<PAGE>

                                                                  EXHIBIT 3.5(a)

                          CERTIFICATE OF INCORPORATION

                                       OF

                          CORAL RESERVES ENERGY CORP.



STATE OF OKLAHOMA        )
                         )    ss:
COUNTY OF OKLAHOMA       )


TO:  THE SECRETARY OF STATE OF THE STATE OF OKLAHOMA:

                                   ARTICLE I
                                   ---------

     The name of the corporation is Coral Reserves, Inc.

                                   ARTICLE II
                                   ----------

     The address of the corporation's registered office in the State of Oklahoma
is 100 Park Avenue Building, Suite 400, Oklahoma City, Oklahoma 73102.  The name
of the corporation's registered agent at such address is Scott M. Rayburn.

                                  ARTICLE III
                                  -----------

     The purpose of the corporation is to engage in any lawful act or activity
for which corporations may be organized under the general corporation law of the
State of Oklahoma.

                                  ARTICLE IV
                                  ----------

     The total number of shares of stock which the corporation shall have
authority to issue is 10,000 shares, each of the shares having a par value of
$0.10, thereby resulting in the corporation having total authorized capital
stock in the amount of $1,000, all of which shall be Common Stock.
<PAGE>

     The Board of Directors of the corporation shall have full authority, to the
extent permitted by law, to increase, decrease or otherwise adjust the capital
stock of the corporation, to designate the classes or series thereof and to
determine whether all or any part of such stock shall have voting powers, full
or limited, or no voting powers, and to determine such designations, and such
powers, preferences, relative, participating or optional, or other special
rights and qualifications, limitations or restrictions thereof as the Board
shall from time to time determine in duly adopted resolutions.

     At any time and from time to time, when authorized by resolution of the
Board of Directors and without any action by its shareholders, the corporation
may issue or sell any shares of its capital stock of any class or series,
whether out of the unissued shares thereof authorized by the Certificate of
Incorporation of the corporation as originally filed or by an amendment thereof
or out of shares of its capital stock acquired by it after the issue thereof,
and whether or not the shares thereof so issued or sold shall confer upon the
holders thereof the right to exchange or convert such shares for or into other
shares of capital stock of the corporation of any class or classes or any series
thereof.  When similarly authorized, but without any action by its share
holders, the corporation may issue or grant rights, warrants or options, in
bearer or registered or such other form as the Board of Directors may determine,
for the purchase of shares of the capital stock of any class or series of the
corporation within such period of time, or without limit as to time, to such
aggregate number of shares, and at such price per share, as the Board of
Directors may determine.  Such rights, warrants or options may be issued or
granted separately or in connection with the issue of any bonds, debentures,
notes, obligations or other evidences of indebtedness or shares of the capital
stock of any class or series of the corporation and for such

                                       2
<PAGE>

consideration and on such terms and conditions as the Board of Directors in its
sole discretion may determine. In each case, the consideration to be received by
the corporation for any such shares so issued or sold shall be such as shall be
fixed from time to time by resolution of the Board of Directors.

                                   ARTICLE V
                                   ---------

     The name and mailing address of each incorporator is as follows:

          Name                           Mailing Address
          ----                           ---------------

     Scott M. Rayburn                    100 Park Avenue, Suite 400
                                         Oklahoma City, Oklahoma 73102

                                  ARTICLE VI
                                  ----------

     The name and address of each director who will serve until the first annual
meeting of the shareholders or until their successors are elected and qualified
is as follows:

          Name                           Mailing Address
          ----                           ---------------

     Leo Woodard                         3313 Arrowhead Drive
                                         Edmond, OK 73013

     John Penton                         1807 Huntington
                                         Oklahoma City, OK 73116

                                  ARTICLE VII
                                  -----------

     In furtherance and not in limitation of the powers conferred by statute,
the Board of Directors is expressly authorized:

     (a)  To adopt, amend or repeal the Bylaws of the corporation.

     (b)  To authorize and cause to be executed or granted  mortgages, security
          interests and liens upon the real and personal property of the
          corporation.

                                       3
<PAGE>

     (c)  To set apart out of any of the funds of the corporation available to
          dividends a reserve or reserves for any proper purpose and to abolish
          any such reserve in the manner in which it was created.

     (d)  By a majority of the whole Board of Directors, to designate one or
          more committees, each committee to consist of one (1) or more of the
          directors of the corporation.  The board may designate one (1) or more
          directors as alternate members of any committee, who may replace any
          absent or disqualified member at any meeting of the committee.  Any
          such committee, to the extent provided in the resolution or in the
          Bylaws of the corporation, shall have and may exercise the powers of
          the Board of Directors in the management of the business and affairs
          of the corporation, and may authorize the seal of the corporation to
          be affixed to all papers which may require it; provided, however, the
          Bylaws may provide that in the absence or disqualification of any
          member of such committee or committees, the member or members thereof
          present at any meeting and not disqualified from voting, whether or
          not he or they constitute a quorum, may unanimously appoint another
          member of the Board of Directors to act at the meeting in the place of
          any such absent or disqualified member.

     (e)  When and as authorized by the affirmative vote of the holders of a
          majority of the stock issued and outstanding having voting power
          given at a shareholders' meeting duly called upon such notice as is
          required by law, or when authorized by the written consent of the
          holders of a majority of the voting stock issued and outstanding, to
          sell, lease or exchange all or substantially all of the property and
          assets of the corporation, including its goodwill and its corporate
          franchises, upon such terms and conditions and for such consideration,
          which may consist in whole or in part of money or property including
          shares of stock in, and/or other securities of, any other corporation
          or corporations, as its Board of Directors shall deem expedient and
          for the best interests of the corporation.

                                  ARTICLE VIII
                                  ------------

     Whenever a compromise or arrangement is proposed between this corporation
and its creditors or any class of them and/or between this corporation and its
shareholders or any class of them, any court of equitable jurisdiction within
the State of Oklahoma, on the application in a summary way of this corporation
or of any creditor or shareholder thereof, or on the application of any receiver
or receivers appointed for this corporation under the provisions of Section 1106

                                       4
<PAGE>

of Title 18 of the Oklahoma Statutes or on the application of trustees in
dissolution or of any receiver or receivers appointed for this corporation under
the provisions of Section 1106 of Title 18 of the Oklahoma Statutes order a
meeting of the creditors or class of creditors, and/or of the shareholders or
class of shareholders of this corporation, as the case may be, to be summoned in
such manner as the court directs. If a majority in number representing three-
fourths (3/4) in value of the creditors or class of creditors, and/or of the
shareholders or class of shareholders of this corporation, as the case may be,
agree to any compromise or arrangement and to any reorganization of this
corporation as consequence of such compromise or arrangement, the compromise or
arrangement and the reorganization shall, if sanctioned by the court to which
the application has been made, be binding on all the creditors or class of
creditors and/or on all the shareholders of this corporation, as the case may
be, and also on this corporation.

                                   ARTICLE IX
                                   ----------

     Meetings of shareholders may be held within or without the State of
Oklahoma, as the Bylaws may provide.  The books of the corporation may be kept
(subject to applicable law) inside or outside the State of Oklahoma at such
place or places as may be designated from time to time by the Board of Directors
or in the Bylaws of the corporation.  Elections of directors need not be by
written ballot unless the Bylaws of the corporation shall so provide.

                                   ARTICLE X
                                   ---------

     To the extent permitted by law, no contract or transaction between the
corporation and one or more of its directors or officers, or between the
corporation and any other corporation, partnership, association or other
organization in which one or more of its directors or officers are directors or
officers or have a financial interest, shall be void or voidable solely for this
reason, or

                                       5
<PAGE>

solely because the directors or officers are present at or participate in the
meeting of the Board or committee thereof which authorizes the contract or
transaction, or solely because the directors or officers or their votes are
counted for such purpose.

                                   ARTICLE XI
                                   ----------

     The Board of Directors shall have power to indemnify any person who was or
is a party or is threatened to be made a party to any threatened, pending or
completed action, suit or proceeding, whether civil, criminal, administrative or
investigative, other than an action by or in the right of the corporation, by
reason of the fact that he is or was a director, officer, employee or agent of
the corporation, or is or was serving at the request of the corporation as a
director, officer, employee or agent of another corporation, partnership, joint
venture, trust or other enterprise, against expenses, including attorneys' fees,
judgments, fines, and amounts paid in settlement actually and reasonably
incurred by him in connection with such action, suit or proceeding if he acted
in good faith and in a manner he reasonably believed to be in or not opposed to
the best interests of the corporation, and, with respect to any criminal action
or proceeding, had no reasonable cause to believe his conduct was unlawful.  The
termination of any action, suit or proceeding by judgment, order, settlement,
conviction, or upon a plea of nolo contendere or its equivalent, shall not, of
itself, create a presumption that the person did not act in good faith and in a
manner which he reasonably believed to be in or not opposed to the best
interests of the corporation, and, with respect to any criminal action or
proceeding, had reasonable cause to believe that his conduct was unlawful.

     The Board of Directors shall have the power to indemnify any person who was
or is a party or is threatened to be made a party to any threatened, pending or
completed action or suit by

                                       6
<PAGE>

or in the right of the corporation to procure a judgment in its favor by reason
of the fact that he is or was a director, officer, employee or agent of the
corporation, or is or was serving at the request of the corporation as a
director, officer, employee or agent of another corporation, partnership, joint
venture, trust or other enterprise against expenses, including attorneys' fees,
actually and reasonably incurred by him in connection with the defense or
settlement of such action or suit if he acted in good faith and in a manner he
reasonably believed to be in or not opposed to the best interests of the
corporation and except that no indemnification shall be made in respect to any
claim, issue or matter as to which such person shall have been adjudged to be
liable to the corporation unless and only to the extent that the court in which
such action or suit was brought shall determine upon application that, despite
the adjudication of liability but in view of all the circumstances of the case,
such person is fairly and reasonably entitled to indemnity for such expenses
which the court shall deem proper.

     Expenses incurred by an officer or director in defending a civil or
criminal action, suit or proceeding may be paid by the corporation in advance of
the final disposition of such action, suit or proceeding upon receipt of an
undertaking by or on behalf of such director or officer to repay such amount if
it shall ultimately be determined that he is not entitled to be indemnified by
the corporation as authorized by the provisions of this section.  Such expenses
incurred by other employees and agents may be so paid upon such terms and
conditions, if any, as the board of directors deems appropriate.

     The indemnification and advancement of expenses provided by or granted
pursuant to this Article shall not be deemed exclusive of any other rights to
which those seeking indemnification or advancement of expenses may be entitled
under any bylaw, agreement, vote of

                                       7
<PAGE>

shareholders or disinterested directors or otherwise, both as to action in his
official capacity and as to action in another capacity while holding such
office.

                                  ARTICLE XII
                                  -----------

     No director shall be personally liable to the corporation or its
shareholders for monetary damages for breach of fiduciary duty as a director;
provided, however, this provision shall not eliminate or limit the liability of
a director:  (i) for any breach of the director's duty of loyalty to the
Corporation or its shareholders; (ii) for acts or omissions not in good faith or
which involve intentional misconduct or a knowing violation of law; (iii) under
Section 1053 of Title Eighteen of the Oklahoma General Corporation Act; or (iv)
for any transaction from which the director derived an improper personal
benefit.

                                  ARTICLE XIII
                                  ------------

     In furtherance and not in limitation of the powers conferred by the laws of
the State of Oklahoma, the Board of Directors is expressly authorized to adopt,
or amend or repeal the Bylaws of the corporation.

                                  ARTICLE XIV
                                  -----------

     The corporation reserves the right to amend, alter, change or repeal any
provision contained in this Certificate of Incorporation, in the manner now or
hereafter prescribed by law, and all rights conferred upon the shareholders
herein are granted subject to this reservation.

     THE UNDERSIGNED (whether one or more), being the incorporator hereinbefore
named, for the purpose of forming a corporation pursuant to the Oklahoma General
Corporation Act, makes this Certificate, hereby declaring and certifying that
this is the act and deed of the undersigned and that the facts herein stated are
true, as of this 17th day of January, 1990.

                                       8
<PAGE>

                              _____________________________________
                              Scott M. Rayburn, Incorporator

                                       9