FORMATION MECHANISM OF THE SHELL BEACH IN THE EAST SEA OF BRAZIL – A CASE OF THE ITAPEMA FORMATION IN THE SANTOS BASIN

Based on the data of the seismic, drilling, cores, logging curves, this paper analyses the seismic reflection structure, drilling lithological, core deposit structure, core and thin section diagenetic porosity and permeability, puts forward the change Changes and cycle characteristics graded phenomenon of sediment grain size of Itapema shell beach in the Santos basin, reveals its formation mechanisms. Results show that the lithology of the Itapema is dominated by coquina, muddy coquina and argillaceous coquina. Bioclastic granule with calcium and bone shell becomes the material base of the shell limestone. Paleo-geomorphology controls the shell beach's areal distribution, while lake level changes effect the spatial shifting and evolution law of shell beach. The shell beach could be divided into three sedimentary microfacies: main beach, beach wing, and slope of front-beach. Their distribution depends on the ancient landforms. The main beaches are located on the underwater paleo-uplift and consist of bivalve shell limestones with the developed holes and hillock seismic facies. Beach wings surround the uplifts, composed of muddy shell limestone. The slopes of front-beach are in the transitional area between the underwater paleo-uplift and the depocenter with shaly shell limestone and mudstone. The evolution and migration of the shell beach are influenced by the lake level changes. During HST the bivalve shell beach develops in the east uplifts; During TST the shell beaches shift to the periphery of east uplifts. During LST distribution of the shell beach gradually expands and small scale shell beaches begin to develop in the eastern depression. Therefore the eastern uplifts and the bulge in central and eastern depression can be regarded as favorable shell beach zones. The results of this study are helpful for offshore oil and gas exploration in this area.