Abstract:

The burial age of cave fills can provide valuable insights into the formation of ancient caves and the cave filling order and cave reservoir development. The Ordovician palaeokarst caves in the Tahe oilfield are significantly impacted by cave filling, but the burial age of cave fills and its implication for the hydrocarbon reservoir development in the oil field have not been systematically analyzed. Here we present findings on the cave deposits based on analyses of the geologic setting of the oil field as well as core, well-logging, burial history and productivity data. We found the layered, gray-green calcareous sandstone and argillaceous siltstone were deposited via transport filling in the Early Hercynian and fracture-associated, gray-green argillaceous siltstones via seepage filling in the same era; whilst well-rounded/sorted, multicolor conglomerate fills, with complex rock composition and sandy matrix, were deposited in the Late Caledonian by transport filling. Meanwhile, poorly-sorted collapse breccias, chemically homogeneous, mixed with grey-green mudstone via seepage filling, were deposited in the Early Hercynian during early burial; whilst fractured, calcite/oil-bearing collapse breccias, mainly on cave roof, without seepage filling, were deposited in the late burial stage. The reservoir controlling factors of fracture/cavity reservoirs of karst caves were very complex, where cave fills deposited in the early burial stage via transport, seepage and collapse filling were mostly mudstones, with poor reservoir quality; whereas secondary fractures formed during late burial from collapse settlement without mud fill greatly improved reservoir quality, and the upper/top parts of karst caves were favorable for reservoir development. This research provided a valuable reference for the understanding of karst cave formation and exploration of high-quality hydrocarbon reservoirs.

Key words: palaeo-karst caves, fracture-cave reservoirs, filling ages, collapsed fillings, Ordovician, Tahe oilfield