Chesapeake Energy Corp., Oklahoma City, (NYSE: CHK) has amended its Haynesville shale joint venture agreement with Plains E&P Co., Houston (NYSE: PXP).

Chesapeake has granted Plains a one-time option to avoid paying the last $800 million of its $1.65-billion drilling carry obligation to Chesapeake, which represents approximately 25% of the original joint venture transaction. The amendment includes three key features. first, the option may only be exercised by Plains during the two-week period from June 15, 2010 through June 30, 2010. Second, should Plains exercise the option, it will be required to convey 50% of all of its Haynesville shale joint venture assets to Chesapeake as of Dec. 31, 2010, including all investments in leasehold, production and reserves at that date.

Chesapeake estimates Plains’ investment in the Haynesville assets as of Dec. 31, 2010 will likely range from $3 billion to $3.2 billion.

Lastly, until Dec. 31, 2010, Plains’ obligations to both pay for 50% of Chesapeake’s drilling costs in the joint venture and participate in each well in which Chesapeake participates remain unchanged and are mandatory.