Integration of mud gas isotope logging (MGIL) with field appraisal at Horn Mountain Field, deepwater Gulf of Mexico

Prior applications of gas isotopic data to reservoir studies have often suggested that measured differences of a few per mil in gases sampled between wells indicated hydrocarbon compartmentalization concomitant with poor reservoir communication. These conclusions were generally invoked without adequate consultation and integration of geological and engineering well data, and only revealed to be true in the simplest of circumstances where pressure data clearly suggested the same. Not surprisingly, gas isotope data has yet to find widespread production engineering applications as a convincing tool for reservoir formation evaluation. This situation is unlikely to change due to the prohibitive drilling economics of acquiring adequate quantities of physical gas samples from down-hole tools in every zone of interest across multiple wells in a field. Recent application of a new technique, mud gas isotope logging (MGIL), has shown comparable isotopic data to traditional gas samples collected from down-hole tools while providing the petroleum systems analyst with the necessary large datasets needed to make accurate and confident reservoir evaluations. Many additional benefits of MGIL to formation evaluation have also been recognized, and it is envisaged MGIL possesses the potential to develop as a standard protocol on drilling wells.This paper documents the first published application of MGIL in the context of complete field-scale reservoir integration and production appraisal encompassing 18 well penetrations over a 3-yr drilling program. Case data from the Horn Mountain Field (Gulf of Mexico) are used to demonstrate this technology and its impact to field assessment and appraisal. Initial well data and two 3-D seismic surveys were used to assess reservoir continuity and compartmentalization across the field. Subtle pressure and gas-to-oil ratio (GOR) differences from eight appraisal wells suggested that the main economic M-Sand may be divided into at least two compartments, however the data could not be unambiguously relied upon in itself. MGIL data provided the strongest evidence in confirming the existence of two ‘baffled’ field compartments. Significantly for subsequent development wells, MGIL data were able to accurately discern and allocate compartments in wells without reliable wireline or absent MDT pressure data. MGIL data also recognized the significance of geological features such as a shale-filled channel visible on amplitude maps, and inferred to serve in concert with faults as a baffled flow barrier. Data such as these may have important implications for reservoir energy and pressure support during production. The impact of hydrocarbon stratification attributed to geological reservoir structural features and/or hydrocarbon phase geochemical density characteristics are also examined.Interpretation of the Horn Mountain MGIL data, in addition to characterizing and verifying oil/gas shows, was also successful in providing essential data on highlighting all penetrated pay zones, deconvolving multiple pure biogenic and mixed biogenic/thermogenic zones, identifying stratigraphic trapping seals and providing detailed petroleum system evaluation.