DIGITAL ANALYSIS OF PORE SPACE CONTROLS ON THE DEVELOPMENT OF SECONDARY MINERAL ASSEMBLAGES IN BASALTIC LAVAS

The relationship between porosity and permeability controls the behaviors of groundwater, geothermal, and petroleum reservoirs developed in basaltic lavas. Recent numerical modeling suggests that pore shape, size, and spacing all play critical roles in determining the dependence of permeability on porosity. In the present study, development of secondary mineral parageneses are used as a monitor of permeability and fluid access to pores and to obtain a quantitative description and interpretation of the evolution of pore space through time. Digital images of thin sections are acquired and processed to map the distribution of pore space and differentiate the development of various stages of mineral paragenesis within the pores. Image analysis is then performed using Scion Image and ArcView® GIS 3.2a to assess aspect ratios, areas and positions of individual features, as well as to perform spatial statistical analysis of the alteration at the thin section scale. The application of this technique is demonstrated on a zeolite facies metabasalt sample from East Greenland, with a total porosity of 13.2 percent, that exhibits incomplete and heterogeneous infilling of gas vesicles by the zeolite chabazite resulting in some pores being completely filled, whereas others contain only partial to no infilling of chabazite. Average aspect ratios of ellipses fit to individual pores are slightly larger for chabazite-filled pores than empty pores (2.02 v. 1.75) although considerable overlap exists between the populations. Average pore areas are 1.06, 1.51, and 0.92 square mm for chabazite-bearing, partially filled, and empty pores, respectively. The standard deviations for each population are similar in magnitude to the averages illustrating the large variation in pore size within the sample. Nevertheless, the slight greater likelihood of secondary mineralization with increasing pore size and ellipticity are consistent with previous modeling of porosity-permeability relationships in basaltic lavas. Theissen polygons calculated about the geometric center of each pore were calculated to assess pore spacing. No relation was found between the pore spacing and the development of secondary alteration, which is consistent with experimental observations of very limited dependence of permeability on porosity.