Reservoir Petrophysical Properties,Ichnological And Sedimentological Significance Of Bioturbation In Siliciclastic Rocks In Subei Basin,China

This research evaluates the reservoir quality,ichnological and sedimentological attributes of bioturbated intervals from three formations in two depressions within the Subei Basin: 1)the second member Paleocene Funing Formation?E₁f₂?in Jinhu depression;2)the first member Paleocene Funing Formation?E₁f₁?in Gaoyou depression;and 3)the third member Paleocene Funing Formation?E₁f₃?in Gaoyou depression.Eight reservoir facies?Rf1 to Rf8?retrieved from five wells are predominantly burrowed by the Cruziana,Scoyenia and/or Skolithos ichnofacies?BI ? 5?and exhibit diverse ichnofossils that are primary indicators of a fluviallacustrine depositional environment.The reservoir facies?Rf1 to Rf5?retrieved from E₁f₂ and E₁f₁ are tested independently utilizing conventional thin-section petrography,pulse decay permeametry,pressure decay porosimetry,Field Emission Scanning Electron Microscope?FESEM?and Energy Dispersive X-ray Spectroscopy?EDS?whereas E₁f₃ samples?Rf6,Rf7 and Rf8?are tested by thin-section petrography,pulse decay permeametry,pressure decay porosimetry and Total Organic Carbon?TOC %?.The outcome of tests and observations obtained are as follows:1)Rf1 and Rf2 are intensely Ophiomorpha-Macaronichnus and Macaronichnus burrowed dusky yellowish sandstones,respectively retrieved from well B5 in E₁f₂,and collectively indicate good sorting and distribution of grains.FESEM and EDS show mud and clay?kaolinite?displacements from the burrow fills that are directed towards burrow lining and/or host sediment;resulting in enhanced isotropy and augmented burrows porosity of 23 % and 33.28 % and net permeability gains in both samples.2)Cored samples retrieved from wells M7,F12 and H19 in E₁f₁ are brownish-red finegrained siltstone?Rf3?,brown fine-grain sandstone?Rf4?and yellowish-brown finegrained sandstone?Rf5?,respectively,and are intensely burrowed by Taenidium,Scoyenia,Skolithos,Palaeophycus,plant root debris/traces and other diminutive trace fossils.Outcome of tests showed augmented porosity?67.18 and 20.23 %?in Rf3 and Rf4 burrowed by Taenidium,Scoyenia,Skolithos and Palaeophycus whilst Rf5 burrowed by plant root debris/traces recorded a marginal 3.67 % burrows porosity decline.Effective permeability resolution through graphs of bioturbation intensity versus permeability shows harmonic mean of permeability best characterizes Rf3 and Rf4,whilst the arithmetic mean of permeability best evaluates Rf5.Reservoir facies are controlled by depositional factors?sorting,grain size distribution,mud-matrix/burrow content and burrow morphology?and diagenetic modifications?albitization,compaction,dissolution,kaolinization and precipitation?of minerals.3)The samples from E₁f₃ show dark grey siltstone?Rf6?fine-grain brown sandstone?Rf7?and fine-grain greenish-grey sandstone?Rf8?all retrieved from well L5.In Rf6,Palaeophycus,Scoyenia and Taenidium burrows jointly show 34 % enhancement.Rf7 and Rf8 burrowed by Taenidium and Skolithos exhibit 25.17 % and 4.36 % burrowmediated porosity gains,respectively.Matrix and burrows permeabilities are < 0.1 md in Rf6,> 6 md in Rf7 and < 2 md in Rf8 but indicate 37.96 %,51.93 % and 15.10 % burrows net permeability enhancements,respectively.The significance of TOC on studied porosity of Rf6?0.66 % ? TOC ? 0.98 %?,Rf7?0.03 % ? TOC ? 0.05 %?and Rf8?0.04 % ? TOC ? 0.08 %?show that total porosity is controlled by removal of organic matter.4)Porosity estimated from acoustic wireline logs of 2 m each intervals that intersects Rf5,Rf6,Rf7 and Rf8 positions indicate porosity < 1 %;indicative of overburden pressure and compaction within the reservoir.