Accumulation conditions and exploration potential of deep coalbed methane in Benxi Formation, Eastern Yan'an area, Ordos Basin

This paper is the result of oil and gas exploration engineering.

Objective The Benxi Formation coal seams in the eastern Yan'an area of Ordos Basin are characterized by table distribution, sadvanced thermal maturity and high gas content. This study evaluates the gas accumulation conditions and resource potential of deep coalbed methane in the Benxi Formation, aiming to provide guidance for future exploration and to further enrich the geological theory of deep coalbed methane accumulation.

Methods Based on a comprehensive review of deep coalbed methane exploration results in China, this study utilizes abundant drilling data and coal core analysis from the eastern Yan'an area to analyze the distribution, reservoir characteristics, coal quality, and gas-bearing properties of the Benxi Formation coal seams, systematically evaluating their accumulation conditions and exploration potential.

Results (1) The 8# coal seam of the Benxi Formation is widely distributed in the eastern Yan’an area, with an average thickness of 4 m. The coal lithotypes are primarily bright coal and semi-bright coal, with a rank reaching lean coal to anthracite. The average gas content is 16.8 m³/t, indicating favorable conditions for deep coalbed methane accumulation; (2) The study area covers 10500 km², with an estimated coalbed methane resource of 970 billion cubic meters and an average resource abundance of 0.92 × 10⁸ m³/km²; (3) Based on six indicators such as coal seam thickness, gas content, and preservation conditions, criteria for selecting favorable areas for deep coalbed methane were established. This resulted in the identification of 147.9 km² as first-class favorable areas and 1138 km² as second-class favorable areas; (4) Following the favorable area selection, a review of existing wells was conducted. Four vertical wells, including Well C7, were selected for fracturing and production testing, achieving post-fracturing gas rates ranging from 1000 m³/d to 1700 m³/d.

Conclusions The Benxi Formation in the eastern Yan'an area possesses significant exploration potential for deep coalbed methane and is expected to become a successor resource for stabilizing gas field production. Future work should focus on detailed geological studies, classified evaluation, selection of optimal production zones, and advancing the exploration and development process for deep coalbed methane.