Print this pageWell Log Evaluation of Source Rocks -- 15 Years and 3000+ Wells Later
CREANEY, STEPHEN, and QUINN R. PASSEY
The use of well logs to evaluate source rocks has resulted in significant insight into the occurrence of source rocks and their stratigraphic distribution within hydrocarbon systems. The <delta>logR technique, which utilizes routine porosity and resistivity well logs that are available in most wells, continues to prove critical in exploration programs for source rock identification and evaluation. The relatively simple technique, which relies on a physical model of the mineral matrix, solid organic matter (kerogen), and fluid-filled porosity (oil, water, or gas), can be applied to most geologic situations, and continues to be routinely applied throughout the world.
Experience has shown that in mud-dominated source rocks, quantitative prediction of total organic carbon (TOC) is reliable, provided that the well logs are of good quality, and thermal maturity is reasonably well known. The primary keys to reliable interpretation are 1) accurate well log processing (porosity/resistivity overlay), and 2) displaying the overlay as well as the predicted TOC curve. Also, recalibration of the technique is needed for accurate TOC determination in coals, and may be needed for some pure carbonate-matrix (non-marl) source rocks.
The ability to generate a reliable and continuous depth plot of TOC in many wells per basin allows for improved interpretation of source rocks in a sequence stratigraphic framework. Tying the well log observations to seismic allows for a calibrated extrapolation of source occurrence into undrilled areas, thus expanding the regional understanding of the overall hydrocarbon system.