Coastal Morphodynamics and Asymmetric Development of the Godavari Delta: Implications to Facies Architecture and Reservoir Heterogeneity

Facies reconstruction and reservoir modeling of ancient deltas entail possibilities of over, or Under Estimation of reservoir distribution and quality, since such studies are often based on Drill-Hole data from a few locations, which could be potentially unrepresentative of the subsurface conditions. Recent studies indicate significant asymmetries in sedimentation on both sides of the river mouths in the Wave-Influenced systems. The present study is an attempt to analyse the asymmetry in the depositional processes and its implications to the possible reservoir heterogeneity of the Godavari delta.

The Godavari delta on the east coast of India exhibits Wave-Influenced morphologies by the presence of a number of spits and barrier islands at its two active distributary lobes of the Gautami and the Vasishta. The most prominent of them is the 21-km-long Kakinada spit, which in its initial stages in the late nineteenth century had even deflected the Gautami course. As a result, the Gautami has built an extensive bayhead delta behind the spit. The 1965 satellite imagery showed several other smaller spits, especially at the southwestern side of the Gautami mouth and the northeastern side of the Vasishta, whereas barrier islands that are welded to the main coast by the backbarrier mudflats occur on the other sides of the mouths.

A comparison of a series of satellite imagery showed that the 15-km-long shoreline in the central part of the delta in between the Gautami and the Vasishta lobes retreated landward by about 500 m during the past four decades. The material eroded from this part, presumably, drifted in both the Directions - Northeastward toward the Gautami and southwestward toward the Vasishta. As the river effluent plumes act like groynes, the material drifted alongshore from the central part toward both the lobes is deposited at the respective updrift sides of the distibutary mouths forming into spits, whereas the River-Borne sediments are deposited at the downdrift sides, initially as bars that emerge as barrier islands, followed by the Backbarrier-Bay-Filliing with riverine muds. The presence of closely spaced beach ridges on the updrift side, and wide mudflats interspersed with narrow sand bodies on the downdift side, that especially characterize the Vassihta. Iobe, indicate asymmetry of the sedimentation processes and facies architecture. The likely occurrence of better quality reservoir facies on the updrift side than on the downdrift side, besides the presumable Shore-Parallel than Shore-Normal prodelta clinoforms of the bayhead delta in the Kakinada Bay reflect the possible reservoir heterogeneity in the Godavari sedimentary basin, considering that similar asymmetries possibly prevailed in the geological past.