Abstract

This paper is a summary of ongoing work to establish the impact of karst on optimizing oil production from carbonate reservoirs. The paper is based on a review of 44 Middle East carbonate reservoirs and is intended to convey preliminary findings. Key observations from this review are that karst reservoirs in the Middle East form one end-member of a continuum that includes, a) conventional layered reservoirs, b) fractured reservoirs, and c) karst reservoirs. It is also evident that many Middle East reservoirs are hybrid reservoirs having static and dynamic characteristics of all three end-members. In such instances the key challenge is to differentiate any karst components that may impact on achieving optimum oil production. Attempting to differentiate the karst contribution is complicated by the frequent occurrence of permeable tectonic fractures and faults. These lineaments contribute to permeability and production characteristics of many of the studied reservoirs, but because their characteristic flow properties are similar to that produced by karst drainage networks, karst drainage has the potential to be unrecognized. In an attempt to identify and isolate karst drainage system contribution to production, a first pass 'quick look' method has been developed by which carbonate reservoirs/fields can be ranked with the objective of assessing the degree that karst influences are present. The quick look method is intended to identify karst products, or the potential for karst products to be present, from which steps can then be undertaken to establish wheter a karst drainage system impacts optimum oil productivity.

What are karst reservoirs?

The term karst has been applied in a wide variety of conflicting and confusing ways across numerous technical disciplines. Karst has been used to designate both specific landforms (including subterranean landforms) and the geographic regions characterized by those landforms (Pers. comm. M. Esteban 2004). Esteban and Klappa (1983) considered karst to be a diagenetic facies developed as an overprint on subaerially exposed carbonate bodies, whereas Choquette and Pray (1971) equate the term karst to the development of caverns, channels and other specific nonfabric selective porosity. Karst can be associated with porosity/permeability enhancement and also destruction through the processes of carbonate dissolution and carbonate precipitation (Tucker & Wright 1990). The term “karst reservoirs” has therefore the potential to take on a wide range of meanings. For the bulk of industry workers karst reservoirs are those reservoirs that have permeability and or storage properties developed through the combined action of subaerial exposure and meteroric waters. However, such a definition is of limited practical value when considering reservoir characterization from the perspective of optimizing oil production. The reasoning behind such a statement is that karst reservoirs can result from a multitude of processes, not just meteoric dissolution, and distinguishing which process (if any) predominates is difficult and in many occasions not possible. A definition for karst reservoirs which focuses on the flow potential, rather than the process, of karst formation has, therefore, practical benefit. One such definition can be drawn from two key hydrogeological and geomorphological principles (Pers. comm. M. Esteban 2004). These principles are based on work by Mangin (1975, 1978), Bakalowicz (1978) and Ford (1978) and provide a useful starting point for considering karst and its impact on optimizing oil production.