Rock physics and seismic signatures of sub-resolution sand-shale systems [electronic resource]
Keywords: Geophysics

This dissertation aims to improve the interpretation of thinly bedded sand-shale systems that are below resolution of conventional well-log and seismic data by using rock physics and quantitative seismic analysis. The key contributions of this dissertation are (1) incorporation of parameter uncertainties into existing models that are used for estimating petrophysical properties of sub-resolution sand-shale systems, (2) a new method for approximating fluid substitution in thinly bedded sand-shale reservoirs that is applicable at the measurement scale without the need to downscale the measurements, and (3) an application of rock physics, spatial statistics, and feature-extraction based attributes to quantitatively interpret seismic data for sub-resolution reservoir properties such as net-to-gross ratio, saturation, and stacking pattern. Most of rock physics relations are derived for rocks that are considered homogeneous at particular scales. If these relations are applied to measurements at other scales, the relations often fail when the measurements represent average properties of heterogeneous rocks (for example, a stack of interbedded sand and shale layers). First, we investigate the Thomas-Stieber model, a model commonly used for estimating volume fraction of sand and its porosity in thinly bedded sand-shale sequences. We present sensitivity and uncertainty analyses of this model under various scenarios, especially when the model assumptions are violated. We also extend the model by incorporating uncertainties into the model parameters using Monte Carlo simulations in a Bayesian framework. Next, we propose a simple graphical mesh interpretation and accompanying equations for approximating fluid substitution in sub-resolution interbedded sand-shale sequences. The advantages of our method are as follows. Even when it is applied to the measurements at their original scales, our method appropriately changes fluid in the sands only, without the need to downscale the measurements. The interbedded sand layers can be either clean or shaly (i.e., sand with dispersed clay). We illustrate the performance of the model using both synthetic and real well log data and present sensitivity analysis of the model parameters. Estimating reservoir properties of sub-resolution sand-shale reservoirs from seismic data is not straightforward because the relation between seismic signatures and rock properties are not unique. This relation is even further complicated by the spatial arrangement of the sub-resolution layers. This dissertation presents a workflow for seismic interpretation of such thin reservoirs. The workflow consists of four main steps: (1) estimate transition matrices (Markov chain model) at the well location from log data, (2) use the matrices to create various sand-shale sequences with varying reservoir properties such as net-to-gross ratio, saturation, and stacking pattern, (3) generate synthetic seismograms corresponding to the sequences and from these seismograms extract attributes which will be used as a training set, and (4) finally use the training set to estimate reservoir properties of the area away from the well. Most of seismic attributes discussed here are obtained using feature-extraction techniques, which compare amplitudes of seismogram segments and find new representations of these seismograms in a new, smaller set of features. We apply the workflow to both synthetic data and real data from channelized turbidite deposits in West Africa.