Speaker
Description
Abstract: The Permian Gufeng Formation shales in the northern Sichuan Basin exhibit extremely high organic matter abundance, demonstrating significant potential for shale gas exploration. However, the development of organic pores in this unit is remarkably limited. This paradox of "high abundance but low porosity" severely constrains resource evaluation and exploration breakthroughs. Focusing on the calcareous shales of the Gufeng Formation in northern Sichuan, this study integrates multidisciplinary approaches—including sedimentology, organic/inorganic geochemistry, scanning electron microscopy (SEM), nitrogen adsorption, and ReaxFF reactive molecular dynamics simulations—to systematically reveal the mechanisms governing exceptional organic matter enrichment and the controlling factors behind the differential development of organic pores. The main conclusions are as follows:
1. Organic matter enrichment is synergistically controlled by preservation conditions and paleoproductivity. During the transgressive phase, preservation driven by anoxic conditions was dominant. During the maximum flooding period, the highest Total Organic Carbon (TOC) values (up to 34.70%) were achieved through the synergy of sulfidic environments and high productivity driven by intense upwelling. In contrast, the regressive phase was primarily constrained by increased terrigenous input and weakly oxidizing conditions.
2. Biological precursor type dictates the potential for organic pore development. Organic matter derived from benthic algae possesses complex biological structures that can be inherited to form abundant slit-shaped pores (30–150 nm). Conversely, planktonic algae, with their simpler structures, exhibit poor pore development (<50 nm). Given the relatively high proportion of planktonic algae (>30%) in the Gufeng Formation, its overall potential for organic pore development is inherently low.
3. Cementation and bitumen infilling destroy primary pore space. During early diagenesis, micritic calcite precipitated within kerogen-hosted pores, reducing meso- to macropore volumes by over 60%. Upon entering the oil window, generated bitumen further infilled micropores, resulting in an additional 7–12% loss in porosity. This complex diagenetic modification substantially offset the pore volume advantages expected from the high organic matter abundance.
4. Mineral–organic reactions regulate pyrolysis pathways and pore generation efficiency. Clay minerals promote deep cracking of organic matter and the generation of gaseous hydrocarbons via an "adsorption–catalysis" mechanism, thereby facilitating pore development. In contrast, carbonate minerals stabilize intermediate products and scavenge hydrogen atoms to produce H₂O rather than hydrocarbons through a "complexation–hydrogen scavenging" mechanism, thus suppressing gas generation. The extremely high carbonate mineral content in the Gufeng Formation induced an "oil-rich but gas-poor" pyrolysis pathway, significantly reducing pore generation efficiency.
This study establishes a pore evolution model for calcareous shales that couples depositional en-vironments, biological precursor composition, diagenetic modification, and mineral–organic reactions. This model systematically explains the genetic paradox of low porosity in high-TOC calcareous shales, offering a new theoretical framework for predicting pore structure evolution and fluid storage capacity in carbonate-rich source rocks.
Key words： Gufeng Formation, Organic matter enrichment, Organic pore evolution, Mineral–organic reactions, Shale gas.
| Country | China |
|---|---|
| Acceptance of the Terms & Conditions | Click here to agree |
