ABSTRACT: Biosteering in Saudi Arabia is successful in achieving greater contacts with the Khuff C Member pay zones. The procedure supports Underbalanced Coiled Tube Drilling (UBCTD) and has been applied since 2008 to improve gas and condensate recovery from development wells in the Upper Permian Khuff C carbonate of Saudi Arabia. Multiple laterals are drilled by re-entry of the mother-bore with the objective of maintaining the well-path trajectory within selected porous layers. This method of drilling permits the drill string to re-enter the mother-bore of older development wells and new infill wells. The drill commences a lateral path through a window milled in the mother-bore casing and has the advantage of rapid adjustment to the inclination and trajectory as well as enabling continued gas and condensate production while drilling. As wireline logging during slim-bore, coiled-tubing drilling is confined to the gamma tool, which follows the bit by approximately 30 ft, the only "real-time" source of stratigraphic control is provided by micropaleontological analysis, up to two hours ahead of the gamma data.Controlled drilling prohibits the use of rate of penetration (ROP) as an accessory stratigraphic guide. For instance, the use of water as a drilling medium enables cuttings to be received with a lag time of 10 to 20 minutes from 12,000ft, depending on the degree of gas contribution. By comparing the micropaleontology of cuttings samples with those recorded in a cored offset well, the stratigraphic position can be determined within 2 ft vertical thickness. Caving is minimal due to the sliding and non-rotational drilling method. This information enables the well-path trajectory to be monitored, as it approaches the planned layer, and real-time instructions provided to the directional driller whenever deviation from the targeted porosity layer is detected. This process is termed biosteering. The Upper Permian microfossils are typically less than 0.5mmin size and can be well-preserved in small cuttings samples. In addition, themorphology of foraminifera, calcareous algae, bryozoa and other microfossils are sufficiently varied to enable confident species identification in randomly oriented thin-sections. Although of predominantly shallow marine origin, Khuff C depositional environments were highly varied leading to a succession of vertical microfacies variations that provide calibration with biofacies analyzed from cuttings in the "laterals". As the Khuff C environment also varied across the region, no single microfacies scheme can be applied to all wells in the development area and necessitated establishment of a local reference zonation for each biosteered well. Such a local reference section is based on high-resolution micropaleontological and petrographic analysis of cored wells located as close as possible to the intended lateral trajectory. Knowledge of the micropaleontological diversity, abundance and good preservation in all post-Permian carbonate reservoirs provides the potential for biosteering in similar reservoirs in the Middle East and circum-Tethyan regions.