Xuan Qin

Seismic Characters of Pore Pressure Due to Smectite-to-illite Transition

In this study, we strive to understand unloading overpressure caused by smectite-to-illite by using shelf well log data from offshore Louisiana. Two trends of smectite-to-illite transition can be categorized, based on their signatures on the sonic travel time and density crossplot. It corresponds to the cases of fluid expansion and fluid escape during the smectite-to-illite. The fluid expansion can arise high magnitude overpressure and tends to happen when the upper formation has been undercompacted and has less sand content. For the fluid escape case, it has relative deeper overpressure onsets and its overlying formation is less undercompacted and has more sand content. From the synthetic seismic gathers, the two trends of smectite-to-illite have different seismic responses, and can be discerned with AVO technique.

An Effective Reservoir Parameter for Seismic Characterization of Organic Shale Reservoir

Sweet spots identification for unconventional shale reservoirs involves detection of organic-rich zones with abundant porosity. However, commonly used elastic attributes, such as P- and S-impedances, often show poor correlations with porosity and organic matter content separately and thus make the seismic characterization of sweet spots challenging. Based on an extensive analysis of worldwide laboratory database of core measurements, we find that P- and S-impedances exhibit much improved linear correlations with the sum of volume fraction of organic matter and porosity than the single parameter of organic matter volume fraction or porosity. Importantly, from the geological perspective, porosity in conjunction with organic matter content is also directly indicative of the total hydrocarbon content of shale resources plays. Consequently, we propose an effective reservoir parameter (ERP), the sum of volume fraction of organic matter and porosity, to bridge the gap between hydrocarbon accumulation and seismic measurements in organic shale reservoirs. ERP acts as the first-order factor in controlling the elastic properties as well as characterizing the hydrocarbon storage capacity of organic shale reservoirs. We also use rock physics modeling to demonstrate why there exists an improved linear correlation between elastic impedances and ERP. A case study in a shale gas reservoir illustrates that seismic-derived ERP can be effectively used to characterize the total gas content in place, which is also confirmed by the production well.