Evidence from Illite Crystal Evolution: Exposing the Early Phases and Patterns of Hydrocarbon Accumulation in the Pinghu Formation of the Xihu Depression in the East China Sea.

  The conventional hydrocarbon accumulation model in the Xihu Depression is predominantly characterized by “late-stage accumulation.” However, with advancing exploration, the potential occurrence of commercially significant early hydrocarbon charging events in the Paleogene Pinghu Formation has become a subject of considerable debate. This study examines the accumulation mechanisms of natural gas in the Pinghu Formation through an integrated approach incorporating scanning electron microscopy (SEM), systematic fluid inclusion analysis, and natural gas carbon isotope geochemistry, with a particular focus on the evolutionary patterns of authigenic illite within the reservoir.

  SEM observations reveal three distinct morphological types of authigenic illite in the Pinghu Formation reservoirs: honeycomb, bridge-like, and fibrous. The crystallization of these illite types is primarily governed by diagenetic temperature and pore fluid pH: honeycomb illite forms at low temperatures (60 to 110°C) via smectite transformation; bridge-like illite develops at 120 to 140°C in association with acidic dissolution of K-feldspar; and fibrous illite requires temperatures above 140°C and alkaline conditions for the illitization of kaolinite. A key anomaly contradicting conventional diagenetic sequences was identified: in the shallower and cooler Huagang Formation reservoirs, fibrous illite constitutes up to 76% of the illite assemblage, whereas in the deeper and presumably hotter Pinghu Formation reservoirs, honeycomb and bridge-like types dominate (collectively 65%), with markedly reduced overall abundance. This inverse distribution with depth is interpreted as evidence of early hydrocarbon charging during deep burial of the Pinghu Formation. The introduction of acidic hydrocarbons inhibited the transformation of kaolinite to fibrous illite, thereby preserving the earlier illite morphologies and providing direct mineralogical evidence for an early accumulation event during the Huagang Movement.

  Geological analysis further supports the coupling of key elements conducive to early accumulation: during the Huagang Movement, source rocks had reached burial depths sufficient for hydrocarbon generation (Ro ≥ 0.5%), providing a material basis for large-scale expulsion. Concurrently, the superposition of the Yuquan and Huagang movements facilitated the development of structural–lithologic traps. At this stage, the average porosity of the Pinghu Formation reservoirs was approximately 21%, not yet entering the tightening phase, providing high-quality reservoir space for early hydrocarbon filling and accumulation.

  Fluid geochemical data provide additional robust evidence: hydrocarbon inclusions exhibiting yellow fluorescence with homogenization temperatures peaking between 105 and 135°C record an early hydrocarbon charging event. Furthermore, the methane δ¹³C values of Pinghu Formation natural gas (–38‰ to -34‰) are significantly lighter than those of the overlying Huagang Formation (–34‰ to 29‰), consistent with an early-generated, low-maturity gas source, effectively distinguishing fluid origins between early and late accumulation phases.

  Based on the above research, an early accumulation model governed by the combined effects of “paleo-highs and high-quality reservoirs” is established for the Pinghu Formation. This provides a key predictive model for early-stage reservoir exploration in basins with similar geological conditions worldwide, thereby further expanding new exploration frontiers.