What are in pyrolysis S1 peak and what are missed? Petroleum compositional characteristics revealed from programed pyrolysis and implications for shale oil mobility and resource potential

Abstract

This article presents a novel numerical decomposition method based on generation kinetics for studying oil composition characteristics from S1 peak of single heating rate pyrolysis of source rock samples. Furthermore, phase behavior and PVT analysis were introduced in the calculation to make a full correction for evaporative loss of gaseous and light hydrocarbon in S1 during coring and sample storage. Special programed pyrolysis results of twenty-eight core samples, along with a routine source rock screening pyrolysis dataset of 68 samples, from a recent shale oil study well of the Eocene Qianjiang Formation in Jianghan Basin, China were analyzed to demonstrate the application of proposed methods. We showed that (a) by reassembling corresponding activation energies, the original S1 curve can be split into groups of similar thermal evaporative property, resulting in a composition classification of free hydrocarbons in S1 when it was converted back to temperature domain; (b) a full correction on the gaseous and light hydrocarbon loss in S1 permits a more precise estimation of oil composition as well as total resource; (c) the resulting oil composition grouping provides information for inferring the fraction of extractable hydrocarbon; d) oil composition varies considerably among the samples within a stratigraphic interval of 20 m. Samples from intra salt organic-rich lamina show different compositional characteristics with much less abundance in gaseous and volatile oil compounds; while samples from inter salt interval contain large portion of volatile and light oil, indicating a promising production zone.