Lithofacies paleogeography of Hetaoyuan Formation in the southern steep slope zone of Biyang depression

Abstract：Hetaoyuan Formation in the southern steep slope zone of the Biyang depression is a major hydrocarbon-bearing target zone. Based on the multidisciplinary analysis of drilling core data, petrological characteristics and facies marks, this paper divides the Hetaoyuan Formation into four sedimentary systems: fan-delta deposits, braid river delta deposits, meandering river delta deposits and lacustrine deposits. Based on a comprehensive analysis of detrital heavy minerals, well-logs and well logging data, the authors systematically drew lithofacies-paleogeographical maps from 3rd Member to 1st Member of the Hetaoyuan Formation. The paleogeographic distribution characteristics are as follows: Fan delta is developed in the south, braid river delta occurs in the northeast, meandering river delta exists in the northwest and lacustrine sediments are developed at the center. In the paleogeographical evolution process from 3rd Member to 1st Member of the Hetaoyuan Formation, the fan delta and meandering river delta kept shrinking, and the meandering river delta was filled by the shallow lake in 1st Member of the Hetaoyuan Formation. The area of the lake expanded gradually and the braid river delta became bigger from 3rd Member to 2nd Member and began to decrease in area in 1st Member of the Hetaoyuan Formation.