Characteristics of the S80 strike-slip fault zone and its controlling effects on the Ordovician reservoirs in the Tahe oilfield, Tarim Basin

A fine interpretation of the S80 strike-slip fault zone in the Tahe area of Tarim Basin is carried out by comprehensively utilizing core data, logs, the widths and total volume of the lost intervals in individual wells, highprecision 3D seismic data, and multilayer coherence attributes. Its spatial distribution, and activity intensity and stages are determined thereby, and its controlling effects on the Ordovician reservoirs are explored. The findings indicate that the S80 strike-slip fault zone can be divided into the western, central, and eastern subzones along its strike. These three subzones generally show a noticeable increase in the number, length, and width of overlapping segments from the Cambrian to the Ordovician strata vertically. Laterally, the S80 strike-slip fault zone, from SW to NE, exhibits the contraction of its transtensional segments, the expansion of its transpressional segments, and the gradual disappearance of its pure strike-slip segment. Furthermore, the activity of this fault zone proves strong in its central subzone but weak in its western and eastern subzones. The S80 strike-slip fault zone experienced four activity stages, namely the Middle Caledonian, the Late Caledonian-Early Hercynian, the Late Hercynian, and the Indosinian-Early Himalayan, with the former two stages predominating. This fault zone experienced sinistral strike-slip in the former two stages but dextral in the latter two stages. Three types of reservoirs are developed along the S80 strike-slip fault zone: cavernous, compound(fractured-vuggy and vuggy-fractured types), and fractured types. The development of dissolution vugs is closely related to the segmented activity of strike-slip faults, with the transpressional and translational segments featuring strong activity, as well as the periphery of the major faults of the transtensional segment characterized by weak activity, serving as favorable parts for the development of dissolution vugs. The transpressional segment of the Middle Ordovician Yijianfang Formation exhibits a high linear density of fractures, while those of the Middle Ordovician Yijianfang Formation to the Middle-Lower Ordovician Yingshan Formation show a significantly decreased linear density of fractures vertically, leading to a limited dissolution capacity. The transtensional segment in the Middle Ordovician Yijianfang Formation has a moderate linear density of fractures, and those of the Middle Ordovician Yijianfang Formation to the Middle-Lower Ordovician Yingshan Formation exhibit well-developed fractures and dissolution vugs with an elevated number and scale, establishing this interval as a favorable area for reservoir development.