Stratigraphic Traps in Distal Lacustrine Delta, a Case History from Greater Sirikit East Field, Phitsanulok Basin, Central Plain, Thailand

Abstract

Abstract 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. The Lan Krabu M reservoir in Z6 North b lock of Greater Sirikit East area was the first proven stratigraphic trap in the field by deepening the LKU-X11 well. In fact, the stratigraphic trapping in Lan Krabu M has been strongly indicated in Sirikit Main field where well data demonstrated the limited extent of M reservoirs in the northern part of the Sirikit Main Area. However, while the structural closure is present in Sirikit Main field, the oil trapped in M reservoir in the Greater Sirikit East field is in the absence of any structural closure. In the same Z6 North block, by drilling the LKU-Z03 well, Lan Krabu K1 sub member was the second proven stratigraphic trap in the Greater Sirikit East, with similar concept and configuration to the deeper M reservoir. Initial evaluation on the well result based on wireline log and pressure showed marginal oil accumulation in the reservoir. However, two years production data have changed the conclusion. Pinch-out feature in L2 member can be seen on seismic data and hence mapped in the Z6 Central area. A few delineation and development wells were drilled targeting this reservoir. Well correlation, pressure plot and production data analysis provide substantial support to the stratigraphic trap model in L2 member. The K2 sub-member oil and gas bearing reservoir extend from Z6 Central to Z6 South area, probably bounded by volcanic material in the south as represented in the existing 3D seismic data. Opposed to L2 sub-member, no stratigraphic feature can be directly inferred from seismic data in the K2 interval.