Exhibit 99.(a)  DeGolyer and
MacNaughton 5001 Spring Valley
Road Suite 800 East Dallas, Texas 75244   APPRAISAL REPORT as of DECEMBER 31, 2008 on CERTAIN INTERESTS owned by MESA OFFSHORE TRUST prepared for THE BANK of NEW YORK
TRUST COMPANY, N.A.


DeGolyer and
MacNaughton 5001 Spring Valley
Road Suite 800 East Dallas, Texas 75244  APPRAISAL REPORT as of DECEMBER 31, 2008 on 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, 2008, of the estimates of the extent
of the proved, probable, and possible crude oil, condensate, and natural gas
reserves and the estimates of the value of the proved and proved-plus-probable
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.
Also presented are estimates of the extent only of the possible oil,
condensate, and gas reserves. 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.  Estimates of
proved reserves presented in this report have been prepared in compliance with
the regulations promulgated by the United States Securities and Exchange
Commission (SEC). Estimates of probable and possible reserves presented in this
report have been prepared in accordance with the Petroleum Resources Management
System (PRMS) approved in March 2007 by the Society of Petroleum
Engineers, the World Petroleum Council, the American Association of Petroleum
Geologists, and the Society of Petroleum Evaluation Engineers. These reserves
definitions are discussed in detail in the Definition of Reserves section of
this report.


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,
2008. 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 and proved-plus-probable 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.  Authority  This report was
authorized by Mr. Mike Ulrich, Vice President,Bank of New York.  2


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.  3


DEFINITION of RESERVES  Petroleum
reserves included in this report that are classified as proved 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 SEC of the United States. The petroleum reserves are
classified as follows:  Proved
oil and gas reserves Proved oil and gas reserves are the estimated quantities of
crude oil, natural gas, and natural gas liquids which geological and
engineering data demonstrate with reasonable certainty to be recoverable in
future years from known reservoirs under existing economic and operating
conditions, i.e., prices and costs as of the date the estimate is made. Prices
include consideration of changes in existing prices provided only by
contractual arrangements, but not on escalations based upon future conditions.  (i) Reservoirs are considered proved if
economic producibility is supported by either actual production or conclusive
formation test. The area of a reservoir considered proved includes (A) that
portion delineated by drilling and defined by gas-oil and/or oil-water
contacts, if any; and (B) the immediately adjoining portions not yet
drilled, but which can be reasonably judged as economically productive on the
basis of available geological and engineering data. In the absence of
information on fluid contacts, the lowest known structural occurrence of hydrocarbons
controls the lower proved limit of the reservoir.  (ii) Reserves which can be produced
economically through application of improved recovery techniques (such as fluid
injection) are included in the proved classification when successful testing by
a pilot project, or the operation of an  4


installed program in the reservoir, provides
support for the engineering analysis on which the project or program was based.  (iii) Estimates of proved reserves do
not include the following: (A) oil that may become available from known
reservoirs but is classified separately as indicated additional reserves; (B) crude
oil, natural gas, and natural gas liquids, the recovery of which is subject to
reasonable doubt because of uncertainty as to geology, reservoir
characteristics, or economic factors; (C) crude oil, natural gas, and
natural gas liquids, that may occur in undrilled prospects; and (D) crude
oil, natural gas, and natural gas liquids, that may be recovered from oil
shales, coal, gilsonite, and other such sources.  Proved
developed oil and gas reserves Proved developed oil and gas reserves are reserves
that can be expected to be recovered through existing wells with existing
equipment and operating methods. Additional oil and gas expected to be obtained
through the application of fluid injection or other improved recovery
techniques for supplementing the natural forces and mechanisms of primary
recovery should be included as proved developed reserves only after testing
by a pilot project or after the operation of an installed program has confirmed
through production response that increased recovery will be achieved.  Proved
undeveloped reserves Proved undeveloped oil and gas reserves are reserves that are
expected to be recovered from new wells on undrilled acreage, or from existing
wells where a relatively major expenditure is required for recompletion.
Reserves on undrilled acreage shall be limited to those drilling units
offsetting productive units that are reasonably certain of production when
drilled. Proved reserves for other undrilled units can be claimed only where it
can be demonstrated with certainty that there is continuity of production from
the existing productive formation. Under no circumstances should estimates for
proved undeveloped reserves be attributable to any acreage for which an
application of fluid injection or other improved recovery technique is
contemplated, unless such techniques  5


have
been proved effective by actual tests in the area and in the same reservoir.  The probable and possible reserves
presented in this report have been prepared in accordance with the PRMS
approved in March 2007 by the Society of Petroleum Engineers,the World
Petroleum Council, the American Association of Petroleum Geologists, and the
Society of Petroleum Evaluation Engineers. Probable and possible
reserves are based on geoscience and/or engineering data similar to that used
in estimates of proved reserves, but technical or other uncertainties preclude
such reserves being classified as proved.  Probable Reserves Probable reserves are those
additional reserves which analysis of geoscience and engineering data indicate
are less likely to be recovered than proved reserves but more certain to be
recovered than possible reserves. It is equally likely that actual remaining
quantities recovered will be greater than or less than the sum of the estimated
proved plus probable reserves (2P). In this context, when probabilistic methods
are used, there should be at least a 50-percent probability that the actual
quantities recovered will equal or exceed the 2P estimate.  Possible Reserves Possible reserves are those
additional reserves which analysis of geoscience and engineering data suggest
are less likely to be recoverable than probable reserves. The total quantities
ultimately recovered from the project have a low probability to exceed the sum
of proved plus probable plus possible reserves (3P), which is equivalent to the
high estimate scenario. In this context, when probabilistic methods are used,
there should be at least a 10-percent probability that the actual quantities
recovered will equal or exceed the 3P estimate.  The extent to
which probable and possible reserves ultimately may be recategorized as proved
reserves is dependent upon future drilling, testing, and well performance. The
degree of risk to be applied in evaluating probable and possible reserves is
influenced by economic and technological factors as well as the time element.
Probable and possible reserves in this report have not been adjusted in
consideration of these additional risks to make them comparable to proved
reserves.  6


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, 2008, were used to prepare the estimates
shown herein. Gross production through December 31, 2008, was deducted
from the gross ultimate recovery to arrive at estimates of gross reserves.  Gas quantities 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 reduction for fuel usage, flare, and shrinkage
resulting from field separation  7


and
processing. Condensate reserves estimated herein are those to be obtained by
normal separator recovery.  Estimates of the gross and net proved, probable, and possible reserves,
as of December 31, 2008, 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 ReservesProved
  
  79,016
  
  3,034,618Probable*
  
  4,290
  
  1,430,000Possible*
  
  12,270
  
  4,090,000Net ReservesProved
  
  6,082
  
  213,299Probable*
  
  193
  
  64,350Possible*
  
  551
  
  184,050  
 *Probable
and possible reserves have not been risk adjusted to make them comparable to
proved reserves.  8


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 United States Securities and
Exchange Commission (SEC) and the Financial Accounting Standards Board (FASB).
In this report, values for proved and proved-plus-probable reserves were based
on projections of estimated future production and revenue prepared for these
properties with no risk adjustment applied to the probable reserves. Probable
reserves involve substantially higher risks than proved reserves. Revenue
values for proved-plus-probable reserves have not been adjusted to account for
such risks; this adjustment would be necessary in order to make values for
proved-plus-probable reserves comparable with values for proved reserves.  Oil and Condensate Prices  Initial
oil and condensate prices furnished by Bank of New York are $44.60 per barrel
and were held constant for the producing lives of the properties.  Natural Gas Prices  The
natural gas prices furnished by Bank of New York varied from $5.71 to $5.84 per
thousand cubic feet of gas and were held constant for the producing lives of
the properties.  Operating Expenses and Capital
Costs  The
properties appraised are royalties. Therefore, no operating expenses are
incurred.  9


The
estimated future revenue to be derived from the production and sale of MOSTs
net proved and proved-plus-probable reserves, as of December 31, 2008,
under the economic assumptions furnished by Bank of New York is summarized as
follows, expressed in dollars ($):  Proved
  ($)
  
  Proved plus
  Probable*($)Future Gross Revenue
  
  1,493,534
  
  1,878,033Operating Expenses
  
  0
  
  0Capital Costs
  
  0
  
  0Future Net Revenue**
  
  1,493,534
  
  1,878,033Present Worth at 10 Percent**
  
  1,251,878
  
  1,517,518  
 *
  
  Values for probable reserves have not been risk adjusted to make them
  comparable to values for proved reserves.**
  
  Future income tax expenses were not taken into account in the
  preparation of these estimates.  The
appendix bound with this report presents tabulations and projections of revenue
from the proved and non-risk-adjusted proved-plus-probable reserves for the
interests appraised. Also included are summaries of the probable and possible
reserves.  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
estimated the future net revenue and present worth values set forth herein.
Other rules and regulations of the SEC contain specific provisions that
prohibit the reporting of probable and possible reserves; therefore, the
reporting and filing with the SEC of the probable or possible reserves and the
values based on proved-plus-probable reserves contained herein would not be in conformity
with such rules and regulations and should not, under any circumstances,
be used or relied upon to meet the requirements thereof.  10


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.  11


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, probable, and possible reserves, as of December 31, 2008, 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).  Proved
  
  Probable*
  
  Possible*Net Oil and Condensate, bbl
  
  6,082
  
  193
  
  551Net Sales Gas, Mcf
  
  213,299
  
  64,350
  
  184,050  
 *Probable
and possible reserves have not been risk adjusted to make them comparable to
proved reserves.  Estimated
revenue and costs attributable to MOSTs interests in the proved and
proved-plus-probable reserves, as of December 31, 2008, of the properties
evaluated under the aforementioned assumptions concerning future prices and
costs are summarized as follows, expressed in dollars ($):  Proved
  ($)
  
  Proved plus
  Probable*($)Future Gross Revenue
  
  1,493,534
  
  1,878,033Operating Expenses
  
  0
  
  0Capital Costs
  
  0
  
  0Future Net Revenue**
  
  1,493,534
  
  1,878,033Present Worth at 10 Percent**
  
  1,251,878
  
  1,517,518  
 *
  
  Values
  for probable reserves have not been risk adjusted to make them comparable to
  values for proved reserves.**
  
  Future
  income tax expenses were not taken into account in the preparation of these
  estimates.  12


Gas
quantities estimated herein are expressed at a temperature base of 60 °F and a
pressure base of 14.73 psia.  Submitted,DeGOLYER
  and MacNAUGHTONSIGNED:
  March 10, 2009/s/ Paul
  J. Szatkowski, P.E.Paul
  J. Szatkowski, P.E.Senior
  Vice PresidentDeGolyer
  and MacNaughton
  13


TABLE of
CONTENTS  PageFOREWORD
  1Scope of Investigation
  1Authority
  2Source of Information
  3DEFINITION
  of RESERVES
  4ESTIMATION
  of RESERVES
  7VALUATION
  of RESERVES
  9SUMMARY
  and CONCLUSIONS
  12APPENDIX