Aeolian Dunes

Abstract

Technology Today Series articles provide useful summary informationon both classic and emerging concepts in petroleum engineering. Purpose:To provide the general reader with a basic understanding of a significantconcept, technique, or development within a specific area of technology. Aeolian dune deposits of windblown sands are important reservoirs in someparts of the world. The giant Groningen gas field in Holland and the Leman Bankgas field in the North Sea both contain Rotliegendes formation sandstones laiddown on land by winds. These types of deposits require large volumes of drysand and sufficient wind velocities to move the sand. Such conditions areusually found in desert areas (e.g., the Sahara Desert) and along coastalbeaches (e.g., Padre Island, TX). Crescent-shaped, or barchan, dunes are common(see Fig. 1). Other dune shapes are long, linear mounds, which were depositedeither transverse or longitudinal to the wind direction. The sands in the dunesare generally well sorted and range from fine to coarse sizes. However, thedeposit may also contain dry stream beds (wadis) or dry lakes where poorlysorted channels of sand and gravel are found or where thin layers of silts andclay were left. Dunes laid down on deserts may form regional sand sheetshundreds to more than 1,000 ft [300 m] thick covering hundreds of square miles. Those formed on coastal barrier islands may contain individual bars only a fewfeet thick and less than 100 ft [30 m] wide. Lateral and vertical continuity ofsands is generally good but is sometimes severely limited by postdepositionalcementation. Medium- to giant-scale cross-bedding with dips of 20 to 30 degrees[0.35 to 0.52 rad] is commonly observed (see Fig. 2). Horizontal laminae ofclay/silt deposits may be preserved as vertical barriers in some aeolianformations (see Fig. 3). Lateral communication may also be poor in somelow-permeability reservoirs, such as the Leman Bank, as evidenced by the muchlower permeabilities calculated from well tests than those from arithmeticaverages of core data. The geologic models for high- and low-quality aeoliandune reservoirs are shown in Figs. 4A and 4B, respectively. High-qualitydeposits have well-sorted to extremely well-sorted sands. They have widespreadlateral continuity with minor reductions in horizontal permeability resultingfrom cross-bedding. Vertical continuity may be good but occasionallyinterrupted by isolated, horizontal shale beds. Both continuity andpermeability can be poor in low-quality deposits. Development of production, and location and completion of wells in high-quality deposits, are relativelystraightforward. Because of the good continuity of sands, aquifer performanceand recoveries by natural waterdrive may be high. The lack of vertical barriersin high-quality deposits often requires that the wells be completed to minimizeconing of the invading fluid. In low-quality dune deposits where cementationand shale beds are frequent barriers, however, effective drainage of allregions and productivity can be severe problems. In these, an early integratedprogram of coring, logging, and well testing should be carried out indeveloping the field. Analysis of the data by teams of geophysicists, geologists, and engineers can improve understanding of reservoir continuity toaid in locating and completing additional wells and in evaluatingfluid-injection projects. After the field goes on production, surveillanceprojects. After the field goes on production, surveillance programs of periodicpressure surveys and well tests are especially programs of periodic pressuresurveys and well tests are especially important in providing data fordeveloping more-effective depletion plans for maintaining the desiredproduction rates and achieving the highest economical recovery. p. 11