--> --> “Sweet Spots” at the Sub-Lateral (Decameter to Kilometer) Scale: Target-Zone Heterogeneity Seen in Mass Spectrometry of Mud Gasses

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“Sweet Spots” at the Sub-Lateral (Decameter to Kilometer) Scale: Target-Zone Heterogeneity Seen in Mass Spectrometry of Mud Gasses


Direct quadrupole mass spectrometry (DQMS) of gasses liberated by drilling shows that horizontal wells in shale and other unconventional plays commonly show significant changes in the chemistry of geofluids through the lateral. As seen in several examples, this enables laterals to be zoned as prospective oil, condensate, gas, water legs, or non-productive. In contrast, “sweet spots,” with good initial production rates and low decline rates, are usually considered at the lease block to field scale (i.e. 1–10s of kilometers). The controlling factors have been identified as any combination of TOC, payzone thickness, thermal maturity, fracture density and spacing, brittleness, nature of porosity, etc., depending on the play. The DQMS data suggest that the same phenomena occur on a smaller scale, and can guide completion and fracking plans to improve the economics of a well. Examples from different onshore US basins illustrate the zonation. Wells are commonly strongly zoned for indicators of condensates and oil, as defined by differences in relative populations of alkanes and aromatics, methane and propane ratios, etc. Relative abundances of light elements and compounds indicate downhole changes in porosity and permeability. Correlation between the geofluid zoning and porosity/permeability indicators suggests permeability controls zoning (i.e. compartmentalization). This correlation occurs in some, but not all, wells examined; in some laterals it occurs only locally. Consider the factors that can produce the mass spec zonation. Firstly, the fluid difference may be original: that is, the rock isolates different fluid compositions. Alternatively, the fluids may reflect permeability differences (especially pore throat diameters) that fractionate or allow all hydrocarbons. Spatially, fluid heterogeneity may be expressed stratigraphically (i.e. bed-controlled) or laterally. Bed-parallel heterogeneity would likely reflect post-depositional (diagenetic or structural) effects. We can control for stratigraphic effects somewhat by comparing the mass spec with geosteering. However, note that stratigraphic control may occur at scales below the resolution of the gamma tool (as much as 0.4 m at 15 m/h ROP). In at least one well, the hydrocarbon zoning is at least partly non-stratigraphic: the zone and stratigraphic contacts rarely correlate. Whatever the reasons, sweet spots occur below the lateral scale, and should be considered in planning completions.