The pressure regimes of the No. 2 coalbed methane (CBM) reservoir in the Yanchuannan field located in the southeastern Ordos Basin are highly variable and divided into overpressured (pressure gradient >9.80 kPa/m), slightly underpressured (pressure gradient of 8–9.80 kPa/m), and moderately underpressured (pressure gradient of 5–8 kPa/m). The controlling factors for the variable pressure regimes were investigated through the analysis of geological and hydrological characteristics. The pressure regimes are controlled by different mechanisms in different hydrodynamic environments. In the closed hydrodynamic environment characterized by TDS > 10,000 mg/L and NaCl type of groundwater, the pressure regime is dominated by overpressured to slightly underpressured and is controlled by CBM migration. Overpressure was developed by thermogenic CBM generation during the coalification process and is maintained by thermogenic CBM migration from the extended northwestward and deeply buried CBM reservoir during tectonic uplift. The transition from overpressure to slight underpressure and then to moderate underpressure towards the southeast is the result of the progressively weakened migrated thermogenic CBM with increasing migration distance. In the open hydrodynamic environment characterized by TDS < 10,000 mg/L and NaHCO3 type of groundwater, the pressure regime is dominated by slightly to moderately underpressured and is governed by hydrodynamics. Groundwater is fed by meteoric recharge along the structurally upturned basin margin and creates the hydrodynamic framework during tectonic uplift. The transition from moderate to slight underpressure towards the southwest is associated with the minor decrease range in ground elevation from basin margin to basin interior and the gradually weakened runoff intensity of groundwater with increasing distance to meteoric recharge. The idealized models for the pressure regimes are established, which can provide guidance to deep CBM sweet spot identification in CBM fields in the eastern Ordos Basin and elsewhere.

中文翻译：

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盐川南油田煤层气藏压力状态的地质和水文控制——对鄂尔多斯盆地东部深部煤层气开采的启示


位于鄂尔多斯盆地东南部的盐川南油田2号煤层气（CBM）气藏压力状况变化较大，分为超压（压力梯度>9.80 kPa/m）、轻微欠压（压力梯度8–9.80 kPa/m）和中度欠压（压力梯度5–8 kPa/m）。通过地质和水文特征分析，研究了可变压力状态的控制因素。在不同流体动力学环境中，压力状态由不同的机制控制。在以 TDS > 10,000 mg/L 和 NaCl 型地下水为特征的封闭水动力环境中，压力状态以超压到略微欠压为主，受 CBM 迁移控制。超压是由煤化过程中产热煤层气产生的，并在构造隆起期间由从延伸的西北和深埋的煤层气储层的产热煤层气迁移来维持。从超压到轻微欠压，然后向东南方向过渡到中等负压，是随着迁移距离的增加，产热煤层气迁移逐渐减弱的结果。在以 TDS < 10,000 mg/L 和 NaHCO3 型地下水为特征的开阔水动力环境中，压力状态以轻度至中度欠压为主，受流体动力学控制。地下水沿着结构上翘的盆地边缘通过流星补给，并在构造隆起期间形成水动力框架。 向西南方向从中度到轻微的负压过渡与从流域边缘到流域内部的地面高程略有减小以及地下水径流强度随着流星补给距离的增加而逐渐减弱有关。建立了压力状态的理想化模型，可为鄂尔多斯盆地东部等地煤层气田的深层煤层气甜点识别提供指导。