A GEOLOGICAL APPROACH TOWARD DEVELOPING A MUDROCK-DURABILITY CLASSIFICATION-SYSTEM

Relationships between durability and lithologic characteristics of 61 mudrock samples from North America were investigated with the objectiv e of developing a mudrock-durability classification based on lithologi c characteristics. Second-cycle slake-durability index (Id2) was used as a measure of durability, whereas clay content, clay-mineral composi tion, texture, microfracture frequency, absorption, adsorption, dry de nsity, void ratio, and Atterberg limits were used to characterize mudr ock lithology. Based upon the amount of clay-size material, the presen ce or absence of laminations, and the degree of induration, the mudroc ks were subdivided into claystones, mudstones, siltstones, shales, and argillites. The relationships between durability and lithologic chara cteristics were investigated separately for each class of mudrocks. Th e results show that different lithologic characteristics best correlat e with the durability of each class of mudrocks. The durability of cla ystones correlates best with the amount of expandable clay minerals an d that of mudstones with the frequency of microfractures. The degree o f consolidation, as expressed by absorption, influences the durability of both siltstones and shales. The durability of argillites is relate d to their crystalline texture. Lithologic characteristics can be quan titatively related to a durability classification proposed herein that recognizes three classes of durability: low (Id2 < 50%), medium (Id2 = 50%-85%), and high (Id2 > 85%).