Inclusive reservoir characterization from rock physics and sequence stratigraphic analyses of Middle Miocene, Niger delta

Abstract

The estimation of seismic reservoir properties faces difficulties such as delineating deceptive fluid contacts, over-estimating reserves and drilling dry holes. To overcome these problems, sequence stratigraphic and rock physics analyses that were constrained by the compaction trend were implemented in the Sigma Field. Sigma field is located in the Coastal Swamp depobelt, in the distal part. Seismic data from the field and a total of three wells, Sigma-30, 26, and 25 were provided for the analyses. Biostratigraphic data was also given for the field and dated to shale marker Cassidulina-7. The stratigraphic sequence analysis of the wells in field indicate they were deposited in a regressive, transgressive and aggradational episodes, whereas the entire depositional history shows predominantly regressive episode. Three field reservoirs, SM-14 (LST), SM-04 (TST), and SM-02 (HST) were delineated. Depositional analysis of the landscape from sequence stratigraphic analyses of the field indicates that Sigma field reservoirs were deposited in a front region of the delta. Trend to compaction has been generated for the well. Sigma wells showed a normal trend toward compaction. Rock physics analysis of the reservoirs of interest reveals that LST and TST reservoirs have been difficult to distinguish against HST reservoirs which are easy to classify regardless of their depth for fluids in a Vp/Vs crossplot versus acoustic impedance. In the analysis, a pattern is suggested for TST reservoirs. Therefore, this research work shows that fluid discrimination becomes simpler as we go basin ward as the sorting of the reservoir rocks increases with finer grain sand sizes.