Geochemical source analysis of rock cuttings based on major elemental geochemistry

In recent years, shale logging has played a crucial role in the grouting reinforcement engineering of the floor limestone strata in “North China-type”coalfields. However, existing shale logging techniques are limited to the recognition of physical indicators such as color, particle size, and morphology. This limitation makes an accurate stratification challenging, and the “along-stratum rate” in borehole design is difficult to be ensured. These constraints hinder the effectiveness of water damage control in the floor limestone strata areas. This study, based on the elemental geochemical differences in the Carboniferous Taiyuan Formation thin-layered limestone and its clastic interlayers, selected the Taoyuan Coal Mine in the Huaibei Coalfield as the research area. Some vertical core samples were obtained from drill holes in the upper part of the Taiyuan Formation, including L1 limestone to L4 limestone (with L3 limestone as the grouting control target layer). Using the X-ray fluorescence spectrometry (XRF), the major elemental background values of thin-layered limestone and its interlayers were quantitatively determined. Also, a major elemental geochemical identification model was established for thin-layered limestone using mathematical statistical methods such as cluster analysis and factor analysis. Simultaneously, major elemental testing was conducted on shale samples from directional drilling horizontal branch holes. The identification pattern validation was completed through shale stratigraphic source analysis based on the established geochemical identification model. The results showed that high CaO content and Loss on Ignition (LOI) value could serve as characteristic indicators for the target layer L3 limestone in grouting and reinforcement. Abundances of MgO (0.5%±), MnO (0.03%±), and P2O5 (0.08%±) could be used as recognition indicators for the overlying marker layer J3. The cluster analysis identification model effectively distinguished the Taiyuan Formation L1 to L3 limestone. The model established using element factor scores effectively differentiated clastic interlayers from thin-layered limestone in the Taiyuan Formation. The Fisher discriminant equation obtained by running the model with rock geochemical background values achieved an accuracy rate of 100% in the source analysis of cuttings from grouting target layer L3 limestone. Based on the above findings, the feasibility of applying the elemental geochemical source analysis method, aimed at ensuring the “in-layer rate” in horizontal branch hole cuttings, was demonstrated. This, in turn, led to the proposal of a ground directional drilling “in-layer rate” control technology scheme. This study, grounded in elemental geochemical theory, identified the target layer stratification through characteristic geochemical information carried by cuttings from directional drilling. With the support of on-site rapid quantitative testing techniques, the technology offers new insights into addressing the challenge of grouting and modification at bottom limestone water hazard areas in the North China-type coalfields. © 2024 China Coal Society. All rights reserved.