Was Cain's offering unacceptable to the Lord because it was not an animal sacrifice without blemish?

Question

Genesis 4
1And Adam knew Eve his wife; and she conceived, and bare Cain, and said, I have gotten a man from the LORD.

2And she again bare his brother Abel. And Abel was a keeper of sheep, but Cain was a tiller of the ground.

3And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD.

4And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the LORD had respect unto Abel and to his offering:

5But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell.

6And the LORD said unto Cain, Why art thou wroth? and why is thy countenance fallen?

7If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him.

Answer 1

I'd say more about the spiritual condition of his heart<><

Answer 2

God gives us specific instructions. It is the obedience He desires, not the sacrifice.

1 Samuel 15:22

Answer 3

the sacrifice was to be a blood offering, it is only through the shedding of blood that that the sins of the past are are covered.
however the blood of Jesus covered the sins of the world, past, present, and future.