Significance of pyrobitumen in the evaluation of gas generation from oil cracking in the Sichuan Basin, SW China: A review

Pyrobitumens, abundant in the Sichuan Basin, SW China, serve as indicators of past oil reservoirs. Currently, estimates of gas generation during oil cracking rely solely on pyrobitumen content with a uniform conversion rate. This study delves into reservoir fluid heterogeneity influenced by factors such as gravitational segregation, deasphaltene processes, biodegradation, and thermochemical sulfate reduction. It recommends a comprehensive geological analysis of pyrobitumens for precise estimation. The research underscores oil heterogeneity within the reservoir, with light oil prevalent in the upper column and heavy oil accumulating at the base. Oil cracking initiates in polar-enriched zones due to deep burial, high temperatures, and low activation energy requirement, leading to concentrated pyrobitumen zones at the column base and/or in highly permeable zones. Pyrobitumens are primarily found in dissolution pores and fractures, aiding in identifying their precursors through optical structure and elemental analysis. Methane production per unit weight of pyrobitumen is lower in pyrobitumen concentrated zones, typically at the column base, compared to dispersed pyrobitumen in the upper oil column. Relying solely on residual pyrobitumen content and a single conversion ratio for oil cracking formed methane estimation may result in misleading interpretations. The study proposes a mass balance calculation method, incorporating the atomic H/C ratio, to examine the gas yield-pyrobitumen relationship. Insights gained from the Sichuan Basin can offer valuable information for resource and environmental impact assessments in other basins with thermally cracked oils.