Abstract

Traditional methods of determining volume of clay (Vcl) and effective porosity (Φe) using well log data (SP, Gamma Ray, and/or Neutron-Density) all depend on data from adjacent zones in their formulas. This dependence on data from adjacent zones is illustrated below:

Vcl(sp) = 1.0 – PSP/SSP

Vcl(gr) = IGR/[3.0-(2.0*IGR)] Steiber

Vcl(gr) = 1.7*/[3.38*(IGR+0.7)^2]^0.5 Clavier

Vcl(gr) = 0.083*[2^(3.7*IGR)-1.0] Larionov

(unconsolidated)

Vcl(gr) = 0.33*[2^(2*IGR)-1.0] Larionov

(consolidated)

Where: IGR = (GRlog – GRss)/(GRshale – GRss)

Vcl(nd) = (Φn – Φd)/(Φnsh – Φdsh)

Vcl(pe) = (Pelog – Pess)/(Pesh – Pess) Pess = 1.81

The use of log data from adjacent zones presents two problems in determining reliable Vcl and Φe values that are critical in the log analysis of shaly sandstones.

1. The incompatible clay problem (i.e. clay in the adjacent zone different from the clay in the shaly sandstone reservoir).

2. The absence of suitable values of SSP, GRss, GRsh, Fnsh, Fdsh, and/or Pesh due to changes in adjacent rock type or the adjacent zones are very thin.

The proposed method uses bulk density (ρb) and neutron porosity (Φnls) data from the zone of interest, and determines the total porosity (Φtotal), volume of quartz (Vqtz), and volume of clay (Vcl) using the following simultaneous equations:

Vcl + Vqtz + Φtotal = 1.0

(ρcl*Vcl) + (ρqtz*Vqtz) + (ρf*Φtotal) = ρb

(Φncl*Vcl) + (Φnqtz*Vqtz) + (Φnf*Φtotal) = Φnls

Effective porosity (Φe) then is calculated.

Φe = Φtotal – CBW CBW = Vcl * Φclay

Vcl from this new method will be compared to volume of clay determined in the laboratory [Vcl(lab)] from two formations: 1.) Cretaceous Pictured Cliffs Sandstone in the San Juan Basin and 2.) Cretaceous Olmos Sandstone in South Texas. The use of the New Method will also be illustrated in the Cretaceous Frontier Sandstone in the Rock Mountains and the Permian Spraberry Sandstone in the Midland Basin.