Multi-layered ordovician paleokarst reservoir detection and spatial delineation: A case study in the Tahe Oilfield, Tarim Basin, Western China

Paleokarst systems are major carbonate reservoirs that can form significant and very large oilfields. The Ordovician carbonate reservoirs of the Tahe Oilfield represent a special type of hydrocarbon reservoir consisting primarily of paleokarst caves and fractures. Detailed characterization of paleokarst reservoirs in the Tahe area is challenging because of their heterogeneous distribution and, in particular, because these reservoirs are at depths exceeding 5500 m. This paper describes a detailed approach for integrating data from cores, imaging and conventional well logs, and high-quality seismic datasets to delineate the complex paleokarst systems in the Tahe Oilfield, particularly the paleocaves in the run-off zone. Based on the hydrologic genetic relationships between the karst and paleocaves observed in outcrops, we divided the paleocaves in the run-off zone into chambers, main channels, and branch channels. The chambers, which are generally taller than 15 m, are generally collapsed. The main channels, mostly 5–15 m tall, were the main passages for underground water flow and display good connectivity. The branch channels, typically less than 5 m tall, are relatively dispersed and still have a considerable volume. This method was applied to the area around Well T615, and two layers of paleocaves in the run-off zone were interpreted. Seismic mapping and three-dimensional modeling of the paleocaves were performed, and the lengths, widths, areas and volumes of these three types of paleocaves were statistically analyzed. Although most of the chambers have been penetrated by existing wells, the areas where the main channels and a dense network of branch channels developed are favorable targets for future drilling in the Tahe Oilfield. Our method of detecting and analyzing the paleokarst reservoir structure is applicable to similar paleokarst reservoirs.