4D Seismic Analysis of Reservoir Sands Overlying a Salt Structure

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