Characterization of the Secondary Porosity of Some Cretaceous-Recent Jordanian Building Limestones

MATERIALS AND METHODS

The geology of Jordanian building stones is summarized in Table 1 and the petrography of those selected for the present study is shown in Table 2 (Moh’d, 2002). The petrographic nomenclature is following one or more of the following: Folk (1959, 1962), Dunham (1962) and Fookes and Higginbottom (1975).

The porosity, degree of saturation and sound velocity (Vp) tests were carried out at BRE (UK) and NRA (Jordan) following the BRE testing procedures (Ross and Butlin, 1989).

Derived properties include
Modified saturation: This was obtained by multiplying total porosity with degree of saturation.

Cementation exponent m: This was calculated using Archie formula and assuming that water resistivity as 0.005 where m = log (0.005/water saturation squared)/log total porosity. This parameter can also be estimated from total and sonic porosity for fractured (Rasmus, 1983) and vuggy carbonates (Nugent, 1983).

Permeability: Was obtained using Jorgensen (1988) equation by multiplying 84105 by porosity index = Φ^m+2/(1-Φ)². The obtained values were found to correlate well with measured air permeability using API standards.

Sonic porosity: Is equivalent to velocity of sound-141/ (28.59); where 28.59 is the inverse of 100/(3000-141); 141 and 3000 are transit time (in μ s/m) in calcite crystal and air, respectively.

Vug porosity and Fracture porosity: Are estimated from the dual porosity chart of Aguilera and Aguilera (2003).

Matrix porosity: Is the total porosity minus the sum of vug and fracture porosities.

Dry density: Is calculated using the following equation
Dry density = grain density (1-fractional porosity)

RESULTS

Test results of measured and derived properties are summarized in Table 3. The studied stones are arranged in ascending order based on the value of their cementation exponent m. The lower part of Table 3 is a correlation matrix for the different studied properties.

Table 1:	Stratigraphy of Cretaceous to early Tertiary in Jordan
*Units with good potential as sources of building stones

Table 2:	Petrographic classification of the stones studied
? Classes not known in Folk or Dunham’s classifications

Table 3:	Measured and derived properties of the studied stones listed and correlated

Table 4:	A list of significant relationships seen in the correlation matrix and illustrated in Fig. 1 to 17

Fig. 1:	Deriving sonic porosity from Vp

Fig. 2:	Deriving vug porosity from Vp

Fig. 3:	Vp versus total porosity

Fig. 4:	Vp versus modified saturation

Fig. 5:	Vp versus cementation exponent (m)

Fig. 6:	Vp versus matrix porosity

Fig. 7:	Deriving secondary porosity from total porosity

Fig. 8:	Deriving secondary porosity from total porosity

Fig. 9:	Deriving modified saturation from total porosity. Area below best-fit line represents bimodal and polymodal pores

Fig. 10:	Deriving matrix porosity from total porosity

Fig. 11:	Deriving vug porosity from total porosity

Fig. 12:	Deriving sonic porosity from total porosity

Fig. 13:	Deriving fracture porosity from degree of saturation

Fig. 14:	Deriving cementation exponent M from modified saturation

Fig. 15:	Deriving secondary porosity from modified saturation

Fig. 16:	Deriving sonic and matrix porosity from modified saturation

Significant correlations between different parameters as seen in the correlation matrix assume linear best-fit lines (better in case of non-linear relationships) are plotted in Fig. 1 to 17 and summarised in Table 4.

Fig. 17:	Deriving secondary porosity from cementation exponent M

DISCUSSION

When there is no secondary porosity, values of cementation exponent (m) will be equal to or around 2 indicating inter-particle or inter-crystalline porosity. Consequently, the total porosity will be wholly of the matrix type. When m values are less than 2 then part of the total porosity will be secondary and of fracture type, whereas, m values much higher than 2 reflect the existence of secondary porosity in the form of moldic or vuggy porosity. Lucia (1983) considered that the higher the value of m above 2, the higher the ratio of separate vug porosity to total porosity. An idea about the uni- or bi-modality of pore-size distribution can be easily gained from plotting modified saturation against total porosity (Fig. 9). Modified saturation used in the present work is the same as bulk water used in petroleum engineering literature. It is important here to mention that the relationships achieved to in the present work are restricted to clean limestones (with very little dolomite, quartz or clay) and should not be generalized to other lithologies without further experimental work. Also, the database is very small and to be generalized for other areas much larger data bases are needed to check the validity of conclusions arrived at during this work. The importance of the present work lies in pinpointing parameters that have strong correlations with other difficult to measure parameters.

PRACTICAL IMPLICATIONS

In case of clean limestone and when it is difficult to have access to sophisticated equipment the easiest parameter to measure is bulk density, which can be inverted to total porosity (Moh’d, 2002). This can be used to derive cementation exponent, secondary, matrix and vug porosity, as well as, modified saturation. Water saturation can be obtained from total porosity and modified saturation and consequently fracture porosity may be derived. Fracture porosity can also be calculated by subtracting the sum of matrix and vug porosity from total porosity. Permeability can be estimated from total porosity and cementation exponent using Jorgensen’s (1988) equation.

CONCLUSIONS

Despite its limited size, the present database shows that some easily measured parameters can be used in deriving other parameters with correlation coefficients mostly more than 0.9. Thus, Vp can be used to derive sonic porosity and similarly total porosity can give cementation exponent, secondary, matrix and vug porosity, as well as, modified saturation. The latter can be used to derive cementation exponent, secondary and matrix porosities. Degree of saturation is a good indicator of fracture porosity.

There is a need of larger database to check the validity of relationships deduced by the present study and to restrict its use to clean limestone.