1964 Paper D

COMPUTER DELINEATION OF STRATIGRAPHIC TRAPS IN SAND-SHALE SEQUENCES

Robert Daskam General Electric

ABSTRACT

The digital computer can be used for the determination of probable geological boundaries in the exploitation of stratigraphic traps of the shale-out type. Electric log and geologic data from drilled wells in an area are integrated with regional geologic information from computer stored sedimentary basin catalogs to identify and correlate formations; to calculate bed thicknesses, water resistivities, shaliness ratios and measures of displaceable hydrocarbons at each well for all porous zones; and to map these attributes plus a general favorability criterion to delineate the probable economic extent of each bed. Thus, favorable shaly sand traps and trends are identified for closer scrutiny, and quantitative measurements are developed to aid in the ultimate evaluation made by the log analyst.

The general procedure used is as follows: The short normal logs from several wells in a development area are manually correlated and the formations identified. The remaining wells of the area are then correlated by the computer, using the short normal curve, on the basis of the recognition of the patterns established by the manual correlation of the sample wells. Most probable water resistivities are determined for each formation in each well by the statistical fit of Rw values from a magnetic tape catalog of formation water resistivities for the sedimentary basin in question and with suitable interpolation for uncatalogued formations. Spontaneous potential (SP) logs for each well are examined by the computer to determine the tops each well are examined by the computer to determine the tops and bottoms of each sand of interest. The recorded SP values are corrected for the effects of bed thicknesses, formation resistivities and depth of invasion of drilling fluids using standard equations and published correction charts. The water resistivities are used to calculate the static SP values. Shaliness ratios are then computed. Sonic and induction logs are corrected as necessary, and the remaining mappable parameters are calculated.