Depositional, sedimentary, and diagenetic controls on reservoir quality in carbonate successions: A case study from the carbonate gas reservoirs of the Lower Triassic Feixianguan Formation, eastern Sichuan Basin, China

Abstract

The Kaijiang–Liangping trough is an important carbonate gas zone in eastern Sichuan Basin. The western platform of this trough has received less attention than the eastern one due to its lower reservoir potential. The current study mainly investigated depositional, sedimentological, and diagenetic features of the Lower Triassic Feixianguan Formation (T1f) from the Tieshan area located at the western platform, aiming to discuss their controls on reservoir quality. The gentle slope in the Tieshan area (approximately 5°) deposited platform marginal oolitic shoals featured by thin vertical accumulation and long-distance lateral migration, thereby forming small-scale and scattered-distribution reservoirs. Despite intensive diagenesis alterations after sedimentation, the T1f reservoir porosity was controlled by sedimentological factors (i.e., grain size and sorting) at varying degrees. High-quality reservoirs occurred in the dolograinstones with good sorting and coarse grain sizes. The T1f carbonates underwent the near-surface and burial diagenesis in the Tieshan area. Dolomitization and associated syn- and post-dolomitizational dissolutions were important factors for secondary porosity development. Dolomite recrystallization increased intercrystal porosity in dolostones characterized by precursor mud-dominated fabric, thereby resulting in gradual shift from Lucia class 3 to class 1 petrophysical relationships with increasing crystalline sizes. By contrast, the intergrain porosity was not influenced by recrystallization in dolograinstones, which pervasively displayed classes 1 and 2 petrophysical relationships regardless of dolomite crystal sizes. The T1f gas reservoirs evolved from a paleo-oil accumulation as evidenced by pore-filling bitumen, which had negative effects on crystalline dolostones by filling intercrystal pores. However, its influence on dolograinstones was negligible as dissolution in relation to thermochemical sulfate reduction (TSR) reaction occurred during paleo-oil cracking into gas, thereby increasing intergrain porosity and effectively offsetting porosity decrease caused by bitumen filling. TSR dissolution occurred at certain intensity in the Tieshan area, which was evidenced by H2S occurrence in gas.