Hydrogeological and hydrogeochemical evaluation of groundwaters and surface waters in potential coalbed methane areas in Colombia

Abstract

A hydrogeological, hydrogeophysical, and hydrogeochemical study characterized 701 groundwaters and surface waters and 105 electrical resistivity measurements in sedimentary basins with great potential for unconventional hydrocarbon reservoirs, coal bed methane type (CBM). The study site was around the main coal mines of Cerrejon in La Guajira, Calenturitas and La Jagua in Cesar, Guacamayas in Cordoba, and timber type mines located in Catatumbo, Cundinamarca, and Boyacá departments in Colombia.The main hydrochemical facies in groundwater draining the sedimentary strata from Oligocene to Paleocene intervals in the Colombian coal zones were SO4-Ca-Mg, HCO3-Ca, HCO3-Ca-Mg, and SO4-HCO3 mixed waters. Depending on the type of rock drained along with the migration route of the groundwaters, the main anions found were SO42− > HCO3− > Cl−. The cations Ca2+ and Mg2+ (when associated with dolomite) were considered limestone contributions, while Na+ had a sandstone and shale.The geochemical processes explaining the groundwaters and surface waters chemical compositions included dissolution of halite and other salts in the La Guajira area; gypsum, calcite, and dolomite dissolution, plagioclase dissolution, pyrite oxidation, sulfate reduction with releasing of Fe(OH)2, and dissolution and/or precipitation of silica; and mixing waters in different proportions and combinations in Catatumbo, Boyacá, and Cundinamarca. Multivariate statistical analyses were applied to describe similarities between hydrochemical composition, inorganic facies, and spatial distribution.Hydrogeological and hydrophysical results revealed that the Cerrejon area is an aquifer due to its groundwater table. The rest of the producer areas are aquitards. The coal layers in Cerrejon formation, due to its secondary porosity and high fracturing, behave as a semiconfined to confined aquifer with a high-water content, which must be relieved (depressurized) at the time of the exploitation. Because of its composition, particle size, and degree of consolidation, the coal units of the rest of the studied zones do not transmit the groundwater effectively, generating aquitards rather than aquifers. The 105 vertical electrical soundings permitted the determination of the depth and thickness of coal layers in each producer zone associated with its resistivity (Ω.m.).