Exhibit 99.1








January 31, 2011








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, 2010, 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.  We completed our audit on January 31, 2011.  This report has been prepared for PSE's use in filing with the SEC; in our opinion the assumptions, data, methods, and procedures used in the preparation of this report are appropriate for such purpose.





The following table sets forth Pioneer's estimates of the net reserves and future net revenue, as of December 31, 2010, for the audited properties:





Net Reserves

Future Net Revenue (M$)Oil

NGL

Gas



Present WorthCategory

(MBBL)

(MBBL)

(MMCF)

Total

at 10%Proved Developed Producing

22,209

9,520

37,215

966,368

470,429Proved Developed Non-Producing

1,474

446

1,817

82,409

35,256Proved Undeveloped

7,514

2,546

10,566

296,099

62,069Total Proved

31,196

12,512

49,599

1,344,876

567,753





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. (NSAI).  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 the Standards Pertaining to the Estimating and Auditing of Oil and Gas Reserves Information promulgated by the Society of Petroleum Engineers (SPE Standards).  Additionally, these estimates are within the recommended 10 percent tolerance threshold set forth in the SPE Standards.  We are satisfied with the methods and procedures used by Pioneer in preparing the December 31, 2010, 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.  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 2010.  For oil and NGL volumes, the average of Platts Oil DailyWest Texas Intermediate (Cushing) spot price of $79.282 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 $4.373 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.  The average adjusted product prices weighted by production over the remaining lives of the properties are $78.44 per barrel of oil, $35.14 per barrel of NGL, and $3.55 per MCF of gas.





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.  Estimates of reserves may increase or decrease as a result of market conditions, future operations, changes in regulations, or actual reservoir performance.  In addition to the primary economic assumptions discussed herein, estimates of Pioneer and NSAI are based on certain assumptions including, but not limited to, that the properties will be developed consistent with current development plans, that the properties will be operated in a prudent manner, that no governmental regulations or controls will be put in place that would impact the ability of PSE to recover the reserves, and that projections of future production will prove consistent with actual performance.  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.





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.  Our audit did not include a review of PSE's overall reserves management processes and practices.





We used standard engineering and geoscience methods, or a combination of methods, including performance analysis, volumetric analysis, and analogy, that we considered to be appropriate and necessary to establish the conclusions set forth herein.  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 SPE Standards.  We are independent petroleum engineers, geologists, geophysicists, and petrophysicists; we do not own an interest in these properties nor are we 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:  January 31, 2011








GLB:JLW




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