Objective and Methods This study aims to ascertain abnormal hydrocarbon outflows from the surrounding rocks of coal seams in the Jurassic Yan'an Formation within the Xinzhuang mine field in the Longdong area, southwestern Ordos Basin. Based on the fundamental principles of petroleum geology, sedimentology, and coal geology, this study systematically analyzed the sources, occurrence patterns, and primary controlling factors of hydrocarbons in the surrounding rocks of coal seams using methods including core observation, sample analysis, and statistical mapping.
Results and Conclusions In the Xinzhuang mine field, hydrocarbon outflows are primarily identified in the floor of the No. 8 coal seam in the Yan'an Formation. Their hydrocarbons originate from the mature source rocks of the underlying seventh oil group of the Triassic Yanchang Formation (also referred to as the Chang 7 oil group). The hydrocarbons occur primarily as isolated hydrocarbon pockets or gas pockets within the fine- to medium-grained sandstone reservoirs of subaqueous distributary channels in the Chang 4+5 oil groups, exhibiting a distribution characterized by distinct macroscopic heterogeneity and vertical stratification. The reservoir distribution is generally controlled by sedimentary microfacies. Furthermore, the sandstone reservoirs in the Yanchang Formation underwent significant denudation and modification due to the tectonic uplift induced by the Indosinian movement at the end of the Late Triassic. Then, the fine-grained sediments of the Early Jurassic Fuxian Formation were superimposed on the reservoirs, contributing to the formation of potential reservoir-cap rock assemblages in paleogeomorphic highs such as low residual paleo-hills and slope spurs. Structural traps, including nose-like structures and anticlines formed from the Yanshanian to the Himalayan, further intensified the differential hydrocarbon enrichment, while well-developed strike-slip faults and associated fractures provided pathways for hydrocarbon migration. Governed by the coupling effects of sedimentary microfacies, paleogeomorphology, and local structural morphologies, hydrocarbon enrichment occurs primarily in areas with subaqueous distributary channel sands of the Yanchang Formation lying beneath the low residual paleo-hills or slope spurs of the Fuxian Formation. Most especially, in parts with superimposed local structural traps, which exhibit thin cap rocks and are close to coal seam floors, hydrocarbon outflows are prone to occur under mining disturbance. Analysis of dominant factors controlling hydrocarbon occurrence reveals that the risks of potential hydrocarbon outflows can be classified into levels I to III. Furthermore, zones with various risk levels were predicted. This study provides insights and assessment methodology regarding the sources, occurrence patterns, and hazard prediction of hydrocarbons in the surrounding rocks of coal seams, offering a scientific basis for the prevention and control of hydrocarbon outflows in the surrounding rocks of coal seams in the Xinzhuang mine field and other mining areas with similar geological conditions.
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