Characteristics and controlling factors of the Quaternary beach-bar sand bodies in the Tainan area， Qaidam Basin

This study aims to thoroughly assess the prospects for natural gas exploration and exploitation in littoral-shallow lacustrine beach bars within the Tainan area， Qaidam Basin. To this end， we systematically investigate the sedimentary environments， spatial distribution， evolutionary development， and controlling factors of beach bars in sequence Ⅱ of the Quaternary Qigequan Formation in the area using core observations and drilling （log） data analysis， as well as observations of rock thin sections， whole-rock mineral mineralogy via X-ray diffraction， grain-size cumulative probability curves， clay mineral assemblages， chloride content in mudstone cuttings， and net-to-gross （NTG） ratios of limestones （limestones）. The results indicate that the Tainan area primarily exhibits clastic beach bars， with carbonate beach bars occasionally visible. The clastic beach bars consist primarily of lithic-feldspathic fine-grained sandstones and siltstones， with grain-size cumulative probability curves predominantly showing a saltation-suspension dual-segment mode （64.4%）， followed by multi-segment modes （11.9%）. Furthermore， the clastic beach bars exhibit horizontal， wavy， and low-angle cross bedding structures. The carbonate beach bars， on the other hand， are composed mainly of argillaceous and silty micrites with aphanocrystalline to very fine crystalline textures. Clastic beach bars are well developed in sequence Ⅱ of the Qigequan Formation， with an average single-layer thickness of 2.13 m， an average NTG ratio of 0.34， and relatively small sand bodies in individual bars. These clastic beach bars are mainly distributed in a banded pattern in the core of the Tainan anticline. Among these， bars on the east flank of the anticline have a NE-SW strike， while those on the west flank strike NW-SE. The carbonate beach bars in sequence Ⅱ are limited in scale， with an average NTG of 0.37， and are distributed in a ring-like pattern in the anticline core. The lacustrine basin within the Tainan area is dominated by brackish to saline water， with localized occurrences of brine water. During the deposition of the Qigequan Formation， the Tainan area experienced an arid， cold climate with frequent fluctuations. The clay minerals in the area are primarily composed of illite （44.2%）， followed by mixed-layered illite-montmorillonite （22.9%） and chlorite （19.6%）. Therefore， beach bars are well developed in the Tainan area and show noticeable migration and swing， with significant evolutionary differences between the east and west flanks of the Tainan anticline. Factors controlling the beach bar development in the area mainly include the arid climate conditions and the low-amplitude， synsedimentary Tainan anticline.