NET SAND ANALYSIS IN THINLY BEDDED TURBIDITE RESERVOIRS - CASE STUDY INTEGRATING ACOUSTIC IMAGES, DIPMETERS AND CORE DATA

摘要

Acoustic borehole images often do not provide sufficient resolution for thin bed analysis. In this case study good hole conditions, favourable acoustic contrast and moderate consolidation have resulted in excellent quality images in a multi-layered deep-water reservoir interval. Thinly bedded facies dominate in proximal and distal levee, splay and lobe margin settings. Initial core calibration comprised loading of core photos to the workstation, and detailed depth shifting to match acoustic images. Ultraviolet core photos show good contrast between hydrocarbon-stained sandstones and shales. Histograms of UV staining intensity were generated on a bed-by-bed basis, to which thresholds were applied to generate a continuous sand-shale ratio curve. A manual sand count was then performed integrating acoustic images and dipmeter curves, with a minimum bed resolution of 2-3cm. This works well in bedded intervals, but was particularly difficult where shale is dispersed or image quality is poor. Net sand counts from core, acoustic images and using petrophysical clay volume (VCL) cut-offs were compared by facies association. Core and image log data are in close agreement (within 5% error) apart from shale-clast rich channel lag deposits. Image-derived net sand counts in massive sandstone-dominated facies correspond to conventional log cutoffs of 30% VCL, whereas thinly bedded facies calibrate more closely with cut-offs of 35-50% VCL. Log-derived VCL cut-offs are unable to reproduce the true distribution of net sand, due to thin-bed and bed boundary effects. Moreover, cut-offs are very sensitive to variability of bed thickness, overall NTG and the nature of bed boundaries. A common tendency in conventional log analysis is to underestimate the proportion of thinly bedded sands. This study shows that the effect on volumetric calculations is marginal (10-20% NTG underestimation compared to 20-30% quoted in literature examples). Most of this 'missed' sand is in low NTG very thin beds, partly compensated by an overestimation of net sand in higher NTG dm-scale bedded sands. This is accentuated in proximal levee settings where deformation leads to more bed amalgamation.