Abstract

The Lakadong Member of Sylhet Formation comprise of good reservoir facies within Paleocene-Eocene petroleum system of Upper Assam Basin. A regional sedimentological study is very much essential for better understanding of the environment of deposition and distribution of reservoirs within Lakadong. The present study constitutes an early phase of an upcoming regional study with a focus on the paleo-facies distribution within Lakadong. The approach to this study incorporates the observation results of high resolution electrical image logs along with other conventional open-hole log suite to achieve the objective. The present study reveals that in the north eastern part of the basin, Lakadong Member essentially represents deltaic sedimentation in its bottom part with tidal influenced estuarine environment becoming dominant in the middle part and tidal reworked delta mouth bar in the upper part. Fluctuating energy condition leading to periodic alternations of sand and clay /shale along with increasing intensity of bioturbation towards the top observed within bypassed channel-fill sequence on image log are indicative of tidal influence. Rhythmic sand-clay /shale alternations, foreset dip distribution indicative of ebb and flood current, flaser bedding, drape surfaces and sharp contacts between individual layers are well documented on image log and augment the idea of possible reworking of deltaic clastic deposits by tidal activity. Six distinct sedimentary facies have been identified within Lakadong Member based on textural features and dip pattern observation from the image log. A total of twelve sedimentary facies associations have been observed after vertical integration, of which delta mouth bar appears to be the dominant facies. Dip patterns within cross bedded sand units suggest delta mouth bar progradation direction mainly towards ENE to NE with ESE on a few occasions, there by suggesting paleo-shoreline orientation during deposition of Lakadong sequence to be WNW-SSE. Tidal reworked sands show dominant flood current direction towards SSW. The entire interval of Lakadong is representative of transition from a fluvial dominated delta system to an estuarine environment and finally to a tidally reworked delta system. The presence of tidal reworking has possibly given the wide dispersion of sands within Lakadong.