Primary intergranular pores in oolitic shoal reservoir of lower triassic feixianguan formation, Sichuan Basin, Southwest China: Fundamental for reservoir formation and retention diagenesis

Abstract

The Lower Triassic Feixianguan (飞仙关) Formation oolitic shoal reservoir in the Sichuan (四川) basin (Southwest China) is currently an exploration and research highlight in China. The reservoir is widely believed to be formed mainly by burial dissolution and/or dolomitization on the basis of primary intergranular pores. In this study, through a comprehensive geological study on the whole basin, the dissolution and dolomitization are suggested not to be the fundamental factor of reservoir formation and there thus may be a possible new fundamental mechanism-the preservation of primary intergranular pores, i.e., the retention diagenesis. Based on this, a complex and multi-stage reservoir evolution and formation model is proposed. In the model, the depositional environment is the basis of reservoir initial formation. Subsequently, early compaction and shallow burial cementation result in the primary reservoir differentiation. Then, multi-stage burial dissolution alters and adjusts the reservoir. Because the last stage gaseous hydrocarbons have little diagenetic impact, the reservoir is formed finally. Therefore, this study presents a possible new fundamental mechanism and evolution model for the reservoir formation. The results can be applied in the regional reservoir predication and shaping exploration strategies, and provide reference for the study of shoal reservoirs in other areas. KEY WORDS: carbonate reservoir, oolitic shoal, primary intergranular pore, retention diagenesis, burial dissolution, dolomitization, Lower Triassic Feixianguan Formation, Sichuan basin.

Lower Triassic oolites of the Nanpanjiang Basin, south China: Facies architecture, giant ooids, and diagenesis—Implications for hydrocarbon reservoirs

“…Similar to the outcrops described from the Nanpanjiang Basin, the Feixianguan oolites have been interpreted as representing platform or ramp margin shoals that extend into platform interiors, thinning away from the margin (Ma et al, 2006(Ma et al, , 2007Peng et al, 2010). Although we have not found reference to giant ooids in the Sichuan Basin, petrographic fabrics such as dissolved and collapsed ooids, brittle eggshell compaction of micritic cortices, and alternating cortical laminae of well-preserved micrite and coarse neomorphic spar (figured in Ma et al, 2006;Tan et al, 2011) are identical with those described herein for the Nanpanjiang Basin. Oolite reservoirs in the Sichuan Basin are mostly dolomitized, with much of the reservoir potential from secondary moldic, intercrystalline, vuggy, and fracture porosity, although significant interparticle porosity is also important (Ma et al, 2006;Pan et al, 2010;Peng et al, 2010;Tan et al, 2011).…”

Natural Fractures and Their Contribution to Natural Gas Migration and Accumulation in Marine Carbonate Reservoirs: Lower Triassic Feixianguan Formation, Northeast Sichuan Basin, China

“…Studies on natural fractures have been ignored [26], leading to incomplete and limited research on related fractures. However, cores from gas fields such as Puguang, Tieshanpo, and Yuanba have shown the widespread distribution of fractures, as well as phenomena of dissolution enlargement and bitumen fillings along fractures [25,28,29]; these findings confirm that fractures serve as significant storage spaces and fluid flow channels in the northeastern Sichuan Basin [30][31][32]. Exploratory studies have demonstrated a close relationship between gas production and the extent of fracture development in northeastern Sichuan [20,31,33].…”