Thermochronological Constraints on the Synchronisation of Reservoir Densification and Hydrocarbon Emplacement in the Upper Triassic Chang 8 Tight Sandstone, Ordos Basin

Abstract

The Chang 8 sandstone in the Ordos Basin is a typical tight reservoir, with reservoir quality and heterogeneity controlled by sedimentation and diagenesis. This study focuses on petrology, diagenetic processes, and cement origins, exploring their effects on reservoir quality and heterogeneity. It highlights the cementation sequence constrained by formation temperatures and how this influences porosity evolution and hydrocarbon emplacement. High carbonate cement content, especially near the sand-mud interface within 1 m of the margin, significantly reduces reservoir quality, with carbonate cementation likely supplied by adjacent mudstones. Authigenic chlorite growth is characterised by an inner rim and an outer layer, continuously expanding from the grain surface into the surrounding pore space. Dissolution plays a limited role in improving reservoir quality, and pore redistribution during precipitation in an enclosed diagenetic system may even decrease permeability. Cements play a key role in tightening the reservoir, preserving information on diagenetic temperatures and hydrocarbon charging. By using oxygen isotopes, fluid inclusions, chemical compositions, and Raman spectroscopy, the formation temperatures of cements are determined, enabling the reconstruction of cement paragenesis. This allows dynamic coupling of cementation and reservoir densification with hydrocarbon filling. The study finds that during the main hydrocarbon charging period (80°C–120°C), significant porosity reduction leads to tightness, suggesting that hydrocarbon accumulation occurs before the reservoir becomes significantly tight. This work provides new insights into the coupling of diagenesis and hydrocarbon emplacement, offering a theoretical basis for predicting sweet spots in tight reservoirs with similar geological conditions globally.