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This talk presents a geophysical evaluation of an offshore Miocene Prospect in the Taranaki Basin, New Zealand. The relevant regional geology, stratigraphy and paleogeography are presented as a backdrop to the geophysical mapping and analysis of the prospect within the basin. The 2D seismic mapping is discussed and AVO analysis presented which specifically addresses issues of areal extent of the prospect, hydrocarbon presence and trapping mechanisms. This talk demonstrates the challenge of defining these play elements using multiple seismic techniques in a high potential/ high reward environment.
The prospect lies south of the giant Maui Field (gas) and west of the recently developed Kupe South Field (oil and gas) in water depths ranging from 30 to >120 m. A total 137 2D seismic lines, covering 3200 km, were integrated into the seismic interpretation and mapping.
There are no wells drilled on the prospect currently but the seismic grid ties to a well drilled in 1990 south of the prospect. The well encountered fairly clean very-fine-to fine-grained sandstone beds up to 37 m in thickness as defined by sidewall core analysis and sample cuttings. The sandstone is an informal stratigraphic unit which occurs in the middle part of the Middle Miocene Manganui Formation. (Morrison and Rivers, 1991) It is this well and the extensive seismic database that is key to mapping and defining the prospect.
Wells and seismic from the Maari oil field were used in the AVO analysis as a template for the prospect. The Maari Field oil production is also from a Miocene aged formation with similar thicknesses and petrophysical properties as expected in this prospect.
Method and Theory:
A key south-north seismic line tying the control well to the prospect as well as a key west-east dip seismic line through the prospect will be shown.
The synthetic tie to the south-north seismic line forms the basis for the interpretation of the horizons over the seismic dataset. The key horizons of interest were the SS Top and Base and Top Canyon 1 and 2 surfaces. The synthetic tie was used as a basis for phase and bulk shift corrections where necessary to the remainder of the seismic lines in the database. The data was fair to good quality data and had been recently reworked in 2009.
Maps will be discussed that show the closure of the Miocene aged sands against large scale erosional features present in the basin, at variable times throughout the stratigraphic column. The concept of vertical seal is addressed and demonstrated with well control.
The challenge with a stratigraphic play is that the trapping mechanism is more difficult and subtle to define as compared to structural prospects. Historically in the basin, most of the significant hydrocarbon discoveries to date have been defined by structural traps. The use of seismic AVO (Amplitude Versus Offset) analysis is therefore a key tool used to provide a direct indication of hydrocarbon presence and type in the prospect.
Some of the examples shown are results of the AVO analysis within the proposed trap versus outside the trap, at a consistent stratigraphic level. This analysis supports the premise that there is a gas cap present in the trapped portion of the reservoir. Likewise, fluid substitution modelling using well control from the offsetting Maari Oil Field is also is used to support the presence of gas in the trapped updip portion of the reservoir.
The lateral seal for the sandstone is defined by the presence of the cross-cutting Canyon 1 sequence, which has been mapped extensively with a dense grid of 2D seismic over the prospect. Due to this large 2D seismic grid, the areal extent of the prospect is definable.
In order for this to be an effective lateral seal the canyon sediments would have to be filled with relatively impermeable strata. As there is no well control on the license block, there is a risk that the Canyon 1 sequence would also contain some sands and silts which would provide an inadequate lateral seal.
The risk of an inadequate lateral seal was tested by the AVO analysis performed on the seismic lines over the prospect. It was concluded that the AVO anomally present on the prospect is probably caused by a gas leg just below the Base Canyon-1 unconformity. Furthermore, the lack of an AVO anomaly below the apparent gas cap neither confirms nor denies the possibility of an oil leg.
King, P.R., and G.P. Thrasher, 1996. Cretaceous-Cenozoic geology and petroleum
systems of the Taranaki Basin, New Zealand. Institute of Geological and Nuclear
Sciences Monograph 13, 243 p..
Morrison, W.K., and T.L. Rivers, 1991. Final Geological Report, Motueka-1, PPL 38406. Report submitted to New Zealand
Kathleen is a Professional Geophysicist and a member of the CSEG, SEG, CSUR and APEGA.
Kathleen has over 25 yrs experience as a Geophysicist in major operating companies such as Texaco and Conoco, and as an independent geophysical consultant for many junior oil and gas companies. She has been Chief Geophysicist for the Petrel Robertson Consulting Ltd for the last 3 years focusing Canadian and International geophysical projects. She is a past member of the CSEG Executive, served as Session Chair for the 2013 Geoconvention on Unconventional Geophysical applications, Past SEG Convention Technical Program editor, Past Editor of the Canadian Heavy Oil Association Handbook and currently a Mentor for the CSEG Foundation Mentorship Program.