Exhibit 15.6





DeGolyer and MacNaughton

5001 Spring Valley
Road

Suite 800 East

Dallas, Texas 75244



January 19, 2016



CNOOC Limited

c/o Nexen Energy ULC

801-7th Avenue S.W.

Calgary,
AB, T2P 3P7

Canada



Re: DeGolyer and MacNaughton - Report of Third Party
for certain United Kingdom and Nigeria properties owned by CNOOC Limited



Ladies and Gentlemen:



Pursuant to your request,
we have conducted an independent reserves audit of CNOOC Limited’s (CNOOC) proved oil, condensate, natural gas liquids (NGL),
and gas reserves, as of December 31, 2015, for certain fields owned by CNOOC and managed by its wholly owned subsidiary, Nexen
Energy ULC (Nexen), in the United Kingdom and Nigeria as shown in Table 1. This audit was completed on January 19, 2016. CNOOC
has represented that these properties account for 3 percent of its total proved reserves in its United Kingdom and Nigeria properties
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 its estimates of the reserves, as of December 31, 2015, for the same properties as those which
we audited. 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.



Estimates of 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






DeGolyer and MacNaughton2




interests owned by CNOOC after
deducting all interests owned by others. Net reserves are defined as working interest reserves after deductions for royalties.



Estimates
of oil, condensate, and NGL (collectively Liquids), and gas 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.



Data used
in this audit were obtained from reviews with Nexen personnel, from 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 and 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.
| | - | -------------------------------------------------------



| | · | Future development and operating costs.
| | - | ---------------------------------------






DeGolyer and MacNaughton3




Estimates
of reserves were prepared by the use of appropriate geologic, petroleum engineering, and evaluation principles and techniques that
are in accordance with practices 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 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.



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 audited, we believe existing regulations have been applied
appropriately.



Reserves
estimates in this report are based upon 2015 first-day-of-the month fiscal average pricing using benchmark pricing. For the United
Kingdom, the 2015 oil price is based upon Brent benchmark of U.S.$54.25 per barrel, while the gas price is based upon National
Balancing Point benchmark of U.K.£0.44 per therm. For Nigeria, the oil price is based upon the Brent benchmark of U.S.$54.25
per barrel. Specific pricing for each field was adjusted for historical quality and transportation costs. The resulting adjusted
price is referred to as the “realized price.” For total proved reserves, the estimated realized prices were U.S.$53.99
per barrel and






DeGolyer and MacNaughton4






U.S.$5.78 per thousand cubic
feet of gas for the United Kingdom and U.S.$51.68 per barrel of oil for Nigeria based upon a volume-weighted average of the properties
evaluated.



Certain properties
in which CNOOC has an interest are subject to the terms of various profit sharing or joint operating agreements. The terms of these
agreements generally allow for working interest participants to be reimbursed for portions of capital costs and operating expenses
and to share in the profits. The reimbursements and profit proceeds are converted to a barrel of oil equivalent or standard cubic
foot of gas equivalent by dividing by product prices to determine the “entitlement quantities.” These entitlement quantities
are equivalent in principle to net reserves and are used to calculate an equivalent net share, termed an “entitlement interest.”
In this report, CNOOC reserves or interest for certain properties subject to these agreements is the entitlement based on CNOOC’s
working interest.



CNOOC Estimates



Nexen, on
behalf of CNOOC, has represented that its 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, representing 3 percent of its total company
proved reserves on an equivalent basis, are as follows, expressed in thousands of barrels (Mbbl), millions of cubic feet (MMcf),
and thousands of barrels of oil equivalent (Mboe):



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

Select United
Kingdom and Nigeria Fields

Working Interest
Reserves (after royalties) – Net Reserves



Audited by DeGolyer
and MacNaughton

|               |         |       |         |  | Oil       
|-------------- | ------- | ----- | ------- |  | ----------
|               | Liquids |       | Gas     |  | Equivalent
|CNOOC Reserves | (Mbbl)  |       | (MMcf)  |  | (Mboe)    
|Proved         | 115,226 | 3,574 | 115,822



|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.






DeGolyer and MacNaughton5




Reserves Audit Opinion



In our opinion,
the assumptions, data, methods, and procedures used in connection with the preparation of the report are appropriate for the purpose
served by this report.



In our opinion, the
information relating to estimated proved reserves of oil, condensate, natural gas liquids, and gas contained in this report 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
(January2010) 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), (3), (4), (8) of Regulation S–K of the Securities and Exchange Commission; provided,
however, that estimates of proved developed and proved undeveloped reserves are not presented at the beginning of the year.



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.



While the
oil and gas industry may be subject to regulatory changes from time to time that could affect an industry participant’s ability
to recover its oil and gas reserves, we are not aware of any such governmental actions which would restrict the recovery of the
December 31, 2015, estimated oil and gas reserves.






DeGolyer and MacNaughton6




DeGolyer
and MacNaughton is an independent petroleum engineering consulting firm that has been providing petroleum consulting services throughout
the world since 1936. DeGolyer and MacNaughton 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.



Submitted,



/s/ DeGolyer and MacNaughton



DeGOLYER and MacNAUGHTON

Texas Registered Engineering Firm F-716



|CC: | Mr. Craig Paveley, VP – Development, UK                                     
|--- | ----------------------------------------------------------------------------
|    | Mr. Steven Brown, GM –
International Development Sydney Frazer, Deloitte LLP







|       | /s/ Lloyd W. Cade, P.E. 
|------ | ------------------------
|       |                         
|       | Lloyd W. Cade, P.E.     
|[SEAL] | Senior Vice President   
|       | DeGolyer and MacNaughton






DeGolyer and MacNaughton7




CERTIFICATE of QUALIFICATION



I, Lloyd W. Cade Petroleum Engineer
with DeGolyer and MacNaughton, 5001 Spring Valley Road, Suite 800 East, Dallas, Texas, 75244 U.S.A., hereby certify:





| | 1. | That I am a Senior Vice President with DeGolyer and MacNaughton, which company
did prepare the letter report addressed to CNOOC dated January 19, 2016, and that I, as Senior Vice President, was responsible
for the preparation of this letter report.
| | -- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



| | 2. | That I attended Kansas State University, and that I graduated with a Bachelor
of Science degree in Mechanical Engineering in the year 1982; that I am a Registered Professional Engineer in the State of Texas;
that I am a member of the International Society of Petroleum Engineers; and that I have in excess of 33 years of experience in
oil and gas reservoir studies and reserves evaluations.
| | -- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



SIGNED: January 19, 2016





|       | /s/ Lloyd W. Cade, P.E. 
|------ | ------------------------
|       | Lloyd W. Cade, P.E.     
|[SEAL] | Senior Vice President   
|       | DeGolyer and MacNaughton











DeGolyer and MacNaughton



TABLE 1

CNOOC FIELDS AUDITED by
DeGOLYER and MacNAUGHTON as of

DECEMBER 31, 2015



|Properties     |  | Fields                                               
|-------------- |  | -----------------------------------------------------
|United Kingdom |  | Buzzard                                              
|               |  | Golden Eagle Area (Including Peregrine and Solitaire)
|               |  |                                                      
|Nigeria        |  | Usan (Block OML-138)