) 2,141,118 30,879 2,171,997 lease operating costs—1,702,508 1,702,508 GOMEZ PIPELINE OBLIGATION 830,833 877,160 1,707,993 titan fee 5,500,000 10,000,000 15,500,000 atpipfees 1,571,182 1,571,682 3,142,864 GENERAL & ADMINISTRATIVE (See Attached) 437,500 2,264,864 2,702,364 OTHER OPERATING DISBURSEMENTS (See Attached)——_ TOTAL DISBURSEMENTS FROM OPERATIONS 11,420,722 32,084,539 43,505,261 CAPEX CASH DISBURSMENTS (See Attached) -_ 34,933,676 34,933,676 BANKRUPTCY & PROFESSIONAL FEES (See Attached) 620,479 572,818 1,193,297 FINANCING PAYMENTS—PRINCIPAL AND INTEREST (See Attached) 374,275,923 444,444 374,720,367 total disbursements** 386,317,124 68,035,477 454,352,601 17. net cash flow 41,025,138 7,367,814 48,392,952 is. cash-end of month (MOR-2) $ 47,144,872 $ 54,512,686 $ 54,512,686 * Applies to Individual debtors only MOR-7A **Numbers for the current month should balance (match) Revised 07/01/98 RECEIPTS and CHECKS/OTHER DISBURSEMENTS lines on MOR-8

case name: ATP Oil & Gas Corporation CASE NUMBER: 12-36187 ICASH RECEIPTS AND MONTH MONTH MONTH MONTH MONTH MONTH FILING TO idisbursements detail 8/17/12108/31/12 9/30/2012 date j OTHER OPERATING RECEIPTS (Line 3) Miscellaneous $ $ 310,803 $ 310,803 GENERAL & ADMINISTRATIVE (Line 12) Compensation 352,523 1,235,511 1,588,034 Rent—55,812 55,812 Office and general costs : 89,235 89,235 Travel related—26,191 26,191 D&O 82,413 81,233 163,646 Other costs 2,564 776,882 779,446 GENERAL & ADMINISTRATIVE TOTAL 437,500 2,264,864 2,702,364 BANKRUPTCY & PROFESSIONAL FEES (Line 15) Legal advisors 620,479 572,818 1,193,297 Financial advisors—- Operational/Other consultants—— BANKRUPTCY & PROFESSIONAL FEES TOTAL 620,479 572,818 1,193,297 OTHER OPERATING DISBURSEMENTS (Line 13)—— CAPEX CASH DISBURSMENTS (Line 14) Maintenance/Other—— Clipper project 34,933,676 34,933,676 Gomez #9 project—— Plugging and abandonment of oil and gas properties—- CAPEX CASH DISBURSMENTS TOTAL—34,933,676 34,933,676 FINANCING PAYMENTS—PRINCIPAL AND INTEREST (Line 16) 1st lien principal and interest 367,338,823—367,338,823 New facility principal and interest 6,937,100 444,444 7,381,544 FINANCING PAYMENTS—PRINCIPAL AND INTEREST TOTAL 374,275,923 444,444 374,720,367 FINANCING RECEIPTS (Line 4) New facility proceeds $ 406,338,823 $ 40,091,939 $ 446,430,762

case name: ATP Oil & Gas Corporation CASE NUMBER: 12-36187 CASH ACCOUNT RECONCILIATION MONTH OF 9/30/2012 BANK NAME JPMorgan Chase JPMorgan Chase JPMorgan Chase JPMorgan Chase JPMorgan Chase Compass Bank Green Bank ACCOUNT NUMBER 113334321 1000027441 6300050955 58902066488 113400841 2524118079 3300247239 ACCOUNT TYPE Operating Deposit Controlled Disbursement Cafeteria Plan Other Payroll Payroll TOTAL BANK BALANCE $ 66,073,380 $ $ $ 23,936 $ $ $ 217,940 $ 66,315,256 DEPOSITS IN TRANSIT———— 15,800 15,800 OUTSTANDING CHECKS :—11,807,398 10,972—- 11,818,370 ADJUSTED BANK BALANCE $ 66,073,380 $ $ (11,807,398) $ 12,964 $ $ $ 233,740 $ 54,512,686 BEGINNING CASH—PER BOOKS $ 48,012,307 $ 15,299 $ (1,129,899) $ 17,558 $ 229,607 $ $ 47,144,872 RECEIPTS* 74,395,703 1,007,588———- 75,403,291 TRANSFERS BETWEEN ACCOUNTS (2,716,605) (1,022,887) 2,498,889—- (229,607) 1,470,210 -_ (WITHDRAWAL) OR CONTRIBUTION BY INDIVIDUAL DEBTOR MFR-2—————-_ CHECKS/OTHER DISBURSEMENTS* 53,618,025—13,176,388 4,S94—“—1,236,470” 68,035,477 ENDING CASH—PER BOOKS $ 66,073,380 $ $ (11,807,398) $ 12,964 $ $ $ 233,740 _$ 54,512,686 MOR-8 *Numbers should balance (match) TOTAL RECEIPTS and Revised 07/01/98 TOTAL DISBURSEMENTS lines on MOR-7

case name: ATP Oil & Gas Corporation CASE NUMBER: 12-36187 PAYMENTS TO INSIDERS AND PROFESSIONALS Of the total disbursements shown for the month, list the amount paid to insiders (as defined in Section 101(31)(A)-(F) of the U.S. Bankruptcy Code) and the professionals. Also, for insiders, identify the type of compensation paid (e.g. salary, commission, bonus, etc.) (Attach additional pages as necessary). MONTH MONTH MONTH MONTH MONTH MONTH INSIDERS: NAME/COMP TYPE 8/17/12 to 8/31/12 9/30/2012 T. Paul Bulmahn (salary) $ 17,767 $ 36,197 LelandE.Tate (salary) 14,634 29,267 Albert L. Reese Jr. (salary) 11,577 23,154 George R. Morris (salary) 11,848 23,741 John E.Tschirhart (salary) 8,731 17,463 Keith R. Godwin (salary) 11,302 22,604 TOTAL INSIDERS (MOR-1) $ 75,859 $ 152,426 MONTH MONTH MONTH MONTH MONTH MONTH professionals 8/17/12 to 8/31/12 9/30/2012 DIP Lender Legal/Advisors BinghamMcCutchenLLP $ 620,479—Cravath, Swaine & Moore LLP _$ 572,818 TOTAL PROFESSIONALS (MOR-1) $ 620,479 $ 572,818 MOR-9 Revised 07/01/98