Re- Processing of 2D Seismic Line APH-434- The Alpha Basin, Offshore, New Zealand

Abstract

A set of 2-D seismic reflection data from New Zealand was used as a part of this seismic processing procedure. It was known that, at this region a set of petroliferous reservoir situated in an anastomosing faults were present. The objective of this seismic processing is to create an image of the substructure beneath a fault zone known as the Alpha fault zone. The processing involves in removing refractions and direct arrivals, applying true amplitude recovery; correction to account for seismic energy decay and applying a noise filter to remove the random noise recorded whilst data acquisition. At the preliminary stage, a filtered unstacked seismic data was obtained. It is clear that more advanced processing of the seismic data needs to be done to improve the signal-to-noise ratio of the seismic data. In the final processing sequence (migration) an accurate velocity field is obtained for generating the subsurface image. Dip Move out (DMO) and post-stack time migration are the processes which were applied to the APH-434 dataset. A processing workflow has been outlined in this report for future testing and signal analysis of the seismic data. (Keywords:2-D seismic reflection data,seismic processing, signal-to-noise, Alpha fault) I. Introduction The area of study is located in the Alpha basin, the coast of New Zealand as shown in Figure 1. The seismic data was used for investigation of hydrocarbons by the New Zealand Oil and Gas Company (NZOG). The 2D Seismic dataset (Line APH-434) was acquired by Norpac International crew. The dataset was acquired from west to east to cover the main fault zone which is called the Alpha fault. The exploration concepts in the basin relate to an anticlinal-structural trap and a tilting fault block formed during the thrusting period. An overview of the geology, the data processing strategy and the summary of seismic dataset are given in the subsequent sections. Geological Overview The Alpha basin is located in the offshore south western part of the North Island, New Zealand. The eastern part of the basin is bounded by the Alpha fault zone which is a part of the large NE-SW thrust fault system (Holt, 2004) as shown in Figure 1. The potential of hydrocarbon accumulations was investigated in the basin by using several types of data such as seismic data, gravity data and well data. Moreover, the evolution of the Alpha basin was studied to understand tectonics history that might increase possibility of success. The main tectonic events started from the Late Cretaceous to the Paleocene that associated to the extensional regime (Nicol et al., 2004), the Alpha fault was also active during this period. In the early Miocene, the maximum regional subsidence stage occurred. The subsidence extended eastward and seawater occupied the western part of the basin to a depth of 1-1.5 km. In the eastern part, rock in the thrust zone was compressed and exposed to the surface as a result of erosion. In addition, the thickness of the Australian crust increased approximately 1.5 km in the subduction zone, this thickening was associated with topographic build up. Therefore, the thrust front was heavily eroded in the areas that were above sea level. For this reason, large amount of eroded sediment was transported westward to the foreland basin. Fault displacements resulted in westward motion and pushed the eastern hangingwall block overly on the western footwall from the Pliocene to present time as shown in Figure 2