Lithofacies, architecture, and reservoir heterogeneity of tidal-dominated platform marginal oolitic shoal: An analogue of oolitic reservoirs of Lower Triassic Feixianguan Formation, Sichuan Basin, SW China

Abstract

The Yudongliang Outcrop, located in the northern part of the Sichuan Basin, nicely exposes oolitic shoal complexes of the Lower Triassic Feixianguan Formation. The outcrop is an analogue for the subsurface Longgang and Yuanba gas fields and other Permo-Triassic reservoirs with dolomitized and nondolomitized oolite-dominated grainstone bodies as the main producing zones.Based on five measured sedimentological sections, and petrographic and petrophysical analysis of 200 plugs, the architecture of the reservoir bodies has been reconstructed in detail and placed within a high-frequency sequence-stratigraphic framework.The studied interval belongs to the highstand system tract (HST) of a single composite sequence and comprises three shallowing-upward high-frequency sequences, which consist from base to top of oncoid/peloid/ooid grain-dominated packstone, cross-bedded ooid grainstone, dolograinstone, fenestra-laminated dolomite, and crystalline dolomite. The internal architecture of the oolitic shoal is the result of complex tidal-dominated hydrodynamics.Facies and diagenetic partitioning with each high-frequency sequence are the primary controls on pore types and reservoir properties. Original lithofacies texture and subsequent cementation resulted in interbedded thin effective reservoirs and permeability barriers that created a complex internal architecture of reservoir bodies. The shapes of reservoir bodies generally follow those of the lithofacies, but with the addition of intercalated thin flow barriers.