Seismic Frequency Enhancement for Mapping and Reservoir Characterization of Arab Formation: Case Study Onshore UAE
Authors: Mahgoub Mohamed Abd El Ghany , Obaid Khalid Ali , Neves Fernando , Al Mesaabi Saif Ali 
Published: December 2014

Mapping and discrimination of Upper Jurassic Arab reservoirs (Arab A/B/C and D) in this 3D seismic onshore field of Abu Dhabi, is very sensitive to the seismic frequency bandwidth. The alternating sequence of thin layers composed of limestone and anhydrites of the Arab reservoirs requires investigating realistic seismic frequency bandwidth to help in mapping and conducting reservoir characterization. A legacy 3D seismic dataset had frequency boosting in a time invariant fashion which blurred the overall seismic imaging, particularly at the Arab level. The interpretation of legacy data showed severe challenges on the Arab level mapping throughout the whole 3D survey due to considerable remaining seismic noise and artificial boosting of higher frequencies at the expense of the lateral horizon continuity.

The final migrated stack of the 3D legacy seismic data has been subjected to ADNOC in-house post stack reprocessing (PSR) workflow. Three iterations of in-house seismic PSR were applied followed by seismic interpretation and model-based acoustic impedance inversion. Seismic-to-well to tie in several wells was tightly integrated along the processing to evaluate the improvement of synthetic to field seismic data correlation on each data processing stage. The well tie to legacy seismic has seismic correlation of about 60-70% while it reached above 90 % with the in-house PSR outcomes. The PSR lower frequencies honoring cube was helpful in mapping Arab horizon reliably and track it spatially over the whole field. Furthermore, the higher frequency cube was instrumental to invert the seismic data optimally. Mapping the Arab units with confidence will assist in reservoir characterization, therefore the placement of an appraisal well trajectory could be more accurately chosen.

Introduction

The area of study is located in the southwest region of Abu Dhabi, UAE. The reservoir in the field of the study area has multiples pays either light oil in the lower Cretaceous limestone units and non-associated sour gas in the Jurassic Arab Carbonate units. The field is understood to be a salt related elongated structure oriented NE-SW. The Arab Formation is typically subdivided into members A, B, C and D. Despite the recent advent of sophisticated pre-stack inversion algorithms, post-stack model based impedance inversion remains one of the most extensively tested, robust and computationally efficient tools for reservoir characterization, particularly in carbonate reservoirs where pre-stack AVO inversion does remain challenging (Costas G.Macrides et. al., 2006). In addition, isotropic elastic seismic forward modeling has shown that reservoir thickness and porosity are the main controlling factors on the seismic response while fluids play a secondary role in this field.

In the area of study a 3D seismic survey (~ 2,000 km2) was acquired in 2003 with a high (640) nominal fold and with dense acquisition survey parameters with 25x25 m shotpoint and receiver point spacing. The 2003 recorded seismic data quality was quite good and the first pass of data processing was able to produce amenable seismic volume for mapping the late Cretaceous oil reservoirs with high confidence while it was not displaying the same data quality for the Arab formation underneath.

Mapping of Arab was not properly done using the 2003 legacy seismic data because the data was severely contaminated by high frequency noise which strongly cluttered the Arab level and deteriorated its spatial continuity. The input of in-house Post Stack seismic Reprocessing (PSR) was the Van Gogh (VGH) structure oriented filter seismic volume which was cascaded on the top of Seismic Contractor Migrated stack cube.