Seismic Attributes Analysis in a Carbonate Reservoir with a Tar Mat: Application to Satah Oil Field, Abu Dhabi, UAE

  • Salem Nasser Ahmed Al Hammami

Student thesis: Master's Thesis

Abstract

The Satah oil field is located in the northwest of Abu Dhabi with an approximate distance of 180 km in the Arabian Gulf. The Arabian Gulf was subjected to two main tectonic events, the obduction of the Semail Ophiolite and the subduction of the Arabian Plate. These events have impacted the area resulting in the formation of many geological structures. Ultimately some of these structures led to traps of hydrocarbons. Oil and gas are produced from several carbonate reservoir zones such as the Arab units C and D. Moreover, a thick tar mat layer has formed within the carbonate reservoir (Arab D unit). This has resulted in the degradation of the reservoir important properties such as porosity and permeability. Based on well data the thickness of the tar mat varies over the field; it is thick on the flanks and thin at the crest of the structure. However this information is only valid at the well locations. Consequently, well are not adequate for the mapping of the bitumen layer across well locations. In this study a 3D reflection seismic, acquired over the Satah oil field, along with available well will be used to map the extent of the bitumen through attribute analysis. A few attributes will be used along the well information to try to enhance the mapping of the bitumen layer (tar mat) at the bottom of the Arab D reservoir. The seismic post-stack and pre-stack attributes are extracted from 3D seismic volumes to perform inversion for reservoir properties guided by well information. Amplitude Variation with Offset (AVO) analysis, multi-attributes inversion were carried to complement the existing interpretation to better understand the lateral distribution of the tar mat visible at the bottom of the Arab D geological unit. An AVO intercept and gradient volumes were extracted from the two and three term AVO approximations of Aki and Richards and Fatti. These led to cross-plots of these attributes indicating that the tar mat could be mapped and highlighted as class II AVO whereas the top and base of Arab D format exhibits class IV AVO. Oil and water distribution can be discriminated in the reservoir where the water is dominant at the crest of the structure correlating with the water injection wells. P-wave velocity and porosity volumes were calculated through inversion. These were able to highlight, with the help of the initial models constructed from well log information the possible tar mat zone and its extension. These volumes also show agreements with the presence of the tar mat at the reservoir level where the velocity increases at the bottom and the porosity decreases; indicating the tar mat zone at the bottom that extends laterally between the wells. Porosity was predicted using multi-attribute analysis using the post-stack seismic volume, AVO volumes (intercept, gradient, and curvature) as external attributes and calculated internal non-linear attributes. The predicted porosity extracted from sixteen attributes showed a possible interpretation of the tar mat at the bottom of the Arab D unit and its possible lateral extent and thickness. Despite the low resolving power of the reflection seismic 3D data at the reservoir level it is possible to extract valuable information from attributes extracted from seismic when associated to well information in the inversion for reservoir properties. These results carry some uncertainty, due to the low frequency content of the seismic signal and the low number of wells used in the inversion process.
Date of Award2013
Original languageAmerican English
SupervisorYoucef Bouzidi (Supervisor)

Keywords

  • Applied sciences
  • Earth sciences
  • Seismic prospecting
  • Seismic reflection method
  • Geophysics
  • Petroleum Geology
  • Petroleum engineering
  • 0765:Petroleum engineering
  • 0583:Petroleum Geology
  • 0373:Geophysics

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