Abstract: In order to understand the genesis of gypsum zone in the upper Upper part of the fourth member of Shahejie Formation in Bonan sag, comprehensive analyses of drilling data, testing data, the developmental features, and genetic types of gypsium zone were performed. The results show that the gypsum zone is mainly composed of gypsum mudstones and gypsum rocks, and the gypsum rocks mainly occur in the depth range between 2800 m and 3600 m, and as two EW-trending wide belts. The climate was humid and lake was deep during the depositional period of gypsum zone. The distribution of igneous rocks and the gypsum zone appears to be coupled and we speculate that salts substances came from deep thermal brine. The gypsium zone belongs to the type that salt was deposited in a deep water environment. The results may help understand genesis of gypsium zones in other regions.

Key words: gypsum zone, genetic types, igneous rocks, upper Es4, Bonan sag