Genesis 14 v
7 And he said unto him, I am the LORD that brought thee out of Ur of the Chaldees, to give thee this land to inherit it.
8 And he said, Lord GOD, whereby shall I know that I shall inherit it?
9 And he said unto him, Take me an heifer of three years old, and a she goat of three years old, and a ram of three years old, and a turtledove, and a young pigeon.
10 And he took unto him all these, and divided them in the midst, and laid each piece one against another: but the birds divided he not.
11 And when the fowls came down upon the carcases, Abram drove them away.
12 And when the sun was going down, a deep sleep fell upon Abram; and, lo, an horror of great darkness fell upon him.
13 And he said unto Abram, Know of a surety that thy seed shall be a stranger in a land that is not theirs, and shall serve them; and they shall afflict them four hundred years;
14 And also that nation, whom they shall serve, will I judge: and afterward shall they come out with great substance.
15 And thou shalt go to thy fathers in peace; thou shalt be buried in a good old age.
16But in the fourth generation they shall come hither again: for the iniquity of the Amorites is not yet full.
17 And it came to pass, that, when the sun went down, and it was dark, behold a smoking furnace, and a burning lamp that passed between those pieces.
18 In the same day the LORD made a covenant with Abram, saying, Unto thy seed have I given this land, from the river of Egypt unto the great river, the river Euphrates:
A thanksgiving offering is a freewill offering and it is obvious that Jehovah is not offering a sacrifice of thanksgiving to himself by commanding Abraham to kill these animals. So this cannot be a thanksgiving offering. First of all the offerings before the law of Moses have to show how atonement is made before sacrifices can be offered which display thanksgiving for the giving of this blessing. This is the logical order. There could not have been thanksgiving offerings to God from the beginning.