A Case Study for Fractured Reservoir Detection Using Attributes Difference of near/Far Offset Stacked Seismic Data in Jz Oilfield
The lower reservoir of JZ oilfield is a buried hill of metamorphic rock of Archaean age (Ar) with the primary reservoir at depths from 1585-3735m. Both storage and permeability are contained along the natural fracture system. Porosity in that system varies up to 9%. This study describes fracture detection from analysis of 3D pre-stack seismic data from different offsets, coupled with existing geologic data and narrow azimuth seismic. Conducted fractures in the JZ oilfield are usually high angle. So, more fracture information is obtained from seismic data at far offsets than near offsets. Differences in anisotropy are attributed to the natural fracture system. The range of the offsets in the study area is between 176.05-3767.26m, which was divided into three intervals 225-800m, 801-1750m, and 1751-3750m. The intervals from 225-800m and 1751-3750m offsets were stacked separately as near offsets and far offsets.
Many seismic attributes show differences between near and far offset data, for example amplitude, frequency, total energy, P-wave/S-wave impedance and attenuation. Of those attributes we found that the high frequency attenuation gradient attribute is significantly larger in far offset stacks than in near offsets over known fractured intervals. These differences are not seen in non-fractured zones. The fractured intervals are determined from FMI and well performance data.
Based on this analysis, the highest fracture density is found from 10-80ms below the top of the buried hill, consistent with the results of FMI analysis for most wells where those data exist. A secondary reservoir zone is found near the top of the buried hill structure ranging from 0-¬10ms. Below 80ms, there appear to be no zones with reservoir potential.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009