Filling patterns and reservoir property of the Ordovician buried-river karst caves in the Tabei area, Tarim Basin

An integration of outcrop observations, as well as data from drilling, logging, and seismic surveys in an oilfield is applied to analyze the filling types and filling cycle assemblages of karst caves associated with paleokarst buried rivers; accordingly, the filling sequences and patterns of the paleokarst buried rivers, as well as the discussion on their petroleum geological implications. The results show that the Ordovician buried-river karst caves with a filling rate of 89.9% in the Tahe oilfield, are predominantly filled with sandy mudstones and collapse breccias. These karst caves host multiple combination cycles featuring coarse-grained lower parts and fine-grained upper parts, which can be classified into polycyclic sedimentary assemblages and polycyclic collapse-sedimentary assemblages for filling. The former is distributed in the karst slope’s lower reaches of flat landform, where wells with lost circulation and stringers account for small and high proportions, respectively. In contrast, the latter is situated in the karst slope’s upper reaches featuring landform of great drops, where wells with lost circulation are of high proportion together with multiple high-yielding wells. The following conclusions can be reached through analysis: (1) The tortuous spatial structure of buried rivers, combined with their strong runoff transport capacity, facilitate the filling of large amounts of karst detrital materials, resulting in an extremely high filling rate; (2) The seasonal fluctuations in the phreatic surface lead to the formation of cyclic and comparable fillings. This, coupled with water erosion and tectonic activities, gives rise to multi-phase collapses of karst caves. Consequently, polycyclic collapse-sedimentary filling assemblages are formed in the upper reaches, with unfilled spaces developed; (3) The relatively closed underground environment supersaturated with calcium carbonate, results in severe calcareous cementation of fillings, decreasing the intergranular porosity; (4) The unfilled spaces serve as the major targets with potential for oil and gas exploitation.