At 7+ km depths in the Tarim Basin, hydrocarbon reservoirs in Ordovician rocks of the Yijianfang Formation contain large cavities (∼10 m or more), vugs, fractures, and porous fault rocks. Although some Yijianfang Formation outcrops contain shallow (formed near surface) paleokarst features, cores from the Halahatang oilfield lack penetrative paleokarst evidence. Outcrop paleokarst cavities and opening-mode fractures are mostly mineral filled but some show evidence of secondary dissolution and fault rocks are locally highly (∼30%) porous. Cores contain textural evidence of repeated formation of dissolution cavities and subsequent filling by cement. Calcite isotopic analyses indicate depths between ∼220-2000 m. Correlation of core and image logs show abundant cement-filled vugs associated with decameter-scale fractured zones with open cavities that host hydrocarbons. A Sm-Nd isochron age of 400 ± 37 Ma for fracture-filling fluorite indicates cavities in core formed and were partially cemented prior to the Carboniferous, predating Permian oil emplacement. Repeated creation and filling of vugs, timing constrains, and the association of vugs with large cavities suggests dissolution related to fractures and faults. In the current high-strain-rate regime, corroborated by velocity gradient tensor analysis of GPS data, rapid horizontal extension could promote porous and/or solution-enlarged fault rock, fractures, and cavities to connect.
Thematic collection: This article is part of the The Geology of Fractured Reservoirs collection available at: https://www.lyellcollection.org/cc/the-geology-of-fractured-reservoirs