‘And Abraham planted a Tamarisk tree at Beersheba and called there on the name of Yahweh, the Everlasting God - El ‘Olam.'
It may be that the Philistines in Gerar worshipped El ‘Olam whom, because of the significance of his name Abraham accepted as being Yahweh for he knew Yahweh to be God from everlasting to everlasting (there was no concept of ‘eternity'. ‘Olam meant from time past to time future), compare El Elyon (Genesis 14:22).
The Tamarisk tree was native to the area. It was to mark and possibly to provide shelter over the well. Thus the thirsty passer by, needing water, would see the well was there.
“Called there on the name of Yahweh”. As priest of the tribe he originated cult worship there. It became a shrine to the goodness of God, the central place of worship for his family tribe.