Effect of Magmatic Intrusion on In Situ Stress Distribution in Deep Coal Measure Strata: A Case Study in Linxing Block, Eastern Margin of Ordos Basin, China

In situ stress is an important factor of unconventional gas development, and it affects mainly geological structures. In the Linxing block, magmatic activity (in Yanshanian) led to the difference of reservoir property and unique distribution of in situ stress regime, which restricted the extraction of gas in deep coal measure strata. With multiple data sets (including imaging logging, fracturing reports, and seismic profiles), a one-dimensional mechanical earth model was established based on curvature–strain conversion model, and the influence of magmatic activity on in situ stress distribution was analyzed. The results showed that the in situ stress regime of coal seams was dominated by vertical principal stress, while the sandstone reservoirs showed regional differences. With the Zijinshan pluton as the center, the distribution of in situ stress regime showed obvious regularity. Vertically, the mechanical properties of strata around pluton have been strengthened, resulting in a 3-stage upward trend of maximum horizontal principal stress (SHmax) with depth, namely "normal (< 1500 m), slow (1500–1800 m), and rapidly (> 1800 m)", while 2-stage stepwise increases (without slow stage) were found on the limbs of the pluton. Regionally, away from the pluton, in situ stress regime transformed from reverse faulting to normal faulting. The magmatic activity also resulted to clockwise rotation of SHmax orientation (WNW), and it was about 45° relative to the other areas in the eastern margin of Ordos Basin (ENE). The high horizontal stress in deep reservoirs limited the formation of hydraulic fracture network, and so further unconventional gas extraction requires special reconstruction measures or it should be located away from the Zijinshan pluton.