EXHIBIT
99.1





February
4, 2010











Mr. Kerry
D. Scott


Pioneer
Natural Resources Company


5205
North O'Connor Boulevard, Suite 200


Irving,
Texas 75039-3746





Dear Mr.
Scott:





In
accordance with your request, we have audited the estimates prepared by Pioneer
Natural Resources Company (Pioneer), as of December 31, 2009, of the proved
reserves and future revenue to the Pioneer Southwest Energy Partners (PSE)
interest in certain oil and gas properties located in the Spraberry (Trend)
Field, Texas.  It is our understanding that the proved reserves
estimates shown herein constitute all of the proved reserves owned by
PSE.  We have examined the estimates with respect to reserves
quantities, reserves categorization, future producing rates, future net revenue,
and the present value of such future net revenue, using the definitions set
forth in U.S. Securities and Exchange Commission (SEC) Regulation S-X Rule
4-10(a).  The estimates of reserves and future revenue have been
prepared in accordance with the definitions and guidelines of the SEC and, with
the exception of the exclusion of future income taxes, conform to the FASB
Accounting Standards Codification Topic 932, Extractive Activities—Oil and
Gas.  This report has been prepared for PSE's use in filing with the
SEC.





The
following table sets forth Pioneer's estimates of the net reserves and future
net revenue, as of December 31, 2009, for the audited properties:





Net
      Reserves
            
            Future
      Net Revenue (M$)Oil
            
            NGL
            
            Gas
            
            
            
            Present
      WorthCategory
            
            (MBBL)
            
            (MBBL)
            
            (MMCF)
            
            Total
            
            at
      10%Proved
      Developed Producing
            
            19,269
            
            7,293
            
            30,119
            
            484,050
            
            251,522Proved
      Developed Non-Producing
            
            457
            
            103
            
            429
            
            20,533
            
            8,285Proved
      Undeveloped
            
            8,014
            
            2,451
            
            10,116
            
            136,657
            
            3,599Total Proved
            
            27,740
            
            9,847
            
            40,664
            
            641,240
            
            263,407





Totals
may not add because of rounding.





The oil
reserves shown include crude oil only.  Oil and natural gas liquids
(NGL) volumes are expressed in thousands of barrels (MBBL); a barrel is
equivalent to 42 United States gallons.  Gas volumes are expressed in
millions of cubic feet (MMCF) at standard temperature and pressure
bases.





When
compared on a lease-by-lease basis, some of the estimates of Pioneer are greater
and some are less than the estimates of Netherland, Sewell & Associates,
Inc.  However, in our opinion the estimates of PSE's proved reserves
and future revenue shown herein are, in the aggregate, reasonable and have been
prepared in accordance with generally accepted petroleum engineering and
evaluation principles.  These principles are set forth in the
Standards Pertaining to the Estimating and Auditing of Oil and Gas Reserves
Information promulgated by the Society of Petroleum Engineers.  We are
satisfied with the methods and procedures used by Pioneer in preparing the
December 31, 2009, estimates of reserves and future revenue, and we saw nothing
of an unusual nature that would cause us to take exception with the estimates,
in the aggregate, as prepared by Pioneer.













The
estimates shown herein are for proved reserves only.  Pioneer's
estimates do not include probable or possible reserves that may exist for these
properties, nor do they include any value for undeveloped acreage beyond those
tracts for which undeveloped reserves have been estimated.  Reserves
categorization conveys the relative degree of certainty; reserves
subcategorization is based on development and production status.  The
estimates of reserves and future revenue included herein have not been adjusted
for risk.





Prices
used by Pioneer are based on the 12-month unweighted arithmetic average of the
first-day-of-the-month price for the period January through December
2009.  For oil volumes, the average of Platts Oil Daily West Texas
Intermediate (Cushing) spot price of $61.138 per barrel is adjusted by lease for
quality, transportation fees, and regional price differentials.  For
gas volumes, the average Platts Gas Daily Henry Hub spot
price of $3.866 per MMBTU is adjusted by lease for energy content,
transportation fees, and regional price differentials.  All prices are
held constant throughout the lives of
the properties.





Lease and
well operating costs used by Pioneer are based on historical operating expense
records.  These costs include those incurred at and below the district
and field levels.  Additionally, the estimated per-well COPAS overhead
fees paid by PSE have been included.  No headquarters general and
administrative overhead expenses of Pioneer or PSE are
included.  Lease and well operating costs are held constant throughout
the lives of the properties.  Pioneer's estimates of capital costs are
included as required for workovers, new development wells, production equipment,
abandonment, and salvage.  The future capital costs are held constant
to the date of expenditure.





The
reserves shown in this report are estimates only and should not be construed as
exact quantities.  Proved reserves are those quantities of oil and gas
which, by analysis of engineering and geoscience data, can be estimated with
reasonable certainty to be economically producible.  If the reserves
are recovered, the revenues therefrom and the costs related thereto could be
more or less than the estimated amounts.  Because of governmental
policies and uncertainties of supply and demand, the sales rates, prices
received for the reserves, and costs incurred in recovering such reserves may
vary from assumptions made while preparing these estimates.  Estimates
of reserves may increase or decrease as a result of future operations, market
conditions, or changes in regulations.





It should
be understood that our audit does not constitute a complete reserves study of
the audited oil and gas properties.  Our audit consisted primarily of
substantive testing, wherein we conducted a detailed review of all
properties.  In the conduct of our audit, we have not independently
verified the accuracy and completeness of information and data furnished by
Pioneer with respect to ownership interests, oil and gas production, well test
data, historical costs of operation and development, product prices, or any
agreements relating to current and future operations of the properties and sales
of production.  However, if in the course of our examination something
came to our attention that brought into question the validity or sufficiency of
any such information or data, we did not rely on such information or data until
we had satisfactorily resolved our questions relating thereto or had
independently verified such information or data.  We used standard
engineering and geoscience methods, or a combination of methods, such as
performance analysis, and analogy, that we considered to be appropriate and
necessary to establish the conclusions set forth herein.  Our audit
did not include a review of Pioneer's overall reserves management processes and
practices.





In
evaluating the information at our disposal concerning this audit, we have
excluded from our consideration all matters as to which the controlling
interpretation may be political, socioeconomic, legal, or accounting, rather
than engineering and geoscience.  As in all aspects of oil and gas
evaluation, there are uncertainties inherent in the interpretation of
engineering and geoscience data; therefore, our conclusions necessarily
represent only informed professional judgment.





Supporting
data documenting this audit, along with data provided by Pioneer, are on file in
our office.  The technical persons responsible for conducting this
audit meet the requirements regarding qualifications, independence, objectivity,
and confidentiality set forth in the Standards Pertaining to the Estimating and
Auditing of Oil and Gas Reserves Information promulgated by the Society of
Petroleum Engineers.  We are independent














petroleum
engineers, geologists, geophysicists, and petrophysicists; we do not own an
interest in these properties and are not employed on a contingent
basis.





Sincerely,





NETHERLAND, SEWELL & ASSOCIATES,
INC.


Texas Registered Engineering Firm
F-002699








/s/ C.H. (Scott) Rees
III


By:


C.H. (Scott) Rees III,
P.E.


Chairman and Chief Executive
Officer














/s/ G. Lance Binder


By:


G. Lance Binder, P.E.
61794


Executive Vice
President








Date Signed:  February 4,
2010








GLB:JLW




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