<PAGE>
 
               [LETTERHEAD OF RYDER SCOTT COMPANY APPEARS HERE]


                                                                    EXHIBIT 99.1

                                January 9, 1998



Vastar Resources, Inc.
15375 Memorial Drive
Houston, Texas  77079

Gentlemen:

     At your request, we have reviewed Vastar Resources, Inc.'s (Vastar)
estimates of remaining recoverable proved reserves as of December 31, 1997
attributable to the interests owned by Vastar in certain properties.  The
properties reviewed by Ryder Scott Company Petroleum Engineers (Ryder Scott)
consist of 16 fields.  The estimated net reserves as of December 31, 1997
attributable to Vastar's interests in the properties as estimated by Vastar and
reviewed by us, are summarized below and shown by field in Table 1 which is
attached.

                            VASTAR RESOURCES, INC.
                    Estimated Net Remaining Proved Reserves
                Attributable to Leasehold and Royalty Interests
                    For Properties Reviewed by Ryder Scott
                            As of December 31, 1997
             -----------------------------------------------------
                Oil/Condensate - Barrels     51,027,600
                Plant Products - Barrels     12,843,100
                Gas - MMCF                    1,549,653

     Oil, condensate, and natural gas liquids volumes are expressed in standard
42 gallon barrels.  All gas volumes are expressed in millions of cubic feet
(MMCF) at the official temperature and pressure bases of the areas where the gas
reserves are located.

     The proved reserves, which are attributable to the properties reviewed by
Ryder Scott, conform to the definition as set forth in the Securities and
Exchange Commission's Regulation S-X Part 210.4-10 Sec. (a) as clarified by
subsequent Staff Accounting Bulletins, and are based on the following
definitions and criteria:

     Proved reserves of crude oil, condensate, natural gas, and natural gas
liquids are estimated quantities that geological and engineering data
demonstrate with reasonable certainty to be recoverable in the future from known
reservoirs under existing operating conditions using the cost and price
parameters discussed in other sections of this report.  Reservoirs are
considered proved if economic producibility is supported by actual production or
formation tests.  In certain instances, proved reserves are assigned on the
basis of a combination of core analysis and electrical and other type logs which
indicate the reservoirs are analogous to reservoirs in the same field which are
producing or have demonstrated the ability to produce on a formation test.  The
area of a reservoir considered proved includes (1) that portion delineated by
drilling and defined by fluid contacts, if any, and (2) the adjoining portions
not yet drilled that can be reasonably judged as economically productive on the
basis of available geological and engineering data. In the absence of data on 
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Vastar Resources, Inc.
January 9, 1998
Page 2


fluid contacts, the lowest known structural occurrence of hydrocarbons controls
the lower proved limit of the reservoir.  Proved reserves are estimates
of hydrocarbons to be recovered from a given date forward.  They may be revised
as hydrocarbons are produced and additional data become available.  Proved
natural gas reserves are comprised of non-associated, associated and dissolved
gas.  An appropriate reduction in gas reserves has been made for the expected
removal of natural gas liquids, for lease and plant fuel, and for the exclusion
of non-hydrocarbon gases if they occur in significant quantities and are removed
prior to sale.  Reserves that can be produced economically through the
application of improved recovery techniques are included in the proved
classification when these qualifications are met:  (1) successful testing by a
pilot project or the operation of an installed program in the reservoir provides
support for the engineering analysis on which the project or program was based,
and (2) it is reasonably certain the project will proceed.  Improved recovery
includes all methods for supplementing natural reservoir forces and energy, or
otherwise increasing ultimate recovery from a reservoir, including (1) pressure
maintenance, (2) cycling, and (3) secondary recovery in its original sense.
Improved recovery also includes the enhanced recovery methods of thermal,
chemical flooding, and the use of miscible and immiscible displacement fluids.
Estimates of proved reserves do not include crude oil, natural gas, or natural
gas liquids being held in underground or surface storage.

REVIEW PROCEDURE AND OPINION

     In performing our review, we have relied upon data furnished by Vastar with
respect to property interests owned, production and well tests from examined
wells, geological structural and isopach maps, well logs, core analyses, and
pressure measurements.  These data were accepted as authentic and sufficient for
determining the reserves unless, during the course of our examination, a matter
in question came to our attention in which case the data were not accepted until
all questions were satisfactorily resolved.  Our review included such tests and
procedures as we considered necessary under the circumstances to render the
conclusions set forth herein.

     In our opinion Vastar's estimates of future reserves for the properties
reviewed by us were prepared in accordance with generally accepted procedures
for the estimation of future reserves, and we found no bias in the utilization
and analysis of data.  In general, we were in reasonable agreement on an overall
16 field total net equivalent gas basis (6 MCF per barrel) with the estimates
prepared by Vastar.

     Vastar furnished us with its estimates of gross and net remaining reserves
as of December 31, 1997 and we reviewed these data.  There were some slight
variances in the reserves reviewed by Ryder Scott and the Vastar final reserves
report; however, these differences are insignificant in our opinion.  Vastar
assured Ryder Scott that the correct net interests had been utilized to convert
the gross reserves to net reserves.  As a consequence, it is our opinion that
the data presented herein for the properties we reviewed fairly reflect Vastar's
estimated net reserves.

     Certain technical personnel of Vastar are responsible for the preparation
of reserve estimates on new properties and for the preparation of revised
estimates, when necessary, on old properties.  These personnel assembled the
necessary data and maintained the data and work papers in an orderly manner.  We
consulted with these technical personnel and had access to their work papers and
supporting data in the course of our review.
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Vastar Resources, Inc.
January 9, 1998
Page 3


ESTIMATES OF RESERVES

     In general, the reserves for the properties we reviewed are estimated by
performance methods or the volumetric method; however, other methods were used
in certain cases where characteristics of the data indicated such methods were
more appropriate.

     The estimates of reserves by the performance method utilized extrapolations
of various historical data in those cases where such data were definitive.
Reserves were estimated by the volumetric method in those cases where there was
inadequate historical data to establish a definitive trend or where the use of
production performance data as a basis for the reserve estimates was considered
to be inappropriate and the volumetric data were adequate for a reasonable
estimate.

     The reserves presented herein, as estimated by Vastar and reviewed by us,
are estimates only and should not be construed as being exact quantities.
Moreover, estimates of reserves may increase or decrease as a result of future
operations.

GENERAL

     Our opinion on estimated proved reserves is based on a review of data in
Vastar's files; however, we have not made any field examination of the
properties.  In general, the reserve estimates for the properties we reviewed
are based on data available through the third quarter of 1997.  Gas imbalances,
if any, were not taken into account in the gas reserve estimates reviewed by
Ryder Scott.

     At Vastar's request, our review was limited to an examination of reserve
quantities; therefore, we accepted without independent verification Vastar's
representation that they applied economic parameters consistent with the
guidelines of the Securities and Exchange Commission in their estimates of
future income from the reserves presented in this report.  We were not requested
to review Vastar's estimates of future yearly production rates; however, a
number of Vastar's estimates were based on an extrapolation of historical
production trends and, in these cases, future yearly production rates were
reviewed as a part of our reserve review.

     Neither we nor any of our employees has any interest in the subject
properties and neither the employment to do this work nor the compensation is
contingent on our estimates of reserves for the properties which were reviewed.

     This report was prepared for the exclusive use of Vastar.  The data and
work papers used in the preparation of this report are available for examination
by authorized parties in our offices.  Please contact us if we can be of further
service.

                                       Very truly yours,

                                       RYDER SCOTT COMPANY
                                       PETROLEUM ENGINEERS

                                       /s/ JOHN R. WARNER, P.E.
                                       ----------------------------------
                                       John R. Warner, P.E.
                                       Group Vice President

JRW/sw
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                            VASTAR RESOURCES, INC.

                      PROPERTIES REVIEWED BY RYDER SCOTT
                            AS OF DECEMBER 31, 1997
<TABLE> 
<CAPTION> 
                                                                   NET RESERVES
                                     -------------------------------------------------------------------------------
                                     OIL/CONDENSATE              GAS             PLANT PRODUCTS       GAS EQUIVALENT
    FIELD                               BARRELS                 MMCF                 BARRELS               MMCFE
    -----                            --------------             ----             --------------       --------------
<S>                                  <C>                      <C>                <C>                  <C> 
Ignacio Blanco/Fruitland                         0              554,322                     0              554,322
South Pass 60                           20,625,100               65,770             3,342,700              209,577
Hugoton                                      2,100              113,518             2,346,800              127,612
Grand Isle Complex                       9,964,700               63,495               685,700              127,396
Wilburton                                        0              123,236                     0              123,236
Carthage                                 1,209,200               78,027             3,043,000              103,540
West Cameron 66                          2,907,000               67,488             1,304,000               92,754
Basin F. Coal                                    0              138,468                     0              138,468
High Island 24L                            346,000               45,628               431,000               50,291
Joaquin/Logansport/Spider                  150,800               75,813                     0               76,718
Eugene Island 175                       11,036,400               12,789               119,900               79,727
Panoma                                           0               44,270             1,010,000               50,328
South Panola                                     0               46,602                     0               46,602
West Delta 106                           3,856,000               23,234               560,000               49,732
San Juan Conventional                      173,200               52,068                     0               53,107
Golden Trend                               757,100               44,925                     0               49,467
Total                                   51,027,600            1,549,653            12,843,100            1,932,877
</TABLE>