Source-to-sink pattern and sand body distribution in the first member of Permian Lower Shihezi Formation in Fuxian area, Ordos Basin

The Fuxian area in the southeastern Ordos Basin is a crucial area for natural gas exploration in the Upper Paleozoic. The primary exploration target, the first member of the Permian Shihezi Formation (He 1 Member), is significantly influenced by complex tectonic and sedimentary processes, resulting in an unclear understanding of the source-to-sink distribution pattern and sand body distribution. Therefore, in-depth research on the sedimentary fillings, the genetic types of sand bodies, and their distribution patterns under multiple provenance systems is of significant importance for efficient natural gas exploration and large-scale storage enhancement in this region. The study employed field outcrop survey, core analysis, thin-section observation, heavy mineral analysis, and well logging data to systematically analyze the sedimentary provenance of the He 1 Member in the Fuxian area. The study summarized the genetic types, structural characteristics, and the distribution and evolution patterns of sand bodies. During the sedimentary periods, the He 1 Member in the Fuxian area primarily developed a terrigenous clastic debris filling zone in the southern region and a mixed-provenance terrigenous clastic debris filling zone in the northern and southern region. Nine lithofacies types and three lithofacies assemblages were identified within the He 1 Member, with the main sand body genetic types being braided river facies channel filling in braided rivers and channel bars. Along the provenance direction, lithofacies assemblages evolved from incision, aggradation, and lateral accretion in the southern provenance zone to lateral accretion in the mixed-provenance zone of the northern and southern region. The He 1 Member experienced a sedimentary evolution process from terrigenous coarse clastic debris sedimentation during flood periods at an initial stage to muddy sedimentation during low water periods at a later stage. In addition, the sedimentary periods of He 1-1 and He 1-3 sublayers were favorable stages for the development of braided composite channel sand bodies, especially during the sedimentary period of the He 1-1 sublayer. The dominant sand bodies were mainly developed in the southern provenance sedimentary filling area in the central and eastern Fuxian area and the primary mixed-provenance filling area in the northern Fuxian area.