Pore Evolution Mechanisms in the Longmaxi Formation Shale Reservoir of the Anchang Syncline under Basin-Mountain Tectonic Evolution.

Abstract

The pore structure of shale defines the space available for gas generation and storage, with the evolution of these pores being closely linked to shale gas accumulation and migration. Investigating the evolution of shale pores under basin-orogenic tectonic settings is crucial for under-standing the variations in resource potential across different blocks of the Anchang Syncline. This study focuses on the Longmaxi Formation shale within the Anchang Syncline, systematically examining the differences in the lateral pore structure evolution across three distinct blocks during basin-mountain tectonic development. By integrating reservoir property characterization with burial-hydrocarbon generation modeling, and employing experimental techniques including X-ray diffraction (XRD), low-pressure nitrogen adsorption (LN₂GA), and low-pressure carbon dioxide adsorption (CO₂GA), this research reveals the pore evolution mechanisms in contemporaneous shale strata during tectonic uplift from the perspectives of multifractal theory and controlling factor correlation analysis. The results indicate that (1) Pore characteristics differ significantly among the three blocks. Samples from block A (e.g., MAY2) are predominantly microporous, while samples from block C (e.g., MAD4) exhibit a higher proportion of mesopores (2-50 nm). Samples from block B (e.g., MAD3) show an intermediate pore structure. (2) Despite similar thermal maturity evolution levels and TOC contents across the blocks, as well as comparable burial and hydrocarbon generation histories, the primary factors controlling the pore differences are the magnitude of latestage tectonic uplift, the duration of thermal maturity evolution, and preservation conditions. (3) Analysis of fractal dimensions suggests that mineral composition promotes the complexity of microporous structures while inhibiting the complexity of mesoporous structures. (4) Based on the interblock variations, two distinct evolutionary models are established: model 1 ("rapid evolutionsustained overpressureweak uplift modification") features a shorter maturity evolution duration, longer overpressure preservation, and minor late-stage uplift; model 2 ("slow evolutiontransient overpressurestrong uplift modification") is characterized by a longer maturity evolution duration, shorter overpressure preservation, and significant latestage uplift.