Biogenically modified reservoir rock quality: A case from the lowermost member Paleocene Funing Formation, Gaoyou Depression, Subei Basin, China

Abstract

Bioturbation can influence reservoir quality and is consequential to the producibility of a reservoir. The study of samples from the lowermost member of the Paleocene Funing Formation (E1f1), Gaoyou Depression, Subei Basin, shows how bioturbation affects reservoir quality. Techniques used to study the samples include petrography, pressure decay porosimetry, pulse decay permeametry, Field Emission Scanning Electron Microscopy, and Energy Dispersive X-ray Spectroscopy. Sample A is intensely burrowed by Taenidium, Scoyenia, Skolithos, Palaeophycus, and other trace fossils. Increased isotropy in sample A contributes to cleaner well-sorted burrows, relative to its surrounding matrix, and 67.18% augmented burrow porosity. Taenidium and Palaeophycus in sample B indicate 20.23% improved burrows porosity. Plant debris and/or root traces in sample C have a 3.68% reduction in porosity. In samples A and B, the arithmetic mean of permeability describes all horizontal fluid flows within burrows. In sample C, the geometric mean of permeability describes the fluid flow in all directions. Porosity is ≤ 11.2%, permeability ≤1 md in samples, and sample C log-derived porosity ≤0.33%. This study demonstrates that bioturbation together with depositional factors (sorting, grain size distribution, and mud-matrix/burrow content) and diagenetic modifications (albitization, compaction, dissolution, kaolinization, and precipitation) control the quality of the high to intensely bioturbated (Bioturbation Index 4 to 5; 61–99 vol %) sandstone and siltstone reservoir facies of the E1f1.