Lithofacies and Associated Petrophysical Properties

Lithofacies, Porosity, Permeability

Fundamental to construction of the reservoir geomodel is the population of cells with the basic lithofacies and their associated petrophysical properties- porosity, permeability, and fluid saturation. Petrophysical properties vary between the eight major lithofacies classified (see lower left). Mean and maximum porosities increase with increasing lithofacies number for the limestones (mud- to grainstone; histograms below). In situ (stressed) porosities () were either measured or were calculated from routine helium porosity (_routine) values using the developed correlation: =1.00 _routine-0.68. Permeability is a function of several variables including primarily pore throat size, porosity, grain size and packing (which controls pore body size and distribution), and bedding architecture. Equations were developed to predict permeability and water saturation using porosity as the independent variable because porosity data are the most economic and abundant, and because porosity is well correlated with the other variables for a given lithofacies.In situ Klinkenberg (high-pressure gas or liquid-equivalent) gas permeability (k) exhibits a log-log correlation, or power-law, relationship with porosity though the relationship changes in some facies at porosities below ~6%. Each lithofacies exhibits a relatively unique k-f correlation that can be represented using equations of the form:

At porosities below approximately 6% some facies exhibited higher permeabilities than predicted by the power-law function. For these facies the relationship between permeability and porosity was best represented by an equation of the form: logki = A log₃ + B.Standard error of prediction ranges from a factor of 3.3 to 9.1. At > 6% permeability in grainstone/ bafflestones can be 30X greater than mudstones and >100X greater than marine siltstones of similar porosity. Differences in permeabilities between nonmarine silt/ sandstones and shaly siltstones range from 3.3X at 12% porosity to 7X at 18%. Regression analysis required careful data filtering such as to removed data from fractured samples. Full-diameter cores frequently exhibit permeabilities as great as 50X plug permeabilities due to stress relief fracturing.