Lithology porosity

A vast variety of logging tools are In existence and Section 5.4 will cover only those which enable the evaluation of essential reservoir parameters, specifically net reservoir thickness, lithology, porosity and hydrocarbon saturation. 

The formation cuttings that are ohipped off by the bit travel upward with the mud and are caught and analysed at the surface. This provides information about the lithology and qualitative indications of the porosity. 

If appropriate, correlation panels may contain additional information such as depositional environments, porosities and permeabilities, saturations, lithological descriptions and indications of which intervals have been cored. 

The formation bulk density (p ) can be read directly from the density log (see Figure 5.51) and the matrix density (p J and fluid density (p,) found in tables, assuming we have already identified lithology and fluid content from other measurements. The equation can be rearranged for porosity ((])) as follows ... 

With the lithology matching the log scale, and assuming the formation fully invaded by mud filtrate, a neutron porosity and a density porosity can be determined. 

Porosity and Lithology Determination from Litho-Density Log and CNL Compensated Neutron Log... 

Figure 4-303. Porosity and lithology determination from the Litho-Density logs and the CNL Compensated neutron log. Courtesy Schlumberger.) ( trade mark of Schlumberger.)...

The various volumes of rock matrix, shale, porosity, overpressure porosity and hydrocarbons are used to compute the various tool responses according to a model. The responses are compared to the measured values and a volume optimization is made to minimize the errors grouped in an incoherence function. The value of the incoherence function for the best fit determines the quality of the answer. Figure 4-335 is an example of formation pressure calculation as well as formation evaluation for lithology and fluid content. 

The presence of particles in the fluid medium complicates diffusion in a sediment due to the effects of porosity, represented by n, and tortuosity. Since tortuosity of natural sediments is seldom known it is more convenient to use the term "formation factor" or "lithological factor," denoted L, which takes into account everything but porosity. Tick s diffusion constant D is replaced by the whole sediment diffusion constant Ds, where < D. 

A further complication is that the ideal dispersion hemisphere of a gas is prone to distortion. The source may not liberate gas uniformly over time, producing fluctuations in m. The rock and overburden column above the source may comprise lithologies of variable porosity, which may be cut by faults and fractures, and these various voids may be (partially) occupied by liquids, thus producing several different values of p in the column. The voids themselves may be occupied at different times by liquid (usually water) or by gas (usually soil air) of variable barometric pressure, with the result that the capacity of the voids to disperse gases from depth changes with time. 

Physical properties provide a lithological and geotechnical description of the sediment. Questions concerning the composition of a depositional regime, slope stability or nature of seismic reflectors are of particular interest within this context. Parameters like P- and S-wave velocity and attenuation, elastic moduli, wet bulk density and porosity contribute to their solution. 

As the acoustic properties of water-saturated sediments are strongly controlled by the amount and distribution of pore space, cross plots of P-wave velocity and attenuation coefficient versus porosity clearly indicate the different bulk and elastic properties of terrigenous and biogenic sediments and can thus be used for an acoustic classification of the lithology. Additional S-wave velocities (and attenuation coefficients) and elastic moduli estimated by least-square inversion specify the amount of bulk and shear moduli which contribute to the P-wave velocity (Breitzke 2000). 

When compressional, or P, waves excited by a seismic source propagate in an aquifer that is characterized by heterogeneities of different dimensions, such as variations in lithology and vuggy porosity, different regions respond with different fluid pressures. The associated fluid-pressure diffusion attenuates the wave energy. Three types of waves can be identified - a fast and a slow P wave and a shear wave. For the fast P wave, the pore fluid and porous matrix are compressed simultaneously for the slow P wave, the porous matrix relaxes when the pore fluid is compressed. 

Earlier, these lenticular sedimentary features, 50-70 m in thickness, were interpreted as carbonate reefs, although the only basis for this interpretation was the carbonate lithology of the large-size fragments. High porosity of these reservoirs was attributed to fractures and caverns which are so typical of carbonate reefs. 

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