Architecture, depositional pattern of syn rift sediments in the Northern Song Hong basin and its petroleum system association

Abstract

Rifting with syn rift sediments originally was formed during two tectonic phases in three stages. The syn rift deposits were composed of four units that have been identified by distinct seismic facies. The seismic expression of these syn rift units gives an idea about the linkage of their deposition with different stages of rift evolution. The lowermost units have wedge shaped reflection packages and hummocky internal reflection configuration, representing initial rifting in early rift stage. The overlying two units comprising divergent reflection, prograding pattern with aggradations on footwall represent climax rift stage and the topmost unit with sub-parallel reflection configuration represents the late phase. The units deposited during the rift climax stage have a good source rock potential, whereas the unit deposited in the late rift stage possesses favourable reservoir facies making a complete petroleum system within syn rift sediments.

Core data indicates the Late Oligocene deep lacustrine succession of mainly organic‐rich, world class oil prone source rocks interbedded with mudstones and sandstones. The pelagic deposition of mud and organic algae matters with excellent source rock characteristics was frequently interrupted by river‐fed mud flows, bringing mud and terrestrial organic matter to the lake bottom forming mudstones witha low source rock potential. Occasionally, low and high density turbidities, debris and hybrid flows interrupted mud deposition transport sands into the deep lake bottom forming potential carrier beds and reservoir sandstones.

The syn rift petroleum system association by predicting reservoir and source rock intervals are fundamental to exploration and can therefore help formulating a predictive exploration model of the Northern Song Hong basin.