Charge modeling in Fractured Basement. Application to the Otway Basin, Australia

Dubois, Agnes¹; Lefranc, Marie¹; Schenk, Oliver²
¹DCS, Schlumberger, Kuala Lumpur, Malaysia.
²SIS, Schlumberger, Aachen, Germany.

Fractured basement, as a play, has been proven in several Asian countries (Vietnam, China, Indonesia, Australia…). Fractured basement reservoirs are usually expressed by a basement high or uplift and are found beneath a major regional unconformity. Those unconformities act as fluid career bed and express the erosion and weathering phenomenon that has affected the basement. In those type of unconventional reservoirs, hydrocarbon is stored in the fractured network or in a weathered zone rather than in the matrix porosity. Fractured basement can be charged from overlying source rock expelling hydrocarbon during compaction or from topographically lower source rocks that are laterally connected to the basement (directly or via a career bed).

Exploration for fractured basement is focusing mostly on the existence, or not, of fractures in the basement. Little attention is paid to the charge modeling aspect and to the understanding of how the potential fractured network could have been filled with hydrocarbon. This point is however key.

The present paper investigates charge mechanism and timing in a fractured basement. It takes the Sawpit-1 well (Otway basin, Australia) as an example of fractured basement. The Otway basin is associated with rifting and breakup of the Australian and Antarctic plates. It consists in Jurassic to Pliocene sediments. Its thermal history is fairly well constrained at the basin scale. The Sawpit-1 well was drilled in a tilted basement bloc and encountered oil trapped in fractures that have developed within meta-volcanics rocks. Geochemical analyses have been carried out on in this well. The source rock has been identified as being the Casterton Formation which structural position is lower than the Sawpit-1 basement block. Geomechanical forward modeling of the seismic interpretation is carried out in order to constrain the exact timing of the basement deformation and to capture the stress and strain associated with tectonic deformation. Then a basin analysis study is carried out to understand fluids maturation, expulsion and migration in the basin.

This integrated structural and charge modeling analysis is believed to strongly reduce the risk of drilling future exploration wells in fractured basement.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012