Fitting Sumandak Stratigraphy into Sabah Regional Chronostratigraphic Framework

Abstract

Sumandak cluster fields is located in the Samarang Asam Paya PSC within Sub-Block 6s – 12/ 18 of Block SB310 (Figure 1) and operated by Petronas Carigali Sdn. Bhd. The success story begins with the discovery of Sumandak Main by Sumandak-1 well drilled in September 2001. Seven more exploration wells were drilled on the same play in the area between 2001 until 2003, which resulted to the discoveries of Sumandak Tepi and Sumandak Tengah. To date, twenty-eight (28) development wells have been drilled on Sumandak area and the fields are currently on production. In order to further explore the hydrocarbon potential of this area, a regional study with sequence stratigraphic approach was carried out in 2008. The main objective of this study is to generate stratigraphic framework of Sumandak that can be correlated to the Sabah regional chronostratigraphic framework (figure 2). The generated stratigraphic framework will help to facilitate interpretation in the Block SB310 and surrounding areas. In addition, the study was also aimed to identify any upside hydrocarbon potential for further exploration. The approach adopted in this study was based on Exxon’s techniques (Van Wagoner et. al. 1990) which defined Sequence Boundary (SB) as a product of relative falls in sea level. Seismic data and well data (logs, cores & biostratigraphic data were used to identify major bounding surfaces in order to establish a framework in which genetically related facies can be studied and a realistic depositional model can be constructed. Sequence stratigraphic interpretation such as identification of sequence boundaries, maximum flooding surfaces, reflector terminations (onlap, downlap, toplap and truncation) were done on hardcopy of several selected key seismic lines prior to extend the interpretation to the rest of the available seismic data. The tectonic setting and basin evolution of the Sabah Basin is very much related to the closing of the proto-South China Sea/ Rajang Sea. The opening of the South China Sea since Oligocene causing microcontinents of Dangerous Grounds and Reed Bank to drift and collide with Sabah margin. Active tectonic plate movements throughout Eocene and Miocene have resulted in the development of different provinces across Sabah Basin hence creating the Inboard Belt and East Baram Delta where the study area is located. Sumandak Field is located within a series of progradational deltaic system where rapid sedimentation was observed forming the topset, foreset and bottomset facies. In each successive deltaic system, the basin depocenter moved further offshore to the northwest. The study has recognized fifteen sequence boundaries in Sumandak area of which eight are the existing SBs based on previous interpretations and another seven are new SBs introduced in this study (Table 1 & Figure 3). All identified sequences are categorized as the 3rd order sequence and regionally correlatable with Kinabalu, Trusmadi, Glayzer and Labuan-Paisley Syncline areas. This paper shall discuss the result of the study, which is the refinement of previous interpretation on sequence boundaries in this area. Using various data for integration, the study has established a new stratigraphic framework for Sumandak and the correlation of Sumandak sequence stratigraphy with the Sabah Regional Chronostratigraphic Framework. The upside potential for further exploration in this area shall also be highlighted and discussed.