Acoustic and petrophysical properties of a clastic deepwater depositional system from lithofacies to architectural elements' scales

摘要

An analysis of acoustic, petrophysical, and stratigraphic heterogeneities has been completed at three scales for an outcropping/subcropping deep-water stratigraphic sequence: lithofacies (core/plug), lithostratigraphic unit (well log), and architectural element (seismic). Measurement techniques/instruments included outcrop measured sections; behind-outcrop drilling/logging/coring (and subsequent core and log analysis); ground-penetrating radar; shallow seismic reflection; and electromagnetic induction. At the lithofacies scale, four rock types are defined: (1) heterogeneous sandstones and (2) uniform sandstones, which differ in their grain composition and sedimentary structures, but do not differ significantly in average porosity, permeability, and acoustic impedance; and (3) organic-rich shales and (4) organic-poor shales, which exhibit significantly higher acoustic impedance than either sandstone type. There is an inverse relation between porosity and permeability versus acoustic impedance of the lithofacies at this scale. At the lithostratigraphic unit scale, three units of interbedded lithofacies are defined: (1) uniform sandstone prone, (2) heterogeneous sandstone prone, and (3) shale prone. Successive merging of thinner beds with thicker beds results in clear differences in average rock properties between the lithostratigraphic units, but there is insufficient variation about the averages to preclude statistically significant differentiation of the sandstones. Lithostratigraphic unit properties also vary laterally. At the architectural element scale, two architectural elements are channel element and lobe element. Only wellbore acoustic impedance differs significantly between these two elements. However, the internal lateral architecture of these two elements is quite different. The results highlight the difficulty in evaluating stratigraphic patterns away from the wellbore. More research in this area is warranted. Attempts to quantify lateral variability of properties in a geologically realistic manner are encouraged because lateral variability is as important to reservoir characterization and performance as is vertical variability.