Seismic Properties and Rock Physics Analysis of Sabkha

Abstract

Sabkha is one of the most abundant topographic environments along the Arabian Gulf where they overlain gigantic hydrocarbon fields. Sabkha is characterized with high lithological complexity associated concentrations of salts that cause seismic velocity to rise and distortion of the signal. The static correction is an essential step during early stage of seismic reflection processing to accurately remove the effect of low-velocity surface layers. Typically, the velocity of uppermost layers is replaced with an average velocity of 300 m/s called replacement velocity, this technique can result in high error depending on the complexity of these layers. We aim to produce an accurate velocity model using high-resolution seismic (closed-spaced geophones), incorporated with conventional seismic profiles to resolve such a high complexity and investigate the implication for static corrections. Physical properties of sabkha are still not well understood. Thus, different geophysical methods are applied such as electrical resistivity tomography, vertical seismic profiling (VSP), multi-channel analysis of surface waves (MASW), and rock physics to characterize both coastal and inland sabkha settings. Seismic refraction surveys have recognized a layer interface at a similar depth at 1 m depth which is very similar to the water table observed in the field, where this layer had a velocity than expected for saturated sands (>1500 m/s). This low velocity layer (≈850 m/s) in both sabkhas was found to be an effect of partially saturated (Sr > 99%) transition zone caused by Ascending brines and upward leakage of the aquifers beneath the sabkha. The Vp acquired from seismic refraction and Vs from MASW technique are used to compute poisson’s ratio, along with reverse velocity calculation using rock physics modeling that has confirmed that the low velocity layer is an effect of partial saturation. The sands underlain the Sabkha are not expected to have velocities higher than 1800 yet the seismic refraction showed velocities of 1900 and 2250 m/s for these intervals, where the rock physics modeling reveled it is a function of fluid properties (high solutes present) as these sequences were found not yet stiff to be considered as a rock based on Vs from MASW. The coastal sabkha of Abu Dhabi has a hard thin lithified rock with velocity of 1500 m/s, located between two low-velocity layers between 350-900 m/s causing shingling waves arrivals. Static correction implications were investigated using three different approaches: replacement velocity, tomographic correction and High-resolution static correction. The replacement velocity correction using conventional seismic is applicable for inland sabkha, unlike the coastal sabkha where high resolution of the uppermost complexity is much necessary to resolve the high heterogeneity within the uppermost part and for appropriate static correction applications. The results from ERT data from the coastal sabkha had a very low overall resistivity, where the uppermost layer had 0.45 ohm.m followed by less resistive layer of 0.155 ohm.m which is high homogenous porous sands. These sands sequence had a unique feature of finger structure caused by the salts' dissolution within the first few meters and sinking to the deeper parts of the system. The ground water is chemically inhomogeneous that resulted in some parts with being extremely conductive and believed to be a high concentration of salines.

Date of Award	Dec 2022
Original language	American English
Supervisor	Mohammed Ali (Supervisor)