Abstract

In analyzing unconventional organic “shale” reservoirs it is important to determined from conventional petrophysical logs which is the dominate pore type organoporosity (Фom) or mineral matrix porosity (Фmm). It is important because Loucks and others, 2010 have reported the following about the pore sizes of the Ф om versus Фmm (InterP & IntraP):

Approximate Range

InterP (Фmm): 30 – 2000nm

IntraP (Фmm): 10 – 1000nm

OM (Фom): 5 – 750nm

The Maxwell-Garnett Equation is designed to model the low frequency current response in a mixture of spherical pores (vugs) imbedded in finer matrix pores. In an organic shale reservoir the kerogen (Sw in Фom = 0.0) fragments like the hydrocarbon filled vugs (Swvug = 0.0) are insulators imbedded in a conductive matrix (Фmatrix or Фmm). Remember the electro -magnetic wave can only travel through porosity containing water. In order to apply the Maxwell-Garnett Equation to a organic shale reservoir the amount of each porosity (Фom and Фmm) must be separated. The procedure is listed below:

Фtotal, volume of clay (Vcl), and volume of kerogen (Vke) are obtained using the bulk density (ρb) and neutron (Ф Nls) porosity logs, utilizing the simultaneous equation method developed by Rick Lewis with Schlumberger.

Effective porosity (Фe) = Фtotal – CBW

CBW = Vcl*Фclay

Фom = Vke*OM

OM = amount of porosity in the kerogen

[OM = 0.30]

Фmm = Фe – Фom

MAXWELL – GARNETT EQUATION Ct = Cm {1+{2V[(Cv-Cm)/(Cv+2Cm)]}/1-{V [Cv-Cm)/(Cv+2Cm)]}}

Rt = 1/Ct

Where:

Ct = calculated total conductivity

Rt = calculated total resistivity

Cm = matrix conductivity [Фmm]

Cm = [Фmm^2 * Sw^2]/Rw

Rmm (resistivity of Фmm) Rmm = Rlog - Rb

Rb = bound water resistivity Rb = (Ftotal^2) * Rsh

Rsh = 10ohm-m resistivity of adjacent nonorganic shale

Swmm = [(1/Фmm^2) * (Rw/Rmm)]^0.5

and assuming Swmm = 1.0

Cv = vug conductivity

Cv = Sw(vug)/Rw Sw(vug) = 0.0 Cv = 0.0

V = percent kerogen [percent vugs both are insulators]

Two organic shale were selected the Devonian Woodford [gas] in Oklahoma and the Permian Avalon [oil] New Mexico. In both the Woodford and Avalon the resistivity calculated by the Maxwell-Garnett Equation (Rtcalc) matched the log resistivity (Rtlog) when the water saturation (Swmm) of the mineral matrix porosity (Ф mm) was calculated, however if Swmm = 1.0 was assumed Rtcalc did not match Rtlog. Which indicates that both Фom and Фmm are hydrocarbon-bearing. In the Woodford there were very few zone where Rtcalc did not match Rtlog when Smm = 1.0, which indicates Фmm is the dominate pore type. In the Avalon over most of the entire interval Rtcalc did not match RtLOG when Smm = 1.0, which indicates Фmm is the dominate pore type. Woodford OGIPscf/640acres using a 100nD permeability cut-off by pore type was Фom = 8.5BCF/sec and Фmm = 0.9BCF/sec, and the Avalon OOIPstb/160acres using a 100nD permeability cut-off by pore type was Фom = 13mmbo/160acres and Фmm = 17mmbo/160acres. The percent of the pore type in the Woodford and Avalon are listed below:

Woodford: Фmm = 90% Фmm = 10%

Avalon: Фmm = 43% Фmm = 57%

How would these two organic shale reservoirs plot on a Shaly Sandstone Producibility (Q) Plot? A Q Plot is a triangle cross plot of effective porosity (Фe) X -Axis versus Q [(Фtotal – Фe)/Фtotal] Y- Axis. Therefore Фe represents the amount of porosity free of clay (Фe) and Q represents amount of porosity filled with clay. On the Q Plot there is a line below which is shaly sandstone reservoir and above which is non-shaly sandstone reservoir. This line separating shaly sandstone reservoir from non-reservoir is based on field data from U.S. Gulf Coast, New Mexico, Colorado and Wyoming. When plotted on a Q-Plot the Woodford data plot on or above the shaly sandstone reservoir line. Much of the Avalon data when plotted on the Q-Plot, plots well below the shaly sandstone reservoir line. The very important question is why is so much of the Avalon data below the line; could it be because the Avalon contains more (Фmm) porosity with larger pores (Remember: Loucks and others, 2010).