Reservoir Characterization of Alluvial Glutenite in the Guantao Formation, Bohai Bay Basin, East China

Abstract

Alluvial glutenite reservoirs have obviously stronger heterogeneous and more complex control factors than sandstone reservoirs. Taking the Binxian Uplift area in the Boahi Bay Basin as an example, the aim of this study is to clarify the characteristics and control factors of the alluvial glutenite reservoir quality and the influence of reservoir properties on hydrocarbon accumulation. Pore types in the study area mainly include residual intergranular pores, intergranular dissolved pores, intragranular dissolved pores, and mold pores. The residual intergranular pores and intergranular dissolved pores are the main pore types. Most samples have porosity greater than 15% and permeability is mainly concentrated between 50 mD and 500 mD. It is shown that lithology type, microfacies, and diagenesis have significant impact on the reservoir quality. The reservoir qualities of very fine sandstone and fine sandstone are better than those of conglomerate and gravel-bearing sandstone. Instead of grain size, sorting affects the alluvial glutenite reservoir quality significantly. Oil-bearing samples commonly have sorting coefficient less than 2 while non-oil-bearing samples have sorting coefficient larger than 2. There are significant differences in reservoir physical properties of different sedimentary microfacies. The stream flow in mid-alluvial fan (SFMA) and braided channels outside alluvial fans (BCOA) have relatively weaker compaction and better reservoir quality than the overflow sand body (OFSB) and debris-flow in proximal alluvial fan (DFPA). Calcite cementation, the main cement in the study area, commonly developed at the base of SFMA and BCOA and near the sandstone-mudrock contacts. The source of calcium carbonate for calcite cement mainly came from around mudstone. High calcite cement content commonly results in low porosity and permeability. Individual glutenite thickness is also an important influencing factor on reservoir quality. Reservoirs with large thickness (>4 m) have high porosity and permeability. Dissolution occurred in the reservoir, forming secondary dissolution pores and improving reservoir quality. The dissolution fluid for formation of secondary pores is mainly meteoric waters instead of organic acid. The reservoir property has an important influence on hydrocarbon accumulation. The lower limit of physical properties of an effective reservoir is a porosity of 27% and permeability of 225 mD. The findings of this study can be utilized to predict the reservoir quality of alluvial glutenite reservoirs effectively in the Bohai Bay Basin and other similar basins.