Micro-Imaging and Reservoir Characterization: Estimating Rock Properties from Rock-Calibrated Microresistivity Logs

Jim Puckette¹, and Adam DeVries². (1) School of Geology, Oklahoma State University, 105 NRC, Stillwater, OK  74078, phone: (405) 744-6358, fax: (405) 744-7841, (2) Chesapeake Energy Corporation, P.O. Box 18496, Oklahoma City, OK 73118  [email protected]

Borehole micro-imaging logs, used in conjunction with other geophysical logs and subsurface data, are a powerful tool for estimating rock properties in siliciclastic and carbonate rocks. Core-calibrated, static microresistivity imaging logs can be used to discriminate between such subtle features as 1) high-permeability/porosity sandstones and 2) high-porosity sandstones with significantly reduced permeability as the result of pore lining and filling authigenic clays. Dynamic and static presentations of microresistivity logs are effective in resolving thin beds and other lithologic features such as sedimentary structures, burrows, and trace fossils. In carbonate intervals, microimaging logs have effectively identified dolomite fabric with intercrystalline porosity, vuggy and shelter porosity in limestone, fractures, and low-permeability seals. In core- or outcrop-calibrated shale and other mudrocks, microimaging logs can identify subtle changes in lithology and recognize induced and natural fractures.

The shallow depth of investigation for most applications of microimaging tools limits resistivity measurements to the flushed zone. As a result, formation fluids have minimal impact on measurements and variations in resistivity are a function of rock properties.

Examples from sandstone, carbonate, and shale reservoirs from the midcontinent region demonstrate the effectiveness of microimaging in recognizing tightly cemented seal rocks and adjacent porous reservoirs. An example from the Texas Gulf Coast is used to demonstrate the effectiveness of microimaging in identifying thin reservoirs within a thick, low-resistivity, low-contrast interval containing interbedded shale and sandstone. In all examples, microimages were correlated to core or outcrop, providing corroboration of rock properties estimated from microresistivity measurements.