Integrated Seismic Attribute Analysis and Geo-anomaly Mapping of Middle Marrat Reservoirs in Exploration Phase: A Case Study
Prasad, Raghav *1; Nath, Prabir K.1; Arasu, Raju T.1; Singh, Sunil K.1; Al-Adwani, Talal F.1; Abu Ghneej, Ali Faleh N.1
(1) Prospect Evaluation, Kuwait Oil Company, Ahmadi, Kuwait.
The Marrat Formation of Pliensbachian to Toarcian age was deposited in Kuwait as part of regionally widespread carbonate-evaporite platform that gently sloped to the east. The formation occurs at a depth of 15500-17050 feet and its thickness varies from 580 feet to 700 feet in the area of study. It is overlain by Dharuma formation and underlain by Minjur formation.
Four wells are drilled in the area. Based on well logs the oil bearing Middle Marrat section is characterized by low gamma ray compared to Upper and Lower Marrat in the area. The Middle Marrat is divided into Upper and Lower reservoirs. The Upper reservoir consists of thin anhydrite and dolomite alternations and the Lower reservoir, of clean limestone. The porosity of these reservoirs varies from 4% to 16%. The present study addresses the major challenge of deciphering the distribution of reservoir facies over the area.
3D Prestack Time Migrated seismic data with grid interval of 25m X 25m and fold of 151 is interpreted to map the structure. Volume blending of seismic attributes such as Amplitude, Curvature, Semblance, etc., has been effectively used to visualize and map subtle faults in the area. The structure at Middle Marrat level is a doubly plunging elongated anticline oriented in NNW-SSE direction.
Well log based impedance curves show a relative lowering of acoustic impedance in the porous intervals compared to non-porous intervals of the reservoirs. To map the reservoir facies in the entire area of study geostatistical inversion has been carried out. However, geologically meaningful porosity maps could not be generated in the area using inversion results, perhaps, due to various other factors. Subsequently, amplitude based seismic attribute analysis has been carried out to characterize the Upper and Lower reservoirs of Middle Marrat section. Two distinct geo-anomalies, oriented NNW-SSE, are mapped corresponding to these reservoirs.
Based on the analysis the Upper reservoir is likely to be prospective in the eastern flank of the structure while the Lower reservoir, in the western flank. However, in an area in the northern part of the structure, both reservoirs appear to be better developed.
The reservoir architecture brought out in the present study is in conformity with well data. Sweet spots have been identified in the northern side of the structure for further exploration and delineation of these reservoirs.
AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain