The Stybarrow Oil Field is located in Block WA-255-P offshore Western Australia, in an average water depth of 800m. This paper is a work-in-progress case-study of the process of discovery and appraisal of the Stybarrow field, and the influence on appraisal and development decisions of integrating the various types of geological, geophysical, and engineering data and techniques available to the modern oil company and their partners.The influence of a number of data types on appraisal decision making and development planning is discussed. Elaboration is also made on the impact of near-field exploration potential, and the possible influence of this potential on Stybarrow development planning is also discussed.

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Introduction The Zamzama gas field is located in the Kirthar Foldbelt, Sindh Province, Pakistan approximately 200 km north of Karachi and importantly, only 8km west of the existing Sui-Karachi pipeline (Figure 1). Since discovery in early 1998, a number of innovative approaches to field appraisal, gas marketing, infrastructure rationalisation and field development have been undertaken. This has resulted in the successful completion of the initial development phase, Phase 1, with the Zamzama gas plant coming on stream in July 2003. In addition, plans are well progressed towards realising an incremental Phase 2 development. Cornerstone to the success of development to date has been the implementation of an extended well test (EWT) of the exploration (Zamzama-1/ST1) and appraisal well (Zamzama-2), which came on line in early 2001. The EWT mitigated an entire host of perceived risks associated with a full development, with minimal financial exposure. This paper will examine:The evolution of the initial appraisal programme and its unfolding to dramatically narrow gas reserve uncertainty prior to significant production.The results of the successful Phase 1 development drilling programme and associated further field appraisal.Data collection and analysis and how this has been leveraged. Structural And Geologic Overview The Zamzama structure comprises a large north-south orientated, eastward verging thrusted anticline (Figure 2). The Late Cretaceous Pab sandstone forms the primary hydrocarbon reservoir in the Kirthar Foldbelt and the main reservoir in both the Zamzama and nearby Bhit gas field (Figure 3). At Zamzama, gas is also reservoired in sandstones of the overlying Khadro Formation of Paleocene age. Both the Pab and Khadro reservoirs are of relatively uniform thickness across the entire Zamzama structure. The Khadro formation averages 54m in thickness and varies by less than +/− 3m over an area of more than 150 km2. The Pab formation averages 216m in thickness and where fully penetrated varies by +/− 5m. The Pab Formation conformably overlies the Fort Munro Limestone, which is non-net at Zamzama. Top seal for both the Khadro and Pab reservoirs is provided by marine shales of the Girdo (Ranikot) Formation (Figure 4). The Pab is part of a sand-rich fluvio-deltaic, coastal plain and shoreface depositional system (Figure 5). The Khadro is a more heterogeneous reservoir and of poorer quality compared to the Pab due to the presence of volcaniclastics. The Khadro sands were deposited in a marginal marine-estuarine environment following the deposition of red-brown fluvial floodplain mudstones, which comprise the lower two thirds of the Khadro. The majority of the calculated gas volume (80%) is contained within the core area of the hangingwall (Figure 6). The remainder of the volume is distributed equally between the northern and footwall regions. Wells drilled in a more crestal location (Zamzama-2, −4/ST3 and −5) encounter gas on rock where the top Fort Munro, non-net limestone, is above the gas-water contact (GWC). The Pab formation is distributed extensively throughout the region and outcrops to the south of Zamzama in the Laki ranges. A strong active aquifer is expected throughout production life.

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