## 1 Introduction  
The Cretaceous period has emerged as a pivotal focus in paleoclimatology and hydrocarbon exploration research, distinguished by its hallmark features of elevated CO\({}_{2}\) levels, high global sea levels, elevated temperatures, and prolific hydrocarbon resources [1]. Continental sediments dominate China's Cretaceous geological record, hosting numerous lacustrine basins of this age. Prominently, the Songliao Basin in Northeast China stands out as one of the world's largest Cretaceous lacustrine systems. Its sedimentary succession is characterized by continuous lacustrine elastic deposits, which, unlike other continental depositional settings, provide uniquely comprehensive, uninterrupted, and high-resolutionrecords [2]. These lacustrine archives preserve critical paleoclimate proxies and evidence of major geological events in exceptional detail. Through geochemical analysis of lacustrine sediments, researchers can effectively reconstruct paleoenvironmental and paleoclimate conditions with high fidelity. Source rocks represent the geological basis for hydrocarbon formation, with their development governed by tectonic architecture, sedimentary facies zonation, and depositional environments [3,4]. In small fault-depression lacustrine basins characterized by rapid lateral facies transitions, environmental fluctuations exert intensified controls on source rock development [5]. Emerging research linking source rock deposition to paleoenvironmental conditions highlights how cyclic changes in paleoclimate moisture balance, water column salinity, redox states, paleodepth, and lake productivity collectively modulate vegetation assemblages, parent rock weathering processes, sediment transport dynamics, and depositional cyclicity [6]. Elemental behaviors vary systematically with environmental parameters: some elements are preferentially preserved in sediments, while others are prone to diagenetic mobilization. As such, the elemental composition, abundance patterns, and geochemical ratios archived in sediments serve as critical proxies for reconstructing depositional paleoenvironments. Given the fundamental role of depositional systems in controlling source rock quality, reconstructing paleoenvironmental frameworks, establishing high-quality source rock development models, and conducting fine-scale resource potential assessments are pivotal for identifying prospective hydrocarbon plays [7]. The Xujiweizi Fault Depression serves as the principal gas-rich fault depression in the deep strata of northern Songliao Basin [8,9]. Characterized by intricate depositional environments, strong lithological heterogeneity, diverse source rock lithologies, and complex petroleum systems, this structural unit supports multiple gas accumulation types governed by overlapping hydrocarbon supply systems, variable source rock quality, and spatial distribution patterns. Previous investigations have comprehensively addressed its tectonic framework, sedimentary architecture, and reservoir properties, with some studies examining the depositional paleoenvironments of source rock development [10,11]. However, these efforts have been primarily confined to the Shahezi Formation [12,13], leaving the paleoenvironmental controls on source rock formation and enrichment mechanisms in the Yingcheng Formation poorly understood. To bridge this knowledge gap, this study focuses on Member 4 of the Yingcheng Formation (Ying IV). Systematic sampling was conducted from dark mudstone intervals across key wells, covering both deeper and shallower stratigraphic units. Analyses included total organic carbon (TOC) measurements and major/trace element geochemical characterization. This study integrates source rock evaluation with paleoclimate and paleoenvironmental reconstruction, systematically analyzing the controlling factors of source rock development environments. An organic matter enrichment model for Ying IV is established, providing strong evidence for the research direction of targeting gas reservoirs using these primary hydrocarbon kitchens.