## 1 The Lower Limit of Effective Reservoir Burial Depth for Clastic Rocks  
In recent years, China's onshore basins have achieved continuous breakthroughs in ultra-deep oil and gas exploration, making the expansion into deeper layers inevitable [1, 2, 3, 4]. The Tarim Basin contains 6.77 billion tonnes of petroleum and 9.7 trillion cubic meters (tcm) of natural gas in deep and ultra-deep formations. The Junggar Basin holds 2.84 billion tonnes of petroleum and 2.24 tcm of natural gas. The Sichuan Basin contains 161,300 tonnes of petroleum and 38.11 tcm of natural gas, while the Ordos Basin is estimated to contain 0.1 tcm of natural gas resources. Collectively, these four basins account for over 376 million barrels of oil equivalent (MBOE) in recoverable resources (Table 1). These collective data not only highlight China's growing dominance in global deep-layer hydrocarbon reservesbut also reflect the critical role of tertiary basins in reshaping energy geopolitics. In terms of recoverable crude oil, the Tarim and Junggar Basins are comparable to the Mesopotamian Fordeep Basin in Kuwait (66-72 billion barrels) and Iraq's West Qurna Field (15-21 billion barrels). Regarding natural gas, the Sichuan Basin surpasses Qatar's North and Ras Lafan Fields (over 900 tcf), while the Tarim Basin is comparable to Russia's Urengoy Field (385 tcf). Combined, these four Chinese basins represent the largest known natural gas reserves globally, exceeding Russia's total of 1680 tcf. Traditionally, clastic rocks' primary pores--originally composed of clastic particles, pores, and interstitial materials--gradually diminish during burial. The low solubility of these components limits the formation of secondary pores. As a result, petroleum geologists often define basin-specific lower-depth limits for effective clastic reservoirs [2; 5; 6; 7]. This concept plays a dual role in exploration, initially guiding efforts toward key strata for breakthroughs but later potentially restricting exploration beyond established limits. However, throughout the exploration history of China's eastern, central, and western basins, the understanding of these depth limits has evolved. As exploration advances, the lower depth limit for effective clastic reservoirs continues to extend downward, undergoing constant reassessment and revision [8; 9; 10; 11; 12; 13]. In the early stages, scholars generally defined burial depths greater than 3500-4000 m as deep layers [2; 14; 15; 16]. Influenced by this understanding, early oil and gas exploration in the eastern Bohai Bay, Songliao, Subei, and central Ordos Basins assumed that the effective reservoir burial depth for clastic rocks was within the same range [1; 17; 18]. However, intensified exploration in the Tarim, Junggar, and western Sichuan Basins revealed significant structural, geological, and evolutionary differences between the large, superimposed basins with low geothermal gradients in the west and the faulted and down-warped basins in central and eastern China [19; 20; 21; 22; 23; 24; 25; 26; 27]. These differences delayed the diagenetic evolution of clastic rocks, shifted exploration targets to deeper layers, and led to the discovery of deeply buried effective reservoirs. Commercial oil and gas discoveries have since been made at ultra-deep realms (>6000 m) [28; 29; 30; 31]. Key breakthroughs include Yong 1 (2004) (Yong 1 is a key exploration well in the Junggar Basin, located in the northwest of China) at 5860 m in the Jurassic formation of the central Junggar Basin, Shun-9 (2012) (Shun-9 is an exploration well in the Tarim Basin, a vast basin in southwestern Xinjiang, China) at 5600 m in the Silurian of the Shuntuoguole area in the Tarim Basin, Zheng-10 (2022) (Zheng-10 owned and operated by China Sinopec, a major oil and gas company in China.