Exhibit 99.1

Endeavour International Corporation

February 28, 2011

Mr. Derek Neilson

Endeavour International Corporation

114 St. Martin’s Lane

London WC2N 4BE

United Kingdom

Dear Mr. Neilson:

In accordance with your request, we have audited the estimates prepared by Endeavour International Corporation (Endeavour), as of December 31, 2010, of the proved reserves and future revenue to the Endeavour interest in certain oil and gas properties located in the United Kingdom and the United States. Endeavour’s estimates for Bacchus Field, located in the United Kingdom, reflect a working interest of 10 percent, which is the interest owned by Endeavour prior to the February 2011 execution of the Sales and Purchase Agreement that increases their working interest in the field to 30 percent. It is our understanding that the proved reserves estimates shown herein constitute all of the proved reserves owned by Endeavour. We have examined the estimates with respect to reserves quantities, reserves categorization, future producing rates, future net revenue, and the present value of such future net revenue, using the definitions set forth in U.S. Securities and Exchange Commission (SEC) Regulation S-X Rule 4-10(a). The estimates of reserves and future revenue have been prepared in accordance with the definitions and guidelines of the SEC and, with the exception of the exclusion of future income taxes, conform to the FASB Accounting Standards Codification Topic 932, Extractive Activities—Oil and Gas. We completed our audit on February 23, 2011. This report has been prepared for Endeavour’s use in filing with the SEC; in our opinion the assumptions, data, methods, and procedures used in the preparation of this report are appropriate for such purpose.

The following table sets forth Endeavour’s estimates of the net reserves and future net revenue, as of December 31, 2010, for the audited properties:

Endeavour International Corporation

		Net Reserves												Future Net Revenue ($)
		Oil				NGL				Gas								Present Worth
Country/Category		(Barrels)				(Barrels)				(MCF)				Total				at 10%
UNITED KINGDOM
Proved Developed Producing			1,055,465				26,559				497,832				42,386,138				42,264,733
Proved Developed Non-Producing			152,411				5,633				57,333				9,354,294				8,111,145
Proved Undeveloped			2,423,845				0				55,621,793				252,783,202				150,444,221
Total Proved			3,631,722				32,193				56,176,957				304,523,633				200,820,098
UNITED STATES
Proved Developed Producing			14,201				0				13,145,879				40,247,126				28,174,878
Proved Developed Non-Producing			179				0				135,431				259,404				64,602
Proved Undeveloped			44,412				0				18,495,233				28,957,851				9,507,042
Total Proved			58,792				0				31,776,543				69,464,380				37,746,522
ALL PROPERTIES
Proved Developed Producing			1,069,666				26,559				13,643,711				82,633,264				70,439,611
Proved Developed Non-Producing			152,590				5,633				192,764				9,613,697				8,175,747
Proved Undeveloped			2,468,257				0				74,117,025				281,741,053				159,951,262
Total Proved			3,690,514				32,193				87,953,500				373,988,014				238,566,620

Totals may not add because of rounding.

The oil reserves shown include crude oil and condensate. Oil and natural gas liquids (NGL) volumes are expressed in barrels that are equivalent to 42 United States gallons. Gas volumes are expressed in thousands of cubic feet (MCF) at standard temperature and pressure bases. Monetary values shown in this report are expressed in United States dollars ($).

When compared on a field-by-field basis, some of the estimates of Endeavour are greater and some are less than the estimates of Netherland, Sewell & Associates, Inc. (NSAI). However, in our opinion the estimates of Endeavour’s proved reserves and future revenue shown herein are, in the aggregate, reasonable and have been prepared in accordance with the Standards Pertaining to the Estimating and Auditing of Oil and Gas Reserves Information promulgated by the Society of Petroleum Engineers (SPE

Endeavour International Corporation

Standards). Additionally, these estimates, in the aggregate, are within the recommended 10 percent tolerance threshold set forth in the SPE Standards. We are satisfied with the methods and procedures used by Endeavour in preparing the December 31, 2010, estimates of reserves and future revenue, and we saw nothing of an unusual nature that would cause us to take exception with the estimates, in the aggregate, as prepared by Endeavour.

The estimates shown herein are for proved reserves. Endeavour’s estimates do not include probable and possible reserves that exist for these properties, nor do they include any value for undeveloped acreage beyond those tracts for which undeveloped reserves have been estimated. Reserves categorization conveys the relative degree of certainty; reserves subcategorization is based on development and production status. The estimates of reserves and future revenue included herein have not been adjusted for risk.

Oil and NGL prices used by Endeavour are based on the 12-month unweighted arithmetic average of the first-day-of-the-month price for each month in the period January through December 2010. For United Kingdom properties, the average Dated Brent price of $79.37 per barrel is adjusted by field for quality, transportation fees, and regional price differentials. For United States properties, the average West Texas Intermediate price of $75.96 per barrel is adjusted by field for quality, transportation fees, and regional price differentials. All oil and NGL prices are held constant throughout the lives of the properties. The average adjusted product prices weighted by production over the remaining lives of the properties are $78.89 per barrel of oil and $41.70 per barrel of NGL.

Gas prices used by Endeavour are based on the 12-month unweighted arithmetic average of the first-day-of-the-month price for each month in the period January through December 2010. For United Kingdom properties, the average National Balancing Point spot price of 42.49 pence per therm (equivalent to $6.591 per MMBTU) is adjusted by field for energy content, transportation fees, and regional price differentials. For United States properties, the average Henry Hub spot price of $4.376 per MMBTU is adjusted by field for energy content, transportation fees, and regional price differentials. All gas prices are held constant throughout the lives of the properties. The average adjusted gas price weighted by production over the remaining lives of the properties is $6.142 per MCF.

Lease and well operating costs used by Endeavour are based on historical operating expense records. These costs include the per-well overhead expenses allowed under joint operating agreements along with estimates of costs to be incurred at and below the district and field levels. Headquarters general and administrative overhead expenses of Endeavour are included to the extent that they are covered under joint operating agreements for the operated properties. Lease and well operating costs are held constant throughout the lives of the properties. Endeavour’s estimates of capital costs are included as required for

Endeavour International Corporation

workovers, new development wells, production equipment, and abandonment. The future capital costs are held constant to the date of expenditure.

The reserves shown in this report are estimates only and should not be construed as exact quantities. Proved reserves are those quantities of oil and gas which, by analysis of engineering and geoscience data, can be estimated with reasonable certainty to be economically producible. Estimates of reserves may increase or decrease as a result of market conditions, future operations, changes in regulations, or actual reservoir performance. In addition to the primary economic assumptions discussed herein, estimates of Endeavour and NSAI are based on certain assumptions including, but not limited to, that the properties will be developed consistent with current development plans, that the properties will be operated in a prudent manner, that no governmental regulations or controls will be put in place that would impact the ability of Endeavour to recover the reserves, and that projections of future production will prove consistent with actual performance. If the reserves are recovered, the revenues therefrom and the costs related thereto could be more or less than the estimated amounts. Because of governmental policies and uncertainties of supply and demand, the sales rates, prices received for the reserves, and costs incurred in recovering such reserves may vary from assumptions made while preparing these estimates.

It should be understood that our audit does not constitute a complete reserves study of the audited oil and gas properties. Our audit consisted primarily of substantive testing, wherein we conducted a detailed review of all properties. In the conduct of our audit, we have not independently verified the accuracy and completeness of information and data furnished by Endeavour with respect to ownership interests, oil and gas production, well test data, historical costs of operation and development, product prices, or any agreements relating to current and future operations of the properties and sales of production. However, if in the course of our examination something came to our attention that brought into question the validity or sufficiency of any 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. Our audit did not include a review of Endeavour’s overall reserves management processes and practices.

We used standard engineering and geoscience methods, or a combination of methods, including performance analysis, volumetric analysis, analogy, and reservoir modeling, that we considered to be appropriate and necessary to establish the conclusions set forth herein. As in all aspects of oil and gas evaluation, there are uncertainties inherent in the interpretation of engineering and geoscience data; therefore, our conclusions necessarily represent only informed professional judgment.

Supporting data documenting this audit, along with data provided by Endeavour, are on file in our office. The technical persons responsible for conducting this audit meet the requirements regarding qualifications, independence, objectivity, and confidentiality set forth in the SPE Standards. We are

Endeavour International Corporation

independent petroleum engineers, geologists, geophysicists, and petrophysicists; we do not own an interest in these properties nor are we employed on a contingent basis.

						Sincerely,
						NETHERLAND, SEWELL & ASSOCIATES, INC.
						Texas Registered Engineering Firm F-002699
						By:		/s/ C.H. (Scott) Rees III   C.H. (Scott) Rees III, P.E.
								Chairman and Chief Executive Officer
By:		/s/ Derek F. Newton   Derek F. Newton, P.E. 97689				By:		/s/ David E. Nice   David E. Nice, P.G. 346
		Vice President						Vice President
Date Signed: February 28, 2011						Date Signed: February 28, 2011

DFN:KEA