Spo Advisory SC 13D QUICKSILVER RESOURCES / SPO ADVISORY ownership change

EXHIBIT A

Pursuant to Rule 13d-1(k) of Regulation 13D-G of the General Rules and Regulations of the Securities and Exchange Commission under the Securities Exchange Act of 1934, as amended, the undersigned agrees that the statement to which this Exhibit is attached is filed on behalf of each of them in the capacities set forth below.

Dated: August 27, 2010	By:	/s/ Kim M. Silva
	Name:	Kim M. Silva
	Attorney-in-Fact for:
	SPO PARTNERS II, L.P. (1)
	SPO ADVISORY PARTNERS, L.P. (1)
	SAN FRANCISCO PARTNERS, L.P. (1)
	SF ADVISORY PARTNERS, L.P. (1)
	SPO ADVISORY CORP. (1)
	JOHN H. SCULLY (1)
	WILLIAM E. OBERNDORF (1)
	WILLIAM J. PATTERSON (1)
	EDWARD H. MCDERMOTT (1)
	THE ELIZABETH R. & WILLIAM J. PATTERSON FOUNDATION(1)
	(1) A Power of Attorney authorizing Kim M. Silva to act on behalf of this person or entity has been previously filed with the Securities and Exchange Commission.

[Signature Page to Agreement Pursuant to Rule 13d-1(k)]