STP-EPA-0106 contains the Wallal Graben which is interpreted to be over 5.5 km deep in this area. Proven, organic-rich Ordovician source rocks have been penetrated by numerous nearby wells to the east and are interpreted to have been deposited within the Wallal Graben. The possible presence of these source rocks, numerous potential reservoirs coupled with a mid-basin ridge evident on the seismic data make STP-EPA-0106 a prospective block.
The northern extent of the Samphire Graben is also located within the eastern area of STP-EPA-0106. This graben is not as deep as the Wallal Graben at this location but the proposed work program will target this area to determine its prospectivity.
The dominant northwest/southeast structural grain of this area has been set up by extensional tectonics in the Ordovician. The Ordovician extension initiated the growth of a major rotational fault that forms the south-west margin of the Wallal Graben with down to northeast rotation. 2D seismic data interpretation clearly defines a “mid-basin ridge” structure that consists of a tilted fault block from Basement to late Ordovician levels and associated anticlinal drape of the overlying Devonian and Permian sediments.
This structure (Prospect A) extends for in excess of 20 km on the interpreted seismic data with its northern extent located in STP-EPA-0106. With increased data acquisition, this structural trend may reappear in the northern section of the Wallal Graben within STP-EPA-0106.
Potential exists for a “roll-over” structure along the main basin margin fault system and also a sub-crop play along the eastern margin of the half graben. These constitute higher risk plays but will comprehensively interpreted and evaluated.
The Goldwyer Formation unconventional play may exist within STP-EPA-0106. This target is interpreted to be favourably located in the oil/condensate maturity window and is within easy drilling depth (2000-3000m). Horizontal wells with multi-stage fracture stimulation programs may enable the economic extraction of hydrocarbons from this interval.
There are no satisfactory well ties for accurate correlation of stratigraphic units into the Wallal Graben. Jump correlations with regional seismic data infer that a near complete Ordovician sequence is preserved including a thick and deeply buried Goldwyer Formation section (primary source rock).
It is likely that subsidence along the downthrown side of the Wallal Fault enabled a localised marine incursion during the Ordovician to occur within STP-EPA-0106. As open ocean is inferred to have lain to the northwest at this time, it is most likely that this restricted lacustrine/lagoon system extends along the length of this half graben facilitating the deposition of high quality source rocks.
Due to the confined nature of the depositional environment, it is postulated that the source rocks within the Wallal Graben may be richer than those deposited in the other areas of the Canning Basin where increased oceanic circulation and less anoxic conditions prevailed.
The Goldwyer Formation interpreted within the Wallal Graben is modelled to be thermally mature for hydrocarbon generation and is favourably located within the oil/condensate maturity window (figure to the right).
It is both the conventional and unconventional plays within STP-EPA-0106 that constitute its prospectivity.
Identified analogues include the highly successful East Africa Rift plays and the Barmer Basin in Rajasthan, India.