Interpreting Geological Systems from Seismic Attributes: Reasons fo Caution from the Northern Malay Basin

摘要

Seismic attributes are routinely used to interpret depositional patterns, particularly where channel systems form the main reservoir sands. This paper presents an example of an apparently clearly imaged channel-like feature, the size of which did not match that implied by pressure transient analysis. The anomaly in question is a relatively straight, sharp-edged, channel-like feature interpreted at a TWT depth greater than 2 seconds. An appraisal well was drilled through the feature, encountering a gas-charged sand at the interpreted level in the well. A drill-stem test (DST) was con ducted, and gas was produced to surface in a sustainable manner. Based o n the combi nation of seismic attributes and well data, the sa nd body was geologically interpreted to be a relatively clean, tidally reworked sand deposited in a linear incised valley, with thickness-width ratio of around 1:3 0. This ratio is considered narrow for the depositional setting, but i s within published ranges for channel systems. When the DST results were subjected to Pressure Transient Analysis (PTA) and dynamic modeling, the technique implied that the sand had a drainage radius several times that indicated by the seismic attribute. Subsequent re- interpretation of the seismic an omaly has led to t he conclusion that the seismi c attribute did not image the reservoir sand. Rather, the attribute i s now thought to indicate an erosional cut through a coal bed, which by coincidence overlies a gas-filled sand. The discrepancy between the seismi c interpretation and t he well test resul t has implica tions for volumetri c estimates. Long-term production testing is therefore being considered in order to better-define the lateral extent of the reservoir. In the m eantime, a high level of caution will be applied to the interpretation of seismic imag es from deeper reservoir intervals.