Discussion on lithology data conversion methods in studies of seismic sedimentology

摘要

Seismic sedimentology is a new interdisciplinary in oil and gas exploration, and consists of two branches: seismic lithology and seismic geomorphology. The current prevailing seismic lithology method is 90° phase conversion technology, which transforms seismic data to lithology data for the purpose of direct interpretation of seismic events, acting like interpreting logging data. Based on premise of zero-phase wavelet of seismic data, with the geological formations of thin sandstone layers and simple vertical composition of sandstone and mudstone with great impedance contrast, the results of seismic events transformed by 90° phase conversion method correspond to lithology interfaces, and have the capability of effectively distinguishing sandstone from other different lithologies. However, continental sedimentary basins face several geological difficulties, such as diversity of depositional types, fast changeable sedimentary facies, great variable sandstone thickness, complex diagenesis and diagenetic sequence. These problems result in velocity of sandstone from acoustic logging or seismic survey less than velocity of incumbent mudstone, moreover these abnormal velocity discrepancies vary with sediment types and buried depth, which bring great difficulties for utilizing seismic data to predict sedimentary lithology and identify sandstone formations especially thin layers. Therefore, 90° phase conversion technology has limited applications, and cannot guarantee its accuracy for predicting unconventional oil and gas reservoirs. In this paper, we discusses several conversion methods of seismic data to lithology data, including 90° phase conversion, trace integration, poststack inversion (for example, colour inversion and logging constrained inversion) and pre-stack inversion technology. We focused on the advantages and disadvantages of these techniques as well as their feasibility and applicable conditions, so that the optimal method matching strata slice is able to be selected to describe subsurface geomorphology. These lithology data conversion methods, can greatly improve the lithology discrimination ability and lithology data resolution, which would benefit extensive application of seismic sedimentology, especially at the areas with complex lithology and tight hydrocarbon reservoirs in continental sedimentary basin.