Petrographic study of the Therria-Lakadong unit of Upper Assam basin

The Therria - Lakadong unit forms a part of the Upper Assam basin explored by the Oil India Limited. Petrographic study on 47 subsurface samples drawn from 3548.5m to 4377m in five well sections has revealed that the sandston e type is dominantly wacke, minor arenite and calcareous sandstone with qua rtz content varying from 31.4 to 82.7%. The other constituents are feldspar , rock fragments, cement, matrix, mica, opaques, glauconite and fossils. Th e limestone comprises sparry calcite, micrite, bioclasts, peloids, quartz, opaques and glauconite. The heavy minerals recorded are zircon, rutile, tit anite, epidote, garnet, hornblende, enstatite, hypersthene, augite and kyan ite with traces of diopside, sillimanite, brookite and monazite. The heavy mineral assemblage suggests that the sediments were derived from igneous, m etamorphic and sedimentary rocks. Based on IR and XRD studies, the clay min erals were identified as kaolinite, illite, montmorillonite and chlorite, w hich together with glauconite, suggest a nearshore depositional environment . Biosparite indicates high-energy conditions in the environment. Although some of the original porosity is lost at depth due to pressure effects, the sandstone in general has good porosity.