IMPACT OF STRUCTURE EVOLUTION AND KARSTIFICATION PROCESS IN KOKAS FIELD, BINTUNI BASIN WEST PAPUA PROVINCE

• Main Author: Nauw, Marlon
• Format: Theses
• Language: Indonesia
• Subjects: Geologi, hidrologi & meteorologi
• Online Access: https://digilib.itb.ac.id/gdl/view/55382
• Institution: Institut Teknologi Bandung

The Kokas field is located in Bintuni Basin West Papua Province. The areal extent is 6x6 km. The basin is bounded by the Sorong Fault in the north, the Terera Aiduna Fault in the south, the Misool-Onim-Kumawa ridge (MOK) in the west and the Lenguru Fold Belt (LFB) in the east. To develop the field, a detailed risk assessment is required. This study is part of that and addresses the risk for well placement locations. The focus of this research is the influence of the eocene-oligocene carbonate and process of karstification. This carbonate is commonly identified as the Faumai Formation. The Faumai Formation is known as a karstified interval based on well drilling activity both in Kokas field and offset fields. Drilling risks associated with karstification include stuck pipe, stuck bottom hole assembly (BHA), downhole problems such losses, high vibration and weight on bit drop (WOB). These risks will impact the cost of well operations, that’s why it is important to mitigate earlier. The methodology of this research is based on literature, past drilling activity reports, seismic interpretation, 1D burial history, restoration, reconstruction of carbonate evolution and analogue data from the nearest field. To the understand completed geological settings of Faumai Formation the work is extended beyond the Faumai Formation to the interval below Kopai Formation and above the carbonate Sele Formation. Drilling result in research location shows total loses and WOB drop. Based on well K-2 this drilling problem due to well location at fault zone. However, in well K-1 shows total loses but not related with fault zone but from seismic response show in negative seismic reflector or trough. There is uncertainty about the quality of the seismic data. The event could be due to sub-seismic fault or karstified zone. The X-1 well, which is 50km from the Kokas field, was studied to understand age and rock. Petrography data from X-1 well shows porosity 10-13%, diagenesis processes with replacement of cement carbonate with quartz overgrowth, have grain and more porous. Based on this data, the assumption of research location has similar property of Faumai Formation with X-1 well. When fault reactive during plio-pleistosen will increase fracture zona and erosion due to good rock property. Seismic interpretation results show that older faults stop at the Top Kopai Formation, but after the late oligocene strike slip system was reactivated some of older faults continue to Faumai Formation. Late oligosen fault generated strike slip faults in the Faumai Formation. The type of these faults are positive flower structures and negative flower structures. The structural evolution of the Faumai Formation is controlled by two major tectonic events. The first event is strike slip at late oligocene due to movement of pacific plate in North of Papua and second event is plio-pleistocene uplift because of shortening at LFB. These two major events generated a high amount of erosion at the Kokas Field. The first erosion occurred at the late oligocene and the second at the plio-pleistocene, for a total erosion after integration of seismic restoration and 1D model is ~4363 ft/~1330 m. Karstification at the Kokas Field due to structure uplift during late oligocene and plio-pleistocen. This uplift processes due to tectonic activity as a resulted Faumai Formation uplift near surface and connect with surface water so get eroded. Erosion process for karstification is vertical erosion. Karstification processes in Kokas Field is control by tectonic as uplift and another factor such patch reef topography, carbonate thickness ~5000 ft, erosion and rock properties more porous. Result interpretation shows karstification at Kokas Field distributed at faults zone and there are karstification not at fault zone this possibly due to stratigraphy or fault and fracture but bellow seismic resolution or sub-seismic. Response seismic shows cave in negative seismic reflector or seismic trough and isolated.