Buried hill karst reservoirs and their controls on productivity

Abstract

Abstract The concept model of ancient buried hill karst reservoirs was built taking the NWKK carbonate rock oilfield in the South Turgay Basin in Kazakhstan as an example, and the characteristics of karst reservoirs and their controls on productivity were analyzed in vertical and horizontal directions. Vertically, the karst reservoirs can be subdivided into, from top to bottom, weathering crust zone, epikarst zone, caves zone, vadose zone and phreatic zone, in which the development of fractures is strengthened gradually; Six reservoir types, breccia pore, cave, vug, fracture-vug, fracture-pore and pore, were developed; Each karst zone has high oil production and low water cut in the initial stage because of the developed fractures and karst caves, but the production declines rapidly and the water cut rises fast. Horizontally, the karst reservoir characteristics are controlled by structure altitude and ancient lithofacies: the ancient buried hill with high structure altitude and loose lithology suffers from stronger weathering and leaching, has much more solution caves and holes, the main reservoir types are breccia pore, cave, and vug, while the ancient buried hill with low structure altitude and compacted lithology has highly developed fractures and stronger communication with bottom water, the main reservoir types are fracture-vug and fracture-pore, and the reservoirs have better productivity effect but rapid production declines and water cut rising. Different areas in plane should adopt different development techniques and policies.