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