EXHIBIT 99.6

D.F. KING & CO., INC.
77 WATER STREET
NEW YORK, NY 10005-4495
(212) 269-5550

RE: MERGER OF 46 PARKER & PARSLEY LIMITED PARTNERSHIPS

As described in the proxy statement/prospectus dated October 12, 2001 (the
"Prospectus"), of Pioneer Natural Resources Company ("Pioneer Parent") and
Pioneer Natural Resources USA, Inc. ("Pioneer USA"), Pioneer Parent and Pioneer
USA are proposing to merge (the "Transaction") the 46 limited partnerships named
below (the "Partnerships") with and into Pioneer USA. Copies of the Prospectus
have been mailed directly to your clients, along with proxy cards for use by
your clients in voting their units representing limited partnership interests
(the "Units").

TO GIVE EFFECT TO THE VOTES CAST DIRECTLY BY YOUR CLIENTS, YOUR FIRM MUST
SIGN AND RETURN THE ACCOMPANYING OMNIBUS CONSENT TO MR. TOM LONG OF D.F. KING &
CO., INC. AT 77 WATER STREET, 20TH FLOOR, NEW YORK, NY 10005 OR BY FAX TO (212)
809-8839.

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NOTE

In the Transaction, your clients are being asked to vote "FOR", "AGAINST"
or "ABSTAIN" on their Partnerships' participation in the Transaction.

A FAILURE TO SIGN AND RETURN THE ACCOMPANYING OMNIBUS CONSENT MAY
INVALIDATE YOUR CLIENTS' VOTES WITH POTENTIALLY ADVERSE CONSEQUENCES FOR YOUR
CLIENTS. ACCORDINGLY, YOU ARE URGED TO ACT PROMPTLY.

TO ASSURE THAT YOUR OMNIBUS CONSENT IS RECEIVED BY US IN TIME FOR
FORWARDING TO AND RECEIPT BY THE TABULATION AGENT BEFORE THE VOTING DEADLINE, WE
MUST RECEIVE YOUR OMNIBUS CONSENT BY 12:00 NOON, NEW YORK CITY TIME, ON
DECEMBER 19, 2001.

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The Partnerships are:

<Table>

Parker & Parsley 81-I, Ltd. Parker & Parsley 88-A Conv., L.P.
Parker & Parsley 81-II, Ltd. Parker & Parsley 88-A, L.P.
Parker & Parsley 82-I, Ltd. Parker & Parsley 88-B Conv., L.P.
Parker & Parsley 82-II, Ltd. Parker & Parsley 88-B, L.P.
Parker & Parsley 82-III, Ltd. Parker & Parsley 88-C Conv., L.P.
Parker & Parsley 83-A, Ltd. Parker & Parsley 88-C, L.P.
Parker & Parsley 83-B, Ltd. Parker & Parsley Producing Properties 88-A,
L.P.
Parker & Parsley 84-A, Ltd. Parker & Parsley Private Investment 88, L.P.
Parker & Parsley 85-A, Ltd. Parker & Parsley 89-A Conv., L.P.
Parker & Parsley 85-B, Ltd. Parker & Parsley 89-A, L.P.
Parker & Parsley Private Investment 85-A, Ltd. Parker & Parsley 89-B Conv., L.P.
Parker & Parsley Selected 85 Private Investment, Parker & Parsley 89-B, L.P.
Ltd.
Parker & Parsley 86-A, Ltd. Parker & Parsley Private Investment 89, L.P.
Parker & Parsley 86-B, Ltd. Parker & Parsley 90-A Conv., L.P.
Parker & Parsley 86-C, Ltd. Parker & Parsley 90-A, L.P.
Parker & Parsley Private Investment 86, Ltd. Parker & Parsley 90-B Conv., L.P.
Parker & Parsley 87-A Conv., Ltd. Parker & Parsley 90-B, L.P.
Parker & Parsley 87-A , Ltd. Parker & Parsley 90-C Conv., L.P.
Parker & Parsley 87-B Conv., Ltd. Parker & Parsley 90-C, L.P.
Parker & Parsley 87-B, Ltd. Parker & Parsley Private Investment 90, L.P.
Parker & Parsley Producing Properties 87-A, Ltd. Parker & Parsley 90 Spraberry Private
Development, L.P.
Parker & Parsley Producing Properties 87-B, Ltd. Parker & Parsley 91-A, L.P.
Parker & Parsley Private Investment 87, Ltd. Parker & Parsley 91-B, L.P.
</Table>

If you need assistance or have any questions, please call Mr. Tom Long of
D.F. King & Co., Inc. at (212) 269-5550. Thank you for your prompt attention to
this matter.

OMNIBUS CONSENT
(RE: 46 PARKER & PARSLEY LIMITED PARTNERSHIPS)

The undersigned, a registered holder of units representing limited
partnership interests (the "Units") in one or more of the partnerships named
below (the "Partnerships"), hereby appoints as proxy (individually, a "Proxy")
each of the persons, partnerships, associations, corporations and other entities
named as beneficiaries in the registrations of the undersigned on the books of
the Partnerships on September 21, 2001 (the "Record Date"), with full power of
substitution in each, to vote the number of Units of each Partnership held on
the books of that Partnership by the undersigned on the Record Date for the
benefit of each named Proxy, in the corresponding amount or amounts held by the
undersigned on the Record Date for the benefit of each named Proxy, which the
undersigned would be entitled to vote by written consent, during the period
beginning October 22, 2001 through and including December 20, 2001 or such later
date to which the special meeting for the proposals set forth in the Prospectus
(as defined below) may be extended, upon any and all matters which may be
properly acted upon by written consent, including all such matters described in
the proxy statement/prospectus dated October 12, 2001, of Pioneer Natural
Resources Company and Pioneer Natural Resources, USA, Inc., and any amendment or
amendments thereto (the "Prospectus"), relating to said action by written
consent.

The appointment of each Proxy, whether or not coupled with an interest, is
revocable, at any time, by a filing by the undersigned with one or more of the
Partnerships or their duly authorized agents of an instrument similar to this
instrument relating to the Units of the specified Partnership(s), the filing of
which instrument or instruments shall revoke the appointments specified herein
only to the extent specified in such instrument or instruments.

This omnibus consent supersedes and revokes any and all appointments
heretofore made by the undersigned with respect to the voting of the Units
specified herein.

The Partnerships are:

<Table>

CUSTODIAN:

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