Petrophysical properties of deep Longmaxi Formation shales in the southern Sichuan Basin, SW China

摘要

Deep shale layer in the Lower Silurian Longmaxi Formation, southern Sichuan Basin is the major replacement target of shale gas exploration in China. However, the prediction of “sweet-spots” in deep shale gas reservoirs lacks physical basis due to the short of systematic experimental research on the physical properties of the deep shale. Based on petrological, acoustic and hardness measurements, variation law and control factors of dynamic and static elastic properties of the deep shale samples are investigated. The study results show that the deep shale samples are similar to the middle-shallow shale in terms of mineral composition and pore type. Geochemical characteristics of organic-rich shale samples (TOC > 2%) indicate that these shale samples have a framework of microcrystalline quartz grains; the intergranular pores in these shale samples are between rigid quartz grains and have mechanical property of hard pore. The lean-organic shale samples (TOC < 2%), with quartz primarily coming from terrigenous debris, feature plastic clay mineral particles as the support frame in rock texture. Intergranular pores in these samples are between clay particles, and show features of soft pores in mechanical property. The difference in microtexture of the deep shale samples results in an asymmetrical inverted V-type change in velocity with quartz content, and the organic-rich shale samples have a smaller variation rate in velocity-porosity and velocity-organic matter content. Also due to the difference in microtexture, the organic-rich shale and organic-lean shale can be clearly discriminated in the cross plots of P-wave impedance versus Poisson's ratio as well as elasticity modulus versus Poisson's ratio. The shale samples with quartz mainly coming from biogenic silica show higher hardness and brittleness, while the shale samples with quartz from terrigenous debris have hardness and brittleness less affected by quartz content. The study results can provide a basis for well-logging interpretation and “sweet spot” prediction of Longmaxi Formation shale gas reservoirs.