Importance of Micropores in Deeply Buried Tertiary Sandstones Along the Texas Gulf Coast

Loucks, Robert G.¹, Shirley P. Dutton² (1) The University of Texas at Austin, Austin, TX (2) Bureau of Economic Geology, Austin, TX

Significant evolution of pore networks occurs during burial in sandstones along the Texas GOM. In shallow (<~10,000 ft) sands/sandstones, intergranular and moldic pores and micropores (pores with pore-throat radii of 0.5 microns or less) can all be common. At intermediate depths, intergranular pores become cemented, and moldic pores and micropores dominate. At greater depths (>~20,000 ft), moldic pores become cemented, and the pore network is dominated by micropores. Micropores occur in matrix, authigenic clays, and altered grains. They can be formed by weathering at the source or weathering during transport, or they can be created at various depths through alteration of grains or precipitation of authigenic clays. Volcanic and shale-rock fragments and altered feldspars are commonly micropore rich.

Recognition of micropores is important because the transform between porosity and permeability for micropores is different than for intergranular and moldic pores. Micropores produce less permeability per porosity unit than macropores because of smaller pore throats. They also increase water saturation and capillarity and hence affect petrophysical analysis (e.g., fluid saturation). Calculating permeability of deeply buried sandstones using total porosity values derived from wireline-log analysis or numerical modeling may overestimate permeability and lead to overly optimistic economic assessment. Also, if a seismic porosity anomaly is composed dominantly of micropores, a low-permeability value needs to be assigned to the prospect for an accurate economic evaluation. Estimation of the quantity of micropores in deeply buried prospective sandstones is crucial in assigning risk to a prospect.