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Advanced Mud Gas and Fluid Inclusion Analysis—Vaca Muerta Horizontal, Argentina

Abstract

Advanced mud gas and fluid inclusion techniques were applied to a horizontal well within the Vaca Muerta formation, which penetrates the entire unit and produces wet gas and condensate. Source rock characteristics and maturity data are consistent with compositional and isotopic data from mud gas and suggest a predominantly local source for hydrocarbons, potentially with a minor, preserved early biogenic gas component. Direct quadrupole mass spectrometry of mud gas during drilling indicates that hydrocarbon species up to C9 are present, suggesting wet gas to gas-condensate within the reservoir section. Distinct chemical compartments are identified, and highest gas response is noted near the base of the section. The upper section of the reservoir is relatively enriched in light components, including helium. Water saturation appears to be low, with slightly higher values documented above the reservoir and near the base of the penetrated interval. Minor CO2 may have a distinct source. No significant bit-generated gas or sulfur species are identified. Bulk crushing analysis of fluid inclusions and nanopores in cuttings using the fluid inclusion stratigraphy (FIS) technique indicates predominantly wet gas to gas-condensate, and is consistent with mud gas data. Specifically, higher responses are observed in the deeper section, helium enrichment is noted in the upper interval, and slightly elevated water saturation is suggested above and near the base of the reservoir. A multistage generation/migration history is indicated, and a late-mature fluid composition is consistent with other lines of evidence. Bubblepoints of upper-moderate to high gravity petroleum inclusions indicate that fluids are significantly under-saturated at current reservoir temperature and would produce as wet gas to gas-condensate, as indicated by testing. FIS response and visible inclusion abundance does not correlate with calcite veins, present throughout the horizontal well. These healed fractures may have predated the main stages of generation, expulsion, and migration of the dominant hydrocarbon phases documented in the fluid inclusion record. This study demonstrates the value of combined mud gas (molecular and isotopic) and cuttings volatile analysis in predicting fluid distribution and composition in horizontal wells, and the potential for using fluid inclusions in archived samples from vertical wells to establish fluid characteristics prior to horizontal drilling.