EXHIBIT 99.1 APACHE CORPORATION CERTIFICATION OF CHIEF EXECUTIVE OFFICER AND CHIEF FINANCIAL OFFICER I, G. Steven Farris, certify that the Annual Report of Apache Corporation on Form 10-K for the year ended December 31, 2002, fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934 (15 U.S.C. Section 78m or Section 78o (d)) and that information contained in such report fairly represents, in all material respects, the financial condition and results of operations of Apache Corporation. /s/ G. Steven Farris - ----------------------------------------- By: G. Steven Farris Title: President, Chief Executive Officer and Chief Operating Officer I, Roger B. Plank, certify that the Annual Report of Apache Corporation on Form 10-K for the year ended December 31, 2002, fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934 (15 U.S.C. Section 78m or Section 78o (d)) and that information contained in such report fairly represents, in all material respects, the financial condition and results of operations of Apache Corporation. /s/ Roger B. Plank - ----------------------------------------- By: Roger B. Plank Title: Executive Vice President and Chief Financial Officer