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Reservoir Heterogeneities of Lower Triassic Feixianguan Formation Oolitic Reservoir, Sichuan Basin, China


The Outcrops Yudongliang and Qiyueshan located in the north and east parts of the Sichuan Basin respectively, nicely expose oolitic shoal complexes of the Lower Triassic Feixianguan Formation. These outcrops are analogue for the Longgang and Yuanba gasfields, and are also relevant to other Permo-Triassic reservoirs with dolomitized and non-dolomitized grainstone bodies as main reservoir intervals. A combination of sequence stratigraphic and sedimentary analysis on the continuous outcrop wall resulted in a detailed 2D cross section that displays various facies and sedimentary structure within a high hierarchy sequence framework. Based on 15 measured sedimentological sections, and petrographic and petrophysics analysis of 674 plugs, the architecture of the reservoir bodies has been reconstructed in details. The style and amount of vertical and lateral reservoir heterogeneities observed on the outcrop were synthetized and generalized into a 3D reservoir model based on a Digital Outcrop Model. The studied section is in the upper part of a composite sequence HST, and is made of three stacked shallowing upward cycles, which consist from base to top of oncoid/peloid/ooid grain-dominated packstone(GDP), cross-bedded ooid GDP to grainstone(GRN), dolograinstone(DGRN), dolomitized fenestra laminated wackstone with ooids(DFL), and coarse crystalline dolomite(CD). The GDP-GRN facies-association is characterized by uniform thickness bedding stacked as leyer-cake, whereas DGRN-DFL-CD facies-association shows more frequent thickness change associated with large scale bedforms and erosion. Facies and Diagenetic partitioning with each cycle is the primary control on pore-types and reservoir properties. The paragenesis observed in the outcrop resembles the one observed in the subsurface. Due to the meteoric dissolution, the DGRN with interparticle pores and CD with no intercrystal calcite fillings in the upper part of cycles hold the best porosity(POR) and permeability(PERM), whereas GDP, GRN, and DGRN in lower part of cycles tend to be lower POR and PERM owe to cementation, moreover, the DGRN and DFL with oomoldic pores in the top of cycles are inclined to have high POR and comparably lower PERM because of the fabric and calcite fillings. As a consequence, GDP, tightly cemented GRN, and calcite-filled DFL act as a vertical barrier of fluids with the thickness of 20 to 100cm and extent laterally more than 300m. The distribution of POR follows facies boundary laterally.