Exhibit 15.5














January 20, 2016



CNOOC Limited

c/o Nexen Energy ULC

801 – 7th Avenue SW

Calgary, Alberta

T2P 3P7



|Re: | McDaniel & Associates - Report of Third Party for certain Canadian Oil Sands properties
owned by CNOOC Limited
|--- | --------------------------------------------------------------------------------------------------------------



Gentlemen:



Pursuant to your request, McDaniel &
Associates Consultants Ltd. (“McDaniel”) has conducted an independent audit of CNOOC Limited’s (“CNOOC”)
proved synthetic crude oil reserves, as of
December 31, 2015, for certain Canadian Oil Sands properties owned by CNOOC and managed by its wholly-owned subsidiary, Nexen Energy
ULC (“Nexen”), as shown in Table 1. CNOOC has represented that these properties account for 19 percent of its total
company proved reserves on an equivalent barrel basis as of December 31, 2015, and that its reserves estimates have been prepared
in accordance with the United States Securities and Exchange Commission (SEC) definitions. We have reviewed information provided
to us by Nexen on behalf of CNOOC that it represents to be to be its estimates of the reserves, as of December 31, 2015, for the
same properties as those which we audited. The completion date of our report is January 20, 2016. This report was prepared in accordance
with guidelines specified in
Item 1202(a)(8) of Regulation S-K and is to be used for inclusion in certain filings of the SEC.



Reserves included herein are expressed
as reserves as represented by CNOOC. Gross reserves are defined as the total estimated petroleum to be produced from these properties
after December 31, 2015. Working interest reserves are defined as that portion of the gross reserves attributable to the interests
owned by CNOOC after deducting all working interests owned by others. Net reserves are defined as working interest reserves after
the deduction of royalties.



Estimates of synthetic crude oil should
be regarded only as estimates that may change as further production history and additional information become available. Not only
are such reserves 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.







|2200,
    Bow Valley Square 3, 255 - 5 Avenue SW, Calgary AB  T2P 3G6 | Tel:
    (403) 262-5506 | Fax:
    (403) 233-2744 | www.mcdan.com
|--------------------------------------------------------------------- | ----------------------- | ----------------------- | -------------






Nexen Energy ULCJanuary 20, 2016Report of Third Party for certain Canadian Oil Sands Properties owned by CNOOC LimitedPage 2




Data used in this audit were obtained from
reviews with Nexen personnel, Nexen files, 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 Nexen with
respect to property interests, production from such properties, current costs of operation and development, prices for production,
agreements relating to current and future operations and sale of production, and various other information and data that were accepted
as represented. Furthermore, if in the course of our examination something came to our attention, which brought into question the
validity or sufficiency of any of such information or data, we did not rely on such information or data until we had satisfactorily
resolved our questions relating thereto or had independently verified such information or data. A field examination of the properties
was not considered necessary for the purposes of this report.



Methodology and Procedures



The process of estimating reserves requires
complex judgments and decision-making based on available geological, geophysical, engineering and economic data. To estimate the
economically recoverable oil, synthetic crude oil and natural gas reserves, and related future net cash flows, we consider many
factors and make assumptions including:





| | · | expected reservoir characteristics based
on geological, geophysical and engineering assessments;
| | - | ------------------------------------------------------------------------------------------------



| | · | future production rates based on historical
performance and expected future operating and investment activities;
| | - | ----------------------------------------------------------------------------------------------------------------



| | · | future oil and gas prices and quality
differentials;
| | - | ----------------------------------------------------



| | · | assumed effects of regulation by governmental
agencies; and
| | - | -----------------------------------------------------------



| | · | future development and operating costs
| | - | --------------------------------------



Estimates of reserves were prepared using
standard geological and engineering methods generally accepted by the petroleum industry as presented in the publication of the
Society of Petroleum Engineers entitled “Standards Pertaining to the Estimating and Auditing of Oil and Gas Reserves Information
(Revision as of February 19, 2007).” Generally accepted methods for estimating reserves include volumetric calculations,
material balance techniques, production decline curves, pressure transient analysis, analogy with similar reservoirs, and reservoir
simulation. The method or combination of methods used is based on professional judgment and experience.



Discovered oil and natural gas reserves
are generally only produced when they are economically recoverable. As such, oil and gas prices, and capital and operating costs
have an impact on whether reserves will ultimately be produced. As required by SEC rules, reserves represent the quantities that
are expected to be economically recoverable using existing prices and costs. Estimates may change substantially as additional data
from ongoing development activities and production performance becomes available and as economic conditions impacting oil and gas
prices and costs change.






Nexen Energy ULCJanuary 20, 2016Report of Third Party for certain Canadian Oil Sands Properties owned by CNOOC LimitedPage 3




The proved reserves estimates in this report
were based upon 2015 first-of-the month fiscal average pricing using benchmark pricing. Oil prices were primarily based upon West
Texas Intermediate at Cushing crude oil benchmark of US$50.28 per barrel and Western Canadian Select at Hardisty benchmark of US$36.76
per barrel. Specific pricing for each field was adjusted for historical quality and transportation cost differentials, and for
currency exchange rates. The resulting adjusted price is referred to as the “realized price.” For total proved reserves,
the estimated realized prices were US$47.81 per barrel of synthetic crude oil, based upon a volume weighted average of the properties
evaluated.



Generally, operations are subject to various
levels of government controls and regulations. These laws and regulations may include matters relating to land tenure, drilling,
production practices, environmental protection, marketing and pricing policies, royalties, various taxes and levies including income
tax, and foreign trade and investment, that are subject to change from time to time. Current legislation is generally a matter
of public record, and additional legislation or amendments that will affect reserves or when any such proposals, if enacted, might
become effective generally cannot be predicted. Changes in government regulations could affect reserves or related economics. In
the regions that are currently being evaluated we believe we have applied existing regulations appropriately.



CNOOC Estimates



Nexen on behalf of CNOOC has represented
that estimated proved reserves attributable to the audited properties are based on SEC definitions. CNOOC represents that its estimates
of the reserves attributable to these properties represent 19 percent of its total company proved reserves after royalties on an
equivalent basis and are as follows, expressed in thousands of barrels (Mbbl), and thousands of barrels of oil equivalent (Mboe):



CNOOC’s estimate of Reserves as
of December 31, 2015

Certain Canadian Fields Audited by McDaniel
& Associates

Oil Sands

|CNOOC Reserves                              | Synthetic
        Crude Oil
        (Mbbl) | Oil Equivalent
                                                                                (Mboe)
|------------------------------------------- | ------------------------------------------ | -----------------------------------------------------------------------------------------------------
|Working Interest Reserves (after royalties)
|Proved                                      | 815,257                                    | 815,257                                                                                              



Note: Gas is converted to oil equivalent
using a factor of 6,000 cubic feet of gas per 1 barrel of oil equivalent based on an energy equivalent basis.



Reserves Audit Opinion



McDaniel has used all data, assumptions,
procedures and methods that it considers necessary to prepare this report.






Nexen Energy ULCJanuary 20, 2016Report of Third Party for certain Canadian Oil Sands Properties owned by CNOOC LimitedPage 4




In our opinion, the information relating
to estimated proved reserves of synthetic crude oil contained in this opinion has been prepared in accordance with Paragraphs 932-235-50-4,
932-235-50-6, 932-235-50-7 and 932-235-50-9 of the Accounting Standards Update 932-235-50, Extractive Industries – Oil
and Gas (Topic 932): Oil and Gas Reserve Estimation and Disclosures (January 2010) of the Financial Accounting Standards Board
and Rules 4–10(a) (1)–(32) of Regulation S–X and Rules 302(b), 1201, and 1202(a) (1), (2), (3), (4), (5), (8)
of Regulation S-K of the Securities and Exchange Commission.



We have examined the assumptions, data,
methods procedures and proved reserves estimates prepared by Nexen on behalf of CNOOC. In our opinion, the proved reserves for
the reviewed properties as estimated by CNOOC are, in aggregate on the basis of equivalent barrels, reasonable because when compared
to our estimates, or if we were to perform our own detailed estimates, reflect a difference of not more than plus or minus 10 percent.



The analyses of these properties, as reported
herein, were conducted within the context of an audit of a distinct group of properties in aggregate as part of the total corporate
level reserves. Extraction and use of these analyses outside of this context may not be appropriate without supplementary due diligence.



McDaniel is an independent petroleum engineering
consulting firm that has been providing petroleum consulting services throughout the world for over 60 years. McDaniel does not
have any financial interest, including stock ownership, in CNOOC. Our fees were not contingent on the results of our evaluation.
This letter report has been prepared at the request of Nexen on behalf of CNOOC.



This report was prepared by McDaniel &
Associates Consultants Ltd. for the exclusive use of CNOOC. It is not to be reproduced, distributed, or made available, in whole
or in part to any person, company, or organization other than CNOOC without the knowledge and consent of McDaniel & Associates
Consultants Ltd. We reserve the right to revise any of the estimates provided herein if any relevant data existing prior to preparation
of this report was not made available or if any data provided was found to be erroneous.



If there are any questions, please contact
the writer directly at (403) 218-1379.



Sincerely,



McDaniel & Associates Consultants Ltd.



/s/ P. A. Welch

____________________________



P. A. Welch, P. Eng.



|CC: | Mr. Mike Birdgeneau, P. Eng., Team Lead, Oil Sands Resource
Planning                     
|--- | -----------------------------------------------------------------------------------------
|    | Ms. Charishe Amante, P. Eng., Corporate Reserves
Analyst

Ms. Sydney Frazer, Deloitte LLP








Nexen Energy ULCJanuary 20, 2016Report of Third Party for certain Canadian Oil Sands Properties owned by CNOOC LimitedPage 5




Table 1

CNOOC Properties Audited by McDaniel
& Associates

Select Canadian Oil Sands Properties

December 31, 2015



|  | BUSINESS UNIT | FIELD NAME
|- | ------------- | ----------
|  |               |           
|1 | OIL SANDS     | LONG LAKE 
|2 | OIL SANDS     | SYNCRUDE