Analysing On The Petroleum Conditions And Reservoir Rules Of The NW Sabah Basin

NW Sabah Basin, which lies on the north of Borneo, is a foreland basin because of Nan Sha block’s subduction toward the Kalimantan and collision between the two area from Oligocene to Lower Miocene. Structure characteristic in different tectonic zone of Sabah Basin are analysised in this paper, using the basic geology, oil and gas geology and hydrocarbon accumulation analysis as the main line, and the petroleum geology is also researched. We summarize oil-gas reservoir-forming rule and hydrocarbon pooling condition, propose the model of hydrocarbon reservoir forming and the controlling factor for the hydrocarbon pooling and then forecast the potential petroleum exploration area.The basin can be divided into 7 secondary tectonic areas on the basis of different structure styles in different zone. They are Crocker Fold-Thrust Belt(CFTB), Inboard Belt, Outboard Belt, East Baram Delta, Thrusting zone, Sabah through and NW Sabah platform. Among of these areas, the former three zones are the important petrolum exploration targets.In summary, the basin experienced three tectonic evolution processes. They are Rifting, Compressing and Depressing stages. Besides, some authors divided basin’s sedimentary stratigraphy into four units: Stages I,Ⅱ,ⅢandⅣ. The former three units deposited in Rifting stage; StagesⅣA -C formed in compressing stage; the rests formed in depressing stage. The coaly and carbonaceous shales in the forth unit are the main source rock and all of these are dominated by a type III kerogen assemblage excepting that occurring in the Outboard Belt area where type of kerogen indicates the presence of a mixed type III/II assemblage. The sandstone and carbonate in the stageⅣare the main exploring target. The original tectonic and palaeomorphology controls the distribution of source rock and reservoir in this basin. The reservoir was nearby to source rock, sandstone units crossed in vertical and horizontal direction and the trap developed in several periods. Besides, there is no rigional seal for it.The hydrocarbon, coming from the source rock in StageⅣ, migrates along the fault, unconformity surface and the connected sand body into the trap forming all kinds of pooling. They include stratigraphic and structural reservoirs. The former formed by the superimposed sandstone units, stratigraphic overlap and pinch-out, the later formed by the high-relief anticlines often cored by shale diapirs, collapse structure and rollover anticline. Besides, different trap locates in different tectonic zone.The basin’s future exploration potential zone will mainly be located in the deepwater setting of the Outboard Belt and Thrust Sheet Zone, which developed turbidite fan. Besides, the remained potential in the Inboard Belt will be focused on subtle stratigraphic trap.