July 11 (Reuters) - U.S. oil and gas producer Diamondback Energy's average unhedged realized price for natural gas was 10 cents per thousand cubic feet (Mcf) in the second quarter, the company said in a regulatory filing on Thursday.

Natural gas prices have fallen in the reported quarter, hurt by a lower demand forecast, high output and excess inventories, with gas prices turning negative several times so far this year, especially in Texas, Arizona and California.

Negative prices signal there is too much power or gas being produced in a region.

Separately, smaller rival APA Corp said on Thursday it had curtailed about 78 million cubic feet per day (MMcf/d) of natural gas production in the second quarter due to weak prices.

The company also curtailed 7,600 barrels per day of natural gas liquids during the quarter, primarily associated with the voluntary gas curtailments.

APA said it expects U.S. natural gas prices to average 30 cents Mcf in the second quarter.

In the first quarter, APA had curtailed about 35 MMcf/d of natural gas production amid weak prices. (Reporting by Vallari Srivastava in Bengaluru; Editing by Mohammed Safi Shamsi)