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As part of ongoing regional evaluation of tight gas sand resources, computer processing of well log data was used as a tool for revealing gross trends in stratigraphic and diagenetic characteristics. Log data for 20 wells in the northern Green River basin were processed using a DEC LSI-11/23 microprocessor.
The processing technique involved the application of single or multiple logic tests for lithology discrimination and elimination of "bad hole" data. Resulting data for the entire length of the well are plotted vs. depth as x-y plots, thereby displaying large-scale trends that may be present. Cross sections constructed using these crossplots aid in regional evaluation.
This technique has proven to be particularly useful for identifying the regional Cretaceous-Tertiary unconformity, which in many wells shows up as a dramatic shift in gamma-ray response across the boundary from low (Cretaceous) to high (Tertiary). Possible zones of overpressuring are identified by means of shale conductivity crossplots. Porosity reduction trends as well as gas zones are revealed through plots of sandstone sonic travel time.
The primary advantages of this method are that it (1) reduces the sampling bias that can occur with manual plotting and reduces the sensitivity of data trends to discriminator values due the large sample population; (2) reveals data trends that may not be apparent from viewing analog well log prints or that may be masked by the log response due to the presence of other lithologic characteristics; and (3) provides additional insight into problems related to stratigraphy, overpressuring, and porosity, for which other kinds of data are incomplete or absent.
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