ABSTRACT

Sixty percent of the supratidal deposits of a well in the giant Yacheng Gas Field show evidence of pedogenic alteration. Pedogenic features of these Mid-Tertiary sediments include porphyroskelic and lattisepic textures, argillans, embayed quartz grains, peds, horizonation, high carbonaceous content, pedotubules, and mottling. Yacheng sediments presently occur in a high-grade diagenetic setting (depth of approximately 4 km and temperatures of about 180°C). Despite mineralogical transformations induced by burial, much of the pedogenic and burial diagenetic history of Yacheng sediments is revealed by detailed mineralogical and geochemical analysis. This analysis also reveals the co-existence of authigenic illite formed by different reaction pathways over differing timetables.

Analytical data indicate that although the clay mineral suite includes only chlorite and illite, each mineral has formed by a variety of processes. Most chlorite in Yacheng sediments is of detrital origin. This is indicated by the similarity in the amount and distribution of Fe and Al in chlorite from schist cobbles and in the sediment clay fraction as determined by quantitative x-ray diffraction (XRD) and analytical electron microscopy (AEM). Authigenic chlorite, while comparatively rare in the studied interval, grew from solution in pore spaces and tends to be significantly poorer in Fe and Mg than detrital chlorite.

Three varieties of illite were recognized by their textural, morphological, and structural characteristics. Detrital illite, the least abundant component, exhibits 2M₁ stacking order by powder XRD. Macroscopic muscovite flakes, the probable parent of clay-sized detrital illite, were dated by K/Ar methods at 207.35 ± 3.95 Ma. Authigenic illite formed by precipitation in pore spaces and by replacement of smectite of pedogenic origin. These two varieties of illite have distinctive textural, morphological, and chemical characteristics.

Pore-growth illite displays euhedral textures in scanning (SEM) and transmission (TEM) electron microscopes and post-dates authigenic quartz and feldspar formation. This type of illite has 1M stacking order, and electron diffraction images show that individual particles have a single coherently diffracting domain down the c* crystallographic axis. By contrast, authigenic illite particles that formed by replacement of smectite have anhedral morphologies in TEM and exhibit a texture similar to that of smectite in petrographic thin sections, SEM, and lattice fringes imaged by high resolution electron microscopy (HRTEM). Matrix illite also displays 1M order by powder XRD but electron diffraction shows that submicron particles are aggregates of coherently diffracting domains that are rando ly stacked along the c* axis. HRTEM lattice fringe images indicate that these diffracting domains average 8-10 unit cells thick. Anhedral illite particles show, on average, higher Al and Fe and lower Si and K by AEM than co-existing pore-growth illite. K/Ar age dates suggest that the integrated time of formation of pore-filling illite is significantly later than the mean illitization time of smectite-derived illite.