A logging evaluation method for geological engineering "double sweet spot" for shallow marine shale gas: A case study of the Luzhai Formation in the northern Guizhong Depression

Abstract

Objective The marine organic-rich shale of the Luzhai Formation of the northern Guizhong Depression is an important replacement field for shale gas exploration in the Dian-Qian-Gui Basin. At present, great breakthroughs have been made in the initial stage of exploration. So it is very important to evaluate the geological-engineering integration. Methods In this study, petrophysical data (e.g., core, thin section and scanning electron microscopy data) and logging data (e.g., conventional, imaging, nuclear magnetic resonance and array acoustic) are used to study the logging characteristics of geological and engineering "sweet spots" on the basis of core calibration logs and fine delineation of small layers, respectively. By superimposing the hydrocarbon source rock quality, reservoir quality and engineering quality, the coupling and optimization of geological and engineering "double sweet spots" are finally completed. Results The results show that the lithology of the shale in the first member of the Luzhai Formation is dominated by three types: siliceous shale, mixed shale and gray shale. The organic matter abundance and pore permeability are low, and the TOC content is between 1% and 2%. The biogenic silica content is low, and the source of silica is mainly the land source of debris supply. Shale gas is mainly stored in the pore spaces and microfractures associated with clay minerals, and adsorbed gas is the main source, while the free gas content accounts for a small proportion. The brittleness index ranges from 20% to 80% and is basically greater than 40%. The stress coefficient of variation ranges from 0.26 to 0.50, and the direction of the present maximum horizontal in situ stress is NNE-SSW. Conclusion The shale "sweet spot" section at the bottom of the third small layer was selected as the target window of the horizontal well, and the results of horizontal section drilling and gas measurement showed good results. The research results can provide theoretical guidance and methodological support for the evaluation of "sweet spot" logging and the selection of horizontal well target windows for shale gas wells.

Keywords

• shallow shale gas
• "sweet spot"
• logging evaluation
• geology-engineering integration
• luzhai formation
• guizhong depression