Exhibit 10.13

FIRST AMENDMENT
TO
ASSET PURCHASE AND SALE AGREEMENT

between

WILLIAMS REFINING & MARKETING, L.L.C.,
WILLIAMS GENERATING MEMPHIS, L.L.C.,
WILLIAMS MEMPHIS TERMINAL, INC.,
WILLIAMS PETROLEUM PIPELINE SYSTEMS, INC.
AND
WILLIAMS MID-SOUTH PIPELINES, LLC

Sellers

and

THE WILLIAMS COMPANIES. INC.

Sellers' Guarantor

and

THE PREMCOR REFINING GROUP, INC.

Purchaser

and

PREMCOR INC.

Purchaser's Guarantor

This First Amendment is made effective as of January 16, 2003 by and
among the Sellers, Sellers' Guarantor, the Purchaser and Purchaser's Guarantor
(the "Parties").

RECITALS

A. The Parties made and entered into that certain Asset Purchase and
Sale Agreement as of the 25/th/ day of November, 2002 (the "Asset PSA").

B. The Parties now desire to amend the Asset PSA as set forth herein.

In consideration of the matters set forth herein, the Parties agree that
the Asset PSA is hereby amended in the following particulars:

1. Exhibit H - Financing Commitments. First Amended Exhibit H is attached
hereto. It replaces and supersedes Exhibit H to the Asset PSA. Pursuant to
Section 5.l(i) of the Asset PSA, First Amended Exhibit H incorporates copies of
the Purchaser's financing commitment letters from Blackstone Management
Associates III, Occidental C.O.B. Partners, Thomas D. O'Malley, Morgan Stanley
Senior Funding, Inc. and Morgan Stanley Capital Group Inc

2. Schedules 6.2(m) and 6.2(n). First Amended Schedules 6.2(m) and 6.2(n)
are attached hereto and replace and supersede Schedules 6.2(m) and 6.2(n) to the
Asset PSA.

3. Sellers Closing Obligations. The first phrase of Section 3.2(a) is
amended to read as follows: "Sellers shall deliver to Purchaser or to
Purchaser's Affiliates or designees (as specified in writing at least three (3)
business days prior to Closing):"

[THE BALANCE OF THIS PAGE HAS INTENTIONALLY BEEN LEFT BLANK, WITH
SIGNATURES APPEARING ON THE NEXT PAGE]

Signature page to that certain First Amendment to Asset Purchase and Sale
Agreement between Williams Refining & Marketing, LLC., Williams Generating
Memphis, L.L.C., Williams Memphis Terminal, Inc., Williams Petroleum Pipeline
Systems, Inc and Williams Mid-South Pipelines, LLC (Sellers), The Williams
Companies, Inc. (Sellers' Guarantor) and Premcor Refining Group, Inc.
(Purchaser) and Premcor Inc. (Purchaser's Guarantor)

IN WITNESS WHEREOF, the Parties have caused this First Amendment to be
executed by their duly authorized representatives as of the date first above
written.

"Purchaser" "Sellers"

The Premcor Refining Group, Inc. Williams Refining & Marketing, L.L.C.

By: /s/ Michael Gayda By: ____________________________________
-------------------------------------- Name: ______________________________
Name: Michael Gayda Title: _____________________________
---------------------------------
Title: Senior V.P. Williams Generating Memphis, L.L.C.
--------------------------------

"Purchaser's Guarantor" By: ____________________________________
Name: ______________________________
Premcor Inc. Title: _____________________________

By: /s/ Michael Gayda Williams Memphis Terminal, Inc.
--------------------------------------
Name: Michael Gayda
--------------------------------- By: ____________________________________
Title: Senior V.P. and General Counsel Name: ______________________________
-------------------------------- Title: _____________________________

Williams Petroleum Pipeline Systems, Inc.

By: ____________________________________
Name: ______________________________
Title: ____________________________
"Sellers' Guarantor"

The Williams Companies, Inc. Williams Mid-South Pipeline, LLC

By: ______________________________________ By: ____________________________________
Name: ________________________________ Name: ______________________________
Title: _______________________________ Title: _____________________________

Signature page to that certain First Amendment to Asset Purchase and Sale
Agreement between Williams Refining & Marketing, LLC., Williams Generating
Memphis, L.L.C., Williams Memphis Terminal, Inc., Williams Petroleum Pipeline
Systems, Inc. and Williams Mid-South Pipelines, LLC (Sellers), The Williams
Companies, Inc. (Sellers' Guarantor) and Premcor Refining Group, Inc.
(Purchaser) and Premcor Inc. (Purchaser's Guarantor)

IN WITNESS WHEREOF, the Parties have caused this First Amendment to be
executed by their duly authorized representatives as of the date first above
written.

"Purchaser" "Sellers"

The Premcor Refining Group, Inc. Williams Refining & Marketing, L.L.C.

By: ______________________________________ By: /s/ Randy M. Newcomer
Name: ________________________________ ------------------------------------
Title: _______________________________ Name: Randy M. Newcomer
------------------------------
Title: President
-----------------------------

Williams Generating Memphis, L.L.C.
"Purchaser's Guarantor"
By: /s/ Randy M. Newcomer
Premcor Inc. ------------------------------------
Name: Randy M. Newcomer
------------------------------
Title: President
-----------------------------

By: ______________________________________ Williams Memphis Terminal, Inc.
Name: ________________________________
Title: _______________________________ By: /s/ Randy M. Newcomer
------------------------------------
Name: Randy M. Newcomer
------------------------------
Title: President
-----------------------------

Williams Petroleum Pipeline Systems, Inc.

By: /s/ Randy M. Newcomer
------------------------------------
Name: Randy M. Newcomer
------------------------------
Title: Vice President
-----------------------------

"Sellers' Guarantor"
Williams Mid-South Pipeline, LLC
The Williams Companies, Inc.
By: /s/ Randy M. Newcomer
/s/ Philip D. Wright ------------------------------------
By: -------------------------------------- Name: Randy M. Newcomer
Name: Philip D. Wright ------------------------------
-------------------------------- Title: President
Title: Chief Restructuring Officer -----------------------------
-------------------------------