Classification and evaluation of shale oil enrichment: lower third member of Shahejie Formation, Zhanhua Sag, Eastern China

Abstract

As an important unconventional resource, shale oil plays differ from conventional reservoirs in accumulation and enrichment mechanisms of hydrocarbons. The lower third member of the Shahejie (Es3l) Formation in the Zhanhua Sag, Bohai Bay Basin is a typical continental shale oil which is a good candidate for researchers to understand enrichment mechanisms. Therefore, we collected a large number of samples from a single well that was drilled through the Es3l member to analyze mineralogy, geochemical parameters by programmed pyrolysis, thermal maturity by vitrinite reflectance (Ro%), pore structure by N2 adsorption and mercury intrusion capillary pressure (MICP). First, following core observations, four different lithofacies were recognized. Second, based on the S1 vs. TOC plot, we proposed five separate types of resource: enriched and (ir) recoverable resource, potential and (ir) recoverable resource, and finally ineffective resource. The quality of the resource deteriorates sequentially with the enriched and recoverable being the best and ineffective resource the worst. We concluded that the overall oil-bearing property of the Es3l is affected by both hydrocarbon generation and oil retention characteristics. Ultimately, high abundance of OM, kerogen type and thermal maturity, along with the degree of oil-enriched pores abundance will decide the most favorable conditions for shale oil accumulation/enrichment. Considering all of these, the lower sections of the ORBMS–ORBCS (organic-rich bedded mixed/calcareous shale) and OFLCS (organic-fair laminated calcareous shale) facies were found to own the optimal combination of hydrocarbon generation and reservoir property for economical exploitation. We believe, the proposed methodology can be used for continental shale oil assessment around the globe.