Restoring source rock initial quality and quantity with kinetic-based inversion - Applied to the Wolfcamp play in the Permian Delaware Basin

Abstract

To calculate the volume of hydrocarbon generated from a mature source rock, it is necessary to restore the total organic carbon (TOC) and hydrogen index (HI) to their initial conditions before the hydrocarbon generation. Various methods have been proposed to restore source rock properties. Most of these methods require measured HI data and assumptions on initial HI (HIi) to calculate the transformation ratio (TR) and then restore the initial TOC (TOCi) from measured TOC data. Estimating HIi can be challenging for samples with high maturity, for cases in which immature samples and maceral data are lacking, or for heterogeneous source rocks. This study presents a workflow using a kinetic-based inversion procedure to restore the source-rock's HIi and TOCi from measured HI and TOC data. Unlike the other methods, the kinetic-based inversion method uses simulated TR derived from the basin model and independent of assumed HIi to restore the HIi and TOCi from measured HI and TOC data. Thus, a pre-defined basin model with calibrated thermal model and assigned kinetic (organofacies) for the source rock intervals is required before the kinetic-based inversion procedure is performed. The workflow is applied to the Wolfcamp Play in the Permian Delaware Basin using a proprietary dataset covering the whole basin spatially. Based on geochemical interpretation, the organofacies within the Wolfcamp interval vary from predominantly marine clastic (OF “B”) to mixed marine-terrigenous (OF “D/E”) facies. The kinetic-based inversion result shows a considerable range of restored HIi and TOCi, values indicating source rock heterogeneity of the Wolfcamp Play. Generally, Wolfcamp A and B intervals are excellent source rocks with P50 HIi of ca. 594 and 508 mgHC/gTOC and P50 TOCi of 2.84 and 2.60 wt%, respectively. Wolfcamp C and D intervals are also good-very good source rocks with relatively lower potential than Wolfcamp A and B. Despite the limitation and uncertainty as to the consequence of using TR derived from the basin model, the kinetic-based inversion procedure has the advantages of requiring no assumption for HIi and restoring the source rock according to the thermal maturity, source rock organofacies, kinetic model, and measured data, among many others. Interpreted overall kerogen type of the Wolfcamp source rock using restored HIi and TOCi supports the interpretation of predominantly marine to mixed marine-terrigenous typed organofacies source rock using other data. Preliminary calculation of the restored HIi and TOCi and the calculated volumes of hydrocarbon yield also support the massive hydrocarbon volume predicted and produced for the Wolfcamp Play in the Delaware Basin.