AAPG ACE 2018

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Sedimentary Characteristics and Factors Controlling the Shell Beach in the Itapema Group, the Santos Basin

Abstract

The Shell Beaches in the Cretaceous Itapema Group have been one of the most vital reservoirs in the Santos Basin,they formed in the shallow–semi-deep lake environment. Their facies zone and sequence evolution are quite complex and show significant differences from the overlying Baravelha Algal Stromatolite Reef. Based on core and microscope observations and combined with logging and seismic facies analysis, results suggest that three sedimentary microfacies on the shell beach could be indetified as following: main beach, beach wing, and slope of front-beach. The microfacies distribution is controlled by the ancient landforms. The main beaches were located on the underwater paleo-uplift and consist of bivalve shell limestones with the developed holes and hillock seismic facies, their seismic reflection is chartered by middle amplitude and mid-low frequency and intermittent clutter. Beach wings surround the uplifts, composed of muddy shell limestone, their seismic reflections appear as sill-like and wedge-shaped facies, mid-high amplitude, middle frequency and less continuous parallel reflection. The slopes of front-beach are in the transitional area between the underwater paleo-uplift and the depocenter with shaly shell limestone and mudstone, their seismic reflections present middle amplitude, low frequency and continuous parallel reflection. The evolution and migration of the shell beach are influenced by the lake level changes. During HST the shell beach with bivalve shells develop in the east uplifts; During TST the shell beaches shift to the periphery of east uplifts. During LST distribution of the shell beach gradually expand and small scale shell beaches begin to develop in the central depression and eastern depression. Therefore the eastern uplifts and the bulge in central and eastern depression can be regarded as favorable shell beach zones.