A comparative study on reservoir quality of different Paleocene depositional systems in the Lishui-Jiaojiang Sag

The Lishui-Jiaojiang Sag, a Paleocene rift basin, varies significantly in reservoir quality among different depositional systems of multiple provenances. To characterize the reservoirs of various depositional systems and identify favorable exploration prospects, we integrated data from cores, well logs, seismic surveys, and laboratory analyses to identify various depositional systems and compare reservoir quality of similar sandstone types and burial depths. Results show that the Lishui Sub-sag B developed two depositional systems in the Cenozoic: shallow-marine submarine fans and sand bars, and they both originated in the southern Zhemin Uplift. The sandstone types are predominantly litharenite rich in volcanic lithics. At similar burial depths, sand bars are under strong hydrodynamic conditions and repeated winnowing, resulting in better grain roundness and sorting, lower content of plastic volcanic lithics, and consequently superior reservoir quality compared to those of submarine fans. In the Lishui Sub-sag A, Sub-sag C, and Jiaojiang Sag, braided delta and fan delta depositional systems were developed, from the source areas of northern Zhemin Uplift and the Yandang Uplift. The reservoirs are characterized by relatively high porosity and permeability, due to higher contents of metamorphic lithics and felsic minerals that are resistant to mechanical compaction. Under similar burial depths, the reservoir quality of conglomerates and coarse-grained sandstone in the proximal braided delta fronts and fan delta fronts is significantly better than that of fine-grained sandstone in distal delta front settings. Grain size is the pivotal factor for high-quality reservoirs in this region. Therefore, we recommended that in the Lishui Sub-sag B, priority exploration shall be directed to sand bars and amalgamated channels and channelized lobes in proximal submarine fans and relatively high maturity. In the Lishui Sub-sag A, Sub-sag C, and Jiaojiang Sag, the proximal delta front areas, along with distal high-energy coarse-grained “sweet spots”, have superior reservoir quality and hold promising exploration potential.