Exhibit 99(a)

DEGOLYER AND MACNAUGHTON5001 SPRING VALLEY ROAD
SUITE 800 EAST
DALLAS, TEXAS 75244

APPRAISAL REPORT
as of
DECEMBER 31, 2007
on
PROVED RESERVES
of
CERTAIN INTERESTS
owned by
MESA OFFSHORE TRUST
prepared for
THE BANK of NEW YORK TRUST COMPANY, N.A.

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TABLE of CONTENTS

|                        |                        | Page
|----------------------- | ---------------------- | ----
|FOREWORD                |                        | 1   
|                        | Scope of Investigation |     
|                        | Authority              |     
|                        | Source of Information  |     
|DEFINITION of RESERVES  |                        | 2   
|ESTIMATION of RESERVES  |                        | 3   
|VALUATION of RESERVES   |                        | 4   
|SUMMARY and CONCLUSIONS |                        | 5   
|APPENDIX                |                        |     

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DEGOLYER AND MACNAUGHTON5001 SPRING VALLEY ROAD
SUITE 800 EAST
DALLAS, TEXAS 75244

APPRAISAL REPORT
as of
DECEMBER 31, 2007
on
PROVED RESERVES
of
CERTAIN INTERESTS
owned by
MESA OFFSHORE TRUST
prepared for
THE BANK of NEW YORK TRUST COMPANY, N.A.

FOREWORD

Scope of Investigation

This report presents an appraisal, as of December 31, 2007, of the extent and value of the proved crude oil, condensate, and natural gas reserves of
royalty interests in certain properties owned by the Mesa Offshore Trust (MOST) located offshore from Louisiana and Texas in the Gulf of Mexico. The
Managing General Partner is Pioneer Natural Resources USA Inc. (PNR). This report was prepared at the request of The Bank of New York Trust Company, N.A. (Bank of New York),
trustee for MOST.

Reserves
estimated in this report are expressed as gross and net reserves. Gross reserves are defined as the total estimated petroleum to be produced from these properties after
December 31, 2007. Net reserves are defined as that portion of the gross reserves attributable to MOST's interests after deducting royalties and interests owned by others.

This
report presents values that were estimated for proved reserves using initial prices provided by Bank of New York and initial costs provided by PNR. Future price and cost
assumptions were provided by Bank of New York. A detailed explanation of the price and cost assumptions used herein is included in the Valuation of Reserves section of this report.

Values
are expressed in terms of estimated future gross revenue, future net revenue, and present worth. Future gross revenue is that revenue which will accrue from the production and
sale of the estimated net reserves. Future net revenue is calculated by deducting estimated operating expenses and capital costs from the future gross revenue. Future income tax expenses were not
taken into account in the preparation of these estimates. Present worth is defined as future net revenue discounted at a specified arbitrary discount rate compounded monthly over the expected period
of realization. In this report, present worth values using a nominal discount rate of 10 percent are reported in detail and values using nominal discount rates of 5, 15, 20, and
25 percent are reported as totals in the appendix to this report.

Estimates
of oil, condensate, and gas reserves and future net revenue should be regarded only as estimates that may change as further production history and additional information become
available. Not only are such reserves and revenue estimates based on that information which is currently available, but such estimates are also subject to the uncertainties inherent in the application
of judgmental factors in interpreting such information.

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Authority

This report was authorized by Mr. Mike Ulrich, Vice President, Bank of New York.

Source of Information

Information used in the preparation of this report was obtained from PNR's files on behalf of Bank of New York, from records on file with the appropriate
regulatory agencies, and from public sources. In the preparation of this report we have relied, without independent verification, upon such information furnished by PNR and Bank of New York
with respect to property interests, production from such properties, current costs of operation and development, current prices for production, the future plans for development of the properties,
agreements relating to current and future operations and sale of production, and various other information and data that were accepted as represented. A field examination of the properties was not
considered necessary for the purposes of this report.

DEFINITION of RESERVES

Petroleum reserves included in this report are classified as proved and are judged to be economically producible in future years from known reservoirs under
existing economic and operating conditions and assuming continuation of current regulatory practices using conventional production methods and equipment. In the analyses of production-decline curves,
reserves were estimated only to the limit of economic rates of production under existing economic and operating conditions using prices and costs as of the date the estimate is made, including
consideration of changes in existing prices provided only by contractual arrangements but not including escalations based upon future conditions. Proved reserves classifications used in this report
are in accordance with the reserves definitions of Rules 4-10(a) (1)-(13) of Regulation S-X of the United States Securities and Exchange Commission (SEC).
The petroleum reserves are classified as follows:



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ESTIMATION of RESERVES

Estimates of reserves were prepared by the use of standard geological and engineering methods generally accepted by the petroleum industry. The method or
combination of methods used in the analysis of each reservoir was tempered by experience with similar reservoirs, stage of development, quality and completeness of basic data, and production history.

Where
appropriate the volumetric method was used to estimate the original oil in place (OOIP) and original gas in place (OGIP). Structure maps were prepared to delineate each reservoir,
and isopach maps were constructed to estimate reservoir volume. Electrical logs, radioactivity logs, core analyses, and other available data were used to prepare these maps as well as to estimate
representative values for porosity and water saturation.

Estimates
of ultimate recovery were obtained after applying recovery factors to OOIP or OGIP. These recovery factors were based on consideration of the type of energy inherent in the
reservoirs, analyses of the petroleum, and the structural positions of the properties.

For
depletion-type reservoirs or those whose performance disclosed a reliable decline in producing-rate trends or other diagnostic characteristics, reserves were
estimated by the application of appropriate decline curves or other performance relationships. In the analyses of production-decline curves, reserves were estimated only to the limits of economic
production based on current economic conditions.

The
rates used for future oil, condensate, and gas production are estimated to be within the capacity of a well or reservoir to produce. Data available from wells drilled on the
appraised properties through December 31, 2007, were used to prepare the estimates shown herein. Gross production through December 31, 2007, was deducted from the gross ultimate recovery
to arrive at estimates of gross reserves.

Gas
volumes estimated herein are expressed as sales gas at a temperature base of 60 degrees Fahrenheit (°F) and a pressure base of 14.73 pounds per square inch
absolute (psia). Sales gas is defined as the total gas to be produced from the reservoirs, measured at the point of delivery, after

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reduction
for fuel usage, flare, and shrinkage resulting from field separation and processing. Condensate reserves estimated herein are those to be obtained by normal separator recovery.

Estimates
of the gross and net proved reserves, as of December 31, 2007, of the properties appraised are presented as follows. Oil and condensate reserves are expressed in barrels
(bbl) and gas reserves are expressed in thousands of cubic feet (Mcf).

Oil and
Condensate
(bbl)

Sales Gas
(Mcf)Gross Reserves
Proved

135,283

3,955,609Net Reserves
Proved

10,391

276,858


VALUATION of RESERVES

This report has been prepared using initial prices provided by Bank of New York and initial costs provided by PNR on behalf of Bank of New York.
Future prices were estimated using guidelines established by the SEC and the Financial Accounting Standards Board (FASB). In this report, values for proved reserves were based on projections of
estimated future production and revenue prepared for these properties.



The
estimated future revenue to be derived from the production and sale of MOST's net proved reserves, as of December 31, 2007, under the economic assumptions furnished by Bank of
New York is summarized as follows, expressed in dollars ($):

Proved
($)Future Gross Revenue

2,900,239Operating Expenses

Capital Costs

1,476,367Future Net Revenue*

1,423,872Present Worth at 10 Percent*

1,192,446




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The
appendix bound with this report presents tabulations and projections of revenue from the proved reserves for the interests appraised.

In
our opinion, the information relating to estimated proved reserves, estimated future net revenue from proved reserves, and present worth of estimated future net revenue from proved
reserves of oil, condensate, and gas contained in this report has been prepared in accordance with Paragraphs 10-13, 15 and 30(a)-(b) of Statement of Financial Accounting
Standards No. 69 (November 1982) of the FASB and Rules 4-10(a) (1)-(13) of Regulation S-X and Rule 302(b) of
Regulation S-K of the SEC; provided, however, that (i) certain estimated data have not been provided with respect to changes in reserves information and (ii) future
income tax expenses have not been taken into account in estimating the future net revenue and present worth values set forth herein.

To
the extent the above-enumerated rules, regulations, and statements require determinations of an accounting or legal nature or information beyond the scope of our report, we are
necessarily unable to express an opinion as to whether the above-described information is in accordance therewith or sufficient therefor.

SUMMARY and CONCLUSIONS

Evaluated herein are royalty interests in certain properties owned by MOST located offshore from Louisiana and Texas in the Gulf of Mexico. Estimates of MOST's
net proved reserves, as of December 31, 2007, of the properties appraised are presented as follows. Oil and condensate reserves are expressed in barrels (bbl) and gas reserves are expressed in
thousands of cubic feet (Mcf).

ProvedNet Oil and Condensate, bbl

10,391Net Sales Gas, Mcf

276,858


Estimated
revenue and costs attributable to MOST's interests in the proved reserves, as of December 31, 2007, of the properties evaluated under the aforementioned assumptions
concerning future prices and costs are summarized as follows, expressed in dollars ($):

Proved
($)Future Gross Revenue

2,900,239Operating Expenses

Capital Costs

1,476,367Future Net Revenue*

1,423,872Present Worth at 10 Percent*

1,192,446




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Gas
volumes estimated herein are expressed at a temperature base of 60 °F and a pressure base of 14.73 psia.

|                      |  | Submitted,                                                            
|--------------------- |  | ----------------------------------------------------------------------
|                      |  | GRAPHIC                                                               
|                      |  |                                                                       
|                      |  | DeGOLYER and MacNAUGHTON                                              
|SIGNED: March 6, 2008 |  |                                                                       
|GRAPHIC               |  | GRAPHIC                                                               
|                      |  |                                                                       
|                      |  | Paul J. Szatkowski, P.E.
Senor Vice President
DeGolyer and MacNaughton

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