Facies and porosity 3D models constrained by stochastic seismic inversion to delineate Paleocene fluvial/lacustrine reservoirs in Melut Rift Basin, Sudan

摘要

The Paleocene sandstones of the Yabus and Samma formations are the main oil reservoirs in the Melut Rift Basin of interior Sudan. In our study area, we implemented a multi-modeling approach to reveal heterogeneity and reservoir characteristics of this sandstone. It estimates acoustic impedance from seismic data using deterministic and stochastic inversion algorithms. We integrated petrophysical and geostatistical analyses of the well logs to produce facies and porosity 3D models. The study populated and evaluated three different porosity models; one from well logs only, a second from well logs constrained by inversion results, and a third from well logs constrained by inversion results and facies. Stochastic inversion reveals a significant vertical facies heterogeneity as less as two meters in Yabus Sandstone. This heterogeneity is a lithology dependent and facies distribution is structurally controlled by major faults that developed during the third rift phase. The geostatistical integration of seismic inversion, sedimentologic and stratigraphic analyses reveal lateral facies distribution in the study area. The significant correlation between facies distribution and porosity allows reliable mapping of thin sandbodies. The facies and porosity 3D models indicate vertically-stacked channels with isolated sandbodies in Yabus Sandstone. This strongly suggests a meandering origin for this sandstone. However, the thick sandbodies observed in Samma Formation might indicate a braided channel setting. The dominant claystone and mudstone facies deposited in Melut Formation indicate a shallow lacustrine setting. The reservoir quality decreases with depth from the Paleocene Yabus and Samma formations at the top towards the Cretaceous Melut Formation at the bottom. The porosity estimates support this conclusion as detected low porosity (as less as 5%) in Melut Formation with respect to higher porosity (as high as 25%) in Yabus and Samma formations. The fades and porosity 3D models enhanced our understanding in predicting the reservoir heterogeneity, quality and architecture of this sandstone.