Plains Exploration shares down on stock offering

*Stock falls 7 percent in extended trading

*Sees Q1 production at mid-point of prior forecast

HOUSTON, April 21 (Reuters) - Plains Exploration & Production Co (PXP.N) on Tuesday said its plans to offer 10 million of its shares to fund capital expenditures, sending the exploration and production company's shares down 7 percent.

In after-hours trading, Plains' shares traded around $18.65, down from the New York Stock Exchange close of $20.10.

Plains said it will grant underwriters a 30-day option to purchase up to 1.5 million additional shares of common stock to cover over allotments if needed.

In a separate news release, the Houston-based company said its first-quarter production will be "just above" the mid-point of its annual production sales volume forecast for 78,000 to 82,000 barrels of oil equivalent per day.

Plains also said it does not expect to write down the value of of any of its oil and gas assets in the first quarter. (Reporting by Anna Driver in Houston; Editing by Tim Dobbyn)