<PAGE>   1
                        [RYDER SCOTT COMPANY LETTERHEAD]

                                                                   EXHIBIT 99(a)

                               February 19, 1999



PennzEnergy Company
Post Office Box 4616
Houston, Texas  77210-4616

Gentlemen:

                  At your request we have prepared an estimate of the reserves,
future production, and income attributable to all operating and ownership
interests of PennzEnergy Company including PennzEnergy Exploration and
Production, L.L.C., PennzEnergy Company - Bluebell/Altamont, Pennzoil Caspian
Corporation, and Pennzoil Venezuela Corporation, S.A. (collectively referred to
herein as the Company) as of December 31, 1998. In accordance with the
requirements of FASB 69, our estimates of the Company's net proved reserves as
of December 31, 1996, 1997, and 1998, 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 1998
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 1998 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,
1998 are shown below:

<TABLE>
<CAPTION>
                                           Proved Net Reserves
                                         As of December 31, 1998
                                      -----------------------------
                                      Liquid, Barrels     Gas, MMCF
                                      ---------------     ---------
<S>                                   <C>                 <C>    
          Developed and Undeveloped
             United States              125,543,732         849,368
             Foreign                     93,261,474           2,266
                Total Worldwide         218,805,206         851,634

          Developed
             United States              108,226,490         783,990
             Foreign                     12,233,076           2,266
                Total Worldwide         120,459,566         786,256
</TABLE>

                  The "Liquid" reserves shown above are comprised of crude oil,
condensate, and natural gas liquids. Natural gas liquids comprise 23.3 percent
of the Company's developed liquid reserves and 13.5 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


<PAGE>   2


PennzEnergy Company
February 19, 1999
Page 2


hydrocarbon sales gas reserves as of December 31, 1998 do not include 135,504
MMCF of carbon dioxide which is also sales gas. Revenues from carbon dioxide
sales, however, are included in our estimates of future cash inflows as of
December 31, 1998. Our estimates of proved net reserves as of December 31, 1998
include 17,179,701 barrels and 2,266 MMCF for Pennzoil Venezuela Corporation,
S.A. for which the Company has a contract to develop and produce but does not
have actual ownership of the hydrocarbons.

                  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.


<PAGE>   3


PennzEnergy Company
February 19, 1999
Page 3


              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.
              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, tertiary, or pressure maintenance 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, 1998 from this report and as of December 31, 1997 from
our previous report are presented below.

<TABLE>
<CAPTION>
                                                                Total Worldwide
                                                            As of December 31(1)(2)
                                                      ------------------------------------
                                                           1998                  1997
                                                      ---------------      ---------------

<S>                                                   <C>                  <C>            
                Future Cash Inflows                   $ 3,797,956,140      $ 5,807,484,505
                Future Costs
                    Production                        $ 1,504,251,473      $ 1,925,735,766
                    Development                           428,750,500          544,076,586
                                                      ---------------      ---------------
                        Total Costs                   $ 1,933,001,973      $ 2,469,812,352
                Future Net Cash Inflows
                    Before Income Tax                 $ 1,864,954,167      $ 3,337,672,153

                Present Value at 10%
                    Before Income Tax                 $ 1,078,627,124      $ 2,052,998,650
</TABLE>

----------------------------

(1) The cash inflow data for December 31, 1998 include revenues from 135,504 net
MMCF of carbon dioxide reserves which have a future net cash inflow before
income tax of $37,081,330 and present value at 10 percent before income tax of
$11,054,723. 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.

(2) The cash inflow data for December 31, 1998 includes net cash inflow before
income tax of $10,087,639 and present value at 10 percent before income tax of
$1,886,767 attributable to Pennzoil Venezuela Corporation, S.A. 


<PAGE>   4

PennzEnergy Company
February 19, 1999
Page 4


                  Our estimates as of December 31, 1998 and 1997 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 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, 1998 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, 1998 and these prices were held constant to depletion of the
properties. In accordance with SEC guidelines, changes in liquid prices
subsequent to December 31, 1998 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 future
development costs were held constant throughout the life of the properties. The
initial operating costs for each property are based on current operating costs.
In certain cases, these costs were reduced over time as variable costs, such as
well count or production throughput declined as is typical in actual operations.
In most cases, the current operating costs were held constant throughout the
life of the property. 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


<PAGE>   5

PennzEnergy Company
February 19, 1999
Page 5


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 1998.

                  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 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

                                                   /s/ RONALD HARRELL      

                                                   Ronald Harrell, P.E.
                                                   President
RH/plk

<PAGE>   6
                                                                   EXHIBIT 99(a)

                                   TABLE NO.1

                                PENNZOIL COMPANY

                           PROVED NET RESERVE DATA(1)

<TABLE>
<CAPTION>
                                                                ------------------------------   ------------------------------ 
                                                                                                         United States
                                                                         Total Worldwide           Total Onshore and Offshore   
                                                                ------------------------------   ------------------------------ 
                                                                  1998       1997       1996       1998       1997       1996   
                                                                --------   --------   --------   --------   --------   -------- 
<S>                                                             <C>        <C>        <C>        <C>        <C>        <C>      
Net Proved Liquid(3) Reserves, Millions of Barrels
-------------------------------------------------------------- 
Developed and Undeveloped
   Beginning of Year                                               227.5      187.4      191.6      152.3      165.1      174.9 
      12/31/95  Est. Prepared by Outrim Szabo Assoc. Ltd.(4)           0          0        9.6          0          0          0 
      Revisions                                                    (25.1)       3.2       12.2      (13.4)      (0.8)       8.1 
      Extensions and Discoveries                                     9.6       41.6       23.2        8.6       11.7       10.5 
      Improved Recovery                                             31.3        1.0        2.1        1.9        1.0        2.1 
      Estimated Production                                         (19.6)     (19.8)     (21.6)     (19.0)     (19.5)     (20.4)
      Purchase of Reserves In-Place                                  1.5       19.9        8.1        1.5          0        7.0 
      Sales of Reserves In-Place                                    (6.4)      (5.8)     (37.8)      (6.4)      (5.2)     (17.1)
End of Year                                                        218.8      227.5      187.4      125.5      152.3      165.1 

Developed
   Beginning of Year(5)                                            141.8      142.2      152.6      127.8      140.7      151.2 
   End of Year                                                     120.4      141.8      142.2      108.2      127.8      140.7 

Net Proved Gas(6) Reserves, Billions of Cubic Feet
-------------------------------------------------------------- 
Developed and Undeveloped
   Beginning of Year                                               1,059      1,277      1,291      1,054      1,187      1,256 
      12/31/95  Est.  Prepared by Outrim  Szabo Assoc. Ltd.(4)         0          0        178          0          0          0 
      Revisions                                                      (71)         2         61        (68)       (26)        38 
      Extensions and Discoveries                                      83        127        172         83        104        143 
      Improved Recovery                                                1          0          2          1          0          2 
      Estimated Production                                          (167)      (214)      (219)      (167)      (206)      (202)
      Purchase of Reserves In-Place                                    4          0         36          4          0          8 
Sales of Reserves In-Place                                           (58)      (133)      (244)       (58)        (5)       (58)
End of Year                                                          851      1,059      1,277        849       1054      1,187 

Developed
   Beginning of Year(5)                                              968      1,160      1,161        963      1,070      1,132 
   End of Year                                                       786        968      1,160        784        963      1,070 
--------------------------------------------------------------  --------   --------   --------   --------   --------   -------- 
</TABLE>

<TABLE>
<CAPTION>
                                                                ------------------------------
                                                                
                                                                           Foreign(2)
                                                                ------------------------------
                                                                  1998       1997       1996
                                                                --------   --------   --------
<S>                                                             <C>       <C>        <C>
Net Proved Liquid(3) Reserves, Millions of Barrels
-------------------------------------------------------------- 
Developed and Undeveloped
   Beginning of Year                                                75.2       22.3       16.7
      12/31/95  Est. Prepared by Outrim Szabo Assoc. Ltd.(4)           0          0        9.6
      Revisions                                                    (11.7)       4.0        4.1
      Extensions and Discoveries                                     1.0       29.9       12.7
      Improved Recovery                                             29.4          0          0
      Estimated Production                                          (0.6)      (0.3)      (1.2)
      Purchase of Reserves In-Place                                    0       19.9        1.1
      Sales of Reserves In-Place                                       0       (0.6)     (20.7)
End of Year                                                         93.3       75.2       22.3

Developed
   Beginning of Year(5)                                             14.0        1.5        1.3
   End of Year                                                      12.2       14.0        1.5

Net Proved Gas(6) Reserves, Billions of Cubic Feet
-------------------------------------------------------------- 
Developed and Undeveloped
   Beginning of Year                                                   5         90         35
      12/31/95  Est.  Prepared by Outrim  Szabo Assoc. Ltd.(4)         0          0        178
      Revisions                                                       (3)        28         23
      Extensions and Discoveries                                       0         23         29
      Improved Recovery                                                0          0          0
      Estimated Production                                             0         (8)       (17)
      Purchase of Reserves In-Place                                    0          0         28
Sales of Reserves In-Place                                             0       (128)      (186)
End of Year                                                            2          5         90

Developed
   Beginning of Year(5)                                                5         90         29
   End of Year                                                         2          5         90
--------------------------------------------------------------  --------   --------   --------
</TABLE>

------------------------

(1)  Reserve changes due to extensions and discoveries and improved recovery
     during 1996, 1997 and 1998 were categorized by Ryder Scott Company. The
     remaining reserve changes during 1996, 1997 and 1998 were categorized by
     the Company.

(2)  1996 includes 1.4 million barrels and 4 billion cubic feet, 1997 includes
     21.3 million barrels and 4 billion cubic feet and 1998 includes 17.2
     million barrels and 2 billion cubic feet of natural gas in Venezuela for
     which Pennzoil Venezuela Corporation, S.A. 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
                            Total Worldwide        Onshore and Offshore             Foreign
                           As of December 31         As of December 31         As of December 31
                        -----------------------   -----------------------   -----------------------
                           1998         1997         1998         1997         1998          1997
                        ----------   ----------   ----------   ----------   ----------   ----------
<S>                     <C>          <C>          <C>          <C>          <C>          <C>
Future Cash Inflows(2)  $    3,798   $    5,808   $    3,000   $    4,847   $      798   $      961

Future Costs
   Production(3)        $   (1,504)  $   (1,926)  $   (1,215)  $   (1,621)  $     (289)  $     (305)
   Development(4)             (429)        (544)        (370)        (458)         (59)         (86)
                        ----------   ----------   ----------   ----------   ----------   ----------

Total Costs             $   (1,933)  $   (2,470)  $   (1,585)  $   (2,079)  $      348   $     (391)

Future Cash Inflows
   Before Income Tax    $    1,865   $    3,338   $    1,415   $    2,768   $      450   $      570

Present Value @ 10%
   Before Income Tax    $    1,079   $    2,053   $      941   $    1,791   $      138   $      262
----------------------  ----------   ----------   ----------   ----------   ----------   ----------
</TABLE>

---------------

(1)  Data for 1998 and 1997 include cash inflows and costs attributable to
     carbon dioxide reserves located in the United States. The 1998 carbon
     dioxide reserves account for $37.1 million of cash inflows before income
     tax and $11.0 million of present value at 10% before income tax. The 1997
     carbon dioxide reserves account for $70.3 million of future cash inflows
     before income tax and $21.4 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.