Controls and Geological Significance of Macerals in Hybrid Shales: A Case Study on the Gaoyou Sag, Subei Basin, East China.

Abstract

In the Gaoyou Sag located within the Subei Basin, the hybrid shales from the second member of the Funing Formation (E1f2) have been identified as a prolific source of shale oil production, despite their characteristically low organic matter content (TOC < 1.5%). This observation suggests that specific macerals within these hybrid shales demonstrate a pronounced hydrocarbon generation potential, thus unveiling a new frontier for shale oil exploration endeavors. In this study, 16 samples were rigorously extracted from both mudstone and hybrid shale strata within the E1f2. An exhaustive suite of organic and inorganic geochemical analyses was conducted on these specimens. The analyses elucidated several key findings: (1) The maceral composition within the hybrid shales is predominantly comprised of alginite, solid bitumen, and inertinite. Remarkably, the variability of alginite content within the hybrid shales is more pronounced than that observed in high-TOC mudstones. High-TOC mudstones are characterized by a preponderance of lamalginite and a paucity of telalginite, leading to a diminished aggregate hydrocarbon potential. (2) Biomarker ratios (e.g., sterane/hopane, C23 tricyclic terpane/αβ C30 hopane, C24 tetracyclic terpane/C26 tricyclic terpane, etc.) suggest a primary derivation from lower aquatic organisms, with a secondary contribution from terrigenous organic matter within the hybrid shales. (3) The accretion of macerals is governed by an intricate set of factors, including the input of terrigenous detritus, paleosalinity, and paleoproductivity. (4) Alginite is identified as the principal constituent responsible for hydrocarbon genesis in hybrid shales. The proliferation of alginite facilitates the concurrent enrichment of shale oil and organic matter within the hybrid shales of the Subei Basin, illustrating a cooperative mechanism underlying the accumulation of shale oil and organic matter. This indicates that even hybrid shales with scant organic content exhibit considerable potential for shale oil exploration.