COMPARISON OF THE MINERALOGICAL, AND CHEMICAL-COMPOSITION OF 2 SHALESFROM THE WESTERN CANADA SEDIMENTARY BASIN AND THE UNITED-STATES GULF-COAST

The mineralogy and geochemistry of shales reflect the composition of t he initially deposited precursor mud, subsequently modified by diagene tic processes. To see if significant geochemical differences exist bet ween shales that mainly owe their present-day composition to either de position or diagenesis, we compare the published mineralogical, bulk a nd clay-fraction geochemical, and clay-fraction O-isotopic composition s of 2 shales. One shale is from the Western Canada Sedimentary Basin (WCSB), and its composition mainly reflects primary (depositional) che mical and mineralogical variations (smectitic to illitic illite/smecti te) within this unit. The other shale is from the United States Gulf C oast (USGC), and its composition mainly reflects mixed-layer illite/sm ectite (I/S) diagenesis of deposited smectitic clay material. The chem ical and mineralogical trends of WCSB and USGC shares, including one o f increasing illite content in I/S with depth or maturity, are essenti ally indistinguishable, in both bulk shale and clay fraction, despite the contrasting genetic interpretations for the origin of the containe d I/S. Thus, similar mineralogical and chemical trends with depth or t emperature can result either from inherited depositional compositional heterogeneity of the sediment, from burial metamorphism of shale or a combination of both. Interestingly, the O-isotopic compositions of th e clay fractions from the WCSB and USGC are significantly different, a fact that reflects original clay formation from source material and w ater of quite different isotopic compositions. The discrimination betw een depositional and diagenetic contributions to shale composition con tinues to pose challenges, but a combination of bentonite, illite poly type, clay isotopic and trace and rare earth elemental analyses togeth er with illite age analysis holds promise for future work.