Integrating Seismic Rock Properties Modeling and Inversion to Characterize a Complex Biogenic Gas Reservoir with Limited Seismic Information: Onshore Abu Dhabi, Case Study
Authors: Omonigho Emovigho , Ahmed Al Hanaee , Khan Riaz , Khunaiwan Redy , Abdelaal Atef , Mohamed Sameer 
Published: November 2019

Increased demand for gas in the recent years has motivated Exploration companies to revisit erstwhile overlooked Miocene Biogenic gas potentials, onshore Abu Dhabi. This paper detail how advanced geophysics techniques including rock physics forward modeling and inversion has been integrated to understand the distribution of potential gas bearing zones in this complex unconventional setting where inter-well information is limited. The results integrated with understandings from other disciplines support drilling and field appraisal strategy in the area.

The Miocene Formation of interest consists of an Upper, Middle and Lower unit, with varying levels of complexities and hydrocarbon presence identified from drilled wells. We show how we integrated all available data including logs, core, fluids, cuttings, mud-gas, petrophysical and seismic information to constrain the seismic forward model and invert the seismic data to define potential for gas presence in the area. Lithologic boundaries were defined from cuttings and geologic correlations. Half-space rock properties analyses and well ties provided understanding of the seismic responses, and the geologic picks mapped accordingly. Gassmann fluid substitution were carried out using conditioned Vp, Vs and density logs to understand the sensitivity of the lithologies to different pore fluid fill including brine and different gas proportions. AVO forward modeling was also carried out to understand if gas ‘sweet spots’ may be visible from analyses of amplitudes. Rock physics plots were analysed including AI, SI, GI, PI and Lame's parameters to establish relationship to reservoir properties, and optimum discriminators of fluid, porosity and TOC were accordingly determined. The low frequency model was developed from logs, and prestack 2D seismic data up to 40° were inverted for elastic impedances. Bayesian rock type classification scheme was deployed to extract potential gas prolific areas.

Seismic rock properties analyses provided invaluable insight to the reservoir characterisation strategy for the Biogenic gas formation. The analyses showed that delineating gas presence is challenging using conventional amplitude or AVO analyses techniques. Potential for fluid optimization exists from analyses of poisson's impedance (PI) as well as extended elastic impedance at the fluid projection with reasonable certainty. Pre-stack simultaneous inversion of the seismic lines was carried out, followed by Bayesian rock type classification to identify regions of increased gas potential in areas of seismic coverage

This paper represents for the first time integrated seismic rock properties and inversion techniques are applied to delineate an unconventional Biogenic gas reservoir. The results hold potential benefit for well placement and input to distribution of reservoir properties in the geologic model. The method will be extended to analyzing the gas potential from the currently acquired mega 3D seismic over Abu Dhabi.