**Exhibit 10.13(a)**

**AMENDMENT AND JOINDER TO OMNIBUS AGREEMENT**

**THIS AMENDMENT AND JOINDER TO OMNIBUS AGREEMENT**(this “***Amendment***”) is made as of December 18, 2006, among Atlas Pipeline Partners, L.P., a Delaware limited partnership, and Atlas Pipeline Operating Partnership, L.P., a Delaware limited partnership (collectively, the“***MLP***”), Atlas America, Inc., a Delaware corporation (“***Atlas America***”), Resource Energy, LLC, a Delaware limited liability company (formerly Resource Energy, Inc., “***ResourceEnergy***”), Viking Resources, LLC, a Pennsylvania limited liability company (formerly Viking Resources Corporation, “***Viking Resources***”), Atlas Energy Resources, LLC, a Delaware limited liability company(“***Atlas Energy***”), and Atlas Energy Operating Company, LLC, a Delaware limited liability company (“***Energy Operating***”). 

**RECITALS**

A. The MLP, Atlas America, Resource Energy and Viking Resources are parties to the OmnibusAgreement dated February 2, 2000 (the “***Omnibus Agreement***”) and desire to amend the same in accordance with the terms hereof. 

B. Atlas America has formed Atlas Energy and Energy Operating for the purpose of facilitating the initial public offering of Atlas Energy and, pursuant to the Contribution and Assumption Agreement of even dateherewith, is transferring all of the issued and outstanding member interests in Resource Energy and Viking Resources to Energy Operating, and the parties hereto wish to join Atlas Energy and Energy Operating to the Omnibus Agreement. 

**NOW, THEREFORE**, in consideration of the premises, and the mutual covenants and agreements herein set forth, and intending to be legally bound, theparties agree as follows: 

1. Joinder in the Omnibus Agreement. Effective as of the date hereof, each of Atlas Energy and EnergyOperating hereby joins the Omnibus Agreement and accepts and agrees to be bound as a “Resource Entity” thereunder with respect to all of the terms and conditions thereof, except Section 5.2 thereof, and the MLP hereby consents to suchjoinder. 

2. Removal of Atlas America as Primary Obligor; Guaranty. Except to the extent that Atlas America purchases an asset thatis first properly presented to Atlas Energy as a Business Opportunity (as defined and as permitted by the Omnibus Agreement of even date herewith, between Atlas America and Atlas Energy, as the same may be amended from time to time), effective as ofthe date hereof, Atlas America is hereby removed as a primary obligor of the Omnibus Agreement, and the MLP hereby consents to such removal. Atlas America hereby unconditionally and absolutely 

guarantees the performance of the Resource Entities’ obligations arising under the Omnibus Agreement (the “***GuaranteedObligations***”). If the Resource Entities fail or refuse to perform any Guaranteed Obligations, the MLP may make a demand upon Atlas America (a “***Demand***”). A Demand shall be in writing and shall reasonably andbriefly specify in what manner any of the Resource Entities has failed to perform under a Guaranteed Obligation, with a specific statement that the MLP is calling upon Atlas America to perform pursuant to this section. Atlas America agrees thatexcept as expressly set forth herein, it will remain bound under this section notwithstanding any defenses which, pursuant to the laws of suretyship, would otherwise relieve a guarantor of its obligations under a guarantee. Atlas America reservesthe right to assert defenses which any of the Resource Entities may have to performance of any Guaranteed Obligation other than defenses arising from the bankruptcy or insolvency of such Resource Entity and other defenses expressly waived hereby.

3. Definitions. The following definitions set forth in the Omnibus Agreement are hereby amended and restated in their entirety asfollows: 

“***Investment Program***” means a Person principally engaged in the drilling of natural gas and oil wells forwhich a Resource Entity or a subsidiary of a Resource Entity acts as a general partner, managing partner or manager and the securities of which have been offered and sold to investors. 

***“Master Natural Gas Gathering Agreement”*** means the Master Natural Gas Gathering Agreement among the Resource Entities and the MLPdated as of February 2, 2000, as amended from time to time. 

4. Consulting Services. Section 2.5 of the Omnibus Agreementis hereby amended and restated as follows: 

*Consulting Services in Connection with Acquisitions*. The Resource Entities agree toassist the MLP in seeking to identify for possible acquisition Third Party Gathering Systems and to provide consulting services to MLP in evaluating and acquiring any such identified gathering system. Further, the Resource Entities agree to give theMLP prompt written notice of the identification by any of them of any Third Party Gathering System for possible acquisition by such Resource Entity or any Affiliate (each, an “***Identified Third Party Gathering System***”). Suchnotice shall set forth all information available to the Resource Entities about the Identified Third Party Gathering System, including the identity the gathering system and its seller and the proposed sales price, and shall include all writteninformation about the Identified Third Party Gathering System provided to the Resource Entities by or on behalf of the seller as well as any information or analyses compiled by the Resource Entities from other sources (collectively, the“***Business Opportunity Information***”). The Resource Entities shall continue to provide promptly to the MLP any and all Business Opportunity Information subsequently 

 

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received. Within a time period specified by the Resource Entity’s notice to the MLP, which shall be a reasonable time under the circumstances, the MLPshall advise the Resource Entity in writing whether it wishes to acquire the Identified Third Party Gathering System. If the MLP advises the Resource Entity of its intent to acquire the Identified Third Party Gathering System, the Resource Entitiesshall refrain from making an offer for the Identified Third Party Gathering System except as permitted hereunder. If the MLP (i) advises the Resource Entity that (with the approval of the conflicts committee of the General Partner) it does notintend to acquire the Identified Third Party Gathering System, (ii) advises the Resource Entity of its intent to acquire the Identified Third Party Gathering System but does not complete the acquisition or investment within a reasonable timeafter the MLP’s notice of its intent to the Resource Entity or (iii) fails to timely advise the Resource Entity of its intent, any of the Resource Entities shall be free to acquire the Identified Third Party Gathering System. 

5. Construction Management Services. Article 3 of the Omnibus Agreement is hereby amended and restated as follows: 

In the event the MLP expands the Gathering System or constructs a new addition to the Gathering System, whether pursuant to Article 2 or otherwise, theResource Entities agree to cause a wholly-owned corporate subsidiary of Atlas Operating to provide to the MLP oversight of construction management in connection with any such expansion as requested by the MLP. The MLP shall pay all third partyconstruction costs related to any such expansion directly to the service provider for the construction services related thereto. 

6.Other Terms of the Omnibus Agreement. Except as otherwise expressly provided herein, the Omnibus Agreement is not amended, modified or affected by this Amendment. 

7. Counterparts. This Amendment may be executed in any number of counterparts and by different parties hereto in separate counterparts, each of which when so executed and delivered shall be deemed to be anoriginal and all of which taken together shall constitute but one and the same instrument. Delivery of an executed counterpart of this Amendment by facsimile shall be equally as effective as delivery of a manually executed counterpart of thisAmendment. 

 

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**IN WITNESS WHEREOF**, the parties have executed this Amendment to be effective as of the date firstwritten above. 

 
**Shipper:**ATLAS AMERICA, INC.By: 
 
Name: Its: 
 
ATLAS ENERGY RESOURCES, LLCBy: 
 
Name: Its: 
 
ATLAS ENERGY OPERATING COMPANY, LLCBy: Atlas Energy Resources, LLC, its sole memberBy: 
 
Name: Its: 
 
RESOURCE ENERGY, LLCBy: Atlas Energy Operating Company, LLC, its sole memberBy: Atlas Energy Resources, LLC, its sole memberBy: 
 
Name: Its: 
 

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VIKING RESOURCES, LLCBy: Atlas Energy Operating Company, LLC, its sole memberBy: Atlas Energy Resources, LLC, its sole memberBy: 
 
Name: Its: 
 
**Gatherer:**ATLAS PIPELINE OPERATING PARTNERSHIP, L.P.By: Atlas Pipeline Partners GP, LLC, its general partnerBy: 
 
Name: Its: 
 
ATLAS PIPELINE PARTNERS, L.P.By: Atlas Pipeline Partners GP, LLC, its general partnerBy: 
 
Name: Its: 
 

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