Using Mud Gas Logging (MGL) and stable isotope measurements, geoscientists can identify pay zones, assess hydrocarbon type, and evaluate reservoir compartmentalization.
To identify pay zones, wireline, LWD and MWD log analyses have traditionally been supplemented with analyses of hydrocarbon abundances in mud gas. However, traditional mud gas analyses could miss or incorrectly identify hydrocarbon-bearing intervals, due to the effects of drilling conditions on the hydrocarbon concentrations in the mud gas (e.g., Whittaker, 1991). For example, pay zones could be missed where they have been flushed by drilling while overbalanced. In addition, the utility of mud gas analyses was previously limited to pay zone identification, and provided little information on other issues, such as hydrocarbon type, the potential for down-dip oil, or reservoir compartmentalization.
However, in recent years, the suite of analyses performed on mud gases has grown to include isotopic characterization of individual hydrocarbon components (e.g., Ellis et al., 1999; 2003). By removing the dependence on gas concentrations, this advance has greatly increased the ability of mud gas analyses to identify hydrocarbon-bearing intervals. The tremendous advance represented by isotopic analyses of mud gases can best be understood by recalling that gas isotopic analyses can reveal if a gas is bacterial or thermogenic in origin, and also can be used to characterize the thermal maturity of the rocks that generated a thermogenic gas (see Determining the Origin of Gas). Such information on gas origin has a variety of very practical applications. For example, if a gas is found to be entirely bacterial in origin, then the likelihood of a down-dip oil leg is small. However, if a gas is thermogenic, or is mixed thermogenic/biogenic, then it may or may not have an associated oil leg (see Down-dip Oil Prediction). Furthermore, determination of gas origin and gas maturity can reveal differences between the gas charge in adjacent reservoir intervals and can in that way allow reservoir continuity to be assessed (see Assessing Reservoir Continuity; Beeunas et al., 1996, 1999).
Figure 1: Carbon Isotope Composition Log from a development well. Similar methane isotope values within the individual intervals A, B, C, D and E indicate the presence of reservoir connectivity (vertically continuous reservoir) within these intervals. The compositional differences between these intervals suggest the absence of vertical continuity. Note that the MDT stable isotopic data are consistent with those collected during Mud Gas Logging (MGL).
Mud gas isotopes can also be used to assess the presence or absence of lateral reservoir continuity between wells. However, while dissimilarity of isotopic composition is a good indicator of lack of reservoir connectivity, isotopic similarity of two reservoir sections may or may not indicate connectivity.
Figure 2: Mud Gas Diagnostic Ratio Log shows mud gas geochemistry in addition to the stable isotopic data. This depth plot combines standard mud logging data, analyses from down-hole samples (if available) and the Mud Gas results. The diagnostic ratios: GWR (gas wetness ratio), LHR (light-to-heavy ratio) and OCQ (oil character qualifier) are defined by Haworth et al. (1985).
Isotopic analyses of mud gas samples are not performed at the well site. Rather, the gas samples are collected either in disposable, non-pressurized "gas bags" or in a specialized type of non-pressurized metal cylinder. The samples are then analyzed at an off-site laboratory. Sampling gas for analysis (whether it be mud gas, produced gas, or a gas seep) is a straightforward process (see Sampling Techniques).
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Beeunas, M. A., and M. A. McCaffrey, 2004, Proprietary OilTracers Report.
Beeunas, M. A., D. K. Baskin, J. L. Jurgens, A. R. Dincau, and M. Schoell, 1996, Application of Gas Geochemistry for Reservoir Continuity Assessment, Gulf of Mexico : AAPG/EAGE Research Symposium, "Compartmentalized Reservoirs: Their Detection, Characterization and Management (Abstract).
Beeunas, M. A., D. K. Baskin, and M. Schoell, 1999, Application of gas geochemistry for reservoir continuity assessment and identification of fault seal breakdown, South Marsh Island 61, Gulf of Mexico - Abstract, AAPG Hedberg Research Conference "Natural Gas Formation and Occurrence" June 6-10, 1999, Durango, Colorado
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Whittaker, A., 1991, Mud Logging Handbook: Englewood Cliffs, New Jersey, Prentice Hall, 531 p.