Complex genesis of multiperiod fractures in the Middle Triassic Leikoupo Formation in the Pengzhou gas field, western Sichuan Basin, China

The cumulative expression of multistage deformation is complex multiperiod fractures, which are commonly seen in tectonic zones. The Middle Triassic Leikoupo Formation in the western Sichuan Basin Depression, China, is a typical marine carbonate reservoir with natural fractures caused mainly by tec- tonic movements. According to outcrops, drill cores, image logging, and fluid inclusions, the fracture characteristics, types of natural fractures, and interactions of fractures are determined. In total, 419 natural fractures in 493.2 m of cores from 7 wells are investigated, which are mainly shear and tensile fractures with a small number of weathering generated fractures. Meanwhile, the results of the stable isotope analysis of center dot 513C and center dot 518O, as well as the flow fluid inclusion data, reveal four tectonic periods of fractures with different occurrences. Based on the history of regional tectonic evolution, indicating one period of weathering fractures ascribable to stratal uplift and three periods of structural fractures related to the sequential tectonic movements of the Longmenshan fault belt.By analyzing the interaction re- lationships of fractures, three types of fracture interaction relationships are observed: cutting, restraining, and overlapping. The four stages fractures are chronologically assigned to (1) the early Indosinian N-S trending compression, (2) the late Indosinian NW-SE compression, (3) the middle Yan- shanian NE-SE compression, and (4) the early Himalayan E-W compression. The influence of natural fractures on gas migration and well production in marine carbonates is discussed, and indicates that tectonic fractures could provide seepage channels for gas migration and accumulation from near or distant hydrocarbon source rocks into the Middle Triassic Leikoupo Formation. This study utilizes a pragmatic approach for understanding the fracture genesis mechanism in oil and gas field with multi- period fractures.(c) 2022 The Authors. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

Automated summary

This study investigates the complex multiperiod fractures in the Middle Triassic Leikoupo Formation in the western Sichuan Basin Depression. The characteristics and types of natural fractures, as well as their interactions, are determined. The study also identifies four tectonic periods of fractures and discusses the influence of natural fractures on gas migration and well production.