How Sedimentological Variations in Carbonate Cemented Sandstones Affect Seismic Impedance?

Tanima Dutta, Tapan Mukerji, and Gary Mavko
Geophysics, Stanfird University, Stanford, CA

Our goal is two-fold: (1) to identify the sedimentological variations within carbonate cemented sandstones and (2) to quantify their effects on P-impedance. To accomplish this goal, we identify the relationship between carbonate cementation and key stratigraphic surfaces, such as, the incision surfaces and the flooding surfaces. Next, we use rock-physics models to quantify the impact of sediment parameters on P-wave impedance. We find that the carbonate cemented sandstones are extremely heterogeneous in nature, even within a depth interval of ~60 meter in our study area at offshore Equitorial Guinea, West Africa. Their grain-size, sorting, mineralogy, clay-content, amount of cement and degree of leaching vary considerably. However, these sedimentological variations can be classified into two distinct clusters in the P-impedance vs. porosity domain. The carbonate cemented sandstones from the base of incision surface are usually associated with lower shaliness, lower porosity and higher P-impedance. On the contrary, data from the top of flooding surfaces exhibit higher shaliness, higher porosity and lower P-impedance. The porosity- impedance trends of the data are quantitatively interpreted using rock-physics models. For example, the constant-cement model with 1% carbonate cement fit the porosity-impedance trends derived using well data.

In conclusion, the carbonate cements can be classified into two groups with distinct sedimentological properties, porosity and seismic impedance. Rock physics models are useful to quantify these sedimentological properties based on porosity-impedance trends.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas