Nonconventional Medium-Wavelength Static-Correction Method to Overcome Shallow Gas Anomaly Problem

Abstract

Sisi and Nubi are two gas fields operated by Total E&P Indonésie, located 20 km offshore from the Mahakam Delta (East Kalimantan). In order to design the future field development, a 1000 km_ 3D seismic data set was acquired during 2003 that covered the field area and beyond. There are two main challenges in processing this data:Push-down effects. This problem is well known in the Mahakam Delta area and is associated with shallow gas anomalies characterized by very low seismic velocities.Amplitude distortion. Shallow gas anomalies also act as amplitude masks Total E&P Indonésie solved the problem using non conventional medium wavelength static corrections. The processing method was established using WesternGeco technology in cooperation with Total expertise. Introduction Sisi and Nubi are two gas fields operated by Total E&P Indonésie and located 20 km offshore from the Mahakam Delta, where water depth ranges from 60 to 100 m. Sisi and Nubi cover areas of respectively 58 km_ and 78 km_ (Fig.1) and are scheduled to be put into production by 2007. In order to optimize the development, 3D seismic was acquired and processed during 2003–2004 with the objective to perform a complex seismic characterization of reservoirs. This goal requires maximizing the seismic data quality in an area where shallow gas anomalies severely distort the seismic signal. Shallow gas anomalies affect the seismic data in three ways: push down effect, image defocusing and amplitude masking. Total E&P Indonésie in collaboration with WesternGeco addressed this challenging problem using non conventional short and medium wave static corrections. The Sisi Nubi Field Generalities Fig. 1 Sisi Nubi Field Generalities Since their discoveries, in 1986 and 1992 for Sisi and Nubi respectively, several wells have been drilled. The reservoir zone is Upper Miocene in age and includes channels, stacked distal mouth bars, and deltaic low stand wedges. The average thickness of elementary reservoir units is around 10 m for channels and 1 m for bars. In the shallower pay interval, Fresh Water Sands formation, the trap is a fault controlled structural closure. Hydrocarbons are trapped along the up thrown compartment of faulted blocks. In the deeper pay zones, Sisi Main Zone, trapping is stratigraphic and associated with the eastward shaling out of the reservoir (Fig.2). First gas production from the Sisi-Nubi Field is scheduled for 2007. Objective of 3D acquisition and technical challenge During 2003 a 1000 km_ 3D seismic data has been acquired to cover both the fields and surrounding areas, with the objective to be able to perform reservoir characterization in the Fresh Water Sand interval.