Abstract

Depositional environment of a sedimentary sequence plays a vital role in the nature of sediments and distribution pattern of sediments. Environment of deposition is generally determined by sedimentological study, wire line log interpretation from well data of specific locations. The spatial extrapolation in to nearby areas can not be done alone by sedimentological study/ log interpretation. There are a number of techniques / tools available to identify the distribution pattern of sediments deposited in different environments of a stratigraphic sequence. Sequence stratigraphy is now a well established technique for determining sediment distribution pattern in a sedimentary sequence deposited in varied depositional environments. This special technique requires an in-depth knowledge of the subject and sufficient palaeo-geological information about the study area. Often, it becomes difficult to conceptualize the basic framework of the sequence stratigraphy method due to limited regional information and scanty data. As for example in fluvio-deltic-shallow marine conditions with active sea level fluctuation coupled with rapid fluvial sedimentation, sporadic palaeogeographic information hampers conceptualization of geological model. In such a situation another technique can help in solving some of the uncertainties in depositional setting. Basic concepts of Sequence stratigraphy coupled with sand (reservoir) shale (nonreservoir) ratio mapping provide excellent result in identifying depositional pattern of reservoirs units. This is a simple, easy and cost effective way to understand the broad depositional setting of sediments without involving high technology/gadgets.

A case study from an area within the petroliferous Upper Assam Basin of Assam-Arakan geological province of India is presented here.

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