Systematic application of seismic amplitude analysis for exploration in the Upper Cibulakan Formation, example from gas discovery Les-1 Offshore Northwest Java

Abstract

Abstract The Main and Massive members of the Miocene Upper Cibulakan Formation are the primary reservoirs within ARCO Indonesia's Offshore Northwest Java (ONWJ) Production Sharing Contract (PSC) area. Approximately 75% of the known oil and gas reserves are reservoired in these shallow (800 m to 2000 m subsea) marine sandstones. Although ARCO has applied seismic amplitude analysis (bright spot detection) for at least 15 years, it is only since the widespread utilization of workstation applications that these anomalies have become easier to detect, evaluate and analyze. The Upper Cibulakan exploration strategy utilized an integrated approach to identify, evaluate and analyze seismic amplitude anomalies. The approach began with the screening of all available seismic data for amplitude anomalies, and analysis of undrilled anomalies for amplitude strength, phase, AVO (Amplitude Versus Offset) response and structural configuration. In addition, part of the approach was to statistically analyze previously drilled amplitude anomalies and to construct a database relating the anomalies to the presence of gas. A review of 65 amplitude anomalies from 39 wells indicates that bright amplitude anomalies are statistically associated with gas accumulations. This approach has created a positive impact on evaluating the Upper Cibulakan potential in terms of defining new prospects with amplitude anomalies and also in highgrading existing structural prospects. The LES-1 gas discovery, the first test of this approach, resulted in a cumulative flow of 62.2 MMCFGPD and 1,229 BCPD from 5 tested zones. Favourable bright spots identified in the LES prospect were confirmed (by drilling LES-1) to be gas sand responses. The success of the LES-1 well has proven, at least initially, the validity of this approach as it pertains to ONWJ as well as its potential for application in other areas with shallow sandstone reservoirs.