Exhibit 99.1

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January 24, 2011

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 and probable reserves and future revenue, as of December 31, 2010, to the Gastar Exploration Ltd. (Gastar) interest in certain oil and gas properties located in the United States. We
completed our evaluation on January 24, 2011. It is our understanding that the proved 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; in our opinion the assumptions, data, methods, and procedures used in the preparation of this report are
appropriate for such purpose.

We estimate the net reserves and future net revenue to the Gastar interest in these properties, as of
December 31, 2010, to be:



|                               |  |              |        |  |                        |  |            |       |  |  |                     |  |  |  |            | 
|------------------------------ |  | ------------ | ------ |  | ---------------------- |  | ---------- | ----- |  |  | ------------------- |  |  |  | ---------- | 
|                               |  | Net Reserves |        |  | Future Net Revenue ($) | 
|Category                       |  | Oil(Barrels) |        |  | Gas(MCF)               |  |            | Total |  |  | Present Worthat 10% | 
|Proved Developed Producing     |  |              | 35,718 |  |                        |  | 32,825,142 |       |  |  | 66,214,700          |  |  |  | 47,857,400 | 
|Proved Developed Non-Producing |  |              | 8,956  |  |                        |  | 8,746,724  |       |  |  | 18,350,400          |  |  |  | 12,232,300 | 
|Proved Undeveloped             |  |              | 16,658 |  |                        |  | 8,319,614  |       |  |  | 14,163,800          |  |  |  | 7,192,000  | 
|                               |  |              |        |  |                        |  |            |       |  |  |                     |  |  |  |            | 
|Total Proved                   |  |              | 61,332 |  |                        |  | 49,891,480 |       |  |  | 98,728,900          |  |  |  | 67,281,700 | 
|                               |  |              |        | 
|Probable Developed             |  |              | 7,191  |  |                        |  | 7,855,463  |       |  |  | 21,414,500          |  |  |  | 12,977,600 | 
|Probable Undeveloped           |  |              | 0      |  |                        |  | 41,482,338 |       |  |  | 57,018,100          |  |  |  | 19,356,500 | 
|                               |  |              |        |  |                        |  |            |       |  |  |                     |  |  |  |            | 
|Total Probable                 |  |              | 7,191  |  |                        |  | 49,337,801 |       |  |  | 78,432,600          |  |  |  | 32,334,100 | 

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 and probable reserves. As requested, possible reserves that exist for these properties have not been
included. Because of a lack of available data, our study of the Pennsylvania and West Virginia properties was limited to proved only. 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 any income taxes. The future net revenue has been discounted



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at an annual rate of 10 percent to determine its present worth, which is shown to indicate the effect of time on the value of money. Future net revenue presented in this report, whether
discounted or undiscounted, should not be construed as being the fair market value of the properties.

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 shown herein 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 each month in the period January through December 2010. For gas volumes, the average regional spot prices are adjusted by lease for energy content and local price differentials. For oil volumes, the
average regional posted prices are adjusted by lease for quality, transportation fees, and regional price differentials. All 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 $3.532 per MCF of gas and $72.95 per barrel of oil. Average index prices along with the average realized prices for each area are shown in the following table:



|                               |  |                        |  |                           |       |  |                             |  |       |              |  |                              |  |  |                                |  |  |  |       | 
|------------------------------ |  | ---------------------- |  | ------------------------- | ----- |  | --------------------------- |  | ----- | ------------ |  | ---------------------------- |  |  | ------------------------------ |  |  |  | ----- | 
|                               |  | Gas                    |  |                           | Oil   | 
|Area                           |  | PricingIndex           |  | AverageSpotPrice($/MMBTU) |       |  | AverageRealizedPrice($/MCF) |  |       | PricingIndex |  | AveragePostedPrice($/Barrel) |  |  | AverageRealizedPrice($/Barrel) | 
|Texas                          |  | Katy                   |  |                           | 4.318 |  |                             |  | 3.494 |              |  | West TexasIntermediate       |  |  | 75.96                          |  |  |  | 72.83 | 
|Pennsylvania and West Virginia |  | Columbia GasAppalachia |  |                           | 4.502 |  |                             |  | 4.220 |              |  | West TexasIntermediate       |  |  | 75.96                          |  |  |  | 73.07 | 
|Wyoming                        |  | CIG RockyMountains     |  |                           | 3.945 |  |                             |  | 3.488 |              |  | N/A                          |  |  |                               |  |  |  |      | 
|Kansas                         |  | Henry Hub              |  |                           | 4.376 |  |                             |  | 3.298 |              |  | N/A                          |  |  |                               |  |  |  |      | 

Lease and well operating costs used in
this report are based on operating expense records of Gastar. 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. Estimates of reserves may increase or decrease as a result of market conditions, future operations, changes in regulations, or



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actual reservoir performance. In addition to the primary economic assumptions discussed herein, our estimates 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 Gastar to recover the reserves,
and that our 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 this report.

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 the Standards Pertaining to the Estimating and
Auditing of Oil and Gas Reserves Information promulgated by the Society of Petroleum Engineers (SPE Standards). We used standard engineering and geoscience methods, or a combination of methods, including performance analysis, volumetric analysis,
and analogy, that we considered to be appropriate and necessary to categorize and estimate reserves in accordance with SEC definitions and guidelines. 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.

The
data used in our estimates were obtained from Gastar, public data sources, and the nonconfidential files of Netherland, Sewell & Associates, Inc. (NSAI) and were accepted as accurate. Supporting geoscience, performance, and work data are on
file in our office. The titles to the properties have not been examined by NSAI, nor has the actual degree or type of interest owned been independently confirmed. The technical persons responsible for preparing the reserves estimates presented
herein meet the requirements regarding qualifications, independence, objectivity, and confidentiality set forth in the SPE Standards. We are 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-002699By:

/s/ C.H. (Scott) Rees IIIC.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: January 24, 2011 |  | Date Signed: January 24, 2011

WKB:SDB



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