<PAGE>   1
 
                                                                   EXHIBIT 99(a)
 
                                February 3, 1998
 
Pennzoil Company
Post Office Box 2967
Houston, Texas 77001
 
Gentlemen:
 
     At your request we have prepared an estimate of the reserves, future
production, and income attributable to certain leasehold and royalty interests
of Pennzoil Company including Pennzoil Exploration and Production Company,
Pennzoil Caspian Corporation, Pennzoil Venezuela Incorporated and Pennzoil
Products Company (collectively referred to herein as the Company) as of December
31, 1997. In accordance with the requirements of FASB 69, our estimates of the
Company's net proved reserves as of December 31, 1994, 1995, 1996, and 1997, as
contained in this report and our previous reports, are presented in attached
Table No. 1 together with a tabulation of the components of the differences in
the estimates as of such dates. The Company's reserves in the United States are
located in all the main producing states (except Alaska), and in state and
federal waters offshore California, Louisiana, and Texas. The Company's foreign
reserves are located in Venezuela and Azerbaijan.
 
     The estimated reserve volumes and future income amounts presented in this
report are related to hydrocarbon prices. December 1997 hydrocarbon prices were
used in the preparation of this report as required by Securities and Exchange
Commission (SEC) and Financial Accounting Standards Bulletin No. 69 (FASB 69)
guidelines; however, actual future prices may vary significantly from December
1997 prices. Therefore, volumes of reserves actually recovered and amounts of
income actually received may differ significantly from the estimated quantities
presented in this report. Our estimates of the proved net reserves attributable
to the interests of the Company as of December 31, 1997 are shown below:
 
<TABLE>
<CAPTION>
                                                        PROVED NET RESERVES
                                                      AS OF DECEMBER 31, 1997
                                                    ----------------------------
                                                    LIQUID, BARRELS    GAS, MMCF
                                                    ---------------    ---------
<S>                                                 <C>                <C>
Developed and Undeveloped
  United States...................................    152,311,973      1,054,369
  Foreign.........................................     75,184,915          4,418
                                                      -----------      ---------
          Total Worldwide.........................    227,496,888      1,058,787
Developed
  United States...................................    127,838,778        963,554
  Foreign.........................................     14,018,093          4,418
                                                      -----------      ---------
          Total Worldwide.........................    141,856,871        967,972
</TABLE>
 
     The "Liquid" reserves shown above are comprised of crude oil, condensate,
and natural gas liquids. Natural gas liquids comprise 20 percent of the
Company's developed liquid reserves and 13 percent of the Company's developed
and undeveloped liquid reserves. All hydrocarbon liquid reserves are expressed
in standard 42 gallon barrels. All gas volumes are hydrocarbon sales gas
expressed in MMCF at the pressure and temperature bases of the area where the
gas reserves are located. Our estimates of hydrocarbon sales gas reserves as of
December 31, 1997 do not include 177,684 MMCF of carbon dioxide which is also
sales gas. Revenues from carbon dioxide sales are included in our estimates of
future cash inflows as of December 31, 1997. In addition, the Company owns
40,210 long tons of sulfur reserves as of December 31, 1997 which are not shown
above; however, the revenue from these sulfur reserves is included in the cash
inflow data in this report. Our estimates of proved net reserves as of December
31, 1997 include 21,261,444 barrels and 4,418 MMCF for Pennzoil Venezuela, Inc.
for which the Company has a contract to develop and produce but does not have
actual ownership of the hydrocarbons.
<PAGE>   2
February 3, 1998
Page  2
 
     The proved reserves presented in this report comply with the SEC's
Regulation S-X Part 210.4-10 Sec. (a) as clarified by subsequent Commission
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, 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 escalation based on future conditions. Reservoirs are considered
     proved if economic producibility is supported by either actual production
     or conclusive formation test. 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 fluid contacts,
     the lowest known structural occurrence of hydrocarbons controls the lower
     proved limit of the reservoir. 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. 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. Estimates of proved reserves do not include
     crude oil, natural gas, or natural gas liquids being held in underground or
     surface storage. 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 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.
     Developed reserves may be subcategorized as producing or non-producing
     using the SPE/WPC Definitions:
 
        Producing
        Reserves sub-categorized as producing are expected to be recovered from
        completion intervals which are open and producing at the time of the
        estimate. Improved recovery reserves are considered producing only after
        the improved recovery project is in operation.
 
        Non-Producing
        Reserves sub-categorized as non-producing include shut-in and behind
        pipe reserves. Shut-in reserves are expected to be recovered from (1)
        completion intervals which are open at the time of the estimate but
        which have not started producing, (2) wells which were shut-in for
        market conditions or pipeline connections, or (3) wells not capable of
        production for mechanical reasons.
<PAGE>   3
February 3, 1998
Page  3
 
        Behind pipe reserves are expected to be recovered from zones in existing
        wells, which will require additional completion work or future
        recompletion prior to the start of production.
 
     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 reasonable certainty that there is continuity of
     production from the existing productive formation. Estimates for proved
     undeveloped reserves are attributable to any acreage for which an
     application of fluid injection or other improved technique is contemplated,
     only when such techniques have been proved effective by actual tests in the
     area and in the same reservoir.
 
     Because of the direct relationship between volumes of proved undeveloped
reserves and development plans, we include in the proved undeveloped category
only reserves assigned to undeveloped locations that we have been assured will
definitely be drilled and reserves assigned to the undeveloped portions of
secondary or tertiary projects which we have been assured will definitely be
developed.
 
     The Company has interests in certain tracts which have substantial
additional hydrocarbon quantities which cannot be classified as proved and
consequently are not included herein. The Company has active exploratory and
development drilling programs which may result in the reclassification of
significant additional volumes to the proved category.
 
     In accordance with the requirements of FASB 69, our estimates of future
cash inflows, future costs, and future net cash inflows before income tax as of
December 31, 1997 from this report and as of December 31, 1996 from our previous
report are presented below.
 
<TABLE>
<CAPTION>
                                                         TOTAL WORLDWIDE
                                                     AS OF DECEMBER 31(1)(2)
                                                 -------------------------------
                                                      1997             1996
                                                 --------------   --------------
<S>                                              <C>              <C>
Future Cash Inflows............................  $5,807,484,505   $8,958,482,749
Future Costs
  Production...................................  $1,925,735,766   $2,266,664,101
  Development..................................     544,076,586      490,063,774
                                                 --------------   --------------
          Total Costs..........................  $2,469,812,352   $2,756,727,875
Future Net Cash Inflows Before Income Tax......  $3,337,672,153   $6,201,754,874
Present Value at 10% Before Income Tax.........  $2,052,998,650   $3,967,371,608
</TABLE>
 
---------------
 
(1) The cash inflow data for December 31, 1997 include revenues from 177,684 net
    MMCF of carbon dioxide reserves which have a future net cash inflow before
    income tax of $70,271,184 and present value at 10 percent before income tax
    of $21,362,976. The cash inflow data for December 31, 1996 include revenues
    from 181,855 net MMCF of carbon dioxide reserves which have a future net
    cash inflow before income tax of $56,210,214 and present value at 10 percent
    before income tax of $19,770,225.
 
(2) The cash inflow data for December 31, 1997 includes net cash inflow before
    income tax of $28,467,080 and present value at 10 percent before income tax
    of $5,453,598 attributable to Pennzoil Venezuela, Inc.
 
     Our estimates as of December 31, 1997 and 1996 of future cash inflows,
future costs, future net cash inflows before income tax, and present value at 10
percent before income tax are shown individually for total worldwide, total
United States (onshore and offshore), and foreign areas in Table No. 2 which is
attached.
 
     The future cash inflows are gross revenues before any deductions. The
production costs were based on current data and include production taxes in the
United States, certain foreign taxes where applicable, ad
<PAGE>   4
February 3, 1998
Page  4
 
valorem taxes, and certain other items such as transportation and processing
costs, in addition to the operating costs directly applicable to the individual
leases or wells. The development costs were based on current data and include
certain dismantlement and abandonment costs net of salvage for properties where
such costs are relatively significant.
 
     The Company furnished us with gas prices in effect at December 31, 1997 and
with its forecasts of future gas prices which take into account SEC guidelines,
current market prices, contract prices, and fixed and determinable price
escalations where applicable. In accordance with SEC guidelines, the future gas
prices used in this report make no allowances for future gas price increases
which may occur as a result of inflation nor do they account for seasonal
variations in gas prices which may cause future yearly average gas prices to be
somewhat different than December gas prices. For gas sold under contract, the
contract gas price including fixed and determinable escalations exclusive of
inflation adjustments was used until the contract expires and then was adjusted
to the current market price for the area and held at this adjusted price to
depletion of the reserves.
 
     The Company furnished us with liquid prices in effect at December 31, 1997
and these prices were held constant to depletion of the properties. In
accordance with SEC guidelines, changes in liquid prices subsequent to December
31, 1997 were not considered in this report.
 
     Operating costs for the leases and wells in this report were based on
information provided by the Company and include only those costs directly
applicable to the leases or wells. When applicable, the operating costs include
a portion of general and administrative costs allocated directly to the leases
and wells under terms of operating agreements. Development costs were furnished
to us by the Company and are based on authorizations for expenditure for the
proposed work or actual costs for similar projects. The current operating and
development costs were held constant throughout the life of the properties. At
the request of the Company, their estimate of zero net abandonment costs after
salvage value for onshore properties was used in this report. Ryder Scott has
not performed a detailed study of the abandonment costs nor the salvage value
and makes no warranty for the Company's estimate. The estimated net cost of
abandonment after salvage was included for offshore properties where abandonment
costs net of salvage are significant. The estimates of the offshore net
abandonment costs furnished by the Company were accepted without independent
verification. No deduction was made for indirect costs such as general
administration and overhead expenses, loan repayments, interest expenses, and
exploration and development prepayments. The Company supplied data on
accumulated gas production imbalances which were taken into account in our
estimates of future production and income.
 
     The estimates of reserves presented herein are based upon a detailed study
of the properties in which the Company owns an interest; however, we have not
made any field examination of the properties. No consideration was given in this
report to potential environmental liabilities which may exist nor were any costs
included for potential liability to restore and clean up damages, if any, caused
by past operating practices. The Company has informed us that they have
furnished us all of the accounts, records, geological and engineering data and
reports, and other data required for this investigation. The ownership
interests, prices, and other factual data furnished by the Company were accepted
without independent verification. The estimates presented in this report are
based on data available through December 1997.
 
     The reserves included in this report are estimates only and should not be
construed as being exact quantities. They may or may not be actually recovered,
and if recovered, the revenues therefrom and the actual costs related thereto
could be more or less than the estimated amounts. Moreover, estimates of
reserves may increase or decrease as a result of future operations.
 
     In general, we estimate that future gas production rates limited by
allowables or marketing conditions will continue to be the same as the average
rates for the latest available 12 months of actual production until such time
that the well or wells are incapable of producing at these rates. The well or
wells were then projected to decline at their decreasing delivery capacity rate.
Our general policy on estimates of future gas production
<PAGE>   5
February 3, 1998
Page  4
 
rates is adjusted when necessary to reflect actual gas market conditions in
specific cases. The future production rates from wells now on production may be
more or less than estimated because of changes in market demand or allowables
set by regulatory bodies. Wells or locations which are not currently producing
may start producing earlier or later than anticipated in our estimates of their
future production rates.
 
     While it may reasonably be anticipated that the future prices received for
the sale of production and the operating costs and other costs relating to such
production may also increase or decrease from existing levels, such changes
were, in accordance with rules adopted by the SEC, omitted from consideration in
making this evaluation.
 
     Neither we nor any of our employees have any interest in the subject
properties and neither the employment to make this study nor the compensation is
contingent on our estimates of reserves and future cash inflows for the subject
properties.
 
                                            Very truly yours,
 
                                            RYDER SCOTT COMPANY
                                            PETROLEUM ENGINEERS
 
                                            Ronald Harrell, P.E.
                                            President
 
RH/sw
<PAGE>   6
 
                                  TABLE NO. 1
 
                                PENNZOIL COMPANY
                           PROVED NET RESERVE DATA(1)
 
<TABLE>
<CAPTION>
                                                                    UNITED STATES
                                                                  TOTAL ONSHORE AND
                                           TOTAL WORLDWIDE            OFFSHORE              FOREIGN(2)
                                        ---------------------   ---------------------   -------------------
                                        1997    1996    1995    1997    1996    1995    1997   1996    1995
                                        -----   -----   -----   -----   -----   -----   ----   -----   ----
<S>                                     <C>     <C>     <C>     <C>     <C>     <C>     <C>    <C>     <C>
Net Proved Liquid(3) Reserves,
  Millions
  of Barrels
  Developed and Undeveloped
  Beginning of Year...................  187.4   191.6   206.2   165.1   174.9   204.5   22.3    16.7    1.7
    12/31/95 Est. Prepared by Outrim
      Szabo Assoc. Ltd.(4)............      0     9.6       0       0       0       0      0     9.6      0
    Revisions.........................    3.2    12.2   (14.9)   (0.8)    8.1   (14.4)   4.0     4.1   (0.5)
    Extensions and Discoveries........   41.6    23.2    20.4    11.7    10.5    18.9   29.9    12.7    1.5
    Improved Recovery.................    1.0     2.1     2.7     1.0     2.1     2.7      0       0      0
    Estimated Production..............  (19.8)  (21.6)  (22.1)  (19.5)  (20.4)  (21.9)  (0.3)   (1.2)  (0.2)
    Purchase of Reserves In-Place.....   19.9     8.1    22.2       0     7.0     8.0   19.9     1.1   14.2
    Sales of Reserves In-Place........   (5.8)  (37.8)  (22.9)   (5.2)  (17.1)  (22.9)  (0.6)  (20.7)   Neg
                                        -----   -----   -----   -----   -----   -----   ----   -----   ----
End of Year...........................  227.5   187.4   191.6   152.3   165.1   174.9   75.2    22.3   16.7
Developed
  Beginning of Year(5)................  142.2   152.6   177.7   140.7   151.2   176.1    1.5     1.3    1.6
  End of Year.........................  141.8   142.2   152.6   127.8   140.7   151.2   14.0     1.5    1.3
Net Proved Gas(6) Reserves, Billions
  of Cubic Feet
Developed and Undeveloped
  Beginning of Year...................  1,277   1,291   1,376   1,187   1,256   1,341     90      35     35
    12/31/95 Est. Prepared by Outrim
      Szabo Assoc. Ltd.(4)............      0     178       0       0       0       0      0     178      0
    Revisions.........................      2      61      51     (26)     38      56     28      23     (5)
    Extensions and Discoveries........    127     172     219     104     143     211     23      29      8
    Improved Recovery.................      0       2       1       0       2       1      0       0      0
    Estimated Production..............   (214)   (219)   (220)   (206)   (202)   (218)    (8)    (17)    (2)
    Purchase of Reserves In-Place.....      0      36      26       0       8      26      0      28      0
Sales of Reserves In-Place............   (133)   (244)   (162)     (5)    (58)   (161)  (128)   (186)    (1)
                                        -----   -----   -----   -----   -----   -----   ----   -----   ----
End of Year...........................  1,059   1,277   1,291    1054   1,187   1,256      5      90     35
Developed
  Beginning of Year(5)................  1,160   1,161   1,273   1,070   1,132   1,242     90      29     31
  End of Year.........................    968   1,160   1,161     963   1,070   1,132      5      90     29
</TABLE>
 
---------------
 
(1) All reserve changes in 1995 were categorized by Ryder Scott Company. Reserve
    changes due to extensions and discoveries and improved recovery during 1996
    and 1997 were categorized by Ryder Scott Company. The remaining reserve
    changes during 1996 and 1997 were categorized by the Company.
 
(2) 1995 includes 1.2 million barrels and 5 billion cubic feet, 1996 includes
    1.4 million barrels and 4 billion cubic feet and 1997 includes 21.3 million
    barrels and 4 billion cubic feet of natural gas in Venezuela for which
    Pennzoil has a contract to develop and produce but does not have actual
    ownership of the hydrocarbons.
 
(3) Liquid reserves shown above are comprised of crude oil, condensate, and
    natural gas liquids.
 
(4) These estimates were prepared by Outrim Szabo Associates, Ltd. for Pennzoil
    Petroleums, Ltd. as of December 31, 1995. The reserves attributable to
    Pennzoil Petroleums, Ltd. were sold during 1996. These estimates are
    included at the Company's request in order to reconcile reserve revisions.
    Ryder Scott Company has not reviewed these reserve estimates and therefore
    expresses no opinion as to their validity.
 
(5) Beginning of year proved developed gas and liquid reserves for 1996 excludes
    proved developed reserves estimated by Outrim Szabo Assoc. Ltd. for Pennzoil
    Petroleums, Ltd. as of December 31, 1995.
 
(6) Excludes carbon dioxide reserve and production data.
<PAGE>   7
 
                                  TABLE NO. 2
 
                                PENNZOIL COMPANY
                          CASH INFLOW AND COST DATA(1)
                           (MILLIONS OF U.S. DOLLARS)
 
<TABLE>
<CAPTION>
                                                                 UNITED STATES          FOREIGN
                                          TOTAL WORLDWIDE    ONSHORE AND OFFSHORE        AS OF
                                         AS OF DECEMBER 31     AS OF DECEMBER 31      DECEMBER 31
                                         -----------------   ---------------------   -------------
                                          1997      1996       1997        1996      1997    1996
                                         -------   -------   ---------   ---------   -----   -----
<S>                                      <C>       <C>       <C>         <C>         <C>     <C>
Future Cash Inflows(2).................  $ 5,808   $ 8,958    $ 4,847     $ 8,270    $ 961   $ 688
Future Costs
  Production(3)........................  $(1,926)  $(2,267)   $(1,621)    $(2,055)   $(305)  $(212)
  Development(4).......................     (544)     (490)      (458)       (445)     (86)    (45)
                                         -------   -------    -------     -------    -----   -----
Total Costs............................  $(2,470)  $(2,757)   $(2,079)    $(2,500)   $(391)  $(257)
Future Cash Inflows Before Income
  Tax..................................  $ 3,338   $ 6,201    $ 2,768     $ 5,770    $ 570   $ 431
Present Value @ 10% Before Income
  Tax..................................  $ 2,053   $ 3,967    $ 1,791     $ 3,697    $ 262   $ 270
</TABLE>
 
---------------
 
(1) Data for 1997 and 1996 include cash inflows and costs attributable to carbon
    dioxide reserves located in the United States. The 1997 carbon dioxide
    reserves account for $70.3 million of cash inflows before income tax and
    $21.4 million of present value at 10% before income tax. The 1996 carbon
    dioxide reserves account for $56.2 million of future cash inflows before
    income tax and $19.8 million of present value at 10% before income tax.
 
(2) Gross revenues are before any deductions.
 
(3) Includes production taxes in the U.S.A., certain foreign taxes where
    applicable, ad valorem taxes, and certain other items such as transportation
    and processing charges.
 
(4) Includes future dismantlement and abandonment costs net of salvage for
    offshore properties where such costs are relatively significant.