4D Seismic Analysis of Reservoir Sands Overlying a Salt Structure

PETRONAS Research has recently embarked on a study to determine and understand<br>pressure-saturation variations in a field through the use of 4D seismic technology. Changes in<br>hydrocarbon pressure and saturation due to production produce noticeable changes in amplitude<br>response. 4D seismic or the use of repeated time-lapse 2D/3D seismic surveys enables detection of<br>4D signal indicative of pressure or saturation changes.<br>The C field is located some 80 km due east of the coastline of West Africa. Water depth<br>ranges from 739.3 to 823.6 m with area of 60km2. The oil and gas in the field are trapped in Early to<br>Mid-Miocene reservoir sands above a faulted dome caused by underlying salt intrusion. The dome is<br>faulted by low angle (45-60°) radial faults. A dominant east-west fault set subdivides the structure<br>into an uplifted area in the north and a downthrown area to the south, which is itself separated into<br>east and west fault blocks by a large north-south fault. Hence, the faults compartmentalize the field<br>and hydrocarbons into three main fault blocks. The reservoir represents mid-slope turbidites which<br>consist of a series of fining upwards sequences.<br>Conducting 4D seismic analysis and understanding pressure-saturation variations involve<br>integrating several geophysical technologies. These technologies include well-synthetic-seismic<br>correlation, Rock Physics, Production Scenario Seismic Modeling at selected injector/producer wells, 2D/3D seismic modeling, 4D AVO Modelling/Interpretation and correlating seismic attributes with<br>engineering data, pressure history and saturation measurements.