Characterization of Seal Potential and Distribution using Pre-Stack Seismic Inversion: A Field Case Study of Onshore Abu Dhabi
The outputs of seismic inversion are broadly used to characterize different lithofacies with variant rock properties and to reduce the uncertainties of target reservoir. The P-wave impedance provides the initial basis of acoustic properties from post-stack seismic data and S-wave impedance provides additional elastic properties from pre-stack seismic data. Regional indications, such as mudlogging hydrocarbon shows, high oil saturation log, high porosity log values and production test results, have shown high prospectivity of the wide-spread shallow-water Mauddud carbonate reservoir. However, characterizing the efficiency and distribution of seal for the Mauddud reservoir is a challenge. The main objective of this study is to develop a workflow utilizing advanced geophysical techniques including rock physics analysis and seismic inversion, to understand the potential and distribution of seal above the target reservoir. The overlying Shilaif Formation in the study area was deposited in an intrashelf basin and three zones are identified: S1 (lower part), S2 (middle part) and S3 (upper part). The bitumen-rich zone in S1 of Lower Shilaif shows sealing potential through well log analysis and it may act as a satisfactory seal for the underlying reservoir. In an oil-bearing source rock play, total organic carbon (TOC) is a mixture of kerogen and liquid hydrocarbon. In this context, bitumen is considered a viscous, soluble hydrocarbon that is an early product of kerogen maturation. Due to its high viscosity, bitumen is less likely to migrate from the kerogen-hosted pore system and is often left behind as a residue after light oil expulsion. The well logs have shown a good indication of the existence of non-movable fluids by the large separation between the porosity from neutron density (PHIT_ND) and the porosity from nuclear magnetic resonance measurements (TCMR). The core analysis also confirmed that the non-movable fluids are with high content of bitumen. Three lithofacies can be identified from well logs: • Clean mudstone characterized with inorganic porosity and low total organic content (TOC), • Mudstone with porosity partly filled with elevated TOC, • Bitumen-rich mudstone. Seismic elastic property analysis (P-impedance and velocity ratio Vp/Vs) with reservoir properties (TOC and separation of PHIT_ND and nuclear magnetic resonance measurements) provides valuable insights to the characterization of target seal. The clean mudstone with inorganic porosity and low TOC is characterized with high P-impedance, the mudstone filled with movable oil is characterized with low Vp/Vs and low P-impedance, the target bitumen-rich mudstone is characterized with high Vp/Vs and low P-impedance. Pre-stack simultaneous seismic inversion and Bayesian-based lithofacies classification were carried out to predict the distribution of target seal in study area. The results show that there is a distribution of relative low P-impedance values in the northeastern and southern parts of study area. However, high Vp/Vs values are only present in the northeastern part of the field. The target seal in S1 unit of Shilaif Formation with high TOC and non-moveable bitumen at an early maturity is most probably present in the northeast part of the study area, consistent with well logs correlation results and present-day vitrinite reflectance map. For the first time, the integration of rock properties and seismic inversion results to predict the potential and distribution of bitumen-rich seal in S1 unit of Shilaif Formation is addressed in this study. The obtained results are valuable information to further rock physics modelling and well placement.
AAPG Datapages/Search and Discovery Article #90370 ©2020 AAPG Middle East Region Geoscience Technology Workshop, 3rd Edition Carbonate Reservoirs of the Middle East, Abu Dhabi, UAE, January 28-29, 2020