Executive Summary The objective of this study is to compare the performances of Underbalanced Coiled Tubing Drilling (UBCTD) versus conventional drilling with stimulation in a Permian carbonate target within a field in Saudi Arabia. The focus of this study is to ultimately investigate the effectiveness of biosteering in UBCTD compared to geosteering and subsequent stimulation in conventional drilling of vertical and horizontal wells within the same target. Biosteering is an application of biostratigraphy used to navigate and maintain well paths within the desired zone. This technique has been employed successfully in UBCTD wells in the Middle East. Biosteerers utilize the characteristic distribution of microfossil assemblages obtained from cuttings samples with the identification of biozones based on existing schemes. The most effective way to geosteer these well is to use this technique in conjunction with lithology descriptions, GR log trends, visible porosity and drilling parameters. UBCTD wells are not stimulated as there is minimal induced formation damage during drilling operations. Geosteering utilizes logging while drilling (LWD) in horizontal wells for lateral placement and to determine perforation intervals. Conventional wells in this study are all stimulated, employing multistage acid fracturing techniques. The dataset in this study comprises 50 wells, all of which are compared by normalized post-completion performance. Post-completion performance from the 50 well dataset was normalized to achieve a representative comparison. UBCTD biosteered wells show indicative superiority when compared to horizontal and vertical conventionally drilled and stimulated wells. In addition to the minimal formation damage in underbalanced wells, footage in the target is higher in biosteered UBCTD wells compared to geosteering. This is due to several reason with the main reason being is the ability of UBCTD to drill multiple laterals in each well, thus maximizing target contact. A detailed look into two wells targeting the same formation and field proved effective UBCTD utilization, yielding a production increase of several orders of magnitude. Despite these results, the normalized production performance may not account for significant LWD merits. In terms of steering, UBCTD slim-hole nature limits applications of available LWD technologies that provide essential information for reservoir characterization and development plans.