Seismic Response of Unconsolidated Reservoirs as Function of Gas Saturation (Shallow Gas: Rock Physics and AVO)

Abstract

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?