FIRST AMENDMENT TO CREDIT AGREEMENT

THIS FIRST AMENDMENT TO CREDIT AGREEMENT (this "Amendment") is entered into as of March 29, 2002, among
MARKWEST RESOURCES CANADA CORP. (previously known as MarkWest Acquisitions Corp.), an Alberta corporation (the "Borrower"), the undersigned Guarantors
(the "Guarantors"), BANK OF AMERICA, N.A. [acting through its Canadian branch], successor to Bank of America Canada, as Canadian
Administrative Agent (the"Administrative Agent")for the Lenders parties to the hereinafter defined
Credit Agreement, and the undersigned Lenders.

Reference
is made to the Credit Agreement dated as of October 12, 2001, (the"Credit Agreement") among Borrower, Administrative
Agent, and the Lenders parties thereto. Unless otherwise defined in this Amendment, capitalized terms used herein shall have the meaning set forth in the Credit Agreement; all section and
schedule references herein are to sections and schedules in the Credit Agreement; and all paragraph references herein are to paragraphs in this Amendment.

RECITALS

A.On the Effective Date as herein defined and immediately prior to the effectiveness of this Amendment, Bank of America,
N.A. [acting through its Canadian branch], is assigning a portion of its Commitment and its Loans to Royal Bank of Canada pursuant to a Lender Assignment Agreement.

B.The Borrower has requested certain amendments to the Credit Agreement in order to, among other things, (i) decrease
the amount of the facility, (ii) add a borrowing base, and (iii) amend certain financial covenants.

C.Subject to the terms and conditions of this Amendment, the Administrative Agent and the undersigned Lenders are willing to
agree to such amendments.

Accordingly,
for adequate and sufficient consideration, the parties hereto agree, as follows:



1.1Definitions.

(a)  The
definition of "Applicable Margin" is amended in its entirety to read as follows:



Applicable MarginPricing
Level

Leverage Ratio

LIBO Rate Loan

Letter of Credit

Canadian
Prime Rate
Loan

Commitment Fee1

Less than or equal to 1.0X

175.0 b.p.

175.0 b.p.

37.5 b.p.

25.0 b.p.2

Greater than 1.0X, but less than or equal to 2.0X

200.0 b.p.

200.0 b.p.

62.5 b.p.

30.0 b.p.3

Greater than 2.0X, but less than or equal to 2.75X

225.0 b.p.

225.0 b.p.

87.5 b.p.

37.5 b.p.4

Greater than 2.75X, but less than or equal to 3.25X

250.0 b.p.

250.0 b.p.

112.5 b.p.

50.0 b.p.5

Greater than 3.25X

275.0 b.p.

275.0 b.p.

137.5 b.p.

50.0 b.p.




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(b)  The
following definition of "Available Borrowing Base" is inserted alphabetically into Section 1.1:



(c)  The
following definition of "Borrowing Base Period" is inserted alphabetically into Section 1.1:



(d)  The
following definition of "Canadian Borrowing Base" is inserted alphabetically into Section 1.1:



(e)  The
definition of "Canadian Dollar Equivalent" is hereby amended to read as follows:



(f)    The
following definition of "Canadian Effective Amount" is inserted alphabetically into Section 1.1:



(g)  The
definition of "Canadian Resident Lender" is amended by deleting the phrase "that is not" and replacing it with the
phrase "whose Canadian business income is not treated as income of".

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(h)  The
definition of "Fixed Charge Coverage Ratio" is amended by (i) deleting the term "12 months" and
replacing it with the term "four Fiscal Quarters" and (ii) deleting each reference to the term "12-month" and replacing each such reference with the term "four-Fiscal
Quarter."

(i)    The
following definition of "Global Semi-Annual Borrowing Base" is inserted alphabetically in Section 1.1:



(j)    The
definition of "Letter of Credit Sublimit" is amended in its entirety to read as follows: "Letter of Credit Sublimit"
means the amount of Cdn. $7,828,500.

(k)  The
definition of "Leverage Ratio" is amended by (i) deleting the phrase "twelve most recently
ended months" and replacing it with the phrase "four Fiscal Quarters most recently ended" and (ii) deleting each reference to the term "12 months" and replacing each such
reference with the term "four Fiscal Quarters."

(l)    The
following definition of"Midstream Business"is inserted alphabetically into Section 1.1.:



(m)  The
following definition of"Midstream Business"is inserted alphabetically into Section 1.1.:



(n)  The
definition of "Revolving Loan Commitment Amount" is amended by deleting the phrase "the Canadian Dollar Equivalent of
U.S. $35,000,000" and replacing such phrase with the term "Cdn. $54,799,500."

(o)  The
following definition of "U.S. Borrowing Base" is inserted alphabetically in Section 1.1:



(p)  The
definition of "U.S. Dollar Equivalent" is hereby amended to read as follows:



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(q)  The
following definition of "U.S. Effective Amount" is inserted alphabetically into Section 1.1:



1.2Section 2.1.1(a). Section 2.1.1(a)is deleted in its
entirety and amended to read as follows:



1.3Section 2.1.3. Section 2.1.3is deleted in its entirety and amended to read as
follows:



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1.4.Section 2.1.5. Section 2.1.5is amended by deleting each reference to the term
"Revolving Loan Commitment Amount" and replacing each such reference with the
term "Canadian Borrowing Base."

1.5Section 2.1.6.Section 2.1.6is
amended by (i) deleting the number "$20,000,000" and replacing it with the number "$10,000,000" and (ii) deleting the number "$110,000,000" and replacing it with the number
"$100,000,000."

1.6Section 3.1(c). Section 3.1(c)is deleted in its entirety.

1.7Section 3.1.2. Section 3.1.2is deleted in its entirety and
amended to read as follows:



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1.8Section 3.3.2. Section 3.3.2is amended to add the following phrase immediately
prior to the period at the end of such section:



1.9Section 7.1.1(c). Section 7.1.1(c)is amended by deleting the term "month" and
replacing it with the term "Fiscal Quarter."

1.10Section 7.1.1(k). Section 7.1.1(k)is amended to read as follows:



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1.11Section 7.1.8. Section 7.1.8is amended to add the
following phrase immediately prior to the period at the end of subsections (a) and (b):



1.12Section 7.2.2. Section 7.2.2is amended by:
(i) deleting the term "the Canadian Dollar Equivalent of U.S. $2,700,000" and replacing it with the term "Cdn.. $4,227,390," and (ii) deleting the term "the Canadian Dollar Equivalent of
U.S. $10,000,000" and replacing it with the term "Cdn.. $15,657,000."

1.13Section 7.2.4(a). Section 7.2.4(a)is amended by (i) deleting each reference to
the term "calendar month" and replacing each such reference with the term
"Fiscal Quarter" and (ii) deleting the term "after Closing" and replacing it with the term "after July 1, 2001."

1.14Section 7.2.4(b). Section 7.2.4(b)is amended by deleting the term "calendar month" and
replacing it with the term "Fiscal Quarter."

1.15Section 7.2.4(c). Section 7.2.4(c)is deleted in its entirety and amended to read as
follows:



Maximum Leverage Ratio

As of the Date of
each Fiscal Quarter Ending
as Set Forth Below4.00

12/31/013.75

3/31/023.50

6/30/023.25

9/30/023.00

12/31/02 and Fiscal Quarters ending thereafter"


1.16Section 7.2.4(d). Section 7.2.4(d)is deleted in its entirety and amended to read as
follows:



Minimum Fixed Charge Coverage Ratio

As of the Date of
each Fiscal Quarter Ending
as Set Forth Below1.40

12/31/011.50

3/31/02 through 12/31/021.75

3/31/03 and thereafter"


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1.17Section 7.2.9(b). Section 7.2.9(b)is deleted in its
entirety and amended to read as follows:



1.18Section 7.2.15. A new Section 7.2.15is hereby inserted to read as follows:



1.19Section 10.1(a). Section 10.1(a)is amended by deleting the word "or" appearing
immediately before subclause (e) thereof and inserting the following phrase
immediately after such subclause (e): "or (f) increase the Canadian Borrowing Base pursuant toSection 2.1.3of the U.S. Credit Agreement
without the consent of the Lenders or increase the Global Borrowing Base pursuant toSection 2.1.3of

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the U.S. Credit Agreement without the consent of the Global Lenders (as defined in the U.S. Credit Agreement),provided, the Required Lenders may
maintain or decrease the Canadian Borrowing Base pursuant toSection 2.1.3of the U.S. Credit Agreement and the Required Global Lenders (as
defined in the U.S. Credit Agreement) may maintain or decrease the Global Borrowing Base pursuant toSection 2.1.3."

1.20Schedule V. Schedule Vto the Credit Agreement is hereby amended to read as set forth
inSchedule Vattached hereto.

1.21Exhibit F. Exhibit F to the Credit Agreement is hereby
amended to read as Exhibit F attached hereto.

Paragraph 2. Effective Date.Notwithstanding any contrary provision, this Amendment is not
effective until the date (the"Effective Date") the following conditions are satisfied.

(a)  The
Administrative Agent shall have received counterparts of this Amendment, executed by the Borrower, the Guarantors, and the Required Lenders.

(b)  The
Administrative Agent shall have received counterparts of the Guaranty (MarkWest Energy Appalachia, L.L.C.), the Pledge and Security Agreement (MarkWest Energy
Appalachia, L.L.C.), and the Acknowledgment of Pledge, each executed by MarkWest Energy Appalachia, L.L.C. and the related financing statement

(c)  The
Administrative Agent shall have received from the Borrower and each Guarantor a certificate dated as of the Effective Date of its secretary, assistant secretary,
manager or general partner as applicable (i) as to resolutions of its board of directors or managers or their equivalent authorizing the execution and performance of this Amendment, and
(ii) if the officer executing this Amendment is not named in the incumbency certificate delivered at the time of execution of the Credit Agreement, as to the incumbency and signature of said
officer.

(d)  The
Administrative Agent shall have received a legal opinion from counsel to the Borrower and each Guarantor as to the due authorization, execution and delivery of this
Amendment.

(e)  The
"Effective Date" shall occur simultaneously as defined in the First Amendment to the U.S. Credit Agreement.

(f)    Fees
and expenses required to be paid pursuant to Paragraph 5 of this Amendment have been paid to the extent invoiced prior to the Effective Date.

(g)  The
Administrative Agent shall have received such other assurances, certificates, documents and consents as the Administrative Agent may require.

Paragraph 3. Successor Canadian Administrative Agent.Pursuant to Section 9.4of the Credit Agreement, Bank of America Canada has resigned as Canadian Administrative Agent and the Lenders hereby appoint Bank of
America, N.A. [acting through its Canadian branch], successor to Bank of America Canada as the successor Canadian Administrative Agent for the Lenders under the Credit
Agreement for all purposes of the Credit Agreement, the Intercreditor Agreement, the Notes and each other Loan Document.

Paragraph 4. Acknowledgment and Ratification.As a material inducement to the Administrative
Agent and the Lenders to execute and deliver this Amendment, each of the Borrower and the Guarantors (a) consents to the agreements in this Amendment and (b) agrees and acknowledges that
the execution, delivery, and performance of this Amendment shall in no way release, diminish, impair, reduce, or otherwise affect the respective obligations of the Borrower or any Guarantor under the
Loan Documents to which it is a party, which Loan Documents shall remain in full force and effect, and all rights thereunder are hereby ratified and confirmed.

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Paragraph 5. Representations.As a material inducement to the Administrative Agent and the
Lenders to execute and deliver this Amendment, each of the Borrower and the Guarantors represents and warrants to the Administrative Agent and the Lenders that as of the Effective Date of this
Amendment and as of the date of execution of this Amendment, (a) all representations and warranties in the Loan Documents are true and correct in all material respects as though made on the
date hereof, except to the extent that any of them speak to a different specific date, and (b) no Default or Event of Default exists.

Paragraph 6. Expenses.The Borrower shall pay within ten (10) days of demand all
reasonable costs, fees, and expenses paid or incurred by the Administrative Agent incident to this Amendment, including, without limitation, the fees and expenses of the Administrative Agent's counsel
in connection with the negotiation, preparation, delivery, and execution of this Amendment and any related documents and filing and recording costs, and costs of title insurance endorsements if
required.

Paragraph 7. Miscellaneous.This Amendment is a"Loan
Document"referred to in the Credit Agreement. The provisions relating to Loan Documents inArticle 10of the Credit
Agreement are incorporated in this Amendment by reference. Unless stated otherwise (a) the singular number includes the plural andvice versaand
words of any gender include each other gender, in each case, as appropriate, (b) headings and captions may not be construed in interpreting provisions, (c) this Amendment must be
construed, and its performance enforced, under Alberta law and applicable Canadian federal law, (d) if any part of this Amendment is for any reason found to be unenforceable, all other portions
of it nevertheless remain enforceable, and (e) this Amendment may be executed in any number of counterparts with the same effect as if all signatories had signed the same document, and all of
those counterparts must be construed together to constitute the same document.

Paragraph 8. Entire Agreement.This Amendment represents the final
agreement between the parties about the subject matter of this Amendment and may not be contradicted by evidence of prior, contemporaneous, or subsequent oral agreements of the parties. There are no
unwritten oral agreements between the parties.

Paragraph 9. Parties.This Amendment binds and inures to the benefit of the Borrower, the
Guarantors, the Administrative Agent, the Lenders, and their respective successors and assigns.

Paragraph 10. Further Assurances.The parties hereto each agree to execute from time to time
such further documents as may be necessary to implement the terms of this Agreement.

The
parties hereto have executed this Amendment in multiple counterparts to be effective as of the Effective Date.

Remainder of Page Intentionally Blank.
Signature Pages to Follow.

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| |  | BORROWER                                                                   
| |  | ---------------------------------------------------------------------------
| |  | MARKWEST RESOURCES CANADA CORP.                                            
| |  | By:                                                                        
| |  | GUARANTORS                                                                 
| |  | MARKWEST RESOURCES, INC., a Colorado corporation                           
| |  | By:                                                                        
| |  | MARKWEST MICHIGAN, INC., a Colorado corporation                            
| |  | By:                                                                        
| |  | WEST SHORE PROCESSING COMPANY, L.L.C., a Michigan limited liability company
| |  | By: Markwest Michigan, Inc., its Manager                                   
| |  | By:                                                                        
| |  |                                                                            
| |  | Name:                                                                      
| |  |                                                                            
| |  | Title:                                                                     
| |  |                                                                            
| |  | BASIN PIPELINE, L.L.C., a Michigan limited liability company               
| |  | By: Markwest Michigan, Inc., its Manager                                   
| |  | By:                                                                        
| |  |                                                                            
| |  | Name:                                                                      
| |  |                                                                            
| |  | Title:                                                                     
| |  |                                                                            

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| |  | MATREX, L.L.C., a Michigan limited liability company                                                                                             
| |  | -------------------------------------------------------------------------------------------------------------------------------------------------
| |  | By: Markwest Resources, Inc., its Manager                                                                                                        
| |  | By:                                                                                                                                              
| |  |                                                                                                                                                  
| |  | Name:                                                                                                                                            
| |  |                                                                                                                                                  
| |  | Title:                                                                                                                                           
| |  |                                                                                                                                                  
| |  | MARKWEST HYDROCARBON, INC.
a Delaware corporation                                                                                                
| |  | By:                                                                                                                                              
| |  |                                                                                                                                                  
| |  | Name:                                                                                                                                            
| |  |                                                                                                                                                  
| |  | Title:                                                                                                                                           
| |  |                                                                                                                                                  
| |  | MARKWEST CANADIAN MIDSTREAM SERVICES, INC. an Alberta corporation                                                                                
| |  | By:                                                                                                                                              
| |  |                                                                                                                                                  
| |  | Name:                                                                                                                                            
| |  |                                                                                                                                                  
| |  | Title:                                                                                                                                           
| |  |                                                                                                                                                  
| |  | MARKWEST ENERGY APPALACHIA, L.L.C., a Delaware limited liability company                                                                         
| |  | By: Markwest Hydrocarbon, Inc., a Delaware corporation, its sole member                                                                          
| |  | By:                                                                                                                                              
| |  |                                                                                                                                                  
| |  | Name:                                                                                                                                            
| |  |                                                                                                                                                  
| |  | Title:                                                                                                                                           
| |  |                                                                                                                                                  
| |  | BANK OF AMERICA, N.A.
[acting through its Canadian branch], successor to Bank of America Canada, as Canadian Administrative Agent and as a Lender
| |  | By:                                                                                                                                              
| |  |                                                                                                                                                  
| |  | Name:                                                                                                                                            
| |  |                                                                                                                                                  
| |  | Title:                                                                                                                                           
| |  |                                                                                                                                                  

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| |  | U.S. BANK, NATIONAL ASSOCIATION, as a Lender
| |  | --------------------------------------------
| |  | By:                                         
| |  |                                             
| |  | Name:                                       
| |  |                                             
| |  | Title:                                      
| |  |                                             

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