Seismic Amplitude Driven Marine Deltaic Facies Exploration—Examples of Stratigraphic Depositional Analogs in Southwestern Louisiana

Abstract

An integrated study was conducted on an area onshore southwestern Louisiana to understand seismic amplitudes and their relationship with sandstone reservoirs. Amplitudes are often an indication of lithology present in the subsurface. The lithology corresponding to amplitudes is generally a matrix of porous and permeable sandstone reservoir material. The buildup of sandstone will exhibit seismic amplitude compared to on trend that lack the sand body. The amplitudes however can often be misleading as an indication of hydrocarbons. This presentation will discuss the data applications used with a focus on understanding analog amplitudes to potential new drill sites.

This study integrated various 3D seismic volumes and tied critical subsurface information. The various data types include amplitude, coherency/similarity, AVO (amplitude variations with offset) on seismic gathers, and synthetics (Fig. 1).

Particular importance is placed on coherency cubes when studying areas of variable stratigraphy. Barrier bars, channel fills, delta mouth bars, channel levees, and crevasse splays can be seen on the time-sliced coherency volume. The coherency/similarity volumes identified stratagraphic accumulations by way of discontinuities in the data. Phase change on either side of the sand reflector event causes a discontinuity in the coherency volume and as such can readily be identified. Stratigraphic seal on three sides and the fault to the northwest provided the trapping mechanism for oil in this case (Fig. 2).

A second of several analogs to be presented is shown in Figure 3. Strong amplitude was discovered updip of an oil field one fault removed. Sufficient well control tied a sand buildup to the amplitudes seen on the seismic. Where the objective sand was shaled out, no amplitude could be seen on the data.

Arbitrary seismic lines across three areas of a meandering channel are shown. Well A was wet and had the weakest far-offset angle-stack amplitude. Well C was an oil producer and had a strong amplitude extraction. Well B exhibited a shale out and was not associated with amplitude. The proposed well location is prospective from this comparison of amplitude displays.

AVO analysis of proven hydrocarbon-bearing reservoirs comparison to prospective drills high grades a list of exploratory projects. Not all cases of prospective AVO indications result in successful completions. The subject prospect had a favorable AVO response when compared to the oil analog shown in Figure 4. Though the prospect had many similarities to the analog oil field response, the prospect's objective proved to be thick wet sand. Post-drill analysis concluded there was sub-resolution connection of the channel sand that allowed leakage of the trap to the north.

This presentation will review analog reservoirs and their seismic characteristics with pre-drill prospects. Not all of the prospects were drilled. The reviewers will have a chance to decide if they would drill the prospect with humorous sidebars shared by the author. A complete utilization of all data can reduce the inherent risk of drilling new ideas; however as will be pointed out, shortcuts or pre-conceived mental motivations can lead to less than lucrative outcomes.

AAPG Datapages/Search and Discovery Article #90196 © 2014 GCAGS, Lafayette, Louisiana, October 5-7, 2014