Architecturally Constrained Spatial Modeling of Permeability within a Fluvial Sandstone Body, Cretaceous Acu Formation, Brazil: ABSTRACT

Abstract

An outcrop of the Cretaceous Acu Formation was investigated as an analog to a heterogeneous group of reservoirs that have significant potential for reserve growth. In the Potiguar Basin of Brazil, the Acu Formation is a 1000-m-thick clastic unit that accumulated within a series of northeast-trending half-grabens during Cenomanian time as the African and South American continental plates separated. The interval consists of an upward-fining cycle of laterally heterogeneous mudstones, sandstones, and conglomerates deposited by a system of braided to meandering streams during an overall transgressive cycle of sedimentation. Architectural and petrophysical attributes of a sandstone body within this succession were investigated and the information used to construct a geologically realistic model of permeability. The outcrop examined exposes a 10-m-thick, 300-m-wide fluvially deposited sandstone body composed of multiple truncating channel storeys. Storeys are 1- to 5-m-thick, 20- to 240-m-wide, and composed of gently dipping beds. A visual comparison of permeability profiles to stratal architecture indicates that (1) permeabilities are reduced one to three orders of magnitude near bed and channel-storey bounding surfaces and (2) beds are characterized by upward-increasing permeability trends. A spatial model of permeability, consistent with the outcrop observations, was constructed using stratal surfaces to define a two-dimensionalmore » chronostratigraphic framework of grids (sequences). Permeability was interpolated between measured values using the sequence boundaries to confine the interpolation. The model will be used as a basis for flow simulation to maximize recovery in the vertically stratified and laterally heterogeneous Acu reservoirs.« less