Summary The low permeabilities of Cretaceous and Paleocene tight gas sands in Wyoming and Colorado commonly are related to the presence of high-surface-area authigenic clays. These clays include chlorite, illite, and mixed-layer smectite/illite formed by regeneration of detrital clays or by diagenetic alteration of chemically unstable framework components such as volcanic rock fragments, some feldspars, and ferric oxides and hydroxides. The success of exploration and exploitation drilling programs in tight sands can be enhanced significantly through an understanding of the origins of these clays. Introduction Establishing a successful exploration and production strategy for tight gas sands requires an understanding of why these sandstones are tight. Not surprisingly, the low permeabilities of tight gas sands can be attributed directly to the relative abundance of very small pores. Small pore size can be related to one or more of these factors: fine grain size, low porosity, and the presence of dispersed clays. Siltstones are good examples of the first control. Low porosities most commonly are produced by extensive cementation and are the dominant factor reducing permeability locally in such units as the "J" sand of the Wattenberg field and Hosston formation of Louisiana and Mississippi. In most tight gas sands with porosities in the range of 8 to 15 %, low permeabilities can be attributed to the presence of dispersed clays. Some tight gas sands contain large pores, most of which represent dissolved framework grains, but have low permeabilities because these larger pores are interconnected only through very small pores. Sandstones in the Mesaverde group of western Colorado commonly exhibit this characteristic. This discussion focuses on tight gas sands in which clays are the major mechanism permeability reduction and emphasizes the origin of these clays and how an understanding of their origin can help in the evaluation of exploration and Production potential. Influence of Clays on Permeability Clays vary widely in the morphologies of the individual particles and in their aggregate morphology. These properties affect surface area and relative abundance of micropores (a micropore is defined arbitrarily as a pose less than 1 um in diameter). Kaolinite (upper left, Fig. 1) exhibits a compact book-type morphology and, in many sandstones, tends to occur as clusters present in only a fraction of the pores. Most chlorite (upper right, Fig. 1) displays cardhouse arrangements having significantly higher surface area and microporosity than kaolinite. Mixed-layer smectite/illite (lower left, Fig. 1) commonly develops as honeycomblike aggregates of curled flakes with stubby spines projecting from the edges. It has a surface area and microporosity intermediate between smectite, which has cellular morphology and is not shown since it is uncommon in tight gas sands, and illite, which develops as curled flakes from whose edges highly elongate ribbonlike spines project (lower right, Fig. 1). The morphologies just described directly affect reservoir quality. Fine-grained, well-sorted sandstones without clays exhibit porosity/permeability relationships exemplified by the upper trend in Fig. 2. Sandstones with minor amounts (2 to 10%) of kaolinite, chlorite, and illite follow trends that increasingly diverge from that for clay-free sandstones. Adequate data are not available with which to plot accurately a trend for smectite and mixed-layer smectite/illite, but it is estimated that smectite follows the trend for chlorite and that the trend for mixed-layer smectite/illite is transitional between the trends for chlorite and illite. Clay Types There are two types of clays in sandstones, detrital and authigenic. Detrital clays are introduced into a sandstone by physical processes at the time of deposition or by biogenic processes shortly after deposition. JPT P. 2871^