Exhibit 99.1

LOGO

February 3, 2010

Mr. J. Russell Porter

Gastar Exploration Ltd.

1331 Lamar Street, Suite 1080

Houston, Texas 77010

Dear Mr. Porter:

In
accordance with your request, we have estimated the proved reserves and future revenue, as of December 31, 2009, to the Gastar Exploration Ltd. (Gastar) interest in certain oil and gas properties located in the United States. It is our
understanding that the reserves estimated in this report constitute all of the proved reserves owned by Gastar. The estimates in this report have been prepared in accordance with the definitions and guidelines of the U.S. Securities and Exchange
Commission (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. Definitions are presented immediately following this letter.
This report has been prepared for Gastars use in filing with the SEC.

We estimate the net reserves and future net revenue to the Gastar
interest in these properties, as of December 31, 2009, to be:



|                               |  |              |  |                        |  |            |  |                    
|------------------------------ |  | ------------ |  | ---------------------- |  | ---------- |  | -------------------
|                               |  | Net Reserves |  | Future Net Revenue ($)
|Category                       |  | Oil(Barrels) |  | Gas(MCF)               |  | Total      |  | Present Worthat 10%
|Proved Developed Producing     |  | 16,733       |  | 28,758,595             |  | 42,929,400 |  | 31,661,500         
|Proved Developed Non-Producing |  | 17,110       |  | 6,768,533              |  | 9,380,200  |  | 7,095,200          
|Proved Undeveloped             |  | 32,882       |  | 13,005,418             |  | 13,644,400 |  | 6,866,100          
|                               |  |              |  |                        |  |            |  |                    
|                               |  |              |  |                       
|Total Proved                   |  | 66,725       |  | 48,532,546             |  | 65,954,000 |  | 45,622,800         

The oil reserves shown include
crude oil and condensate. Oil 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.

The estimates shown in this report are for proved reserves. As requested, probable and possible reserves that exist for these properties have not been
included. This report does not include any value that could be attributed to interests in 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.

Future gross revenue to the Gastar interest is prior to deducting state production taxes and ad valorem taxes. Future net revenue is after deductions for these taxes, future capital costs, operating
expenses, and abandonment costs but before consideration of federal income taxes. The future net revenue has been discounted at an annual rate of 10 percent to determine its present worth. The present worth is shown to indicate the effect of time on
the value of money and should not be construed as being the fair market value of the properties.

LOGO



---

LOGO

For the purposes of this report, we did not perform any field inspection of the properties, nor did we examine the
mechanical operation or condition of the wells and facilities. We have not investigated possible environmental liability related to the properties; therefore, our estimates do not include any costs due to such possible liability. Our estimates of
future revenue do not include any salvage value for the lease and well equipment but do include Gastars estimates of the costs to abandon the wells and production facilities. Abandonment costs are included as capital costs.

Prices used in this report are based on the 12-month unweighted arithmetic average of the first-day-of-the-month price for the period January through
December 2009. For gas volumes, the average regional spot prices are adjusted by area for energy content and local price differentials. As a reference, for the same time period the average Platts Gas Daily Henry Hub spot price was $3.866 per
MMBTU. For oil volumes, the average West Texas Intermediate posted price of $57.65 per barrel is adjusted by area for quality, transportation fees, and regional price differentials. All prices are held constant throughout the lives of the
properties.

Lease and well operating costs used in this report are based on operating expense records of Gastar and Pinnacle Gas Resources,
Inc., the operators of the properties. 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 Gastar 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.
Capital costs are included as required for workovers, new development wells, and production equipment. The future capital costs are held constant to the date of expenditure.

We have made no investigation of potential gas volume and value imbalances resulting from overdelivery or underdelivery to the Gastar interest. Therefore, our estimates of reserves and future revenue do
not include adjustments for the settlement of any such imbalances; our projections are based on Gastar receiving its net revenue interest share of estimated future gross gas production.

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; probable and possible reserves are those additional reserves which are sequentially less certain to be recovered than proved reserves. 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 this report. Estimates of reserves may increase or decrease as a result of future operations, market conditions, or changes in regulations.

For the purposes of this report, we used technical and economic data including, but not limited to, well logs, geologic maps, seismic
data, well test data, production data, historical price and cost information, and property ownership interests.The reserves in this report have been estimated using deterministic methods; these estimates have been prepared in accordance with generally accepted petroleum engineering and evaluation
principles. We used standard engineering and geoscience methods, or a combination of methods, such as performance analysis, volumetric analysis, and analogy, that we considered to be appropriate and necessary to establish reserves quantities and
reserves categorization that conform to SEC definitions and guidelines. In evaluating the information at our disposal concerning this report, we have excluded from our consideration all matters as to which the controlling interpretation may be legal
or accounting, rather than engineering and geoscience. 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.



---

LOGO

The titles to the properties have not been examined by Netherland, Sewell & Associates, Inc. (NSAI), nor has the
actual degree or type of interest owned been independently confirmed. The data used in our estimates were obtained from Gastar; Pinnacle Gas Resources, Inc.; public data sources; and the nonconfidential files of NSAI and were accepted as accurate.
Supporting geoscience, field performance, and work data are on file in our office. The technical persons responsible for preparing the reserves estimates presented herein meet the requirements regarding qualifications, independence, objectivity, and
confidentiality set forth in the Standards Pertaining to the Estimating and Auditing of Oil and Gas Reserves Information promulgated by the Society of Petroleum Engineers. We are independent petroleum engineers, geologists, geophysicists, and
petrophysicists; we do not own an interest in these properties and are not 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/ DAN PAUL SMITH         |  |                               |  | By:                                                                                          |  | /s/ WILLIAM J. KNIGHTS              
|                              |  | Dan Paul Smith, P.E. 49093 |  |                               |  |                                                                                              |  | William J. Knights, P.G. 1532       
|                              |  | Senior Vice President      |  |                               |  |                                                                                              |  | Vice President                      
|                              |  |                           
|Date Signed: February 3, 2010 |  |                            |  | Date Signed: February 3, 2010