Analysis of Shale for Shaly-Sand Porosity Computation and Sedimentary Interpretation in Deepwater Sediments

摘要

The shale line on neutron-density crossplots is a linear trend representing the varying ratio of quartz and wet clays in shale. The slope is used to estimate the effective porosity in shaly sands. Total porosity is calculated using a dry-shale line converted from the wet-shale line with the input of core total porosity and dry grain density of the shale samples. The validity of this porosity computation relies on the integrity of the zero-effective-porosity shale rocks and the consistency of the shale line in the reservoirs. In the continuum of shale and shaly sands, such as the thin-beds and debrites presented in this article, integrated geological and petrophysical analysis reduces the uncertainties and errors in formation evaluation and also in sedimentological interpretation using low-resolution logs. In a Gulf of Mexico oilfield, log and borehole image facies interpretations are calibrated in the cored well andextrapolated into the noncored wells. Thick- and thin-bed sands, debrites, conglomerate complexes and a rare sand injectite are interpreted based on image textures, composite log patterns and neutron-density-crossplot analysis. Although the debrites and conglomerate complexes may be sand-rich and heterogeneous, most of them are distributed along the same shale line as the laminated shale, which suggests that they are matrix-supported sediments with negligible effective fluid and that the shale line is consistent in all rocks in the field. In the offshore Brazil example, the MTDs are the only shaly rocks that may represent the shale line. Most of the shale-clast conglomerates are porous oil-bearing reservoirs. The log-curve patterns and crossplot distribution of the sedimentary facies dramatically change due to the low-density wet clays with possibly high clay-bound water.