Bedding-parallel fractures in ultradeep tight sandstone reservoirs in Jurassic and Cretaceous of Yongjin Oil Field, Junggar Basin, China

Abstract

Bedding-parallel fractures are fractures that are parallel to rock bedding structure planes and have been widely accepted as a key factor for oil and gas production in tight sandstone and shale reservoirs. However, the formation mechanisms of these parallel-bedding fractures are still under debate. In this study, bedding-parallel fractures in Yongjin Oil Field were analyzed using methods including core and microscopic observations, element geochemistry, and carbon and oxygen isotope analysis. Their origin and relations to reservoir quality, faults, and rock mechanical properties were examined. The discovery of bedding-parallel fractures in both the Upper Jurassic and Lower Cretaceous formations indicates that the BPFs are generated later than the early Cretaceous. The filling state of bedding-parallel fractures that with no bitumen and carbonate cement indicate that they formed after oil charging and carbonate cementation. The tensile fracture characteristics in core and thin section observations, and the fact that overburden stress exceeds the pore pressure indicate that the bedding-parallel fractures were neither generated from tectonic compression nor overpressure. The most likely generation mechanism is stress relief during core drilling under high in situ stress conditions. High in situ stress and low tensile strength lead to thinner fracture spacing. The existence of high bedding-parallel fracture density is an indicator of good reservoir quality and result in high oil/gas production.

Keywords

Bedding-parallel fractures / ultradeep reservoirs / tight sandstone / fracture origin / Junggar Basin

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