Sedimentologic architecture, permeability structure, and fractal characteristics of braided fluvial deposits, Cretaceous Acu formation, Potiguar Basin, NE Brazil.

摘要

The focus of this dissertation is on quantification and prediction of depositional variability of reservoir properties in braided fluvial rocks at the oil field interwell scale. The method of investigation consisted of describing outcrop-defined lithoficies architecture, permeability structure, and fractal attributes and applying these characteristics to simulate an analogous reservoir. Outcrops and an oil reservoir in the fluvial (Cretaceous) Acu sandstone of the Potiguar Basin, NE Brazil were the natural laboratories for this research. Rapid local subsidence, high aggradation rates, and episodic avulsions preserved sealing and baffle facies in this bed-load depositional system.;Megascale heterogeneity includes three hierarchical levels of depositional elements: (1) a sandstone sheet-mudstone layer level, (2) a story level, and (3) a bedset/bed level. At this scale, the higher the hierarchical position of the boundary surface among these elements, the thicker, more continuous and, consequently, more effective the reservoir flow constraint. At the macroscale, heterogeneity is formed by both element boundary and internal lithofacies variation.;Arrangement of depositional elements inside the sandstone sheets defined two styles of sedimentologic architecture: (a) the uninterrupted, fining-upward sandstone sheet style (98 km roadcut) and (b) the sandstone sheet style internally truncated by a channel-belt avulsion surface, with no upward textural tread (99 km roadcut). Sandstone sheets displaying these two architectural styles composed the subsurface laboratory.;The close relationship of lithofacies architecture and permeability structure was confirmed by facies-related fluctuations in semivariograms of sandstone sheet and storey-scale elements. The outcrop permeability dataset allowed categorization of two groups of lithofacies that characterized statistically different reservoir (flow) units. Average porosity and permeability in the subsurface laboratory are, in absolute terms, significantly lower than those in the exposures. In relative terms, similar contrasts between the lithofacies groups were also observed in the subsurface reservoir.;The geometry of the distribution of the two reservoir-units on the exposures (squared panels), which mimics the oil occurrence at reservoir conditions, showed fractal characteristics. The 99 km exposures have a fractal dimension of around 1.70, whereas the 98 km exposures reach 1.94. Fractal simulation algorithms were used to transfer the complex reservoir heterogeneity to unquantified but analogous locations. Conditioned fractal simulations successfully predicted reservoir-unit architecture in braided fluvial reservoir rocks. Qualitative and quantitative appraisals indicated that conditioning is essential for obtaining geologically acceptable simulations.