Geochemical Analyses and Lab Data Corrections in Jurassic Carbonate Source Rock: Implications on Resource Assessments

Abstract

An intrashelf depo-center hosts the Jurassic carbonate source rock, which contains one of the richest hydrocarbon producing source rock intervals in the world. The understanding of kerogen distribution and maturity across this frontier unconventional basin is key for focused prospectivity and future development. A correlation between total organic carbon (TOC) and porosity and key reservoir parameters has been observed. The porosity, measured using the Gas Research Institute (GRI) methodology, shows a poor TOC values correlation, as measured by LECO. A substantial amount of liquid hydrocarbon has been recorded during pyrolysis in the S1 peak, constituting a significant portion of the measured TOC. This effect has to be quantified in order to avoid issues when calibrating logs to predict TOC, saturation and porosity. An analytical workflow was developed to remove the free hydrocarbon and bitumen from samples. Each sample was split into two parts; and sent for LECO-TOC and SRA pyrolysis. The initial run was conducted using an industry standard sample preparation method. In the second run, sample preparation included a solvent extraction. An organic solvent was used to remove the free hydrocarbons and mobile bitumen, thus taking off the S1 and oil shouldering effects on the S2 emission peak on the pyrograms. The two resulting data sets were then compared, and showed a net reduction of TOC in addition to increasing in Tmax values for most of the post-extraction samples. Results of this detailed analysis yielded a new set of thermal maturity values. Furthermore, log-derived TOC calculations have been recalibrated resulting in a more accurate petrophysical model. These corrections generated more reliable results that greatly enhanced our understanding of basin maturity variations and organic matter distribution; thus decreasing uncertainty and refining in-place resource estimations across the basin.