Greater Sirikit East (GSE) Field represents recent step-out exploration success in the east of Sirikit Main Field. Sirikit Main was discovered in 1981 and remains the largest field in the basin, with surrounding smaller fields: Sirikit West (northwest, 1983), Thap Raet (north, 1988), Sirikit East (northeast, 1992) and Nong Jig (southwest, 1999). GSE comprises of multiple blocks defined by structural and stratigraphic closures. It shares common reservoir and source rock with Sirikit Main. Some of the trap types and seals are also common. However, some trap types and seal elements are not typical in Sirikit Main. More detailed subsurface evaluations were conducted to explore all the working petroleum systems. This is the key and challenge to the GSE that had been overlooked in the past due to the absence of considerable structural closure in the key horizon maps. Aggressive step-out exploration in GSE started in 2006. More than 12 exploration/delineation wells have been drilled to date testing multiple blocks and plays. The exploration concepts have been continuously developed by taking the feedback from the well post drill evaluation. The new developed or refined concepts were then applied to the subsequent exploration/delineation drilling. This cycle allows better risk mitigation as well as more accurate well targeting. To minimize exploration cost, time and risk, following strategies have been adopted whenever feasible: To use existing drilling surface location, to combine multiple targets and to provide back-up side-track target. Utilizing existing surface location made significant time and cost saving by avoiding costly land acquisition, access road building and time-consuming environmental impact assessment. Continuous exploration and delineation drilling in 5 years has changed the GSE area. The previously overlooked large area that looks like a monocline in main marker maps turned out to hold multiple hydrocarbon accumulations involving both stratigraphic and structural traps. Introduction Greater Sirikit East Field is located just to the east of the Sirikit Field in the S1 concession, Phitsanulok Basin (Figure 1), Central Plain Onshore Thailand, approximately 400 kilometers north of Bangkok. The concession is 100% owned and operated by PTT Exploration and Production. Sirikit Field was discovered in 1981 and remains the largest field in the basin. Up to 2000, 5 adjacent smaller fields have been discovered. Sirikit West Field in the northwest was discovered in 1983. Thap Raet Field in the north, discovered in 1988, Sirikit East in the Northeast, discovered in 1992, Nong Jig oil in the southwest, discovered in 1999, and Nong Pluang gas in the south-southwest, discovered in 2000. The first-discovered Sirikit Field is normally called as Sirikit Main Field in order to distinguish it from the neighboring fields bearing 'Sirikit' name.

Greater Sirikit East oil and gas field is located just to the east of the Sirikit Main field in the S1 concession, Phitsanulok Basin, Thailand. The main reservoirs are fluvio-deltaic Lan Krabu formation members of K, L and M that interfinger with the open lacustrine Chumsaeng formation. Hydrocarbon traps in the field can be grouped into structural and stratigraphic traps. Numerous small structural closures have been proven to be hydrocarbon bearing. Delineation and development well drillings have also proven the working stratigraphic trap system in the absence of structural trap. In some structural closure, observed hydrocarbon column heights from well data exceed their relevant structural spill point, invoking the larger working stratigraphic trap system responsible for the hydrocarbon accumulation.Various examples of proven stratigraphic traps in the field will be presented in this paper. Structural maps, well correlation, pore-pressure plot, production data and existing internal studies on sequence stratigraphy and reservoir facies were incorporated in this evaluation. Reservoir characterization from seismic data is not considered feasible due to resolution limit. Most of stratigraphic traps are within the distal sub-members of Lan Krabu Formation such as M, L2, K4, K2 and K1 which are dominated by mouthbar facies. The trapping is formed by combination of deposition and structure geometries. The structure is East-Northeast dipping, while the depositional direction is from the Northwest to Southeast direction. A trap system is hence formed where reservoir sand pinch-out to the southeast direction that is structurally up-dip, bounded by north-south trending fault in the west.

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Central Faulted Region (CFR), East Flank Region (EFR), and Greater Sirikit East (GSE) are the main production areas in the PTTEP S1 Consession, part of Sirikit Field, the biggest onshore oil field in Thailand. The Lan Krabu Formation is the biggest contributor for hydrocarbon production, defined as a thick fluvio-lacustrine deltaic sequence consisting of thin alternations of mouthbar deposits, channels and Chum Saeng lacustrine claystone. The CFR, EFR and GSE regions are structurally complex, each of which is separated by west dipping, basement rooted main fault. The secondary faults are east dipping antithetic and west dipping synthetic faults which run almost parallel to the main fault. For modelling purposes, detail geological correlation across CFR, EFR and GSE has been established. The correlation utilizes the sequence stratigraphy concept by correlating the flooding surface of the fifth order sequence. The shale interval associated with fifth order flooding surface may inhibit vertical pressure communication or vertical oil movement during production time frame. Given the highly heterogeneous reservoirs in fluvio deltaic environment together with complex faulting system, it is very challenging to build a geological model that represents the detailed geological features of these areas. Selecting the 3D modelling tools is essensial to generate a model with complicated geological features, in a way that reservoir simulation can be performed in an efficient manner. This paper will share challenges prior to and during construction of the 3D geocellular model, as well as various modelling approaches that have been generated with multi-scenarios and multi-realizations tasks. It also demonstrates how pixel, object and multi-points statistics (MPS) based approaches in facies modelling along with the application of continuous and discrete N/G provide alternative input for the reservoir modelling. The result of this study will be used for flow performance evaluation and further dynamic modelling. Introduction Sirikit Field is located in Phitsanulok Basin, Central Plain, onshore Thailand, approximately 400 km north of Bangkok (Figure 1). The Field is the country's largerst onshore oil field located in the S1 concession, currently operated and wholly owned by PTT Exploration and Production. The Field was discovered in 1981 by LKU-A01 well drilled by TSEP (Thai Shell Exploration and Production). In effort to increase the oil recovery, Waterflood has been implemented within the Sirikit Area. The East Flank Region was the first region to be waterflooded starting in 1995. The EFR was selected for a waterflood candidate based on the fact that it had the least amount of faulting among the regions. Water injection in Central Faulted Region just started end of 2010. The Greater Sirikit East is a relatively new area, discovered in 2006, and it is still producing under primary recovery.