Analysis of the controlling factors for sweet spots in lacustrine shale based on core experiments and 2D NMR logging.

Abstract

The Gulong shale in the Songliao Basin, characterized by its high clay content, poses significant challenges for the identification of sweet spots. To address this, a novel temperature-dependent retorting and 2D NMR joint experimental approach was developed to investigate the fluid characteristics of the Gulong shale under varying occurrence states. The experimental results demonstrated that the pore fluids in Gulong shale are composed of multiple components, including free water, capillary bound water, clay bound water, bound oil in small inorganic pores, adsorbed oil in organic pores, free oil in large inorganic pores, and bitumen. In addition to these fluid components, the solid components consist of clay mineral assemblages (hydroxyl groups) and kerogen, collectively making up a total of eight distinct components. Furthermore, a comprehensive T₁-T₂ interpretation framework was established based on the retorting and NMR joint experiments, providing a robust tool for guiding fluid type classification using 2D NMR logging data. Analysis of advanced NMR T₁-T₂ logs indicated the absence of free water in the Gulong shale. A strong correlation was observed between the normalized annual production of the horizontal section per kilometer and the free oil porosity, with the ten horizontal wells targeting high free oil content layers demonstrating the highest productivity. Additional investigations using elemental spectroscopy logging and field emission-scanning electron microscopy (FE-SEM) revealed that layers with high free oil content are associated with larger particle sizes, leading to the development of enlarged inorganic macropores. These findings provide critical insights into the key geological controls governing sweet spots in the Gulong shale, offering valuable guidance for reservoir evaluation and development.