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Characterization of Deep Water Tidalites: Case Study from Cauvery and Krishna-Godavari Basins, India

Abstract

Tidalites and tidal-rhythmites are considered as exclusive signatures of vertically accreted tidal facies originated in marginal marine (intertidal-subtidal) depositional realm. Similar deposits, represented by mud-rich and sand-rich heterolithic strata to sand-rich unidirectional, bidirectional tidal-bundles are also observed in deep water tidally influenced bottom currents. Literatures depicting characterization of shallow marine tidalites from ancient and modern settings are plenty, but their deep water ancient counterparts are rarely reported. Also such deep marine tidal bottom currents in submarine canyons are extensively documented in modern geologic processes. Despite advancements in recent times, process based understanding of deep water sediment is biased on ‘mass transport deposits’ (MTD), though tidalites constitute a significant part of deep-water deposits. Recognition of such deep-water tidalites using process sedimentology is thus critical in reservoir characterization as facies models are the ultimate goal for deep water exploration. The present paper is focussed on documentation and process based facies interpretation of conventional cores from drilled wells from two different settings, viz. the Early Cretaceous syn-rift to passive margin Cauvery Basin (CB) and the Plio-Pleistocene post-rift passive margin of Krishna-Godavari Basin (KGB). Process interpretations are based on direct examination of the cores that depict overwhelming presence of deep water tidal bottom currents. The syn-rift to passive margin sediments in CB, represented by Andimadam and Bhuvanagiri formations, have been deposited by gravity driven depositional processes (MTD, viz. debris flow, slide-slumps and minor turbidites) and their subsequent reworking by tidal bottom currents in deep water. The syn-rift sequence depicts coarsening-up progradational units, correlated with different rifting episodes that indicated sudden episodic accommodation space creation by mechanical subsidence (faulting) followed by longer periods of tectonic quiescence, unlike the passive margin fills. The syn-rift sediments are thus tectonically induced slope controlled deposits without development of shelf-slope system during Cretaceous and hence are texturally and mineralogically immature. The deep-water tidalites in CB are represented by heterolithic facies, tidal rhythmites, thick (spring)/thin (neap) bundles, alternation of parallel and cross-laminae, double mud layers, climbing ripples, cross-beds with mud-draped foresets, bidirectional and sigmoidal cross-bedding with mud drapes and alternating traction-suspension structures. The Plio-Pleistocene Godavari Clay, on the other hand, has been deposited in a passive margin set-up in KGB with fully developed shelf-slope system and has been interpreted as deposited in a deep water continental shelf-slope setting that incorporates different sandstone units at various stratigraphic levels. These sandstones are deposited as MTD (sandy/muddy debrites with minor slides, slumps and turbidites) and subsequently reworked by tidal currents with development of heterolithic facies consists of flaser, lenticular and wavy beds, tidal bundles and sandstone with double mud layers. The two different settings in CB and KGB thus show ubiquitous presence of deep water tidalites. As the depositional processes are the primary controls on reservoir dimension, geometry and quality in deep-water realm, identification of deep water facies is critical. Due to reworking and winnowing away of finer clastics, tidally influencd bottom currents generate well-sorted sandstones with good porosity and permeability and thus controls the reservoir quality.