Controls on the occurrence of beach‐bar sandstone in a Neogene saline lake basin, southwestern Qaidam Basin, China

Abstract

To identify the developmental characteristics and controlling factors of the beach‐bar sandstone in saline lake basins in the Qaidam Basin, China, the lithology and bedding structure of some representative outcrops of the Neogene saline lake basins were comprehensively investigated by means of experimental test data of local drilling cores, casting thin sections, and grain size, together with field outcrops in the basin and the modern salt lake deposition record of Qinghai Lake. Two types of sand bodies in the saline lake basins, that is, delta sand bodies and shore‐shallow lake beach‐bar facies were revealed. Neogene saline lake basins were found to host widespread beach‐bar sand bodies. Lithologically, the sand bodies were composed of fine‐grained sandstone and siltstone, with wavy bedding, low‐angle cross‐bedding, and bimodal cross‐bedding. The beach‐bar sandstone was characterized by multi‐layer superimposition, lateral migration, and NW–SE wide‐coverage continuation. Drilling data showed that beach‐bar sand bodies extended from 10 to 35 km in width and most of them occurred 3–15 km away from braided river delta front. It has been predicted that the total area of beach‐bar sand bodies of 14 sand groups in the Neogene spread across 5,000 km2. Four factors, that is, lake water salinity, terrigenous input, palaeogeomorphology, and monsoons, controlled the construction of the beach‐bar sand bodies developed in the Neogene saline lake of the Qaidam Basin. We can conclude that the settling velocity and transport distance of the sand bodies were affected by the water body salinity in such saline lake basins. Also, the distribution range of the beach‐bar sand bodies was affected by the clastic input from a terrigenous provenance. Furthermore, the wide‐coverage distribution and local enrichment of the beach‐bar sandstone were affected by palaeogeomorphology, and the distribution direction of the beach‐bar sandstone was affected by the driving effect of NW monsoons. We propose a deposition model of the beach‐bar sandstone in saline lake basins that can predict favourable beach‐bar sand body regions, to support the major discoveries of lithologic traps in southwestern Qaidam, China.

Genesis of Neogene Formation Waters in the Central Qaidam Basin: Clues from Hydrochemistry and Stable D‐O‐S‐Sr Isotopes

“…11; Fan et al, 2018;Li et al, 2020Li et al, , 2021. The S isotopes of these waters are also higher than those of the gasfield waters and Quaternary graveloccurring brines, and comparable with the high values of brines in Tertiary oil-bearing strata, whereas the Sr isotopes of these waters are highest among the different fluids in the QB (Wang et al, 1997;Fan et al, 2009;Tan et al, 2011;Li et al, 2021a). The above comparison implies a unique origin and evolutionary processes for the formation waters in this study.…”

“…Regarding the S isotopes, mechanisms causing high S isotopes and low SO 4 concentrations (0.63-1.45 g/L) of formation waters are generally interpreted as a bacterial (Fan et al, 2007;Tan et al, 2011); river waters in the study area and Yahu Lake water (Li et al, 2021a); the brines in the Yiliping-Xitai-Dongtai salt lakes (Fan et al, 2018;Li et al, 2021a); Quaternary gravel-occurring brines in the western Qaidam Basin (unpublished data); Pleistocene gasfield waters (Zhang et al, 2013;unpublished data); and the celestite deposits in the QB (Yang et al, 1993;Wang et al, 1997).…”

Geochemistry of Mudstones/Silty Mudstones from the Qigequan Formation and Shizigou Formation in Yuejin-II Area, Southwestern Area of the Qaidam Basin: Implications for Sedimentary Environment and Sandstone-Type Uranium Mineralization

“…The lithologies of Qigequan Formation are composed of upper white halite and lower dominant gray conglomerate and gravel sandstone, with light gray, grayish-yellow mudstone and sandy mudstone (Figure 3). [69,71,72]). Detailed regional tectonic events see [9].…”

Microbial Communities and Sulfur Isotopes of the Cenozoic Sulfurous Oil Reservoirs in the Southwestern Qaidam Basin, Western China

“…It can be structurally divided into the northern fault block, the western depression and the eastern depression. Huge natural gas resource potential has been discovered in the Qaidam Basin (Chen et al, 2020b;Guo et al, 2022;He et al, 2021;Ma et al, 2018), including the Jurassic coal-type gas at the northern margin, the Paleogene-Neogene oil-type gas in the southwest, and the Triassic biological gas in the Sanhu area (Chen et al, 2019b;Guo et al, 2018b;Li et al, 2021b;Liu et al, 2021a;Shao et al, 2022;Wang et al, 2022;Yin et al, 2021a). The natural gas in the southwest was mainly originated from four sources: oil-associated gas from sapropel-type kerogen (type I kerogen), mixed oil-associated gas, coal-type gas and mixed gas (Guo et al, 2020;Liu et al, 2013;Zeng et al, 2022).…”