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Coal Geology & Exploration

Abstract

Objective Significant breakthroughs have been achieved in the exploration and exploitation of coalbed methane (CBM) in the deep No. 8 coal seam within the east-central Ordos Basin. However, a comprehensive quantitative evaluation system at the regional scale is yet to be established for reservoirs in the No. 8 coal seam. Methods Based on logs, coal quality, and measured physical property data from over 270 wells in the east-central Ordos Basin, this study developed a comprehensive evaluation system for deep coal reservoirs using the fuzzy analytic hierarchy process combined with the entropy weight method. Specifically, the evaluation system consisted of nine key indicators (i.e., burial depth, minimum horizontal principal stress gradient, vitrinite reflectance (Rmax), coal thickness, total proportion of bright and semi-bright coals, gangue proportion, ash content, porosity, and permeability) of three aspects, namely occurrence conditions, coal seam characteristics, and reservoir physical properties. Accordingly, this study conducted a quantitative evaluation of high-quality deep coal reservoirs and further determined their developmental characteristics and distribution patterns. Resultsand Conclusions The high-quality reservoirs in the No. 8 coal seam within the east-central Ordos Basin exhibit a range of distinct characteristics, including coal seam thicknesses of greater than 6 m, total proportions of bright and semi-bright coals exceeding 85%, gangue proportions of less than 5%, and ash content of below 10%. The reservoirs manifest coal seam burial depths ranging between 1800 m and 3000 m, minimum horizontal principal stress gradients of less than 1.65 MPa/hm, and Rmax values varying from 1.3% to 2.5%, suggesting medium- to high-rank coals. Additionally, their porosity and permeability exceed 5% and 0.1 × 10–3 μm2, respectively, indicating high gas storage and fluid flow capacities. The comprehensive evaluation system reveals that the No. 8 coal seam can be divided into classes I, II, and III zones. Among these, class I zones, with comprehensive evaluation index (I) values of > 0.65, account for approximately 50%–60%, primarily distributed in the Shenmu-Jiaxian-Yulin and Mizhi-Suide-Zizhou areas. Class II zones, with 0.45 ≤ I ≤ 0.65, account for 30%–35% and are principally identified in the north-central and southwestern parts of the study area. In this class of zones, the No. 8 coal seam exhibits relatively high ash content, large burial depths, and poor physical properties. Class III zones, with I < 0.45, represent approximately 10% of the total area of the No. 8 coal seam, primarily distributed in the Yimeng uplift and the southern Jingbian area.

Keywords

Ordos Basin, deep No.8 coal, reservoir evaluation, high-quality reservoir, play fairway, analytic hierarchy process, entropy weight method

DOI

10.12363/issn.1001-1986.25.12.0913

Reference

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