Shallow Gas: Rock Physics and AVO (2015)
This MSc thesis has been written by former intern Martijn Janssen.
As a result of the presence of hydrocarbons, ample shallow seismic anomalies are observed in sediments of Cenozoic age in the northern part of the Dutch North Sea area. In The Netherlands, which was the first of all North Sea countries that started producing the play, there are currently three successfully producing shallow fields. The sand production that was expected is effectively controlled by the use of mechanical sand control methods and no water breakthrough is seen so far. With almost eight years of production, the shallow play has proven to be a valuable resource in The Netherlands. There are, besides the three producing fields, still many possibilities for the production of shallow gas in the Dutch offshore section. Therefore a better understanding of its seismic signature is required. The main challenge in assessing these shallow anomalies, or bright spots, is that amplitude brightening already occurs at very low (non-producible) gas saturations or it might even have a lithological origin. Therefore, gas saturation is seen as the largest pre-drill uncertainty. In this study, a detailed forward model, which contains parameters that are used to describe these shallow reservoirs, is compared with the amplitude versus offset (AVO) behavior of state-of-the-art pre-stack seismic data of the study area: the F04/F05 licence blocks in the Dutch offshore section. Attention is paid to the relationship between the AVO response and the gas saturation; is distinction between low and high gas saturations possible by analyzing the seismic response of shallow gas in the pre-stack domain?
In order to obtain representative reservoir and seal parameters for the modelling work, petrophysical analyses are performed using data from three different wells within the study area. A total of eight wells were drilled in the F04/F05 blocks. Although all the wells drilled show limited data availability of the shallow subsurface, datasets from wells F04-01, F05-01 and F05-04 are used due to their data accessibility for both shallow sand and shale layers. The outcomes of wells F04-01 and F05-01 show potential net-to-gross (N/G) ratios of 50%. A clean sand interval with overlying shale layer, resulting from the petrophysical analysis of well F04-01, is marked as a base-case scenario due to the estimated net-to-gross ratios for both seal and reservoir of respectively 20% and 50%. Log data (compressional wave velocity) and estimated density and shear wave velocity data are used to construct a model of elastic properties for the water-bearing situation; the base-case. Next, Gassmann’s algorithm is applied to derive new elastic properties for different gas saturations. The complete Vp-Vs-ρ sets act as input parameters for the Zoeppritz equations. The full Zoeppritz equations are used to model the variation in amplitude with offset for various gas saturations within the reservoir, corresponding to the top-reservoir reflector. A stochastic approach has been applied to the Zoeppritz equations. Modelling results show that in most of the cases the AVO trends for gas-bearing sediments indicate increasing negative amplitudes with increasing offset (e.g. class 3 anomalies). The pre-stack seismic data is initially analyzed using the intercept-gradient method. This method yields five AVO anomalies that correspond to the bright spots observed on full-stack seismic data. Next, the AVO behavior of some of the anomalies has been analyzed in more detail, both off- and on-structure. The top-gas sand reflectors of all the analyzed bright spots show decreasing negative amplitudes with increasing offset (e.g. class 4 anomalies). A difference is observed between the modelled AVO responses and the results of the pre-stack data analysis.
Partly due to the large uncertainties in rock properties and in the methods used (e.g. Gassmann’s equations), it is very challenging to model the seismic response of these shallow sediments in the pre-stack domain. Since the model parameters are based on the observed combination of seal and reservoir in well F04-01 and the analyzed anomalies lie at different locations and depths with respect to this particular seal-reservoir combination, comparison between the model and the real data itself has a significant uncertainty level due to lateral variations in lithology, temperature and pressure. The outcome of this study shows that, based on the dataset used, the observed AVO behavior on pre-stack seismic data cannot be modelled yet. Whether studying the AVO response of shallow gas accumulations contributes to minimizing the gas saturation uncertainty is therefore still inconclusive.
