IDENTIFICATION OF SHALLOW GAS POTENTIAL BY SEISMIC ATTRIBUTE AND AMPLITUDE VERSUS OFFSET (AVO). A CASE STUDY: TUNU, MAHAKAM BLOCK AREA

摘要

Tunu is a giant field delivering about 1000 MMScfd of gas and 18.000 bpd of condensate with about 80 wells drilled per year and almost 20 years of production from the Main Zone (2.500 — 4.500 TVDSS). Shallow gas is defined as biogenic gas which may be encountered at depths close to the surface and a good quality of gas with >98% content of CH4, low S and CO2 content. The presence of shallow gas comprises almost 20% of the entire natural gas resources in Indonesia. In an article "Total E&P Temukan Lapangan Gas Raksasa Tunu di Mahakam" that was posted by Fachry Latif at MigasReview.com on March 6, 2015, the natural gas resources potential discovered at the giant field of Northwest Tunu 3D (NWT 3D) at the Mahakam Block amount to about 700 million cubic-feet of gas. This potential has not been explored in a large-scale and sustainable manner. This paper focuses on seismic methods both for attribute and AVO to identify shallow gas resources. Seismic attribute and Amplitude versus offset (AVO) are an advanced geophysical analysis method carried out to confirm the presence of gas. From both analysis, we can distinguish between coal and gas from the shallow zone. The specific analysis used are attribute amplitude and AVO classified by intercept with gradient curve. The result is Tunu has a gas reservoir which is indicated by low impedance and low reflection coefficient. Shallow gas in Tunu is classified by class III (gas sand) in the group of AVO class. It is characterized by a drastic increase in amplitude towards a trough parallel with increasing in offset (angle). Furthermore, it is a zone characterized by bright spot because acoustic impedance is decreasing which shows in seismic data. Bright spot is one of the direct indicators to identify a presence of hydrocarbons. So, shallow gas identification is not only for hazard mitigation when performing drilling activities but also for calculating economic reserves of shallow gas as a source of unconventional energy.