Multistage fluid activity as a control on the Ediacaran carbonate reservoirs in the Sichuan Basin, SW China

Abstract

The Ediacaran Dengying Formation (Z₂dn) of the Sichuan Basin, Southwestern China, experienced multi-stage fluid flow, which accompanied the reduction and modification of reservoir porosity. Porosity in the Z₂dn carbonate reservoir thus varies in different structural units. Fluid inclusion and implications for reservoir porosity were investigated by combining petrographic, geochemical, fluid inclusion, in-situ U-Pb geochronological, and basin modelling data. Three stages of dolomite cements (CD-1, CD-2, and CD-3) and one stage of dolomite veining (FD) were petrographically, geochemically, and geochronologically distinguished in reservoir rocks, and respectively dated at c. 453 Ma, 336 Ma, and 173 Ma. Two stages of dolomite and quartz cement formation (CD-1, CD-2, Qtz-1, and Qtz-2) were subsequently formed at c. 453 Ma, 298 Ma, 182 Ma, and 165 Ma. CD-1, CD-2 cements and FD veins were interpreted as diagenetic fluids and formed in the sealed environment during the Late Ordovician-Early Permian; they led to multi-stage cementation of reservoir intervals and a reduction in porosity. Importantly, the diagenetic sequences between bitumen and dolomite generation phases indirectly define the dates of two oil charging events. The first episode only occurred in the central Sichuan Basin during the Silurian, and its timing is constrained by the absolute ages of CD-1 and CD-2 cements. The second oil charging episode was the most important in the Sichuan Basin and occurred during Indosinian tectonics according to the trapping temperatures of bitumen-bearing inclusions in Qtz-1 veins. Also significantly, oil charge was able to inhibit the precipitation of authigenic minerals in reservoir intervals. Oil cracking in the Z₂dn reservoir occurred at 173-165 Ma, accompanied by weak overpressure generation in the southeastern Sichuan Basin and medium-intensity overpressure in the central Sichuan Basin. Once again, it prevented compaction and solution compaction to maintain reservoir porosity. Moreover, CD-3 and Qtz-2 cements were possibly derived from hydrothermal fluid, which led to the dissolution of the Z₂dn dolomite reservoir. As corollary, the numerous stages of dolomite cementation recorded here, the recognition of a single episode of oil charge, and weak reservoir overpressures, are the main factor contributing to a lower porosity development in the southeastern Sichuan Basin when compared to the central Sichuan Basin. These results have profound implications to the economic potential of similar oil and gas prospects deep in lower Paleozoic rocks across the world.