Genesis 4:8-9

Genesis 4:8-9

[8] And Cain  talked  with  Abel  his brother:  and it came to pass, when they were in the field,  that Cain  rose up  against  Abel  his brother,  and slew him.  [9] And the LORD  said  unto Cain,  Where  is Abel  thy brother?  And he said,  I know  not: Am I my brother's  keeper? 

What is the context of Genesis 4:8-9?