Geological mechanisms of the accumulation of coalbed methane induced by hydrothermal fluids in the western Guizhou and eastern Yunnan regions

Abstract

Based on the geological characteristics of hydrothermal fluid in the western Guizhou and eastern Yunnan regions, this paper discusses the geological mechanisms of hydrothermal fluid on the coalbed methane (CBM) system. From the perspective of hydrothermal fluid activity in the regional strata, we analyze the geological processes by which coalbed methane is enriched and accumulated in this region. Our results demonstrate that two stages of regional thermal fluid activity occurred in the western Guizhou and eastern Yunnan regions. In the earlier stage, calcium fluid acted as the carrier, but in the later stage, it was replaced by siliceous fluid with a small amount of calcium fluid. The former resulted from thermal cycling of fluid through the strata, and the latter was caused by injection of deeply sourced high temperature fluids. We propose three geological mechanisms by which hydrothermal fluid affected coalbed methane accumulation: stimulation for hydrocarbon generation, transformation for physical properties and fluid supercharging. The hydrothermal fluid impels coalbed methane to generate and migrate at high intensity; it also produces dense swarms of tubular pores that coincide with tectonic stress. As a result, the coal body is broken, and there are changes to the porosity and permeability of the coal. Pressure development in the regional gas reservoir depends on the intensity of the fluid effect and the differential leakage of fluid pressure in the coal reservoir. The relatively closed and stranded hydrothermal fluid environment of coal measure strata tends to lead to a high pressure coalbed methane system. The platform uplift in northwestern Guizhou is one such environment, in which the geological configuration of the high temperature fluid provided favorable conditions for the enrichment and accumulation of coalbed methane.

Multi-Phase Tectonic Movements and Their Controls on Coalbed Methane: A Case Study of No. 9 Coal Seam from Eastern Yunnan, SW China

“…The tectonic movement was dominated by the continuous subsidence movements after the coal-bearing strata deposited in the Late Permian until the Late Jurassic. According to the previous numerical simulation research about the burial and thermal histories of coal seams based on the analysis of vitrinite reflectance and homogenization temperature of fluid inclusion, the largest ancient burial depth of the coal seams can reach 3800 m in the research areas [45][46][47]. Under the effect of paleotemperature, the coal seams experienced deep metamorphism and reached the fat coal stage, accompanied with the massive CBM generation [45,46].…”

“…According to the previous numerical simulation research about the burial and thermal histories of coal seams based on the analysis of vitrinite reflectance and homogenization temperature of fluid inclusion, the largest ancient burial depth of the coal seams can reach 3800 m in the research areas [45][46][47]. Under the effect of paleotemperature, the coal seams experienced deep metamorphism and reached the fat coal stage, accompanied with the massive CBM generation [45,46]. The influence of the Indosinian movement was very weak and the Indosinian geological structure was rare in the Bumu region.…”

Controlling factors of coalbed methane well productivity of multiple superposed coalbed methane systems: A case study on the Songhe mine field, Guizhou, China

“…Regional tectonic map of the Songhe mine field. Tang et al, 2016;Yuan, 2014), forming the upper, middle, and lower sections, which were deposited in lagoon-tidal flat, lower delta plain, and lagoon-tidal flat sedimentary environment, respectively. The Longtan Formation includes 30 to 47 coal seams, and has 18 minable coal seams, with a minable coal thickness and burial depth of approximately 14-32 m and 500-960 m, respectively ( Figure 2).…”

“…The sedimentary environment of the research region is marine-continental transitional facies. With eustatic cycles, the thin coal seam group was formed with discontinuous deposition in the Upper Permian Longtan Formation (Peng et al., 2005; Qin et al., 2008; Tang et al., 2016; Yuan, 2014), forming the upper, middle, and lower sections, which were deposited in lagoon-tidal flat, lower delta plain, and lagoon-tidal flat sedimentary environment, respectively. The Longtan Formation includes 30 to 47 coal seams, and has 18 minable coal seams, with a minable coal thickness and burial depth of approximately 14–32 m and 500–960 m, respectively (Figure 2).…”

“…However, most CBM wells have a short high yield period and are faced with declining gas production because of separate fracturing of each layer or improper combinations of multiple layers for commingled drainage. Recently, the coal-bearing basins in the western Guizhou region have attracted greater attention because of the development of multiple coal seams, although the thickness of a single coal seam is small (Gao et al., 2009; Li et al., 2015; Tang et al., 2016; Tian et al., 2008; Yang et al., 2011).…”