Diagenetic characteristics, evolution, controlling factors of diagenetic system and their impacts on reservoir quality in tight deltaic sandstones: Typical example from the Xujiahe Formation in Western Sichuan Foreland Basin, SW China

Abstract

Deeply buried (3000–5000 m), deltaic sandstones of the Upper Triassic Xujiahe Formation are important tight gas reservoirs in the Sichuan Foreland Basin, China. The diagenesis of these tight sandstones was examined using a variety of petrographic and geochemical techniques, including thin section description, X-ray diffraction (XRD), whole-rock chemical analysis, scanning electron microscopy (SEM), Cathodoluminescence (CL) imaging, electron probe analysis, fluid inclusions and isotopic analysis. These integrated petrographic and geochemical techniques were used to determine the diagenetic history of the sandstones and its impact on the reservoir quality. The tight deltaic sandstones of the T3x2 and T3x4 (second and fourth members of Xujiahe Formation) have undergone a significant and complicated series of diagenetic alterations and changes in geochemical composition. Strong mechanical and chemical compaction together with carbonate cementation destroyed almost all the primary pores and the secondary dissolution pores now dominate the pore space. The T3x4 sandstones experienced a more open diagenetic system at near-surface and eodiagensis resulted in higher porosity than seen in the T3x2 sandstones. Both the T3x2 and T3x4 sandstones experienced closed-system diagenesis during middle-late mesodiagenesis. The early diagenetic dissolution, which mainly occurred in the open geochemical system, produced secondary pores and provided kaolinite and some K+ needed for the subsequent illitization of kaolinite and K-feldspar. The late dissolution of K-feldspar and illitization of K-feldspar in T3x4 sandstones and T3x2 sandstones during the mesodiagenesis, produced some effective secondary pores in the closed geochemical system or in the focused fluid flow zone along fractures. The diagenetic characteristics, size and evolution of (open vs closed) diagenetic system, which were constrained by the depositional environment, deep burial depth and tectonic activity, can be used to predict the reservoir quality ahead of drilling.