Page 120 - Petrophysics

POROSITY 93

Secondary Porosity
Secondary porosity is the result of geological processes (diagenesis
and catagenesis) after the deposition of sediment. The magnitude, shape,
size, and interconnection of the pores may have no direct relation to
the form of original sedimentary particles. Induced porosity can be
subdivided into three groups based on the most dominant geological
process:

1. Solutionporosity: channels due to the solution of rocks by circulating
warm or hot solutions; openings caused by weathering, such as enlarged
joints and solution caverns; and voids caused by organisms and later
enlarged by solution.
2. Dolomitization: a process by which limestone is transformed into
dolomite according to the following chemical reaction:

limestone dolomite
(3.2)
2CaCo~ + Mg2+ -+ CaMg(Co3) + Ca2+

Some carbonates are almost pure limestones, and if the circulating pore
water contains significant amounts of magnesium cation, the calcium
in the rock can be exchanged for magnesium in the solution. Because
the ionic volume of magnesium is considerably smaller than that of the
calcium, which it replaces, the resulting dolomite will have greater
porosity. Complete replacement of calcium by magnesium can result
in a 12-13% increase in porosity [5,6].

3. Fracture porosity: openings created by structural failure of the
reservoir rocks under tension caused by tectonic activities such as folding
and faulting. These openings include joints, fissures, and fractures. In
some reservoir rocks, such as the Ellenburger carbonate fields of West
Texas, fracture porosity is important. Porosity due to fractures alone in
the carbonates usually does not exceed 1% [7].
4. Miscellaneous secondary voids: (1) saddle reefs, which are openings
at the crests of closely folded narrow anticlines; (2) pitches and flats,
which are openings formed by the parting of beds under gentle slumping;
and (3) voids caused by submarine slide breccias and conglomerates
resulting from gravity movement of seafloor material after partial
lithification.
In carbonate reservoirs, secondary porosity is much more
important than primary porosity: Dolomites comprise nearly 80%
of North American hydrocarbon reservoirs [6]. Primary porosity is
dominant in clastic-also called detrital or fragmental-sedimentary rocks