A strong 3-D seismic amplitude anomaly, thought to be associated with the Upper Jurassic Hanifa<br>reservoir, was interpreted to be due to an increase in the porosity of Hanifa carbonates. The Hanifa<br>formation overlain by the Jubaila source rocks is present in the Jafurah basin, east of the Ghawar Field<br>in Saudi Arabia. After drilling exploratory well 1, the targeted Hanifa carbonates proved tight with<br>porosity generally less than five percent. A post drilling study was taken to evaluate the rock properties<br>and the cause of the seismic bright anomaly. The study investigated the effect of reservoir thickness,<br>porosity, lithology, pore fluid type, and total organic content (TOC) on acoustic impedance; in both the<br>Hanifa and Jubaila formations. Synthetic normal incident traces were generated to understand the<br>effect of changes in these reservoir properties on the seismic. This paper summarizes the results of<br>investigations for finding the cause of seismic amplitude anomalies as seen at the top of the Hanifa<br>carbonates.<br>It was concluded that porosity is the dominant factor in the strength of the observed anomalous<br>seismic amplitude in study area. The porosity was mainly caused by high TOC in the Jubaila source<br>rock. The study finds an inverse proportional relation of TOC with the acoustic impedance. As the<br>acoustic impedance decreases with increasing TOC within Jubaila, the impedance contrast between the<br>base Jubaila and the top Hanifa increases. Hence, amplitude brightening would result at the Hanifa top<br>due to increased TOC within the Jubaila formation.


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