This paper presents results of a study evaluating the accuracy of well log analyses calibrated with rock properties measured from core and drill cuttings compared to a calibration using rock properties from just drill cuttings. The study wells are producing from a liquids-rich resource play in the Permian Basin of West Texas, USA. The results of our comparative study suggest that, when core data are not available, an acceptable alternative approach is to substitute limited but selected rock properties that can be measured accurately from drill cuttings. While the preferred method is to use whole core, logs calibrated in our study using cuttings-derived rock properties compare favorably to those calibrated using core measurements. Further, the abundance of representative cuttings available from most wells combined with the small rock sample volumes required for accurate laboratory measurements ensure the practical applicability of this method. Finally, the laboratory techniques for measuring the selected cuttings-derived rock properties used in our calibration process are well established by most commercial laboratories, thereby making this alternative approach both technically viable and cost effective.
Resource plays are defined by Jarvie (2006) as ?hydrocarbon systems where the source and reservoir are the same rock unit or formation.? Jarvie further states that these ?source-reservoir units are generally continuous and represent areas of organic matter preservation as reflected in organic richness.? Resource plays or ?shale reservoirs? (a generic term frequently applied to many resource plays) are essentially self-sourced systems in which all of the requisite petroleum system elements (i.e., source rock, reservoir, and seal) to form a hydrocarbon reservoir are present. Therefore in order to exploit shale oil reservoirs effectively, we must be able to characterize each of these basic petroleum system elements separately as well as understand their relative inter-dependencies.
The most common method for quantifying fluid storage mechanisms as well as estimating potential productivity in shale oil reservoirs is a petrophysical analysis using well logs. Because these resource plays are typically very complex systems with significant variations in rock composition and properties over multiple vertical and horizontal scales, we believe accurate log-based petrophysical models require calibration to properties measured directly from rock samples. For example, calculating porosity from logs in shale oil systems is difficult due to the presence of low-density organic materials, variable mineralogy with associated variations in grain density, and different fluid storage properties. Log-based porosity estimates are particularly difficult when organic matter is present since organics tend to affect the distribution of pores and fluids (Sondergeld, et al., 2010). However, significant improvements in the accuracy of log-based calculations are possible if we can calibrate or ?tune? the log interpretation equations to actual rock property measurements.