Vertical Heterogeneity of Deep Carboniferous Coal Reservoirs in the Northern Part of the Eastern Margin of the Ordos Basin

Abstract

Deep coalbed methane (CBM) is an important frontier for expanding unconventional natural gas reserves and production in China. Significant vertical heterogeneity in coal reservoirs is a primary constraint on exploration and development, and the conventional interval subdivision based solely on burial depth is insufficient for refined evaluation of deep coal reservoirs. This study targets the No. 8 coal seam of the Carboniferous Benxi Formation in the northern part of the eastern margin of the Ordos Basin. Based on core and logging data from wells F and S, this study applies the Milankovitch cycle identification method and uses the short-eccentricity cycle (∼109 kyr) as a working timescale for high-resolution isochronous stratigraphic subdivision of coal seams within a single well. Combined with coal petrology and reservoir property analysis and the Analytic Hierarchy Process (AHP), this study characterizes vertical heterogeneity and quantitatively identifies favorable intervals in the coal reservoir. The results indicate that: (1) the No. 8 coal seam in Well F was divided into three sections, and Well S into two sections. These subdivisions provide a high-resolution cyclostratigraphic framework for evaluating vertical reservoir variations within each well, with higher resolution than conventional depth-based segmentation (spectral confidence > 99%, p < 0.01); (2) coal quality, pore structure, and gas potential vary significantly with depth, and these variations represent geological responses to periodic changes in depositional environment and sediment supply during peat accumulation; (3) AHP-based quantitative evaluation identifies the upper section of Well F (score: 0.336) and the lower section of Well S (score: 0.755) as the favorable targets for the two investigated wells, characterized by good coal quality, favorable pore structure, and high gas storage capacity. These findings provide geological support for refined deep coalbed methane exploration and development in the study area, and also offer a new technical approach for high-precision stratigraphic subdivision of coal reservoirs.