Development characteristics and oil-gas geological significance of slope break zone of Xujiahe Formation in Sichuan Basin

Abstract

Previous studies have delved into slope break zones in faulted basins, yet foreland basins, with their gentle slopes toward the forebulge, underdeveloped large-scale structures, and small terrain undulations, have received limited research on slope-fault belts. During the deposition periods of the 4th and 5th members of the Late Triassic Xujiahe Formation, a foreland basin developed in the Sichuan Basin. Scholars have noted the control exerted by the gentle slope break zone on the sedimentary lithofacies in the central and western Sichuan, particularly in the west of the Luzhou-Kaijiang Paleo-uplift, but related studies remain limited. By interpreting the tectonic slope break zones in seismic reflection profiles of the Sichuan Basin and incorporating existing regional geological data, this study employed particle flow numerical simulation to clarify the types of slope break zones in the central and western Sichuan regions, as well as their relationship with gravity sliding structures. It also analyzed the characte-ristics of new types of oil and gas traps associated with gravity-sliding structures. The findings are as follows: (1) The study area developed a fault slope break zone, with each side bounded by a "front squeezing and rear extension" tectonic combinations formed by gravity sliding. The tectonic deformation is weaker than those of gravity-sliding structures at passive continental margins. (2) The edge fault of the tectonic slope break zone controls the sedimentary facies and sand body types. (3) In the gentle slope zone, the stretching area of gravity-sliding structures lead to the uplift of the footwalls of normal faults, forming syndepositional anticlines with developed sandstones. These sandstones, juxtaposed with organic-rich mudstones within the contemporaneous fault depression, form "side-generated and lateral storage" hydrocarbon reservoirs. (4) In the steep slope zone, gravity sliding forms syndepositional anticlines in the 5th member of the Xujiahe Formation, which are bounded by flank mudstones and post-sliding deposited mudstones, forming in-source sand body reservoirs. (5) Syndepositional anticline hydrocarbon reservoirs related to gravity-sliding structures represent a new type of in-source hydrocarbon accumulation. The 5th member of the Xujiahe Formation in the study area likely contains such reservoirs, distributed in a NE-trending pattern parallel to the slope break zone.

Keywords

• gravity-sliding structures
• tectonic slope break zones
• in-source reservoir
• xujiahe formation
• upper triassic
• sichuan basin