Architectural reinterpretation and acoustic facies of the lower portion of the Apiúna unit (Cambrian deep-water depositional system, Santa Catarina State, Brazil)

Abstract

An architectural model of an ancient sand-rich and mixed mud-sand ramp deep-water sedimentary environment, outcropping near Apiuna, Santa Catarina State, Brazil, is presented. This research contributes in the following way to the exploration sector of the petroleum industry: i) with a conceptual model of reservoirs in ramp deep-water systems and ii) with synthetic seismograms, based on the outcrops. Two types of ramp are recognized. One type, sand-rich, is characterized by extensive sandy sheet reservoirs, not linked to proximal channels and showing an apparent casualness of construction. The mechanisms that generated the sandy-sheets were bipartite: a granular flows (debris flows s.l.), in the lower part, and a turbulent flow (turbidity currents), in the upper part. The different velocity and efficiency of the two flows produced a distinct lithological and architectural distribution of the resultant deposits: medium grain size, amalgamated and thick sandstone in the proximal part; fine-very fine grain size and thin sandstone in the distal part. (2) The second type, mixed sand-mud, is characterized by a channel-levee complex and overbank deposits. The two ramp systems represent a vertical variation of the depositional environment, controlled probably by tectonicmorphologic factors. Linked to the architectural model we produced synthetic seismograms. The properties needed for constructing the synthetic seismograms (densities and P-wave velocities) are evaluated from theoretical petrophysical tables. The synthetic seismograms allow defining a seismic forward model and characterizing the acoustic facies of ramp systems.