Sandy Debrites and Tidalites of Pliocene Reservoir Sands in Upper-Slope Canyon Environments, Offshore Krishna-Godavari Basin (India): Implications

摘要

A depositional model is proposed for deep-water petroleum reservoir sands (Pliocene) in the Krishna–Godavari Basin, Bay of Bengal, India. Based on examination of 313 m of conventional cores from three wells, five depositional facies have been interpreted: (1) sandy debrite, sandy slump, sandy slide, and sandy cascading flow, (2) muddy slump and debrite, (3) sandy tidalite, (4) muddy tidalite, and (5) hemipelagite. Debrites and slumps constitute up to 99% in one well. Sand injectites are common. Pliocene environments are interpreted to be comparable to the modern upper continental slope with widespread mass-transport deposits and submarine canyons in the Krishna–Godavari Basin. Frequent tropical cyclones, tsunamis, earthquakes, shelf-edge canyons with steep-gradient walls of more than 30°, and seafloor fault scarps are considered to be favorable factors for triggering mass movements. Pliocene canyons are sinuous, exhibit 90° deflections, at least 22 km long, relatively narrow (500–1000 m wide), deeply incised (250 m), and asymmetrically walled. Sandy debrites occur as sinuous canyon-fill massive sands, intercanyon sheet sands (1750 m long or wide and 32 m thick), and canyon-mouth slope-confined lobate sands (3 km long, 2.5 km wide, and up to 28 m thick). Canyon-fill facies are characterized by the close association of sandy debrites and tidalites. Reservoir sands, composed mostly of amalgamated units of sandy debrites, are thick (up to 32 m), low in mud matrix (less than 1% by volume), and high in measured porosity (35–40%) and permeability (850–18,700 mD). Because upper-slope sandy debrites mimic base-of-slope turbidite channels and lobes in planform geometries, use of conventional submarine fan models as a template to predict the distribution of deep-water sand is tenuous.