<PAGE>
 
                                                                    Exhibit 10.4

                         REGISTRATION RIGHTS AGREEMENT

     This REGISTRATION RIGHTS AGREEMENT (this "Agreement") dated as August 31,
1996, is by and among NGC Corporation, a Delaware corporation (the "Company"),
BG Holdings, Inc., a Delaware Corporation ("BG"), NOVA Gas Services (U.S.) Inc.,
a Delaware corporation ("NOVA"), and Chevron U.S.A. Inc., a Pennsylvania
corporation ("Chevron").  BG, NOVA and Chevron may hereinafter be referred to
collectively as "Stockholders".

     WHEREAS, the Company has entered into (I) that certain Combination
Agreement and Plan of Merger (the "Combination Agreement") dated as of May 22,
1996 among NGC Corporation, a Delaware corporation ("Old NGC"), Chevron and the
Company (formerly known as Midstream Combination Corp.), pursuant to which Old
NGC was merged with and into the Company and the Company changed its name to
"NGC Corporation" and (ii) certain other agreements referenced therein;

     WHEREAS, the Combination Agreement contemplates that Stockholders will
receive, at the Effective Time (as such term is defined in the Combination
Agreement), that number of shares of common stock, par value $.01 per share of
the Company (the "Common Stock") and of securities convertible into Common
Stock, set forth beside each stockholder's name on the signature page hereof;
and

     WHEREAS, in connection with the Combination Agreement and the Merger
contemplated thereby, the Company has agreed to grant to each Stockholder
certain registration rights set forth below.

     NOW, THEREFORE, in consideration of these premises and other good and
valuable consideration, the receipt and sufficiency of which are hereby
acknowledged by each Stockholder and the Company, the parties hereto agree as
follows:


                                   SECTION 1.
                                  DEFINITIONS

     1.1  SPECIFIC DEFINITIONS.  The following capitalized terms shall have the
meanings ascribed to them in this Section 1.1:

     "Affiliate" shall have the meaning set forth in Rule 12b-2 under the
Exchange Act.
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                                       2


     "Agreement" shall have the meaning set forth in the preamble hereto.

     "BG" shall have the meaning set forth in the preamble hereto.

     "BG Holder" means BG and each transferee of Registerable Common Stock
directly or indirectly (in a chain of title) from BG to whom the right to cause
one or more demand registrations under Section 2.1 has been expressly assigned
in writing directly or indirectly (in a chain of title) from BG.

     "Chevron" shall have the meaning set forth in the preamble hereto.

     "Chevron Holder" means Chevron and each transferee of Registrable Common
Stock directly or indirectly (in a chain of title) from Chevron to whom the
right to cause one or more demand registrations under Section 2.1 has been
expressly assigned in writing directly or indirectly (in a chain of title) from
Chevron.

     "Combination Agreement" shall have the meaning set forth in the recitals
hereto.

     "Common Stock" shall have the meaning set forth in the recitals hereto.

     "Company" shall have the meaning set forth in the preamble hereto.

     "Effective Time" shall have the meaning set forth in the Combination
Agreement.

     "Exchange Act" shall mean the Securities Exchange Act of 1934, as amended.

     "Existing Registration Rights" shall mean the registration rights provided
pursuant to Section 3.1 of and Exhibit A to the Amended and Restated
Stockholders Agreement entered into April 15, 1993 by and among Old NGC and
certain of its stockholders, a copy of which has been provided to the other
parties hereto.

     "Former Trident Holders Registration Rights" shall mean the registration
rights provided pursuant to Section 4.1 of the Stockholders Agreement dated as
of October 21, 1994 among Old NGC, Hicks, Muse, Tate and Furst Incorporated, the
Major Holders (as defined therein) and the other parties thereto.

     "Indemnified Party" shall have the meaning set forth in Section 8.3.

     "Indemnifying Party" shall have the meaning set forth in Section 8.3.
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                                       3

     "Inspectors" shall have the meaning set forth in Section 5.1(l).

     "Loss"or "Losses" shall have the meaning set forth in Section 8.1.

     "Merger" shall have the meaning set forth in the Combination Agreement.

     "NOVA" shall have the meaning set forth in the preamble hereto.

     "NOVA Holder" means NOVA and each transferee of Registerable Common Stock
directly or indirectly (in a chain of title) from NOVA to whom the right to
cause one or more demand registrations under Section 2.1 has been expressly
assigned in writing directly or indirectly (in a chain of title) from NOVA.

     "person" shall mean any business entity (including, without limitation, a
corporation, partnership (limited or general), limited liability company or
business trust) or a natural person.

     "Prospectus" shall have the meaning set forth in Section 8.1.

     "register" "registered" and "registration" and words of similar import
refer to a registration effected by preparing and filing with the SEC a
registration statement in compliance with the Securities Act, and the
declaration and ordering by the SEC of effectiveness of such registration
statement or document.

     "Registrable Common Stock" shall mean any Common Stock of the Company held
or acquired by any Stockholder (or its permitted assigns) at or after the
Effective Time (including, without limitation, shares of Common Stock issued
upon conversion of shares of Series A Preferred Stock of the Corporation), and
any securities issued or issuable in respect of any Registrable Common Stock by
way of any stock split or stock dividend or in connection with any combination
of shares, recapitalization, merger, consolidation, reorganization or otherwise.

     "SEC" shall mean the United States Securities and Exchange Commission.

     "Securities Act" shall mean the Securities Act of 1933, as amended.

     "Stockholders" shall have the meaning set forth in the preamble hereto.

     1.2  OTHER DEFINITIONS.  Other capitalized terms used herein but not
defined in Section 1.1 shall have the respective meanings ascribed to them
throughout this Agreement.
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                                       4

                                   SECTION 2
                              REGISTRATION RIGHTS

     2.1  DEMAND REGISTRATION RIGHTS.  (a)  Upon receipt of a written request
from a BG Holder, a NOVA Holder or a Chevron Holder to register under the
Securities Act (whether for purposes of a public offering, an exchange offer or
otherwise) all or part of the Registrable Common Stock held by such BG Holder,
NOVA Holder, or Chevron Holder, as the case may be, the Company shall as
expeditiously as reasonably possible (but in any event not later than sixty (60)
days after receipt of such request) prepare and file, and use its best efforts
to cause to become effective as soon thereafter as practicable, a registration
statement under the Securities Act to effect the offering of such Registrable
Common Stock in the manner specified in such request.

     (b) The BG Holder, the NOVA Holder or the Chevron Holder, as the case may
be, shall be entitled to select and retain one or more investment bankers or
managers reasonably acceptable to the Company in connection with any
underwritten offerings made pursuant to this Section 2.1.

     (c) Subject to the terms and conditions set forth in Section 2.2, any BG
Holder, NOVA Holder or Chevron Holder may request the Company to register
Registrable Common Stock under the Securities Act pursuant to this Section 2.1
at any time and from time to time; provided, however, that neither a BG Holder,
a NOVA Holder nor a Chevron Holder may request the Company to register
Registrable Common Stock pursuant to this Section 2.1 more than once in any 365-
day period.

     2.2  TERMS AND CONDITIONS OF DEMAND REGISTRATION RIGHTS.  Notwithstanding
anything to the contrary contained elsewhere herein, the registration rights
granted to the BG Holders, the NOVA Holders and the Chevron Holders in Section
2.1 are expressly subject to the following terms and conditions:

     (a) The BG Holders, collectively, the NOVA Holders, collectively and the
Chevron Holders, collectively, shall each only be entitled to three (3) requests
to register Registrable Common Stock under the terms of Section 2.1.  A
"request" as it is used in this Section 2.2(a) shall be deemed to have occurred
only upon completion of a requested registration and the subsequent sale of
Registrable Common Stock.

     (b) In no event shall the Registrable Common Stock to be offered under a
registration statement prepared and filed pursuant to Section 2.1 constitute
less than five percent (5%) of the then outstanding shares of Common Stock.  For
purposes of meeting the five percent (5%) threshold of this Section 2.2(b), the
BG Holders, the NOVA Holders and the Chevron Holders may aggregate their shares
of Registrable Common Stock to be included therein.
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                                       5

     (c) The Company shall be entitled to defer for a reasonable period of time,
but not in excess of ninety (90) days, the filing of any registration statement
otherwise required to be prepared and filed by it under Section 2.1 if the
Company notifies the BG Holder, the NOVA Holder or the Chevron Holder, as the
case may be, within five (5) business days after such BG Holder, NOVA Holder or
Chevron Holder requested the registration under Section 2.1 that the Company (i)
is at such time conducting or about to conduct an underwritten public offering
of its securities for its own account and the Board of Directors of the Company
determines in good faith that such offering would be materially adversely
affected by such registration requested by the BG Holders, the NOVA Holders or
the Chevron Holders or (ii) would, in the opinion of its counsel, be required to
disclose in such registration statement information not otherwise then required
by law to be publicly disclosed and, in the good faith judgment of the Board of
Directors of the Company, such disclosure might adversely affect any material
business transaction or negotiation in which the Company is then engaged.  If
the Company elects to defer the filing of a registration statement pursuant to
this Section 2.2(c), the BG Holder, the NOVA Holder or the Chevron Holder, as
the case may be, may withdraw its request, in writing, during the time of such
deferral and such request shall not be counted toward the limit set forth in
Section 2.2(a).

     (d) Neither the BG Holders, the NOVA Holders nor the Chevron Holders shall
exercise their rights pursuant to Section 2.1 during the 60-day period
immediately following the effective date of any registration statement filed by
the Company under the Securities Act (other than on Form S-8 or another similar
form) in respect of an offering or sale of securities of the Company by or on
behalf of the Company or any other stockholder of the Company.


                                   SECTION 3
                         PIGGYBACK REGISTRATION RIGHTS

     3.1  PIGGYBACK REGISTRATION RIGHTS.  If at any time or from time to time
the Company shall propose to register any Common Stock for public sale under the
Securities Act, the Company shall give each Stockholder prompt written notice of
the proposed registration and shall include in such registration on the same
terms and conditions as the other securities included in such registration such
number of shares of Registrable Common Stock as any Stockholder shall request
within five (5) business days after the giving of such notice; provided,
however, that the Company may at any time prior to the effectiveness of any such
registration statement, in its sole discretion and without the consent of
Stockholders, abandon the proposed offering in which a Stockholder had requested
to participate; and provided further that any Stockholder shall be entitled to
withdraw any or all of its shares of Registrable Common Stock to be included in
a registration statement under this Section 3.1 at any time prior to the date on
which the registration statement with respect to such shares of Registrable
Common Stock is 
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                                       6

declared effective by the SEC. The Company shall be entitled to select the
investment bankers and/or managers, if any, to be retained in connection with
any registration referred to in this Section 3.1, provided such investment
bankers and/or managers are reasonably acceptable to the Stockholders who hold a
majority of the stock held by all Stockholders who elect to participate in such
offering.

     3.2  RESTRICTIONS ON PIGGYBACK REGISTRATION RIGHTS.

Notwithstanding anything to the contrary contained elsewhere herein, the
registration rights granted to stockholders in Section 3.1 are expressly subject
to the following terms and conditions:

     (a) The Company shall not be obligated to include shares of Registrable
Common Stock in an offering as contemplated by Section 3.1 if the Company is
advised in writing by the managing underwriter or underwriters of such offering
(with a copy to each Stockholder), that the success of such offering would in
its or their good faith judgment be jeopardized by such inclusion (after
consideration of all relevant factors, including without limitation, the impact
of any delay caused by including such shares); provided, however, that the
Company shall in any case be obligated to include such number of shares of
Registrable Common Stock in such offering, if any, as such underwriter or
underwriters shall determine will not jeopardize the success of such offering.

     (b) The Company shall not be obligated to include any shares of Registrable
Common Stock in any registration by the Company of any Common Stock in
connection with any merger, acquisition, exchange offer, or any other business
combination, including any transaction within the scope of Rule 145 promulgated
pursuant to the Securities Act, subscription offer, dividend reinvestment plan
or stock option or other director or employee incentive or benefit plan.

     (c) The Company shall use all commercially reasonable efforts to cause the
managing underwriter or underwriters of a proposed underwritten offering to
permit the Registrable Common Stock requested to be included in a registration
of Company Common Stock pursuant to this Section 3 to be included on the same
terms and conditions as any similar securities included therein.
Notwithstanding the foregoing, the Company shall not be required to include any
Stockholder's Registrable Common Stock in such offering unless such Stockholder
accepts the terms of the underwriting agreement between the Company and the
managing underwriter or underwriters and otherwise complies with the provisions
of Section 8 below.  If the managing underwriter or underwriters of a proposed
underwritten offering advise the Company in writing that in its or their good
faith judgment the total amount of securities, including securities requested to
be included in a registration of Company Common Stock pursuant to this Section 3
and other similar securities, to be included in such offering is 
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                                       7

sufficiently large to jeopardize the success of such offering, then in such
event the securities to be included in such offering shall be allocated first to
the Company and then, to the extent that any additional securities can, in the
good faith judgment of such managing Underwriter or Underwriters, be sold
without creating any such jeopardy to the success of such offering, pro rata
among each Stockholder participating in the offering based upon the number of
shares of Common Stock requested to be included in such registration by each
such holder.

     (d) In the event that some but less than all of a Stockholder's shares of
Registrable Common Stock are included in an offering contemplated by a
registration statement pursuant to Section 3, such Stockholder shall execute one
or more "lockup" letters, in customary form, setting forth an agreement by such
Stockholder not to offer for sale, sell, grant any option for the sale of, or
otherwise dispose of, directly or indirectly, any shares of Common Stock, or any
securities convertible into or exchangeable into or exercisable for any shares
of Common Stock, for a period of 90 days from the date such offering commences.


                                   SECTION 4
                      FAVORED NATIONS, CONFLICTING RIGHTS

     4.1  FAVORED NATIONS.  Except as herein provided, the Company shall not
provide registration rights to any other party which, taken as a whole, are more
favorable than those provided to Stockholders hereunder, without also offering
to Stockholders such more favorable rights (other than the Former Trident
Holders Registration Rights or the Existing Registration Rights).  The Company
shall give Stockholders notice within 15 days after the execution of any
agreement (including the terms thereof) between the Company and a third party
which triggers a right of Stockholders to invoke the favored nations provision
of this Section 4.

     4.2   CONFLICTING RIGHTS.  Notwithstanding anything to the contrary set
forth in this Agreement, or any other agreement pursuant to which the Company
hereafter grants any person registration rights, in the event any of the rights
of the Stockholders under the Agreement or such other persons under such other
agreement conflicts with or diminishes any of the rights granted to any of the
persons who are or become parties (the "Trident Parties") to the Former Trident
Holders Registration Rights or the Existing Registration Rights, the rights of
the Trident Parties shall be deemed superior to, and shall take precedence over
the rights of the Stockholders hereunder or any other persons to which the
Company has granted registration rights, and the rights of such Stockholders and
such other persons shall be subject to the rights of the parties to the Former
Trident Holders Registration Rights and the Existing Registration Rights.  The
application of the rights of the Stockholders under this Agreement shall be
reduced, modified or eliminated, as reasonably necessary in order to implement
the provisions of the foregoing sentence.  The Company covenants and agrees that
in the event it grants registration rights to any 
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                                       8

persons, the instrument granting such rights shall contain a provision similar
to this Section 4.2 ensuring the superiority of the Trident Parties' rights in
accordance with this Section 4.2.


                                   SECTION 5
                                   COVENANTS

     5.1  COVENANTS OF THE COMPANY.  In connection with any offering of shares
of Registrable Common Stock pursuant to this Agreement, the Company shall:

     (a) Prepare and file with the Commission such amendments and post-effective
amendments to the registration statement as may be necessary to keep the
registration statement effective for a period of not less than 120 days, or such
shorter period which will terminate when all Registrable Common Stock covered by
such registration statement has been sold or withdrawn at the request of
participating holders of Common Stock; and cause the prospectus to be
supplemented by any required prospectus supplement, and as so supplemented to be
filed pursuant to Rule 424 under the Securities Act;

     (b) Furnish to each Stockholder and to each managing underwriter, if any,
(i) at least two (2) business days prior to filing with the SEC, any
registration statement covering shares of Registrable Common Stock, any
amendment or supplement thereto, and any prospectus used in connection
therewith, which documents will be subject to the reasonable review of such
Stockholders and such underwriter, and, with respect to a registration statement
prepared pursuant to Section 2.1, the Company shall not file any such documents
with the SEC to which any such Stockholder shall reasonably object; and (ii) a
copy of any and all transmittal letters or other correspondence with the SEC or
any other governmental agency or self-regulatory body or other body having
jurisdiction (including any domestic or foreign securities exchange) relating to
such offering of shares of Registrable Common Stock;

     (c) Furnish to each Stockholder and each managing underwriter, if any, such
number of copies of such registration statement, each amendment and supplement
thereto (in each case including all exhibits thereto and documents incorporated
by reference therein) and the prospectus included in such registration statement
(including each preliminary prospectus and prospectus supplement) as such
Stockholder or such underwriter may reasonably request in order to facilitate
the sale of the shares of Registrable Common Stock;

     (d) After the filing of such registration statement, promptly notify each
Stockholder of any stop order issued or, to the knowledge of the Company,
threatened to be issued by the SEC and promptly take all reasonable actions to
prevent the entry of such stop order or to obtain its withdrawal if entered;
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                                       9

     (e) Use its commercially reasonable efforts to qualify such shares of
Registrable Common Stock for offer and sale under the securities, "blue sky" or
similar laws of such jurisdictions (including any foreign country or any
political subdivision thereof in which shares of Common Stock are then listed)
as any Stockholder or any underwriter shall reasonably request and use its
commercially reasonable efforts to obtain all appropriate registrations, permits
and consents required in connection therewith, except that the Company shall not
for any such purpose be required to qualify generally to do business as a
foreign corporation in any jurisdiction wherein it is not so qualified, or to
subject itself to taxation or to file a general consent to service of process in
any such jurisdiction;

     (f) Furnish to each managing underwriter, if any, an opinion of counsel for
the Company addressed to each of them, dated as of the date of the closing of
the offering of shares of Registrable Common Stock, and a "comfort" letter or
letters signed by the Company's independent public accountants, each in
reasonable and customary form and covering such matters of the type customarily
covered by opinions or comfort letters delivered by such parties in underwritten
public offerings, and use its commercially reasonable efforts to have such
opinions and comfort letters addressed to and delivered to each stockholder;

     (g) Furnish unlegended certificates representing ownership of the shares of
Registrable Common Stock being sold in such denominations as shall be requested
by a Stockholder or the managing underwriter, if any, provided such request is
made at least two (2) business days prior to the closing of the sale of such
shares;

     (h) Promptly inform each Stockholder (i) in the case of any offering of
shares of Registrable Common Stock in respect of which a registration statement
is filed under the Securities Act, of the date on which such registration
statement or any post-effective amendment thereto becomes effective and, if
applicable, of the date of filing a Rule 430A prospectus (and, in the case of an
offering abroad of shares of Registrable Common Stock, of the date when any
required filing under the securities and other laws of such foreign
jurisdictions shall have been made and when the offering may be commenced in
accordance with such laws) and (ii) of any request by the SEC, any securities
exchange, government agency, self-regulatory body or other body having
jurisdiction for any amendment of or supplement to any registration statement or
preliminary prospectus or prospectus included therein or any offering memorandum
or other offering document relating to such offering;

     (i) Subject to subparagraph (k) below, until the earlier of (i) such time
as all of the shares of Registrable Common Stock being offered have been
disposed of in accordance with the intended method of disposition by such
Stockholder set forth in the registration statement or other offering document
(and the expiration of any prospectus delivery requirements in connection
therewith) or (ii) the expiration of nine (9) months after such registration
statement or other offering document becomes effective (unless the offering is a
continuous offering of 
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                                       10

securities under Rule 415, in which case until the earliest of the date the
offering is completed and the second anniversary of such effective date), keep
effective and maintain any registration, qualification or approval obtained in
connection with the offering of the shares of Registrable Common Stock, and
amend or supplement the registration statement or prospectus or other offering
document used in connection therewith to the extent necessary in order to comply
with applicable securities laws;

     (j) Use its commercially reasonable efforts to have the shares of
Registrable Common Stock listed on any domestic and foreign securities exchanges
on which the Common Stock is then listed;

     (k) As promptly as practicable, notify each Stockholder at any time when a
prospectus relating to the sale of the shares of Registrable Common Stock is
required by law to be delivered in connection with sales by an underwriter or
dealer, of the occurrence of an event requiring the preparation of a supplement
or amendment to such prospectus so that, as thereafter delivered to the
purchasers of such shares, such prospectus will not contain an untrue statement
of a material fact or omit to state any material fact required to be stated
therein or necessary to make the statement therein, in light of the
circumstances under which they were made, not misleading, and as promptly as
practicable make available to each Stockholder and to each managing underwriter,
if any, any such supplement or amendment; in the event the Company shall give
such notice, the Company shall extend the period during which such registration
statement shall be maintained effective as provided in Section 5.1(i) by the
number of days during the period from and including the date of the giving of
such notice to the date when the Company shall make available to each
Stockholder such supplemented or amended prospectus;

     (l) Make available for inspection during the normal business hours of the
Company by any Stockholder, any underwriter participating in such offering, and
any attorney, accountant or other agent retained by any such Stockholder or any
such underwriter in connection with the sale of shares of Registrable Common
Stock (collectively, the "Inspectors"), all relevant financial and other
records, pertinent corporate documents and properties of the Company as shall be
reasonably necessary to enable them to exercise their due diligence
responsibility, and cause the officers, directors and employees of the Company
to supply all information reasonably requested by any such Inspector in
connection with such registration statement; provided, however, that (i) in
connection with any such inspection, any such Inspectors shall cooperate to the
extent reasonably practicable to minimize any disruption to the operation by the
Company of its business and (ii) any records, information or documents shall be
kept confidential by such Inspectors, unless (1) such records, information or
documents are in the public domain or otherwise publicly available or (2)
disclosure of such records, information or documents is required by a court or
administrative order or by applicable law (including, without limitation, the
Securities Act);
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                                       11

     (m) Enter into usual and customary agreements (including an underwriting
agreement in usual and customary form) and take such other actions as are
reasonably required in order to expedite or facilitate the sale of the
Registrable Common Stock.

     (n) Make "generally available to its security holders" (within the meaning
of Rule 158 of the Securities Act) an earnings statement satisfying the
provisions of Section 11(a) of the Securities Act and Rule 158 thereunder no
later than 45 days after the end of the 12-month period beginning with the first
day of the Company's first fiscal quarter commencing after the effective date of
the registration statement, which earnings statement shall cover said 12-month
period;

     (o) If requested by the managing underwriter or underwriters or the
Stockholder, promptly incorporate in a prospectus supplement or post-effective
amendment such information as the managing underwriter or underwriters or any
participating Holder, as the case may be, reasonably requests to be included
therein, including, without limitation, information with respect to the number
of shares of Registrable Common Stock being sold by the Stockholder to any
underwriter or underwriters, the purchase price being paid therefor by such
underwriter or underwriters and with respect to any other terms of an
underwritten offering of the Registrable Common Stock to be sold in such
offering, and promptly make all required filings of such prospectus by
supplement or post-effective amendment;

     (p) As promptly as practicable after filing with the SEC of any document
which is incorporated by reference in a prospectus contained in a registration
statement, deliver a copy of such document to each Stockholder; and

     (q) Take all other steps necessary to effect the registration of the
Registrable Common Stock contemplated hereby.

     5.2  COVENANT OF STOCKHOLDERS.  Each Stockholder agrees and covenants that,
upon receipt of any notice from the Company of the happening of any event of the
kind described in Section 5.1(k) hereof, such Stockholder will forthwith
discontinue disposition of Registrable Common Stock pursuant to the registration
statement covering such Registrable Common Stock until such Stockholder's
receipt of the copies of the supplemented or amended prospectus contemplated by
Section 5.1(k) hereof, and, if so directed by the Company, such Stockholder will
deliver to the Company all copies, other than permanent file copies, then in
such Stockholder's possession of the most recent prospectus covering such
Registrable Securities at the time of receipt of such notice.
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                                       12

                                   SECTION 6
             RESTRICTIONS ON PUBLIC SALE BY THE COMPANY AND OTHERS

     The Company agrees:

     (a) Not to effect any public sale or distribution of any securities during
the 90-day period commencing on the effective date of a registration statement
filed pursuant to Section 2.1, except in connection with any merger,
acquisition, exchange offer, or any other business combination, including any
transaction within the scope of Rule 145 promulgated pursuant to the Securities
Act, subscription offer, dividend reimbursement plan or stock option or other
director or employee incentive or benefit plan;

     (b) That any agreement entered into after the date hereof pursuant to which
the Company grants registration rights with respect to the Company's securities
shall contain a provision under which holders of such securities agree, to the
extent not inconsistent with applicable laws, not to effect any public sale or
distribution of any such securities (excluding any sale in accordance with Rule
144 under the Securities Act) during the period commencing with the effective
date of a registration statement pursuant to Section 2.1 through the 90-day
period beginning on the date that the registration statement filed pursuant to
Section 2.1 becomes effective.

                                   SECTION 7
                                   EXPENSES

     All expenses incurred in connection with the registration of Registrable
Common Stock, including, without limitation, all filing fees, escrow fees, fees
and expenses of compliance with securities or blue sky laws (including fees and
disbursements of the Company's counsel in connection with blue sky
qualifications of the Registrable Common Stock), rating agency fees, printing
expenses, messenger and delivery expenses, internal expenses (including, without
limitation, all salaries and expenses of the Company's officers and employees
performing legal or accounting duties), the fees and expenses incurred in
connection with the listing of the securities to be registered on each
securities exchange on which similar securities issued by the Company are then
listed, and fees and disbursements of counsel for the Company and the reasonable
fees and disbursements of a single counsel for selling Stockholders and the
Company's independent certified public accountants (including the expenses of
any special audit or "cold comfort" letters required by or incident to such
performance) directly attributable to the registration of securities, Securities
Act liability insurance (if the Company elects to obtain such insurance) , and
the fees and expenses of any special experts or other persons retained by the
Company will be borne by the Company.  The Company shall have no obligation to
pay and shall not pay any underwriting fees, discounts or commissions in
connection with any Registrable Common Stock registered pursuant to this
Agreement or any out-of-pocket expenses of the holders in connection therewith
(except as expressly contemplated by the preceding sentence).
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                                       13

                                   SECTION 8
                                INDEMNIFICATION

     8.1  INDEMNIFICATION BY THE COMPANY.  The Company agrees to indemnify and
hold harmless each Stockholder, its officers, directors and agents, and will
agree to indemnify and hold harmless any underwriter of Registrable Common
stock, and each person, if any, who controls any of the foregoing persons within
the meaning of either Section 15 of the Securities Act or Section 20 of the
Exchange Act, from and against any and all losses, claims, damages and
liabilities (individually, a "Loss" collectively, "Losses") arising from or
caused by (x) any untrue statement or alleged untrue statement of a material
fact contained in any registration statement or prospectus relating to the
Registrable Common Stock (as amended or supplemented if the Company shall have
furnished any amendments or supplements thereto) or any preliminary prospectus,
or caused by any omission or alleged omission to state therein a material fact
required to be stated therein or necessary to make the statements therein not
misleading, and (y) any violation or alleged violation by the Company of the
Securities Act, any blue sky laws, securities laws or other applicable laws of
any state in which shares of Registrable Common Stock are offered and relating
to action or inaction required of the Company in connection with such offering;
and will reimburse each such person for any legal or other out-of-pocket
expenses reasonably incurred in connection with investigating, or defending
against, any such Loss (or any proceeding in respect thereof), subject to the
provisions of Section 8.3, except that the indemnification provided for in this
Section 8.1 shall not apply to Losses that are caused by any such untrue
statement or omission or alleged untrue statement or omission based upon and in
conformity with information furnished in writing to the Company by or on behalf
of any Stockholder expressly for use therein.  Notwithstanding the foregoing,
the Company shall not be liable in any such case to the extent that any such
Loss arises out of, or is based upon, an untrue statement or alleged untrue
statement or omission or alleged omission made in any preliminary prospectus if
(i) a Stockholder failed to send or deliver a copy of the prospectus included in
the relevant registration statement at the time it became effective (the
"Prospectus") with or prior to the delivery of written confirmation of the sale
of Registrable Common Stock to the person asserting such Loss or who purchased
such Registrable Common Stock which are the subject thereof if, in either case,
such delivery is required by the Securities Act and (ii) the Prospectus would
have corrected such untrue statement or omission or alleged untrue statement or
alleged omission; and the Company shall not be liable in any such case to the
extent that any such Loss arises out of, or is based upon, an untrue statement
or alleged untrue statement of a material fact or omission or alleged omission
to state a material fact in the Prospectus, if such untrue statement or alleged
untrue statement or omission or alleged omission is corrected in any amendment
or supplement to the Prospectus and if, having previously been furnished by or
on behalf of the Company with copies of the Prospectus as so amended or
supplemented, a Stockholder thereafter fails to deliver such Prospectus as so
amended or supplemented prior to 
<PAGE>
 
                                       14

and concurrently with the sale of Registrable Common Stock if such delivery is
required by the Securities Act.

     8.2  INDEMNIFICATION BY STOCKHOLDERS.  Each Stockholder agrees to indemnify
and hold harmless the Company, its officers and directors, and each person, if
any, who controls the Company within the meaning of either Section 15 of the
Securities Act or Section 20 of the Exchange Act to the same extent as the
indemnity made pursuant to clause (x) of Section 8.1 above from the Company to
such Stockholder, but only with reference to information furnished in writing by
or on behalf of such Stockholder expressly for use in any registration statement
or prospectus relating to shares of Registrable Common Stock, or any amendment
or supplement thereto, or any preliminary prospectus.

     8.3  CONDUCT OF INDEMNIFICATION PROCEEDINGS.  In case any proceeding
(including any governmental investigation) shall be instituted involving any
person in respect of which indemnity may be sought pursuant to Section 8.1 or
8.2, such person (the "Indemnified Party") shall promptly notify the person
against whom such indemnity may be sought (the "Indemnifying Party") in writing,
provided that the omission to so notify the Indemnifying Party will not relieve
the Indemnifying Party of any liability it may have under this Agreement or
otherwise except to the extent of any loss, damage, liability or expense arising
from such omission.  The Indemnifying Party, upon the request of the Indemnified
Party, shall retain counsel reasonably satisfactory to such Indemnified Party to
represent such Indemnified Party and any others the Indemnifying Party may
designate in such proceeding and shall pay the fees and disbursements of such
counsel related to such proceeding. In any such proceeding, any Indemnified
Party shall have the right to retain its own counsel, but the fees and expenses
of such counsel shall be at the expense of such Indemnified Party unless (i) the
Indemnifying Party and the Indemnified Party shall have mutually agreed to the
retention, (ii) the Indemnifying Party shall have failed to comply with its
obligations under the preceding sentence or (iii) the Indemnified Party shall
have been advised by its counsel in writing that actual or potential differing
interests exist between the Indemnifying Party and the Indemnified Party.  The
Indemnifying Party shall not be liable for any settlement of any proceeding
effected without its written consent, which consent shall not be unreasonably
withheld.  The Indemnifying Party shall not agree to any settlement as the
result of which any remedy or relief, other than monetary damages for which the
Indemnifying Party shall be fully responsible, shall be applied to or against an
Indemnified Party without the prior written consent of such Indemnified Party.

     8.4  CONTRIBUTION.  If the indemnification provided for in this Section 8.4
from the Indemnifying Party is unavailable to an Indemnified Party hereunder in
respect of any losses, claims, damages, liabilities or expenses referred to
therein, then the Indemnifying Party, in lieu of indemnifying such Indemnified
Party, shall contribute to the amount paid or payable by such Indemnified Party
as a result of such losses, claims, damages, liability or expenses in such
proportion as is appropriate to reflect the relative fault of the Indemnifying
Party and 
<PAGE>
 
                                       15

Indemnified Party in connection with the actions which resulted in such losses,
claims, damages, liabilities or expenses, as well as any other relevant
equitable considerations. The relative fault of such Indemnifying Party and
Indemnified Party shall be determined by reference to, among other things,
whether any action in question, including any untrue or alleged untrue statement
of a material fact or omission or alleged omission to state a material fact, has
been made by, or relates to information supplied by, such Indemnifying Party or
Indemnified Party, and the parties' relative intent, knowledge, access to
information and opportunity to correct or prevent such action. The amount paid
or payable by a party as a result of the losses, claims, damages, liabilities
and expenses referred to above shall be deemed to include, subject to the
limitations set forth in Section 8.3, any legal or other fees or expenses
reasonably incurred by such party in connection with any investigation or
proceeding. No party shall be liable for contribution with respect to any action
or claim settled without its written consent, which consent shall not be
unreasonably withheld.

     Notwithstanding the provisions of this Section 8.4, no Stockholder shall be
required to contribute any amount in excess of the amount by which the total
price at which the Registrable Common Stock of such Stockholder was offered to
the public exceeds the amount of any damages which such Stockholder has
otherwise been required to pay by reason of such untrue or alleged untrue
statement or omission of alleged omission.  The parties hereto agree that it
would not be just and equitable if contribution pursuant to this Section 8.4
were determined by pro rata allocation or by any other method of allocation
which does not take into account the equitable considerations referred to in the
immediately preceding paragraph.  No person guilty of fraudulent
misrepresentation (within the meaning of Section 11(f) of the Securities Act)
shall be entitled to contribution from any person who was not guilty of such
fraudulent misrepresentation.

                                   SECTION 9
                                  TERMINATION

     This Agreement shall terminate with respect to any Stockholder upon the
earlier of (i) the first such instance as such Stockholder ceases to own any
shares of Common Stock or (ii) the first such instance after the fifth
anniversary of the Effective Time as such Stockholder ceases to own at least
five percent of the outstanding shares of Common Stock.  For the purposes of
this Section 9, a Stockholder shall be deemed to own any and all Common Stock
owned by (i) such Stockholder and (ii) its Affiliates.  Notwithstanding the
foregoing, the Company's and Stockholders' rights, duties and obligations under
Section 7 and Section 8 shall survive the termination of this Agreement.
<PAGE>
 
                                       16

                                   SECTION 10
                             AVAILABLE INFORMATION

     The Company shall take such reasonable action and file such information,
documents and reports as shall be required by the SEC as a condition to the
availability of Rule 144 and Rule 144A, or any successor provisions.


                                   SECTION 11
                              ASSIGNMENT OF RIGHTS

     11.1  ASSIGNMENT OF RIGHTS.  Subject to Section 11.2 and the provisions of
the Stockholders Agreement (in the form attached as Exhibit 3.2 to the
Combination Agreement), the Registrable Common Stock and rights of any
Stockholder under this Agreement with respect to any Registrable Common Stock
owned by such Stockholder may be assigned to any person who acquires Common
Stock from a Stockholder, except that any person who acquires such Common Stock
(x) pursuant to a public offering registered under the Securities Act, or (y)
pursuant to a transfer made in accordance with Rule 144 under the Securities Act
(or any similar successor provision) may not be assigned rights hereunder with
respect to such Common Stock.  Notwithstanding the foregoing, rights to cause
one or more demand registrations under Section 2.1 may only be assigned if such
rights are expressly assigned in writing from BG, NOVA, Chevron, a BG Holder, a
NOVA Holder or a Chevron Holder.  Any assignment of registration rights pursuant
to this Section 11.1 shall be effective upon receipt by the Company of written
notice from such assigning Stockholder (i) stating the name and address of any
assignee, (ii) describing the manner in which the assignee acquired Common Stock
from such Stockholder and (iii) identifying the Registrable Common Stock with
respect to which the rights under this Agreement are being assigned.

     11.2  SCOPE OF ASSIGNMENT.  The rights of an assignee under Section 11.1
shall be the same rights granted to the assigning Stockholder under this
Agreement, except that in no event shall the Company's obligations hereunder be
increased due to any such assignment, In connection with any such assignment,
the term "Stockholder" as used herein shall, where appropriate to assign the
rights and obligations of the assigning Stockholder hereunder to such assignee,
be deemed to refer to the assignee.  After any such assignment, the assigning
Stockholder shall retain its rights under this Agreement with respect to all
other Registrable Common Stock owned by such Stockholder.
<PAGE>
 
                                       17

                                   SECTION 12
                                 MISCELLANEOUS

     12.1  PROVISION OF INFORMATION.  Each Stockholder shall, and shall cause
its officers, directors, employees and agents to complete and execute all such
questionnaires as the Company shall reasonably request in connection with any
registration pursuant to this Agreement.

     12.2  INJUNCTIONS.  Irreparable damage would occur in the event that any of
the provisions of this Agreement were not performed in accordance with its
specified terms or were otherwise breached.  Therefore, the parties hereto shall
be entitled to an injunction or injunctions to prevent breaches of the
provisions of this Agreement and to enforce specifically the terms of provisions
hereof in any court having jurisdiction, such remedy being in addition to any
other remedy to which they may be entitled at law or in equity.

     12.3  SEVERABILITY.  If any term or provision of this Agreement is held by
a court of competent jurisdiction to be invalid, void, or unenforceable, the
remainder of the terms and provisions set forth herein shall remain in full
force and effect and shall in no way be affected, impaired or invalidated, and
the parties hereto shall use their best efforts to find and employ an
alternative means to achieve the same or substantially the same result as that
contemplated by such term or provision.

     12.4  FURTHER ASSURANCES.  Subject to the specific terms of this Agreement,
each Stockholder and the Company shall make, execute, acknowledge and deliver
such other instruments and documents, and take all such other actions, as may be
reasonably required in order to effectuate the purposes of this Agreement and to
consummate the transactions contemplated hereby.

     12.5  ENTIRE AGREEMENT; MODIFICATION.  This Agreement contains the entire
understanding of the parties with respect to the transactions contemplated
hereby and supersedes all agreements and understandings entered into prior to
the execution hereof, including the rights, if any, of the Parties hereto under
the agreements providing for the Existing Registration Rights and the Former
Trident Holders Registration Rights.  This Agreement may be modified only by a
written instrument duly executed by or on behalf of (i) the Company, (ii) each
BG Holder, (iii) each NOVA Holder and (iv) each Chevron Holder.  No breach of
any covenant, agreement, warranty or representation shall be deemed waived
unless expressly waived in writing by or on behalf of the party who might assert
such breach.
<PAGE>
 
                                       18

     12.6  COUNTERPARTS.  For the convenience of the parties hereto, any number
of counterparts of this Agreement may be executed by the parties hereto, but all
such counterparts shall be deemed one and the same instrument.

     12.7  NOTICES.  All notices, consents, requests, demands, and other
communications hereunder shall be in writing and shall be given by hand or by
mail (return receipt requested) or sent by overnight delivery service, cable,
telegram, or facsimile transmission to the parties at the address specified
beside each party's name on the signature pages hereto or at such other address
as shall be specified by the parties by like notice.

     Notice so given shall, in the case of notice so given by mail, be deemed to
be given and received on the fourth business day after posting, in the case of
notice so given by overnight delivery service, on the day after notice is
deposited with such service, and in the case of notice so given by cable,
telegram, facsimile transmission or, as the case may be, personal delivery, on
the date of actual delivery.

     12.8  GOVERNING LAW.  THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED AND
ENFORCED IN ACCORDANCE WITH THE LAWS OF THE STATE OF DELAWARE WITHOUT REGARD TO
ANY CHOICE OF LAW PRINCIPLES WHICH MIGHT REQUIRE OR PERMIT THE APPLICATION OF
THE LAWS OF ANOTHER JURISDICTION.

     12.9  SUCCESSORS AND ASSIGNS.  This Agreement shall be binding upon and
shall inure to the benefit of and be enforceable by and against the successors
and permitted assigns of the parties hereto.  Except as provided herein, the
parties may not assign their rights under this Agreement and the Company may not
delegate its obligations under this Agreement.  Any attempted assignment or
delegation prohibited hereby shall be void.

     12.10  PARTIES IN INTEREST.  Except as otherwise specifically provided
herein, nothing in this Agreement expressed or implied is intended or shall be
construed to confer any right or benefit upon any person, firm or corporation
other than Stockholder and the Company and their respective successors and
permitted assigns.

     12.11  SHARES SUBJECT TO THIS AGREEMENT; EFFECTIVE TIME.  All shares of
Registrable Common Stock owned or acquired by any Stockholder at or after the
Effective Time shall be subject to, and entitled to the benefit of this
Agreement, such rights to be effective from and after the Effective Time.  This
Agreement shall be null and void and of no effect upon the termination of the
Combination Agreement in accordance with its terms.
<PAGE>
 
                                       19

     IN WITNESS WHEREOF, each Stockholder and the Company have caused this
Agreement to be duly executed as of the date first above written.

                                       NGC CORPORATION


                                       By:________________________________
Address: 13430 Northwest Freeway       Name:______________________________
         Houston, TX 77040             Title:_____________________________


                                       BG HOLDINGS, INC.

Number of Shares of
  Common Stock: 38,623,211
                                       By:________________________________
Address:  1100 Louisiana, Suite 2500   Name:______________________________
          Houston, TX 77002            Title:_____________________________
          Telecopier: (713) 754-7785
with a copy to:


                                       NOVA GAS SERVICES (U.S.) INC.

Number of Shares of
  Common Stock: 38,623,211
 
Address:  690 Mechanic Street          By:________________________________
          Leominster, MA 01453         Name:______________________________
          Attn: Dave Carpenter,        Title:_____________________________
                Secretary  
          Telecopier: (508) 840-6683

with copies to: Jack S. Mustoe
                Senior Vice President and General Counsel
                NOVA Corporation
                801 Seventh Avenue, S.W.
                Calgary, Alberta CANADA T2P 3P7
                Telecopier: (403) 261-3557

<PAGE>
 
                                       20

and:      Alan Talkington
          Orrick Herrington & Sutcliffe
          400 Sansome Street
          San Francisco, CA 94111
          Telecopier:  (415) 773-5759


                                       CHEVRON U.S.A. INC.

Number of Shares of
  Common Stock: 38,623,211
Number of Shares of Series A
  Preferred Stock: 7,815,363
 
Address:  1301 McKinney Street         By:________________________________
          Houston, TX 77010            Name:______________________________
          Attn: President of Chevron   Title:_____________________________
          U.S.A. Production Company
          Telecopier:

with copies to: Harvey D. Hinman
                Vice President and General Counsel
                Chevron Corporation
                575 Market Street
                San Francisco, CA 94105
                Telecopier: (415) 894-6017

and:      Terry Michael Kee
          Pillsbury Madison & Sutro LLP
          235 Montgomery Street
          San Francisco, CA 94104
          Telecopier: (415) 982-1200