Stratigraphic analysis of reflectivity data, application to gas reservoirs in the Burgos Basin, Mexico
Reflectivity data derived from the inversion of post-stack seismic data can be used to improve the detection and spatial delineation of stratigraphic sequences. The enhancement in spatial resolution inherent to post-stack inversion can often substantially improve standard delineation results based on seismic amplitudes. In this research, a study is described of the use of reflectivity data to delineate stratigraphic sequences associated with gas-producing sands in a mature basin. Stratigraphic units in the Burgos Basin consist of Oligocene, wavedominated deltaic sequences within delta front and prodelta facies. Post-stack reflectivity data helped to define the lateral continuity of coast-parallel sand bodies deposited by wave action and reworking of sediments supplied by the ancient Rio Grande. An attempt was made to discriminate high-porosity, clean sands from shaly sands within a given sand unit. This was done using reflectivity data derived from pre-stack inversion. Extensive petrophysical analysis confirmed that prestack inversion could provide a quantitative method that discriminates highquality from low-quality sands. In particularly favorable situations, pre-stack inversion results could even help to discriminate between water and gas. Pre-stack inversion yielded angle-dependent reflectivities as well as density, P-wave and Swave velocities. Transformations of these properties into Lamé’s lambdarho and murho parameters were also explored to assess the influence of shale volume, total porosity, and rock type within sand and shale units delineated by post-stack inversion. Spatial delineation of sand units is presented near two key exploration wells. Success in the location of development wells is significantly constrained by the spatial delineation of reservoir units.