A Forward Stratigraphic Modelling Approach to Determine the Evolution of an Oligocene Syn-Rift Sequence in West Arthit Area, Gulf of Thailand

Abstract

The successful discovery of petroleum exploration primarily depends on the understanding of the basin evolution and sedimentary filling though geological time. Well data also play a key role for reservoir presence and quality analysis; however, none of well fully penetrated the Oligocene Syn-rift sequence in the West Arthit area. Therefore, this study aims to overcome the challenge of limited well information by performing the Forward Stratigraphic Modeling (FSM) to determine basin evolution, depositional setting, and reservoir distribution in this area. The FSM model is constructed with the inputs of paleo-bathymetry, subsidence, sediment supply, water level, and climatic cycle. In addition, the stratigraphic sequence is reproduced based on field observations such as rock samples, seismic mapping, well-log responses, and publications from nearby areas. The main uncertainty of building the FSM model is the initial age of rifting phase due to a lack of well penetration that fully covered the Syn-rift sequence and the limited biostratigraphic data. Therefore, two different age scenarios are examined in this study analogue from the age model as it was published in the Malay Basin locating to the south of study area. Once the FSM model was built, the last step was to calibrate the prediction result with the actual well result and the conventional seismic data to achieve the best accuracy and to increase the confidence on using the model. The FSM model was successfully reproduced the stratigraphic successions of the Syn-rift sequence in West Arthit area. The base case model was chosen from the age scenario of 27.0-23.1 Ma which exhibited four major cyclicities and matched with seismic mapping. The study area had two depocenters, one in the northwest and another one in the southeast. The northern sub-basin was deepened earlier during the first rifting phase whereas the southern sub-basin was subsided later after the second rifting period. With the increase in sedimentation rate and subsidence rate during the third rifting phase, both depocenters were shallowed up and then become a shallow lake covering the whole study area. The last lifting phase coincided with the thermal subsidence that occurred and affected across the region; therefore, the regional extensive lacustrine accumulated in the study area. The results from this study provided a crucial information on petroleum system especially depositional architecture, reservoir distribution, and potential source rock identification, which were incorporated into the planning of future exploration targeting in this field. This study demonstrates the new innovative approach to determine the basin evolution and to understand the variation on depositional setting in the study area with limited well data. This approach also creates the project value by supporting the planning of future exploration and development wells. Furthermore, this technique can be applied to all projects to increase the discovery rate and to add the field reserves.